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e roles in different organs that may differ somewhat between species, and also change with developmental time within t

he same species.

Since genes are pleiotropic (genes expressed in the brain are nearly always also expressed in other parts of the body) de

velopmental disorders of genetic origin will inevitably be systemic. While we should not expect to find simple and direc

t mappings between particular genes and specific aspects of behavior, cognition, or brain function, there is no doubt that

the powerful new methods for analyzing segments of the epigenetic pathway from gene to behavior will open new door

s of discovery about the emergence of human brain functions.

Cap. 4 Building a brain

Four-fold increase in volume between birth and teenage years.

An overview of primate brain anatomy

Neocortex: thin flat sheet, layered structure, with sulci and gyri. It is related to the higher cognitive functions.

Thalamus: most of the sensory inputs pass trough it; information between thalamus and cortex is bidirectional.

Glial cells: more common than neurons, important role in the development of the cortex. –

80% of neurons in the cortex are Pyramidal cells (large apical dendrite runs tangential to the surface of the cortex inpu

t processing), neurons with long axons (output processing) going towards cortical and subcortical regions.

Laminar structures of neocortex 6 layers:

[top] 1) long horizontal white fibers, linking one area to others some distance away

2) / 3) horizontal connections, projecting to neighbouring areas of cortex

4) here, most of the input fibers terminate; high proportions of stellate cells on which these projections terminate

[bottom] 5) / 6) major outputs to subcortical regions of the brain (pyramidal cells with long descending axons)

There is no single pattern that can be said characteristic of all cortical regions and regions may vary in the timing of key

developmental events.

Prenatal brain developmental

Conception → rapid process of cell division → blastocyst (proliferating cells) → (few days later) the blastocyst differen

tiates in the embryonic disk.

Embryonic disk:

endoderm → internal organs

1. mesoderm → skeletal and muscular structures

2. ectoderm → skin surface and nervous system

3. a portion of ectoderm folds in on itself and form the neural tube.

Neural tube differentiates along three dimensions:

• → major subdivisions of the central nervous system (forebrain, midbrain and spinal cord).

length

The front end of the neural tube organizes with a series of bulges and circonvolutions forming:

1. cortex (telencephalon)

2. thalamus and hypotalamus (diencephalon)

3. midbrain (mesencephalon)

4. cerebellum (metencephalon)

5. medulla (myelecephalon

• → the dorsal dimension corresponds to the sensory cortex, while ventral dimension corresponds

circumference

to motor cortex.

• → proliferative zones: close to the hollow (cava)

radius portion of the neural tube

Neurons and glial cells are produced by division of proliferating cells within the proliferative zones to produce clones: n

euroblasts produce neurons, glioblasts produce glial cells.

Two form of migration, of neurons, from the proliferative zones to the particular region where they will be employed in

the mature brain:

• passive cell displacement: new cells are pushed away from the proliferative zones by more recently born cells.

• active migration: new cells move past previously generated cells (laminar structure).

Interaction between cells is critic, from the earliest stages (transmission of electric signal between neurons).

Postnatal brain development

“Rise and fall” developmental pattern during postnatal life.

The formation of neurons and their migration to appropriate brain regions takes place almost entirely within the period

th

of prenatal development in the human: the vast majority of neurons are present by around the 7 month of gestation.

While an increase in synapses (synaptogenesis) begins around the time of birth in humans for all cortical areas studied t

o date, the most rapid bursts of increase, and the final peak density, occur at different ages in different areas. In the visua

l cortex there is a rapid burst at 3 to 4 months, and the maximum density of around 150% of adult level is reached

between 4 and 12 months. A similar time course is observed in the primary auditory cortex. In contrast, while synaptoge

nesis starts at the same time in a region of the prefrontal cortex, density increases much more slowly and does not reach

its peak until after the first year.

There is a postnatal growth of synapses, dendrites and fiber bundles, and nerve fibers become covered in a fatty coverin

brain (myeliniation) → increase in size and complexity of the dendritic tree and in

g that adds further to the bulk of the

measures of density of synaptic contacts between cells.

Myelination: increase in the fatty sheath that surrounds neuronal pathways, increasing the efficiency of transmission (sp

eed), although there are under-myelinated or without-myealination connections. Sensory areas tend to myelinate earlier

than motor areas.

PET studies: rise in overall resting brain metabolism after the first year of life, with a peak 150% above adult levels aro

und 4–5 years (spontaneous and intrinsic brain activities in absence of any overt task).

Processes of selective loss have a significant influence on postnatal primate brain development: the absolute rates of glu

cose metabolism rise postnatally until they exceed adult levels, before reducing to adult levels after about 9 years of ag

e. Several regions of the human cortex follow the increase in density of synapses and, then, a period of synaptic loss [H

uttenlocher]. The timing of the reduction in synaptic density varies between cortical regions (synaptic density in the vis

ual cortex returns to adult levels between 2-4 years, while the same point is not reached until between 10-20 years for th

e prefrontal cortex). This initial overproduction of synapses may have an important role in the apparent plasticity of the

young brain, but there's no strong evidence for this pattern of rise and fall in humans. However, in rodents and other vert

ebrates cell loss may be more significant.

glucose reflects the decrease in synaptic contacts → Developmental study with cats [Chugani

[Johnson] The decrease in

et al.]: the peak of glucose uptake (=assorbimento) in a cat’s visual cortex was found to coincide with the peak in overpr

oduction of synapses in this region. In human visual cortex, the peak of glucose uptake lags behind synaptic density.

the same activity may require less “mental effort” once a certain level of skill has been attained.

Another hypothesis:

A number of neurotransmitters in rodents and humans also show the rise and fall developmental pattern: the excitatory i

ntrinsic transmitter glutamate, the inhibitory intrinsic transmitter GABA, and the extrinsic transmitter serotonin all show

this same developmental trend.

The distinctive “rise and fall” developmental sequence is seen in a number of microscopic and metabolic measures of st

ructural and neurophysiological development in the human cortex.

[Gogtay et al.] Cortical gray matter development from 4 to 21 years → heterogeneity between different individuals, and

between different cortical regions; cortical grey matter shows the characteristic “rise and fall” pattern and the eliminatio

n of excess connections between neurons. For some cortical regions most of the rise occurs before puberty, and most of

the decline after puberty going into early adulthood.

The primary sensory areas of cortex, along with the frontal and occipital poles, show the fastest growth (and decline) cu

rves. Most of the remainder (=resto) of the cortex develops in a back-to-front direction, with the prefrontal cortex showi

ng the most delayed curve. The posterior-superior temporal cortex, a critical part of the social brain network that integra

tes information from different sensory modalities, develops last → controversial hypotesis

data on the volume of white matter (myelinated fiber bundles) → general linear increase with age through to early

MRI

adulthood. The lack of a later decline may reflect the ongoing lifelong myelination of fibers that adds to the overall volu

me of the brain.

Although different authors have found the rise and fall pattern, not all measures show it (e.g., myelination, white matte

r), measures such as synaptic density are static snapshots of a dynamic process in which both additive and regressive pr

ocesses are continually in progress (not distinct and separate phases) and there are a lot of individual differences within

the normal range (both in structure and function).

[Tramo and his colleagues] reconstructed the cortical areas of two identical twins from MRI scans (genetically identical

individuals), finding differences in their brain structure and functioning.

[Schneider and colleagues] (fMRI) studied the areas of activation following upper or lower visual field stimulation. Whi

le it had classically been assumed that the upper and lower visual field mapped on to the regions above and below the su

lcus, they found a lot of variation, with some typical subjects showing an upper/lower visual field cut that straddles this

structure. a timetable for “normal” postnatal brain development in humans must be interprete

In typical adults, efforts to construct

d with caution.

[Shaw and colleagues] (MRI) → the trajectory of change in cortical thickness (=spessore), and not the thickness itself b

est predicts a measure of intelligence (IQ): more intelligent children went through a larger and clearer pattern of rise and

years than did those with more average intelligence scores → differences in

fall in their cortical thickness between 7-19

the dynamic changes during development are critical for our understanding of individual differences in intelligence and

cognition in adults.

The Development of Cortical Areas: Protomap or Protocortex?

We can consider cortical differentiation in regions as being like the slices of a cake some of which may contain thicker j

am or cream layers. In the adult primate most of the cortical areas can be determined by very detailed differences in the

laminar structure, such as the precise thickness of certain layers. Often, however, the borderlines between areas are indis

tinct and controversial: these anatomically defined areas have particular unique functions (early sensory and motor area

s), but there are many cases of functional regions or borders that do not neatly correspond to known neuroanatomical di

visions.

“The majority of typical adults tend to have similar functions within approximately the same areas of cortex → the divis

ion of the cerebral cortex into structural and functional areas is tightly (strettamente) genetically prespecified”

This assumption is only partially correct.

The two dimensions of cortical structure (laminar and areal) are not entirely independent, since structural areal divisions

are partly specified by detailed differences in laminar structure. While the basic architecture of a network is innate (basi

c circuitry, learning rules, type and number of cells, etc.), the detailed patterns of (dendritic and synaptic) connectivity a

re dependent upon experience. While the network imposes architectural constraints on the representations that emerge w

ithin it, there are no innate representations.

Several aspects of the structure of the cerebral cortex, including the general laminar structure and large-scale regions, do

not require neural activity to be established → much of the fine-scale division into functional areas involves activity-de

pendent processes.

[Pasko Rakic]'s Radial Unit Model

Most cortical neurons in humans are generated outside the cortex itself, in the “proliferative zones”: these cells must mi

grate to take up their locations within the cortex. The laminar organization of the cerebral cortex is determined by the fa

ct that each proliferative unit (in the subventricular zone) gives rise to about one hundred neurons. The progeny from ea

ch proliferative unit all migrate up the same radial glial fiber, (the latest travelling past their older relatives), which act li

ke a climbing rope to ensure that cells produced by one proliferative unit all contribute to one radial column within the c

ortex. a neuron reaches its final location: a cell “knows” what type of ne

Differentiation into particular cell types occurs before

uron it will become (pyramidal, spiny stellate, etc.) before it reaches its adult location within the cortex. Some evidence

suggests that cells do indeed begin to differentiate before they reach their final vertical location (the information require

d for differentiation is present at the cell’s birth in the proliferative zones, it is not dependent upon their distance from th

e proliferative zones, or on the characteristics of the neighborhood in which that cell ends up).

Although in many cases a cell’s identity may be determined before it leaves the proliferative zones, some of the properti

es that distinguish cell types may form later.

[Marin-Padilla] the distinctive apical (=al vertice/apicale) dendrite of pyramidal cells, which often reaches into layer 1, i

s a result of the increasing distance between this layer and other layers resulting from the inside-out pattern of growth. T

he increasing separation between layer 1 and the subplate zone which results from young neurons moving into what will

become layers 2 to 6 means that cells which have their processes attached to layer 1 will become increasingly “stretche

d”; their leading dendrite will become stretched tangential to the surface of the cortex, resulting in the elongated apical

dendrite so typical of cortical pyramidal cells. This long apical dendrite allows the cell to be influenced by large number

s of cells from other (more superficial) layers.

[Blakemore and colleagues]: the termination of projections from the thalamus in layer 4 is governed by molecular mark

ers. Slices of brain tissue are able to survive and grow in a Petri dish for several days under the appropriate conditions. I

ndeed, pieces of thalamus of the appropriate age will actually grow connections to other pieces of brain placed nearby.

When a piece of thalamus (LGN) and a piece of visual cortex were placed close together in the dish, projection fibers fr

om the LGN not only invaded a piece of visual cortex of the appropriate age, but also terminated in the appropriate laye

r 4. Thus, layer 4 appears to contain some molecular stop signal that tells the fibers to stop growing and form connectio

ns at that location.

[Molnar and Blakemore] The visual thalamus had a piece of visual cortex and some other piece of brain placed nearby.

The thalamic fibers turned out to dislike cerebellum, rarely penetrating it, but they did grow into hippocampus (the gro

wth into hippocampus was not spatially confined in the way it had been for visual cortex).

The laminar structure of cortex probably arises from local cellular and molecular interactions, and it's not a result of thal

fibers “know” in

amic and sensory input (the identity and location of neurons are determined before birth and incoming

which layer to stop and make synaptic contacts).

Division of cortex into areas (opposing possibilities):

• The areal differentiation of cortex is due to a protomap (Rakic, 1988): differentiation into cortical regions occu

rs early in the formation of the cortex, is due to intrinsic factors, and the activity of neurons is not required. The

cortex is a mosaic from the start: each cortical area has individually specified features particularly suitable for t

he input it will receive or the functions it will perform.

• The different areas of cortex arise out of an undifferentiated protocortex: differentiation occurs later in the deve

lopment of cortex, and it depends on extrinsic factors like input from other parts of the brain or sensory system

The activity of neurons is required (Killackey, 1990; O’Leary, 2002). The division of cortex into areas in the

s.

adult brain is influenced by information relayed from the thalamus, and from interactions with other areas of c

ortex via inter-regional connectivity.

Literature on the areal differentiation of neocortex

Support to the protomap view:

• The newborns of a strain of “knockout” rodent that genetically lack connections between the thalamus and the

cortex still have normal, well-defined regional gene-expression boundaries within their cortex and some other

characteristics of wild-type mice.

• In vitro studies in which cortical tissue is maintained in culture, and thus isolated from potential extrinsic patter

ning cues, still show patterns of gene expression consistent with the development of the hippocampus.

Support to the protocortex view:

• Regionalization of cortex could emerge from a combination of different gradients of gene expression. [Kingsbu

ry and Finlay] → “hyperdimensional plaid” (vs “mosaic quilt” – protomap view): the patterning that emerges i

n a plaid is the result of small changes in many threads.

While more than one hundred genes may show graded differential expression between two different cortical areas durin

g development, genetic influence over the layered structure of cortex is much more direct: an autonomous and dissociab

le genetic pathway coordinates the development of the deeper layers of cortex from that which contributes to the mid an

d upper layers.

• Primary sensory regions, despite being the best candidates for genetic prespecification, can have their propertie

s significantly changed through experience: sensory input may be vital at least for the maintenance of cortical d

ivisions.

• Evidence for cortical differentiation prior to birth does not allow us to conclude that neuronal activity is not im

portant: spontaneous neural activity within the brain is important for differentiation.

Recent reviews: mid-way views between the protomap and protocortex hypotheses.

Most agree that graded patterns of gene expression create large-scale regions with combinations of properties that may

better suit certain computations. Within these large-scale regions, smaller-scale functional areas arise through the activit

y-dependent mechanisms associated with the protocortex view (one region may receive particular thalamic input project

ions, overlaid with a certain pattern of neurotransmitter expression, and the presence of specific neuromodulators: this c

ombination of circumstances, combined with neural activity, may induce unique features such as particular patterns of s

hort-range or long range connectivity). Differentiation into smaller areas within the larger regions may occur through th

e selective pruning (=cutting) of connections.

[O’Leary and colleagues] “Cooperative concentration” model: some different gradients of gene expression may act as o

pposing forces in shaping cortical regions.

According to Rakic’s protomap hypothesis, regional differentiation occurs largely through the different numbers of cells

generated in different layers in different regions of the cortex: the primary visual cortex (area 17) in primates has a large

r number of neurons than neighboring regions (such as area 18). This difference results from the number of divisions in

the progenitor cells that are assumed to constitute part of the protomap.

However, recent research shows that fibers from the thalamus enter the zone with the progenitor cells and have a “mitog

enic” effect (the rate of cell division to create new cells is enhanced): fibers from the retina that project only to a specifi

c part of the zone containing cortical progenitor cells may influence the rate of production (and therefore number) of ne

w neurons; the cortical protomap itself may be shaped by input from other regions known to show spontaneous intrinsic

waves of neural activity.

Primary sensory areas receive direct input from the primary sensory thalamic nuclei, and are less susceptible to change

between species than most of the rest of the cortex: the primary visual cortex in primates has unique characteristics that

have led some to propose that it is the most recently evolved part of cortex.

Summary: the basic laminar structure of the cerebral cortex in mammals appears to be very general. Cellular and molecu

lar level interactions determine many aspects of the layered structure of cortex and its patterns of connectivity. Cortical

neurons are often differentiated into specific computational types before they reach their destination. However, this does

not mean that cells in a particular area are prespecified for processing certain kinds of information.

“Barrel fields” in the somatosensory cortex of rodents: each barrel field is an anatomically definable functional groupin

particular whisker (=baffo) on the animal’s snout. Barrel fields are an aspect of the areal stru

g of cells that responds to a

cture of cortex that emerges postnatally, and are sensitive to whisker-related experience over the first days of life. For ex

ample, if a whisker is removed, then the barrel field that normally corresponds to that whisker does not emerge, and nei

ghboring ones may occupy some of the cortical space normally occupied by it. The areal divisions of the cortex arise as

a result of similar divisions in structures closer to the sensory surface. The sensory surface imposes itself on to the brain

stem, thence to the thalamus, and finally on to the cortex itself. The barrel field compartments emerge in sequence in the

se areas of the brain, with those closest to the sensory surface forming first, and the cortex patterns emerging last.

Cortical Plasticity

The nature of the information entering a region of cortex is important in ensuring the maintenance and further progressi

on of this differentiation: neural activity driven by inputs may be able to change the function and detailed neuroanatomy

of a region. Regions of the mammalian cerebral cortex can support a variety of different representations early in develo

pment.

Evidences:

• Reducing the extent of thalamic input to a region of cortex early in life influences the subsequent size of that re

gion. Changing the quantity of cortex available for thalamic innervation changes the overall pattern of cortical

differentiation and not just the affected region.

[Dehay, Kennedy, & Bullier] When the thalamic input to an area of cortex is surgically reduced in newborn macaque m

onkeys, the thalamic projections to the primary visual cortex (area 17) are reduced by 50% → corresponding reduction i

n the extent of area 17 in relation to area 18 (the border between areas 17 and 18 shifts such that area 17 becomes small

er, but its laminar structure remains normal).

The area which is still area 17 looks identical to its normal structure, and the region which becomes area 18 has only ch

associated with that area → Even those cortical areas for which there is some evidence for prespe

aracteristics normally

cification can be subsequently modified.

Even the outputs characteristic of areas 17 and 18 follow the shift in border between them (e.g.: the region which is nor

mally area 17, but becomes area 18 in the surgically operated animals, has the callosal projection pattern characteristic o

f normal area 18) → the region of cortex that would normally mature into area 17 develops properties that are characteri

stic of the adjacent area 18.

[Huffman] When the cortical sheet is surgically reduced (in the opossumembryo), it produces a complete, although smal

ler and distorted, area map.

[Bishop et al.] In genetically altered strains of mice that lack certain regulatory genes, the resulting area map is distorte

d: regions where these genes are normally expressed at high levels become “compressed”while other regions expand →

Gradients of gene expression set up a scaffolding (=intelaiatura) which then interacts with thalamic input to result in fun

ctional areas. However, in atypical situations resulting from surgical or genetic manipulation, thalamic inputs can also s

hape regions that are not their normal targets.

• When thalamic inputs are “re-wired” such that they project to a different region of cortex from normal, the new

recipient region develops some of the properties of the normal target tissue.

Cross-modal plasticity of cortical areas has been demonstrated in several mammalian species: in the ferret [=furetto], da

maging the normal visual cortex and the lateral geniculate (thalamic target of retinal projections), projections from the r

etina can be induced to project to auditory thalamic areas, and thence to auditory cortex (auditory inputs do not innervat

e their normal thalamic target, the medial geniculate). Under these pathological conditions, retinal projections will spont

aneously reroute themselves to the medial geniculate nucleus (MGN) and then to the auditory cortex as normal. Auditor

y cortex becomes visually responsive (cells in what would have been auditory cortex also become orientation- and direc

tion-selective, and some become binocular), but there's no full evidence that the auditory cortex as a whole becomes fun

ctionally similar to the visual cortex (e.g.: visually driven cells in the auditory cortex fire in isolation from the activity o

f their colleagues; there may be no organization above the level of the individual neuron.

[Sur and colleagues] trained adult ferrets, re-wired in one hemisphere at birth, to discriminate between visual and audito

ry stimuli presented to the normal hemisphere. After this they probed the functioning of the re-wired hemisphere by pres

enting visual stimuli that activated only the re-wired pathway. The ferrets reliably interpreted the visual stimulus as visu

al rather than auditory → visual inputs can direct the construction of the appropriate processing circuitry in a region that

does not normally handle visual information.

• When a piece of cortex is transplanted to a new location, it develops projections characteristic of its new locati

on.

[O’Leary and Stanfield] Rodents, pieces of cortex transplanted from one region to another, early in development (visual

cortex neurons transplanted into the sensorimotor region and vice versa) → the projections and structure of such transpl

ants develop according to their new spatial location rather than their developmental origins. Visual cortical neurons tran

splanted to the sensorimotor region develop projections to the spinal cord, a projection pattern characteristic of the sens

orimotor cortex, but not the visual cortex. Similarly, sensorimotor cortical neurons transplanted to the visual cortical reg

ion develop projections to the superior colliculus, a subcortical target of the visual cortex, but not characteristic of the se

nsorimotor region.

[Schlaggar and O’Leary] Barrel fields develop during postnatal growth, and can be prevented from appearing in the nor

mal cortex by cutting the sensory inputs to the region from the face (barrel structure is sensitive to the effects of early ex

perience such as repeated whisker stimulation, or whisker removal).

Pieces of visual cortex were transplanted into the part of the somatosensory cortex that normally forms barrel fields in t

he rodent. When innervated by thalamic afferents, the transplanted cortex developed barrel fields very similar to those n

ormally observed.

Thus, not only can a transplanted piece of cortex develop inputs and outputs appropriate for its new location, but the inp

uts to that location can organize the internal structure of the cortical region.

Limitations:

◦ Most of these studies have involved primary sensory cortices (high cortex may not behave in the same wa

y) and it may be that cortex is only equipotential within a lineage (e.g., primary-to-primary or secondary-t

o-secondary).

◦ While transplanted or rewired cortex may look very similar to the original tissue in terms of function and s

tructure, it is rarely absolutely indistinguishable from the original.

Differential Development of Human Cortex

The main phylogenetic changes in cortical development in primates are in the extent of cortical tissue produced, and the

protracted time course of development in humans. About the developmental of the laminar of the cortex, although most

neurons are in their appropriate locations by the time of birth in the primate, the “inside-out” pattern of growth

cortical

observed in prenatal cortical development extends into postnatal life.

[Conel] The postnatal growth of cortex proceeds in an “inside-out” pattern with regard to the extent of dendrites, dendrit

ic trees, and myelination. The maturation of layer 5 (deeper) in advance of layers 2 and 3 (superficial) seems to be a ver

y reliably observed sequence for many cortical regions in the human infant: for example, the dendritic trees of cells in la

yer 5 of primary visual cortex are already at about 60% of their maximum extent at birth. In contrast, the mean total len

gth for dendrites in layer 3 is only at about 30% of their maximum at birth. Higher orders of branching in dendritic trees

are observed in layer 5 than in layer 3 at birth, but this inside-out pattern of growth is not evident in the later occurring r

ise and fall in synaptic density.

Differential development in human postnatal cortical growth is also evident in the areal dimension. In postnatal develop

ment, there's a clear evidence of a difference in the timing of postnatal neuroanatomical events between the primary vis

ual cortex, the primary auditory cortex, and the frontal cortex in human infants, with the latter reaching the same develo

pmental landmarks considerably later in postnatal life than the first two → this differential development within the cere

bral cortex has not been reported in other primate species. The most likely difference between human and macaque is th

at the protracted postnatal development of humans means that regional differences are more evident. In the macaque, ho

wever, possible regional differences are compressed into a much shorter time, making them harder to detect: the differe

nces between the macaque and human results may be due to the neuroanatomical techniques used; the decline in synapti

c density in the human brain may not differ between regions, but occur simultaneously at puberty.

of the human brain: differential development between regions of cortex →

PET study about functional development

in infants under 5 weeks of age, glucose uptake was highest in sensorimotor cortex, thalamus, brain stem, and the cereb

ellar vermis. By 3 months of age there were considerable rises in the parietal, temporal, and occipital cortices, basal gan

glia, and cerebellar cortex. Maturational rises were not found in the frontal and dorsolateral occipital cortex until approx

imately 6–8 months.

In addition to the formation of dendritic trees and their associated synapses, most fibers become myelinated during post

natal development → Structural MRI images can reveal a clear gray–white matter contrast, and this allows quantitative

volume measurements to be made during development.

6 months → higher than adult water content in both gray and white matter.

Infants under

The appearance of brain structures is similar to that of adults by 2 years of age, and all major fiber tracts can be observe

d by 3 years of age.

After a rapid increase in gray matter volume up to about 4 years of age, there is then a prolonged period of slight declin

e that extends into adult years.

While increases in white matter extend through adolescence into adulthood, particularly in frontal brain regions, the mo

st rapid changes occur during the first 2 years: myelination appears to begin at birth in the pons and cerebellar peduncle

s, and by 3 months has extended to the optic radiation and splenium of the corpus callosum. Around 8–12 months the w

hite matter associated with the frontal, parietal, and occipital lobes becomes apparent.

Postnatal Brain Development: Adolescence

One period of later development that has commonly been associated with “dips” in development is adolescence.

Adolescence is as a phase of human brain development in its own right. At the onset of puberty significant changes begi

n to occur in brain structure and chemistry: continuing myelination of connections and changes in the density of synaps

es, particularly within the prefrontal region of the cortex (a spurt of growth of synapses followed by a period of prunin

g).

Flow of hormones: higher levels of testosterone in boys may result in reduced synaptic pruning and a consequential gre

ater volume of gray matter in certain frontal regions in men (inconsistent evidence: other studies are required).

behavior → Lack of inhibitory c

Adolescence is commonly described as a period of increased impulsive and risk-taking

ontrol, possibly mediated by a “dip” in the functioning of prefrontal cortex? Changes in the brain’s reward network? Inc

reased activity in the brain’s reward network is associated with more risky choices in adults when they are involved in g

ambling-type tasks. Adolescents show greater activity in their reward network (nucleus accumbens) than younger childr

en or adults.

While both impulsive behavior and risk-taking behavior are prevalent in adolescents, they show different developmental

trajectories and have partially different brain bases. Impulsive behavior (lack of inhibition) is related to PFC (pre-frontal

cortex) development and gradually diminishes from childhood to adulthood. Individuals prone to risky behavior (reward

network) are at further risk during adolescence when the brain systems involved in the anticipation of reward are underg

oing developmental changes.

During adolescence many other “executive functions” such as selective attention, working memory, problem solving, an

d multi-tasking improve steadily. While such executive functions are commonly related to the prefrontal cortex, fMRI st

udies indicate that broad networks of cortical regions are involved in these changes.

Some subcortical regions also change their response properties during adolescence (reflection of their interactions with t

he cortex): in adults, amygdala is activated by the perception of fearful facial expressions. While this structure also resp

onds to fearful faces in children of 11 years, it responds equally to neutral faces at this age, suggesting a less finely tune

d function. Gender differences over the adolescent period: in females the response of the amygdala to fearful faces decr

eased during adolescence whereas it did not in males. The opposite pattern was observed for a region of the prefrontal c

ortex → greater increase in the regulation of emotion by females (prefrontal systems).

There are detectable changes in brain structure, particularly in the prefrontal cortex, during and after adolescence: the lo

ss of gray matter in frontal cortex continues up to the age of 30, and white matter volume continues to increase up to 60

years or beyond.

Postnatal Brain Development: The Hippocampus and Subcortical Structures

Hippocampus and cerebellum also show some postnatal development and have been associated with cognitive changes i

n infancy and childhood. There is much behavioral and neural evidence to indicate that subcortical structures are functi

oning at birth, but they all show some evidence of postnatal development and/or functional reorganization.

The neocortex develops postnatally, and its interactions with subcortical regions undergo certain changes. Subcortical re

gions may influence cortical processing more heavily early in development, with cortical networks gradually gaining th

eir independence from subcortical influence during the course of childhood.

The limbic system (amygdala, hippocampus, cingulate gyrus and parahippocampal gyrus) follows the same developmen

tal timetable as other regions of the cortex, but it is differentiated from the rest of the cortex at an early stage, and doesn'

t show the same degree of plasticity: gyral development (folding) associated with the cingulate region is discernible as e

arly as 16–19 weeks of gestational age in humans and the parahippocampal gyrus within the temporal lobe at 20–23 we

eks gestational age. In contrast, other prominent gyri in the cortex do not emerge until 24–31 weeks.

The major nuclear components of the limbic system, such as the hippocampus, start to differentiate from the developing

temporal lobe around the third and fourth months of fetal development.

of the hippocampus → it becomes a rolled structure tucked inside the temporal lobe and surrounded by t

Differentiation

he dentate gyrus. In humans the production (throughout adulthood) of granule neurons is influenced by hormones and, i

n rats, some types of learning enhance the number of new neurons produced.

The cerebellum is a brain structure thought to be involved in motor control, but which probably also plays a role in som

e aspects of “higher” cognitive functioning. Within 2 months after conception the cerebellum has formed its three prima

ry layers, the ventricular (V), intermediate (I), and marginal (M) layers. However, its development is prolonged and neu

rogenesis in this region continues postnatally with only about 17%of the final number of granule cells present at birth, a

nd neurogenesis possibly continuing until 18 months. Despite being one of the few regions of the human brain to show

postnatal neurogenesis, cerebellar functional development as measured by resting PET shows high glucose metabolic ac

tivity as early as 5 days old (postnatal), the same schedule as other sensorimotor regions such as the thalamus, brain ste

m, and sensorimotor cortex.

Neurotransmitters and Neuromodulators

There are developmental changes in the chemicals involved in the transmission and modulation of neural signals too. N

eurotransmitters in the cerebral cortex may be classified into those that arise within the cortex (intrinsic), and those that

arise from outside the cortex (extrinsic).

The intrinsic transmitters can be divided according to whether they have an excitatory effect or inhibitory effect on post

synaptic sites.

Glutamate (intrinsic excitatory transmitter): important role in the axons of pyramidal cells that project to intrinsic cortic

al microcircuits, other cortical regions, and subcortical regions. With postnatal age, the receptors for the transmitter (not

the quantity of the transmitter) increase, following the rise and fall pattern.

GABA (gamma-aminobutyric acid) is probably the most important intrinsic inhibitory transmitter in the mammalian brai

n. In the human, the same overall pattern of rise and fall seen for glutamate is also observed for GABA: the density of G

ABA receptors increases rapidly in the perinatal period and doubles over the first few weeks before later declining. Lev

els of GABA can be influenced by the extent of sensory experience.

Extrinsic neurotransmitters arise from a number of different subcortical locations. fibers follows the “ins

Acetylcholine originates mainly from the basal forebrain: the innervation of cortex by cholinergic

pattern of growth described earlier (deeper cortical layers innervated before the more superficial ones). In hum

ide-out”

ans, this cholinergic innervation begins prenatally, though adult levels are not reached until about 10 years old. Howeve

r, the binding sites within the cortex for this transmitter decrease from birth onwards, possibly due to synaptic pruning.

Norepinephrine (or noradrenaline) which originates in locus coeruleus, has been associated with cortical plasticity: ther

e is an extensive network of noradrenergic fibers in the cortex at birth which may be more dense than in adults.

Serotonin originates in the raphe nuclei, its level increases rapidly over the first few weeks of life (later decrease in hum

an cortex and hippocampus). It is found mainly in the deeper cortical layers at birth, consistent with the structural inside

-out gradient of development discussed earlier.

Dopamine (extrinsic cortical transmitter, which originates in the substantia nigra), likewise shows an inside-out pattern

around the time of birth, at least in rats. Dopaminergic fibers show the adult pattern of projection into the frontal and cin

gulate cortex through extended postnatal development in rats.

Summary:

• Most intrinsic and extrinsic transmitters are present in the cortex at birth, at least in rats and probably also in hu

mans, but show changes in distribution and overall levels for some time after birth.

• Several intrinsic and extrinsic transmitters show the characteristic rise and fall evident in some measures of str

uctural neuroanatomical development. Paucity of human data → it is currently not possible to say to what exte

nt these developmental patterns overlap.

• Several extrinsic transmitters show the same inside-out gradient of cortical development observed in structural

measures.

• Neurotransmitters may play multiple roles during development (e.g.: noradrenaline and cortical plasticity).

• Some transmitters show a differential distribution throughout the cortex, which may play some role in the subs

equent specialization of regions of cortex for certain functions.

What Makes a Brain Human?

Most researchers are interested in how the human mind arises from its underlying brain development, what is unique ab

out the human brain and the developmental processes that give rise to it. Primates generally have a much more prolonge

d timetable for brain development than other mammals. Even between Homo sapiens and other primates there is a wide

difference in timing (differential laminar and regional cortical differentiation). The order of landmarks of brain develop

ment is conserved across a wide range of species. Disproportionately large growth occurs in the late-generated structure

s such as the neocortex when the overall timetable is slowed. The structure most likely to differ in size in the relatively s

lowed neurogenesis of primates is the neocortex.

[Clancy, Finlay and Darlington] Model about the evolution of the brain, including human prenatal development: the mo

re delayed the general time course of brain development in a species, the larger the relative volume of the later developi

ng structures will be → the slowed rate of development in humans is associated with a relatively larger volume of corte

x, and an especially large frontal cortex.

A single round of additional symmetric cell division at the proliferative unit formation stage would have the effect of do

ubling the number of ontogenetic columns, and hence expanding the area of cortex. In contrast, an additional single rou

nd of division at a later stage, from the proliferative zones, would only increase the size of a column by one cell (about

1%). There is very little variation between mammalian species in the layered structure of the cortex, while the total surf

ace area of the cortex can vary by a factor of 100 or more between different species of mammal. It seems likely, therefor

e, that species differences originate in the timing of cell development: the increased extent of cortex in our brains, and p

articularly prefrontal cortex, is at least in part a product of slowing down the overall timetable of brain development.

The delayed time course of human brain development allows a prolonged postnatal period during which interaction wit

h the environment can contribute to the tuning and shaping of circuitry.

General Summary and Conclusions

While many of the landmarks of pre- and postnatal development are similar between humans and other mammals, the ti

ming of human brain development is characterized by being slower and more protracted. According to some theories, th

is slowed development allows the building of relatively more cortex, and particularly frontal cortex. One major feature

of human postnatal development is that brain volume quadruples between birth and adulthood, mainly due to increases i

n nerve fiber bundles, dendrites, and myelination.

Several measures of structure and neurophysiology, such as the density of synaptic contacts, show a characteristic “rise

and fall” during postnatal development. The issue was raised as to whether the differentiation of the neocortex into anat

omical and functional areas is prespecified: the “protomap” hypothesis states that the differentiation of the cortex into ar

the “protocortex” hypoth

eas is determined by intrinsic molecular markers or prespecification of the proliferative zones;

esis states that an initially undifferentiated protocortex is divided up largely as a result of input through projections from

the thalamus and is activity-dependent. A review of currently available evidence supports a middle-ground view in whic

h large-scale regions are prespecified, while the establishment of small-scale functional areas require activity-dependent

processes.

The very extended period of postnatal development seen in the human brain reveals two differential aspects of cortical d

evelopment not as clearly evident in other primates: an inside-out pattern of development of layers and differences in th

e timing of development across regions.

CAP. 5 Vision, Orienting, and Attention

The Development of Vision

Immaturities in peripheral infant's sensory systems place limits on the perceptual capacities of young infants (e.g.: imma

turity of the retina limits spatial acuity). Some have even argued that such limitations are necessary to prevent overwhel

ming developing visual circuits with too much data too soon. Central nervous system pathway development is an import

ant contributing factor in the development of vision and spatial acuity.

Sedated and sleeping infants respond to visual stimulation in some of the same visual cortical regions as adults.

[Taga et al.] (NIRS) In 2- to 4-month-olds viewing dynamic schematic face-like patterns, a localized area of the occipita

l cortex responded to brief changes in luminance contrasts (event-related changes in blood oxygenation) similar to those

seen in adults.

The central visual field of most primates is binocular, requiring integration of information between the two eyes, achiev

ed in the primary visual cortex.

Functional and anatomical structures observed in layer 4 of the primary visual cortex (ocular dominance columns) are i

mportant for binocular vision. These columns arise from the segregation of inputs from the two eyes (neurons in a singl

e ocular dominance column are dominated by input from one eye in adult mammals). They are the stage of processing n

ecessary to achieve binocular vision and, subsequently, detection of disparities between the two retinal images; they hav

e a sensitive period for their formation and are sensitive to the differential extent of input from the two eyes: binocular v

ision develops at the end of the fourth month of life in human infants (visually evoked potential measures found evidenc

e for binocularity around 3 months).

One of the abilities associated with binocular vision, stereoacuity, increases very rapidly from the onset of stereopsis, re

aching adult levels within a few weeks. This is in contrast to other measures of acuity, which increase much more gradu

ally. This very rapid spurt in stereoacuity requires some equally rapid change in the neural substrate supporting it.

The inputs from the two eyes to the cortex are initially mixed so that they synapse on common cortical neurons in layer

4 (projecting to disparity-selective cells). During ontogeny, geniculate axons originating from one eye withdraw (=si riti

ra) from the region, leaving behind axons from the other eye. These events give rise to the sudden increase in stereoacui

ty observed by behavioral measures in the human infant.

There will be a certain degree of integration between the eyes that will decline once each neuron receives innervation fr

om only one eye.

[Held and colleagues] Infants under 4 months can perform certain types of integration between the two eyes that older i

nfants cannot. They presented a grating (=reticolo) to one eye of an infant, and an orthogonal grating to the other eye. In

fants under 4 months perceived a single grid-like representation instead of two sets of gratings that were orthogonal to e

ach other (synaptic inputs from the individual eyes have not segregated into ocular dominance columns). A given cortic

al layer 4 neuron may have synaptic inputs from each eye and will effectively “see” the image from each eye simultaneo

usly. Information from the two eyes would be summed in layer 4, resulting in an averaging of the two signals. Since the

inputs from each eye summate, in the case of the orthogonal gratings a grid was perceived. Older infants do not perceiv

e this grating because neurons in cortical layer 4 of striate cortex only receive inputs from one eye. The inputs to layer 4

have been segregated from one another, and a given layer 4 neuron will receive input from one eye or the other. The los

s of these connections is probably due to the refinement of synapses by selective loss.

Visual processing in the primate brain is initially divided into a subcortical route involving structures such as the superi

or colliculus, pulvinar and amygdala, and several cortical routes that spread out from the LGN (Lateral Genicolate Nucl

eus) and primary visual cortex.

The Development of Visual Orienting

Visual orienting involves moving the eyes and head in response to, or in anticipation of, a new sensory stimulus. Most o

f the tasks involve stimuli that are within the visual capacities for infants and involve a form of action (movements of th

e eyes) which infants can readily accomplish. While there is continuing development of both sensory and motor process

ing throughout infancy, we can focus on the integration between sensory input and motor output.

year of life, is the human infant’s primary method of gathering information from its envir

Visual orienting, over the first

onment. These shifts of gaze allow the infant to select particular aspects of the external world for further study and learn

ing.

[Bronson, 1980s] The early development of vision and visual orienting could be attributed to a transformation from sub

cortical visual processing to processing in cortical visual pathways over the first 6 months of life: the primary (cortical)

visual pathway is not fully functioning until around 3 months postnatal; there is some limited cortical activity in newbo

rns, and the onset of cortical functioning probably proceeds by a series of graded steps, rather than in an all-or-none ma

nner.

There are multiple pathways involved in oculomotor (eye movement) control and attention shifts in the primate brain.

Four brain pathways:

eye → superior colliculus: this subcortical pathway has input mainly from the temporal visual field (peripheral

1. half of the field of each eye), and is involved in the generation of rapid reflexive eye movements to easily discr

iminable stimuli.

Eye → LGN → primary visual cortex/middle temporal area (MT) –

2. superior colliculus: important role in the d

etection of motion and the smooth tracking of moving objects.

– –

3. V1 other parts of the visual cortex frontal eye fields (FEF): thought to be involved in complex aspects of ey

e movement planning (anticipatory saccades and learning sequences of scanning patterns).

4. Inhibition of the superior colliculus (via substantia nigra and basal ganglia): regulate the activity of the collicul

us, forms an integrated system with the frontal eye fields and parietal lobes and plays some role in the regulatio

n of the subcortical oculomotor pathway by the other cortical pathways.

Maturational view: the sequential development of different pathways is related to the onset of new functions.

Skill learning view: the infant’s brain needs to acquire the sensorimotor skill of generating accurate and informative sac

cades.

Interactive specialization view: some of the pathways will initially have poorly defined borders and functions (lack of s

pecialization) and only with experience will they become dissociable.

Sequence of development of the pathways → [Johnson] The characteristics of visually guided behavior of the infant at p

articular ages are determined by which of the pathways are functional, and this is influenced by the developmental state

of the primary visual cortex. The primary visual cortex is the major (but not exclusive) gateway for input to most of the

cortical pathways involved in oculomotor control, it shows a postnatal continuation of prenatal inside-out pattern of cort

ical growth (deeper layers showing greater dendritic branching, length, and extent of myelination than more superficial l

ayers around the time of birth) and there's a restricted pattern of inputs and outputs from the primary visual cortex.

[Johnson]'s hypothesis of development of cortical pathways underlying oculomotor control:

subcortical pathway from the eye directly to the superior colliculus (cortical projections from the deeper layers of V1

to superior colliculus) → cortical projection which inhibits the superior colliculus pathway → pathway through c

ortical structure MT → pathway involving the frontal eye fields and related structures.

Only the deeper layers of the primary visual cortex support organized information-processing activity in the human new

born. While some of the newborn’s visual behaviour can be accounted for in terms of processing in the subcortical path

way, there is also information processing occurring in the deeper cortical layers at birth.

The ability of newborns infants to track a moving stimulus in the first few months of life has two characteristics: the eye

movements follow the stimulus in a saccadic (step-like) manner, as opposed to the smooth pursuit found in adults and ol

der infants, and the eye movements tend to lag behind the movement of the stimulus, rather than predicting its trajector

y. When a newborn infant visually tracks a moving stimulus, it performs a series of saccadic eye movements (subcortica

l control of orienting).

Newborns orient toward stimuli in the temporal, as opposed to the nasal visual field: midbrain structures such as the coll

iculus can be driven most readily by temporal field input. Evidence from studies of infants in which a complete cerebral

hemisphere has been removed (to alleviate epilepsy) indicate that the subcortical (collicular) pathway alone is capable o

f generating saccades toward a peripheral target in the cortically “blind” field.

of age infants show “obligatory attention”: they have great difficulty in disengaging

Around 1 month their gaze from a s

timulus in order to make a saccade to another location, due to the development of tonic inhibition of the colliculus via th

e substantia nigra. By around 2 months of age infants begin to show periods of visual tracking, although their eye move

ments still lag behind the movement of the stimulus; they become more sensitive to stimuli placed in the nasal visual fie

ld and to coherent motion, coinciding with the functioning of the pathway involving structure MT. Associated with furth

er dendritic growth and myelination within the upper layers of the primary visual cortex strengthening the projections fr

om V1 to other cortical areas, around 3 months of age the pathways involving the frontal eye fields may become functio

may increase the ability to make “anticipatory” eye movements and to learn sequences of looking

nal. This development

patterns (frontal eye fields). In this period, infants' eye movements predict the movement of the stimulus in an anticipato

ry manner. They are able to develop expectancies for non-controllable spatiotemporal events

Marker tasks for several cortical regions thought to play a role in oculomotor control, the parietal cortex, frontal eye fiel

ds (FEF), and dorsolateral prefrontal cortex (DLPFC), have been developed, and show rapid development between 2 an

d 6 months of age in human infants.

Marker tasks for the FEF: frontal cortex damage in humans results in an inability to suppress involuntary automatic sacc

ades toward targets, and an apparent inability to control volitional saccades. Since 4-month-olds are able to inhibit sacca

des to a peripheral stimulus, it is reasonable to infer that their frontal eye field circuit is functioning by this age.

The Johnson model of the development of visual orienting could be described as a maturational hypotesis.

In adults ERPs experiments reveal characteristic pre-saccadic components recorded over the parietal cortex prior to the

“spike potential” (SP), a sharp

execution of saccades. The clearest of these components is the pre-saccadic positive-goin

g deflection which precedes the saccade by 8–20 ms. The spike potential is observed in most saccade tasks in adults, an

d represent an important stage of cortical processing required to generate a saccade.

There was no evidence of this component in infant participants: the target-driven saccades performed by 6-month-olds

were controlled by subcortical routes for visually guided responses mediated by the superior colliculus.

Even when the saccade is generated by cortical computation of the location of the next stimulus, as in the case of anticip

atory eye movements, posterior cortical structures do not seem to be involved in the planning of this action.

While there was a lack of evidence for posterior cortical control over eye movements in experiments with 6-month-olds,

there were effects recorded over frontal leads (saccade-related effects), consistent with the frontal eye field disinhibition

of subcortical circuits when a central foveated stimulus is removed: the frontal eye fields helped to maintain fixation on

to foveated stimuli by inhibiting collicular circuits.

When saccades to peripheral stimuli are made, the ERP evidence indicated that these are largely initiated by collicular ci

rcuits, sometimes as a consequence of inhibition being released by the frontal eye fields. The early involvement of the F

EF pathway could be consistent with a skill learning hypothesis (anterior structures activated earlier than posterior circu

its): there is greater involvement of frontal cortex circuitry in infants because they are still acquiring the skill of plannin

g and executing eye movements.

Interactive specialization: the pathways involved in visual orienting are initially less distinct from both surrounding tiss

a new ability associated with the onset of functioning in a “new” area, there

ue and each other. Rather than the onset of

will be widespread changes across one or more pathways associated with new abilities.

One cortical area involved in oculomotor control (experience-dependent) is the parietal cortex (saccade planning in mon

key, human and neuropsychological studies). It is a region of cortex that undergoes marked developmental changes bet

ween 3 and 6 months of age

These cells code for saccades within an eye- or head-centered frame of reference: their receptive fields respond to comb

inations of eye or head position, on the one hand, and retinal distance from the fovea to the target, on the other. This is i

n contrast to parts of the superior colliculus in which cells commonly respond according to the retinal distance and direc

tion of the target from the fovea.

Do the ability of infants to use extra-retinal frames of reference to plan saccades emerge over the first few months of lif

e?

[Gilmore and Johnson] 4- and 6-month-old infants were exposed to two simultaneously flashed targets on a large monit

or screen. The targets were flashed so briefly that they were gone before the infant started to make a saccade to them. In

many trials they made two saccades, the first of these being to the location of one of the two targets. Having made a sac

cade to one of the two targets, it was examined whether the second saccade that they made was to the actual location of

the second target, or whether it was to the retinal location (the location on the retina at which that target had originally a

ppeared). For 4-month-olds, the majority of second saccades were directed to the retinal location in which the target had

appeared. In contrast, for the 6-month-olds, the majority of second saccades were made to the correct spatial location fo

r the other target. These results suggest that the ability to use extra-retinal cues to plan saccades emerges through the firs

t six months of life. However, saccades based on retinal location (subcortical in origin) are probably present from birth.

In sum, while even newborns are capable of simple target-driven saccades, oculomotor skills continue to develop throug

hout the first year. Influential maturational models have been only partially supported by recent ERP and neuroimaging

evidence. The finding that frontal regions may be more important than more posterior areas presents a challenge to the

maturational approach. It is possible that in early development these pathways are initially less segregated than is obser

ved in adults, and therefore may interact and process information in quite different ways.

Visual Attention

Adults are capable of shifting attention covertly (i.e. without moving the eyes or other sensory receptors), enhancing the

processing of some spatial locations or objects within our visual field, to the exclusion of others.

Effect on detection of cueing saccades to a particular spatial location: a briefly presented cue serves to draw covert atten

tion to the location, facilitating detection of targets at that location. While detection of and responses to a covertly attend

ed location are facilitated if the target stimulus appears very shortly after the cue offset, with longer latencies between c

(“inhibition of return”

ue and target, saccades toward that location are inhibited may reflect an evolutionarily important

mechanism which prevents attention returning to a recently processed spatial location). In adults facilitation is reliably o

bserved when targets appeared at the cued location within about 150 ms of the cue, whereas targets appearing between 3

00 and 1300 ms after a peripheral (exogenous) cue result in longer detection latencies.

Following lesions to the posterior parietal lobe, adults show severe neglect of the contra-lateral visual field (damage to t

he “posterior attention network”: brain circuit which includes not only the posterior parietal lobe, but also the pulvinar a

nd superior colliculus). Damage to this circuit is postulated to impair participants’ ability to shift covert attention to a cu

ed spatial location. The parietal lobe is undergoing substantive and rapid development between 3 and 6 months after birt

h.

?Do infants become capable of covert shifts of attention during this time?

Since infants do not accept verbal instruction and are poor at motor responses used to study spatial attention in adults, th

e only response available to demonstrate facilitation and inhibition of a cued location is eye movements. Overt shifts (=

movimenti espliciti) are used to study covert shifts of attention by examining the influence of a cue stimulus (presented

so briefly that it does not normally elicit an eye movement) on infants’ subsequent saccades toward conspicuous target s

timuli.

[Hood and Atkinson] 6-month old infants have faster reaction times to make a saccade to a target when it appears imme

diately after a brief (100 ms) cue stimulus than when it appears in an uncued location. A group of 3-month-old infants di

d not show this effect.

[Johnson] employed a similar procedure in which a brief (100 ms) cue was presented on one of two side screens, before

bilateral targets were presented either 100 ms or 600 ms later. The 200 ms stimulus onset asynchrony (SOA) was short e

nough to produce facilitation, while the long SOA trials should result in preferential orienting toward the opposite side

(inhibition of return), in 4- month-old infants: infants are capable of covert shifts of attention at this age. These effects w

ere not observed in a group of 2-month-old infants.

Another manifestation of covert attention concerns sustained attention, the ability to maintain the direction of attention t

oward a stimulus even in the presence of distractors.

[Richards] Heart rate marker for sustained attention in infants: the heart-rate-defined period of sustained attention usuall

y lasts for between 5 and 15 seconds after the onset of a complex stimulus.

“Interrupted stimulus method” (investigate the effect of sustained attention on the response to exogenous cues): a periph

eral stimulus (a flashing light) is presented while the infant is gazing at a central stimulus. By varying the length of time

between the onset of the central image and the onset of the peripheral stimulus, the peripheral stimulus was presented ei

ther within the period of sustained covert attention, or outside it. During the periods when heart rate was decreased (sust

ained endogenous attention) it took twice as long for the infant to shift their gaze toward the peripheral stimulus as whe

n heart rate had returned to pre-stimulus levels (attention termination). Those saccades made to a peripheral stimulus du

ring sustained attention are less accurate than normal, and involve multiple hypometric saccades (collicular-generated sa

ccades). The lack of distractibility during periods of sustained attention is likely to be due to cortically mediated pathwa

ys inhibiting collicular mechanisms.

Three transitions from child to adults: greater ability to expand or constrict a field of attention, greater ability to disenga

ge attention from distracting information or invalid cueing, and a faster speed of shifting attention.

[Enns and Girgus] tested school-age children and adults in speeded classification tasks involving a stimulus composed o

f two elements that varied in distance (visual angle). Participants had to classify stimuli on the basis of one of the two el

ements. Younger children (6–8 years) experienced more interference when the elements were closely spaced than did ol

der children (9–11 years) and adults. The same stimuli were used for a second task in which both of the elements had to

be taken into account. In this task the younger children had difficulty when the elements were separated by large visual

angles. The authors conclude that younger children have problems in contracting and expanding the size of attentional f

ocus. An ERP study of auditory attention also concluded that there is a development in the ability to narrow attentional f

ocus during childhood.

Older children and adults are able to shift their attention more rapidly than younger ones.

[Pearson and Lane] Younger infants took longer to covertly shift their attention to more peripheral targets, whereas they

were almost as fast as adults to shift to targets very close to fixation → The speed of shifting, rather than the latency to e

licit a covert shift, improves with age.

In several studies, younger children and infants have been argued to have greater difficulty in disengaging from distracti

ng stimuli or invalid spatial cues.

[Enns and Brodeur] Spatial cueing paradigm to cue either neutrally (all locations cued), unpredictably (random cueing),

or predictably (cue predicts target presentation). While all age groups automatically oriented attention to the cued locati

on, the children processed targets in non-cued locations more slowly than did adults, and did not take advantage of the p

redictability of the cues.

[Tipper et al.] These deficits are due to the relative inability to inhibit irrelevant stimuli.

Research on the neural basis of the development of covert attention in childhood has only just begun, and has been base

d on ERP studies, effects of early cortical damage and developmental disorders of genetic origin.

[Richards] Spatial cueing procedure while recording ERPs from infants, to detect neural signatures of covert attention.

He examined the P1 validity effect in young infants (P1: large positive ERP component around 100 ms after stimulus pr

esentation). In adults the P1 is enhanced in scale in valid trials (where the cue correctly predicts the target): this short lat

ency component reflects early stages of visual processing, demonstrating that shifts of covert attention modulate early s

ensory processing of the target.

While there was little ERP evidence for covert attention shifts in 3-month-old infants, by 5 months the pattern of ERP d

ata resembled that in adults, indicating that infants at this age were shifting attention to the cued location covertly.

[Johnson, Stiles et al.] examined spatial cueing in infants who had unfortunately suffered perinatal damage to the cortex.

Posterior lesions that would normally cause deficits in adults had no effect on the infants, while frontal damage had a m

easurable effect on spatial cueing.

[Craft, Schatz et al.] In spatial cueing tasks with children, there were deficits following anterior (frontal) damage, and n

ot (or less) with posterior damage.

ADHD (attention deficit/hyperactivity disorder) is characterized by inattention, hyperactivity, and impulsivity beginning

before 7 years of age (prevalence 3–5% of school children in the USA, varying across cultures). These children appear t

o have mild difficulties in some tests of sustained and selective attention that may reflect difficulties in processing atten

ded stimuli and/or in maintaining attention in tasks that make demands on cognitive resources.

One of the key symptoms of autism in children and adults is atypical attention.

[Courchesne et al.] Most participants with autism have developmental damage to their cerebellum: this damage was ass

ociated with reduced ability to switch attention, and a slower shifting of covert spatial attention.

Another neural deficit observed is bilateral parietal damage → narrowed (=ridotto) focus of spatial attention: targets pre

sented within the narrowed “spotlight” are detected more rapidly than normal and targets presented at small eccentricitie

s from fixation are responded more slowly since they are outside the narrow attentional focus.

?Are deviant patterns of attention a compounded symptom of original deficits in other domains or do initial problems w

ith attention cause some of the other symptoms of the condition?

During the first year of life infants at risk behave differently from groups of control babies in simple visual orienting par

adigms, and we may even identify those particular infants who will go on to be diagnosed when they are 2 or 3 years ol

d. –

CAP. 6 Perceiving and Acting on the Physical World: Objects and Number

Physical objects are not only recognized and categorized, but are also often manipulated using our hands and feet. Com

plex computations and rich representations need to underlie these processes: for example, objects have to be recognized

from multiple different viewpoints, and under conditions where they are partially obscured (partial occlusion) or in front

of complex backgrounds (object parsing). Our fingers and hands have to be adjusted according to the anticipated size an

d weight of the object, and our wrists oriented appropriately to let us grasp the object in the appropriate place and correc

t orientation.

The Dorsal and Ventral Visual Pathways

Visual information processing about objects is divided into two relati

vely separate streams in the brain:

“Where – – pathway”, from the primary vis

*Dorsal route: or Action

ual cortex to the parietal cortex.

All visually guided actions take place in space, but the spatial process

ing required will differ according to the action to be performed. Multi

ple spatial systems → spatial coding shown by neurons in the dorsal s

tream. Some cells in the parietal cortex anticipate the retinal consequ

ences of saccadic eye movements and update the cortical representati

on of visual space to provide continuously accurate coding of the location of objects in space, while others have gaze(=s

guardo)-dependent responses, marking where the animal is looking. Both of these provide egocentric spatial codings, us

eful over very short periods of time since every time the animal moves the coordinates have to be recomputed.

In the real world target objects are often moving: it is not only necessary for us to track that motion in order to localize t

he object in space, but it is also necessary to anticipate the object’s movement. Some cells in the parietal cortex are invo

lved in the tracking of moving objects and continue to respond after the stimulus has disappeared; there is selectivity in

other parts of the dorsal stream for relative motion and size changes when an object moves toward or away from the vie

wer. Cells in the dorsal pathway also code size, shape, and orientation, necessary for the proper reaching for and graspin

g of an object.

The different spatial-temporal systems described above are partially segregated into different regions and routes within t

he dorsal pathway: the dorsal stream could be viewed as a pathway with many parallel computations of different spatial-

temporal properties occurring at once.

[Milner and Goodale] The cells of the parietal cortex are neither sensory nor motor, but rather sensorimotor, involved in

transforming retinal information (sensory) into motor coordinates (motor), and in transducing perceptual input into mot

or actions. “What – – pathway”, from the primary visual cortex through to

*Ventral route: or Perception parts of the temporal lobe

The properties of neurons in the adult ventral stream seem complementary to those in the dorsal stream.

As one proceeds along the ventral stream, cells respond to more and more complex clusters of features: at the higher lev

els, the complex cells show remarkable selectivity in their firing, are all selective to the figural and surface properties of

objects (i.e. the internal object features). Many of them have very large receptive fields on the retina, developing spatiall

y invariant representations of objects by responding to the presence of a consistent feature cluster independently of its p

osition. Some cells seem to respond maximally to a preferred object orientation (view-centered representation), other c

representation → recognition memory and

ells respond equally to an object in any orientation (transformation-invariant

other long-term representations of the visual world). The properties of neurons along the ventral pathway are useful for

recognizing objects, scenes, and individuals with enduring characteristics rather than the moment-to-moment changes in

the visual array that occur in a natural setting.

Motor actions involve localizing targets within a three-dimensional spatial-temporal world, but recognition or identifica

variability be minimized → 2 different systems → 2 pathways

tion of objects requires that spatial-temporal

Maturational viewpoint: Which of the pathways develops first? Can this explain aspects of behavioural development of

and children can perceive coherent forms prior to judging motion coherence → th

object processing? [Atkinson] Infants

e dorsal pathway develops later than the ventral (no evidence from developmental neuroanatomy). In non-human primat

es, there is evidence of ventral pathway functioning from as young as 6 weeks of age. [Rodman and colleagues] Neuron

s within the superior temporal sulcus are activated by complex visual stimuli, including faces, from 6 weeks of age (equ

ivalent data are not available for dorsal pathway functions). Thus, there is currently very little direct evidence relevant t

o the question available

Skill learning perspective inquire into the acquisition of perceptual recognition skills in the ventral pathway, the emerge

nce of sensorimotor integration skill in the dorsal pathway, and investigate whether interactions between processing in t

he two pathways is acquired during development.

Interactive specialization suggests that as the two pathways become more specialized, and acquire representations appro

priate for either recognition or action, they become less interactive and tend to be co-activated less often; the two pathw

ays initially begin inter-mixed and hard to dissociate, but with development the dissociation becomes more complete. Fr

om this perspective, the complementary specialization of the two pathways emerges during the development of the indi

vidual.

Hidden Objects

Piaget reported that infants fail to retrieve objects partly occluded by a surface until 7–8 months, fail to search manually

for fully occluded objects until several months later, and fail to trace the spatial-temporal trajectory of a hidden object to

its final destination until 18 months of age. These developmental changes reflected a conceptual revolution in the first y

of the first true object representation: an “object concept.”

ears of life that results in the construction

Recent studies, relying on measures of looking, rather than manual reaching, have shown that even 4-month-olds percei

ve that a partly occluded object continues behind its occluder. In recent studies using even more enhanced displays, abili

ties to perceive the complete shapes of partly occluded displays have been extended to 3-week-old human infants. Thus,

human perceptual systems rapidly come to detect one kind of invariance in natural scenes: the invariant shape of an obje

ct over changing patterns of occlusion and background.

Adults detect many other invariant properties of objects as well, including the invariant view-dependent shape of an

object over changes in object size and position (some evidence for size and shape constancy has been obtained even wit

h newborn infants).

Might infants be able to represent objects that they cannot see, when they are tested with looking methods?

In looking tasks infants represent fully occluded objects long before they pass Piaget’s object permanence tasks, althoug

h scattered negative findings also have been reported using this method.

Different cognitive neuroscience theories, to resolve this paradox:

(a) Extent of integration of the dorsal and ventral visual pathways.

[Mareschal and colleagues] The discrepancy between looking and reaching tasks is due to a relative lack of integration

between the dorsal and ventral visual pathways in early infancy: object-directed action toward hidden objects requires a

degree of interaction between the two pathways.

Connectionist model, designed to simulate some of the computational properties and development of the dorsal and vent

ral pathways:  Object recognition route (ventral pathway) develops a spatially invariant representation of an object to

which it is exposed.

 learns to predict the next “retinal” position of the object.

Trajectory prediction route (dorsal pathway)

Through recurrent connections, both pathways have some degree of memory.

 Response integration network (infant’s ability to coordinate and use the information about object posit

ion and object identity) integrates the internal representations generated by the two pathways as requir

ed by a retrieval response task.

A prediction of the model has recently been tested: infants viewed two different objects moving behind, and then reappe

aring from behind, an occluding surface. In some conditions these objects were potentially graspable (=afferrabili) toys,

while other conditions involved stimuli more likely to engage ventral pathway function (faces). When the objects reapp

eared following removal of an occluder, they could have either their features switched (e.g., color, or the identity of a fa

ce), or their spatial location, or both. If infants were surprised (looked longer) when just features were changed, this was

taken to indicate ventral (recognition) pathway processing, whereas if surprise was shown when only the location of the

objects changed, this was taken to indicate dorsal pathway retention.

Results: they were able to encode either the surface features,or the spatial location, but not both; infants are only capabl

e of activating either the dorsal or the ventral pathway, but, unlike adults, not both at the same time.

(b) Changing strength of representations of objects in the brain;

(c) Possible inability of infants to plan the necessary actions to retrieve hidden objects.

Neural Oscillations and Object Processing (animal and human research)

A number of electrophysiological studies in animals and scalp-recorded EEG studies in humans have identified sharply

timed bursts of high-frequency oscillatory activity that relate to aspects of visual processing and cognition.

and colleagues] Burst of “gamma” frequency EEG oscillations (40 Hz) when adults bind together spatia

[Tallon-Baudry

lly separate features to compose a single object → such bursts of Gamma EEG reflect the computational process of “per

ceptual binding.”

?At what age are infants able to bind together separate features to compose a unitary object?

[Johnson and colleagues] Infants viewing a Kanisza figure; a group of 8-month-old infants showed a clear burst of Gam

ma activity over left frontal channels at around the same time after the beginning of stimulus presentation as we would e

xpect to see it in adults. This burst was not evident to the control, “pacmen,” stimulus, which could not be bound into a

single object. oscillations sustained when an object is being “kept in mind”?

?Are these high-frequency

This issue was addressed by recording EEG while presenting infants with visual arrays in which objects appeared or dis

appeared behind occluders. A sustained Gamma response over temporal lobe channels was observed when infants saw a

n object pass behind an occluder without reappearing: the response was enhanced when an occluder was removed and t

he object had unexpectedly disappeared. It's possible that active object representations are enhanced (at least temporaril

y) when the visual pathways of the brain are faced with conflicting visual input. Sustained Gamma is a neural signature

of an “active” object representation that needs to be strengthened or weakened (depending on the extent to which it conf

licts with the current visual input).

Number

Two systems for representing number in several animal species:

1. A diverse collection of animals represent the approximate numerosity of large sets of objects and to respond to

changes in numerosity when other continuous variables are controlled (“analog-magnitude” representation: rep

resent quantity by a representation that reflects physical magnitude proportional to the items being enumerate

d). “Numerical” comparisons are made in a similar way to length or time comparisons. Discriminability is prop

ortional to magnitude (1 and 2 are more discriminable than 7 and 8), successful representations are formed onl

y when all members of a set are perceptually available at once or in immediate succession and these representa

tions can be transferred both across modalities (auditory and visual) and across formats (spatial and temporal).

The same mental magnitude system may represent number, time, and the surface area of objects. While the sys

tem is number-relevant, it is probably not specific to the domain of number.

2. Both trained and untrained birds and primates have been found to represent the exact numerosity of very small

sets of objects. This system is an “object-file” system that originally evolved to allow us to track up to about fo

ur moving objects at a time (Carey, 2001). Representations are limited to set sizes of 3 or 4, successful represen

tations can be formed and maintained even when different members appear successively and then are occluded,

these representations are as abstract as large number representations.

Both these systems exist in humans and emerge early in development. By 6 months of age, infants discriminate between

large numerosities when all other continuous variables are controlled, both in visual spatial arrays and in auditory tempo

ral arrays, only when the ratio difference is large.

Young infants discriminate between small numbers of objects exactly, both in visible arrays and in arrays in which each

object is successively revealed and occluded: young infants and newborns can discriminate between 2 and 3 dots, sound

s, or objects, and 5-month-olds can track simple transformations of object arrays, such as addition and subtraction. How

ever, like monkeys, human infants fail to discriminate between large approximate numerosities when elements are succe

ssively revealed and occluded, fail to discriminate numerosities exactly for arrays of more than 3–4 objects, and they for

m representations that are robust over variations in continuous quantities for large but not small numerosities.

Both infants and other animals have separate systems for representing large approximate numerosity and small exact nu

merosity.

Both the systems exist in human adults and are associated with bilateral activity in the inferior parietal lobes:

Evidence for the analog-magnitude system comes from experiments in which adults must rapidly discriminate be

tween numerosities in visual spatial arrays, visual sequences (light flashes), or auditory sequences.

Evidence for the object-file system comes from experiments in which adults must enumerate or attentively track

several objects.

[Cantlon and colleagues] Sequences of visual arrays that varied either in the number of stimuli or in their local shapes w

ere presented to adults and 4-year-old children: adults showed greater activity around their intra-parietal sulcus when th

e arrays deviated in the number of elements than when they deviated in the shape of the elements (non-symbolic and sy

mbolic numerical processing). Interestingly, 4-year-old children with considerably less symbolic knowledge of number

showed very similar patterns of activation → the cortical tissue sensitive to non-symbolic numerical representations exi

sts in early childhood, and provides a neural basis for the acquisition of symbolic number representations during educati

on.

The “school&arithmetic” system of number representation, that has no upper limit, is not constrained by the Weber fract

to language (number words) has been termed an “integer-list” representation.

ion or by perception, and can be related

?How do human children construct this system?

[Spelke and Carey] Children construct a new concept of number and gain arithmetic skills by bringing together their tw

o initial systems of number representation, and language plays a central role in orchestrating these systems.

Evidence: when children learn the number words and the counting routine, they first map “one” to the object-file system

and all other number words to the analog-magnitude system indiscriminately; next, they coordinate the systems together

to learn the meanings of “two” and “three,” while continuing to use all other number words indiscriminately to mean, ro

ughly, “some”. Finally, children surmise that each word in the count sequence refers to an array that includes one more

entity, and a larger overall set size, than the array picked out by the previous word in the count sequence.

When adults make judgments about number words or symbols (e.g., whether a given number word is larger or smaller t

han five), they activate analog-magnitude representation as well as exact numerosity, and therefore judge more quickly

numbers that are more distant from the comparison number → Approximate number representations are activated in tas

ks requiring representations of the number concepts picked out by the counting words.

When bilingual adults learn new information about exact numerosity, their learning is language-specific (longer respons

e times, when the information is queried in the language not used in training); when they learn new information about e

xact numerosity, their learning shows full transfer across languages → Representations of large, exact numerosities dep

end on language for adults, whereas representations of large approximate numerosities do not.

Sometimes children with otherwise normal intelligence show particular problems with arithmetic (dyscalculia): it contra

sts with cases where deficits in mathematical abilities cooccur with low IQ and other cognitive problems. Dyscalculia m

ay not be as specific as originally supposed since it often co-occurs with dyslexia and attention deficit disorders, or may

be a specific developmental syndrome that can even be artifically induced in typical people by electrical stimulation to r

elevant parts of the parietal lobe.

General Summary and Conclusions

In adults, there is evidence that the dorsal and ventral visual pathways perform different computations on objects (ventra

l pathway: identification and recognition; dorsal pathway: actions on objects).

Bursts of high-frequency neural oscillations have been related to the binding of features to compose objects and to the r

etention of objects following occlusion (direct marker for active representations of occluded objects in infants).

Two number-relevant systems in the primate brain were discussed (analog-magnitude system: active in time or length ju

dgments; object-files system: engaged by tracking small numbers of objects).

Both systems appear to be active in young infants (ability to perform simple numerical computations with small number

s of objects). Some suggest that more sophisticated computations with large numbers require language-mediated

integration between these two systems. Overall, the research to date on number suggests that mathematical thinking, an

d perhaps other forms of uniquely human thinking, results from the coordination of functionally and neurologically

distinct subsystems that emerge early in human development and have homologs in other animals.

Developmental disabilities in the domain of mathematics may result either from impairments to one of the subsystems o

r from impairments to the system, related to language, that coordinates their functioning.

CAP. 7 Perceiving and Acting on the Social World - The Social Brain

Some areas of the brain are specialized for processing and integrating sensory information about the appearance, behavi

of other humans. A variety of cortical areas have been implicated in the “social brain,” including the

or, and intentions s

uperior temporal sulcus (STS), the fusiform face area (FFA) and orbitofrontal cortex.

Mentalistic understanding of others’ behavior (theory of mind) has been associated with various neural structures, includ

ing the amygdala (understanding emotions through empathy), the temporal pole (biological motion and actions), the sup

erior temporal gyrus and the temporoparietal junction, and parts of the prefrontal cortex (mainly orbitofrontal and media

l areas: understanding “intentional” referential mental states, including mental states of the self).

Three perspectives about the origins of the social brain:

• Maturational view: through evolution, specific parts of the brain and areas of cortex have become dedicated to

processing social information; some of the circuits are present and functioning at birth, while other component

s of the network become available through maturation later in development. While the maturational timetable

may be accelerated or decelerated by experience, the sequence of maturation and the domain-specific wiring pa

tterns are not.

• Skill learning view: at least some parts of the social brain are engaged by social stimuli because these tend to b

e the visual inputs with which we are most experienced; we develop a higher level of perceptual expertise for s

ocially relevant visual inputs. By this view we should observe parallels between the development of face proce

ssing in infancy and the acquisition of perceptual expertise for other stimuli in adults.

• Interactive specialization: the social brain will emerge as a network that becomes increasingly finely tuned to r

elevant stimuli and events in an activity-dependent manner. Interactions between more primitive brain systems,

cortical areas and the environment produce the end result of a social brain.

Face Recognition

The ability to detect and recognize faces is a good example of human perceptual abilities (basis of our adaptation as soci

al animals).

Face recognition skills: recognizing a face as such, recognizing the face of a particular individual, identifying facial exp

ressions, using the face to interpret and predict the behavior of others.

[Johnson and Morton 1980s] Two apparently contradictory bodies of evidence: while the prevailing view (most of the

evidence) supported the idea that infants gradually learn about the arrangement of features that compose a face over the

first few months of life (skill learning view: face representations resulted from the information structure of the environm

pattern further than various “scr

ent), at least one study indicated that newborn infants (10 minutes old) track a face-like

ambled” face patterns (maturational view: the brain possesses innate representations of faces).

[Johnson et al.] Newborn infants (30 minutes old) were required to turn their head and eyes to keep a moving stimulus i

n view. The authors were unable to replicate preferential head turning to follow the face pattern, but they successfully re

plicated the preferential response to the face using a measure of eye movements. This experiment confirmed that the bra

in of the newborn human infant contains some information about faces. All of the studies found some evidence for sensi

tivity to face-like patterns.

Three hypotheses about the basis of the newborn preference behavior:

• The sensory hypothesis (skill learning view): the visual preferences of newborns, including face preferences, ar

e determined by low-level psychophysical properties of the stimuli. This hypothesis doesn't assume any domai

n-specific bias early in life.

• Newborns have complex face representations (maturational view): newborns already have complex representat

ions of faces. This hypothesis requires domain-specific circuits to be established prior to experience. Newborns

show preference for attractive faces, they are sensitive to the presence of eyes in a face and prefer to orient tow

ard faces with direct eye gaze (=sguardo) [Farroni et al.].

• the newborn’s brain contains a system that biases it to or

Face-biasing system (interactive specialization view):

ient to faces (Conspec [Johnson and Morton]). In contrast to the maturational hypothesis, the bias is presumed t

o be close to the minimum necessary for picking out faces from a natural environment. In contrast to the sensor

y hypothesis, the spatial relations between face features are thought to be important, even though the represent

ation underlying this preference may not exactly map on to a face.

A primitive face-biasing system (Conspec) can account for the vast majority of data currently available on face preferen

ces with newborns: a simple version of the skill-learning view is an explanation for the development of face processing.

Surprisingly, many other studies which used more conventional infant testing methods have not found a preference for f

ace patterns over others until 2 or 3 months after birth.

Infants gradually construct representations of faces as a result of repeated exposure to them over the first few months of

life. These apparently contradictory findings raised a problem for theories of the development of face recognition that in

volved only one process (either learning or innate face representations).

In an attempt to interpret this apparently conflicting behavioural data, Johnson and Morton turned to evidence from:

• Ethology: filial imprinting in the domestic chick (the process by which young precocial birds, such as chicks, r

ecognize and develop an attachment for the first conspicuous object that they see after hatching). While imprin

ting has been reported in the young of a variety of species, only in precocial species we can measure it using th

e conventional measure of preferential approach.

• Brain development.

Filial Imprinting in Chicks

In the laboratory, 1-day-old domestic chicks will imprint onto a variety of objects such as moving colored balls and cyli

nders. After even a few hours of exposure to such a stimulus, chicks develop strong and robust preferences for the traini

ng object over novel stimuli. In the absence of a mother, any conspicuous moving object larger than a matchbox will ser

ve as an imprinting stimulus, and will come to be preferred over any other.

A particular region of the chick forebrain (mammalian cortex) has been shown to be critical for imprinting. Lesions to I

MM (intermediate and medial mesopallium) placed before or after training on an object severely impair preference for t

hat object in subsequent choice tests, but do not affect several other types of visual and learning tasks.

Although the bird forebrain lacks the laminar organization of the mammalian cortex, the relation of the forebrain to sub

cortical structures is similar following the basic higher vertebrate brain design. The avian forebrain is a site of plasticity,

and not the location of inbuilt, automatic, types of behavior which are located in other structures.

IMM occupies about 5% of chick total forebrain volume. Its main inputs come from visual projection areas, and some o

f its projections go to regions involved in motor control: the area is well placed to integrate visual inputs and motor outp

uts.

In the laboratory a wide range of objects, such as moving red boxes and blue balls, are as effective for imprinting as are

more naturalistic stimuli. However, in the wild, precocial birds such as chicks invariably imprint on their mother, and no

t on other moving objects. A series of experiments in which stimulus-dependent effects of IMM lesions were observed,

showed that groups of chicks trained on an artificial stimulus were severely impaired by IMM lesions placed either befo

re or after training on an object. However, groups of chicks exposed to a stuffed hen (=gallina ripiena) were only mildly

impaired in their preference.

[Johnson and Horn] Naturalistic objects such as hens may be more effective at eliciting attention in young chicks than a

re other objects. Chicks have a predisposition to attend toward features of the head and neck region of the hen.

While this untrained preference seemed to be specific to the correct arrangement of features of the face/head, it was not

specific to the species (e.g.: the head of a duck was as attractive as that of a hen).

[Horn et al.] There are two independent brain systems that control filial preference in the chick:

• The first system controls a specific predisposition: chicks orient toward objects resembling a mother hen, tunin

g to the correct spatial arrangement of elements of the head and neck region. The stimulus configuration trigger

ing the predisposition is not species- or genus-specific. Neural basis currently unknown (hypothesis: optic tectu

m, the homolog of the mammalian superior colliculus).

• The second system acquires information about the objects to which the young chick attends and is supported b

y the forebrain region IMM. In the natural environment, the first brain system guides the second system to acq

uire information about the closest mother hen. Biochemical, electrophysiological, and lesion evidence support t

he conclusion that these two brain systems have largely independent neural substrates (selective lesions to IM

M impair preferences acquired through exposure to an object, but they do not impair the specific predispositio

n).

The two systems influence the preference behavior of the chick independently (there is no internal informational exchan

ge between them): the input to the IMM system is selected simply as a result of the predisposition biasing the chick to o

rient toward any hen-like objects in the environment. Given that the species-typical environment of the chick includes a

mother hen in close proximity and that the predisposition includes adequate information to pick the hen out from other o

bjects in the early environment, the input to the learning system will be highly selected.

The chick imprinting story is inconsistent with the skill learning view (the predisposition is present without prior trainin

g, and the learning involves self-terminating plasticity) and with the maturational view (the IMM emerge from surround

ing tissue as a result of experience): the emergence of a simple vertebrate social brain is consistent with the interactive s

pecialization view (brain plasticity is constrained by simple biases, neural architecture and the early environment).

Brain Development and Face Recognition

Visually guided behavior in the newborn infant is largely (though not exclusively) controlled by subcortical structures (s

uperior colliculus and pulvinar), and not until several months of age cortical circuitry comes to dominate subcortical co

ntrol over behavior. Visually guided behavior in human infants, like that in domestic chicks, is based on activity in two

or more distinct brain systems. If these systems have distinct developmental time courses, then they may differentially i

nfluence behavior in infants of different ages.

The recognition of individual faces in adults involves cortical areas and pathways.

Evidences:

a. Neuropsychological patients with brain damage who are unable to recognize faces (prosopagnosia)

Right or bilateral lesion to the region of cortex between the temporal and occipital (visual) cortex → difficulty in recogn

izing individual faces, but ability to identify other objects. Not all face processing is entirely abolished in these patients:

some patients seem to have intact facial emotion processing, and many show “covert” recognition of familiar faces (sen

sitive measures; e.g.: galvanic skin responses).

b. Neuroimaging studies of face perception

A range of cortical regions within the social brain, including regions of the fusiform gyrus, lateral occipital area, and su

perior temporal sulcus are face-specific regions involved in encoding or detecting facial information.

The “fusiform face area” (FFA), a region more activated by faces than by many other comparison stimuli, may play a m

ore general role in object processing.

There is considerable evidence for specific cortical involvement in face processing among adults and for a face preferen

ce in newborn infants, whose behavior is guided largely by subcortical sensorimotor pathways.

[Morton and Johnson] Theory of infant face preferences:

The first system is accessed via the subcortical visuomotor pathway (preferential tracking of faces in newborns); the infl

uence of this system over behavior declines (inhibition by developing cortical circuits) during the second month of life.

The second brain system depends upon a degree of cortical maturity and exposure to faces: it begins to control infant ori

enting preferences from around 2 or 3 months of age.

The newborn preferential orienting system biases the input set to developing cortical circuitry, configured in response to

to gain control over the infant’s behavior. Once this occurs, the cortical system h

a certain range of input, before it starts

as acquired sufficient information about the structure of faces to ensure that it continues to acquire further information a

bout them.

[Johnson and Morton] The face bias (Conspec) was mediated by subcortical visuo-motor pathways. Even in cases of da

mage to the visual cortex, the bias to orient to faces remained. This route rapidly processes low spatial frequency inform

ation about faces, and then modulates activity in the face-sensitive cortical areas that process fine detailed information a

bout faces.

[Johnson and Morton] identified the superior colliculus as a major visuomotor structure that could be involved in deter

mining Conspec preferences, but another candidate structure is the pulvinar (portions of it receive input directly from th

e superior colliculus and there are reciprocal connections to frontal, temporal, and parietal regions and to the anterior ci

ngulate and amygdala)

N170, a negative-going deflection that occurs after around 170 ms, has been strongly associated with face processing in

a number of studies on adults: the amplitude and latency of this component vary according to whether or not faces are p

resent in the visual field of the adult volunteer under study and its response is highly selective (the different response to

human upright faces than to very closely related stimuli such as inverted human faces and upright monkey faces is an in

dex of the degree of specialization of cortical processing for human upright faces).

In a series of experiments a component in the infant ERP that has many of the properties associated with the adult N17

0, but that is of a slightly longer latency, was identified: it is present from at least 3 months of age (its development cont

inues into middle childhood) and it becomes more specifically tuned to human upright faces with increasing age (while

12-month-olds and adults showed different ERP responses to upright and inverted faces, 3- and6-month-olds did not). T

hus, the study of this face-sensitive ERP component is consistent with the idea of increased specialization of cortical pro

cessing with age.

The interactive specialization view predicts increases in both the degree of specialization and localization of face-evoke

d activity in the cortex during development: certain regions of the cortex show reliable activation to faces from at least

mid-childhood.

[Scherf et al.] passive viewing task with children 5–8 years, adolescents 11–14 years and adults: children exhibited simi

lar patterns of activation of the face processing areas commonly reported in adults (FFA). However, this activation was

not selective for the category of face stimuli (the regions were equally strongly activated by objects and landscapes).

[Golarai et al.] children 7–11 years, adolescents 12–16 years and adults: larger right FFA and left parahippocampal volu

mes of selective activation in adults than in children.

The developmental changes observed in these recent studies provide strong support for interactive specialization as a fra

mework within which to interpret functional brain development.

PET study on 3-month-old infants: a large network of cortical areas were activated when infants viewed faces as compa

red to a moving dot array (bilateral activation of the superior and middle temporal gyrus).

Developmental prosopagnosia: individuals who never develop typical adult face-processing abilities (in particular facial

identity recognition skills) in absence of any obvious sensory or intellectual deficit. These patients activate face-sensitiv

e regions of the cortex but the degree of selectivity of this response remains in doubt: while adult developmental prosop

agnosics show activation in face-sensitive regions, they may also show the lack of specificity reported earlier for typical

ly developing children. In some prosopagnosics face perception recruited additional brain areas (e.g., the inferior frontal

gyrus) that were not commonly found in typical adults, but whom activation has been observed in children.

Behavioural performance, in a developmental prosopagnosic patient, improved the selectivity of cortical processing of f

aces (as measured by the N170 component) increased → increased functional connectivity between face-selective regio

ns as measured by fMRI (right occipital face area and the right FFA).

While 6-month-olds could discriminate between individual monkey faces as well as human faces, 9-month-olds and adu

lts could only discriminate the human faces → predicted competence in young infants not evident in adults.

There is some evidence that infants in the first week of life are able to identify their mother. However, this discriminativ

e ability in newborns is based only on the general shape of the head and hair, and not on facial configuration or features.

With regard to face perception, the available evidence from newborns allows us to rule out the skill learning hypothesis,

while the evidence on the neurodevelopment of face processing over the first months and years of life is consistent with

the kinds of dynamic changes in processing expected from the interactive specialization, and not the maturational, appro

ach.

Perceiving and Acting on the Eyes

Perception of averted gaze can elicit an automatic shift of attention in the same direction in adults, allowing the establis

hment of joint attention, crucial for a number of aspects of cognitive and social development, including word learning.

Mutual gaze (eye contact) provides the main mode of establishing a communicative context between humans, and is bel

ieved to be important for typical social development (mother–infant interaction, foundation for social development).

Superior temporal sulcus (STS) is implied in eye gaze perception and processing: the response properties of this region

are highly specialized (the region responds only to non-biological motion).

Gaze cues are able to trigger an automatic and rapid shifting of the focus of the adult viewer’s visual attention. When th

e target appears in the same location where the cue was directed (congruent position), the participant is faster to look at t

hat target compared to another target at an incongruent position relative to the previous cue. Human infants start to discr

iminate and follow adults’ direction of attention at least from the age of 3 or 4 months.

each trial began with the stimulus face’s eyes blinking (to attract attention), before the pupil

[Hood et al.] 4-month-olds;

s shift to either the right or the left for a period of 1,500 ms. A target stimulus was then presented either in the same dire

as the stimulus face’s eyes were looking (congruent position) or in a location incongruent with the direction of gaz

ction

e. By measuring the saccadic reaction time of infants to orient to the target we demonstrated that the infants were faster

to look at the location congruent with the direction of gaze of the face.

However, it is only following a period of mutual gaze with an upright face that cueing effects are observed: mutual gaze

with an upright face may engage mechanisms of attention. The critical features for eye gaze cueing in infants are lateral

motion of elements and a brief preceding period of eye contact with an upright face.

A network of cortical and subcortical regions are engaged in eye gaze processing in adults: it overlaps with, but does not

completely duplicate, the patterns of activation seen in the perception of motion, and the perception of faces in general.

The eye area of the STS appears to be critical: infants' STS may be less finely tuned than in adults.

Since a period of direct gaze is required before cueing can be effective in infants, several authors have investigated the d

evelopmental roots of eye contact detection. It is already known that human newborns have a bias to orient toward face-

like stimuli, prefer faces with eyes opened and tend to imitate certain facial gestures. Preferential attention to faces with

direct gaze would provide the most compelling evidence to date that human newborns are born prepared to detect

socially relevant information.

[Farroni et al.] Healthy human newborn infants were presented a pair of stimuli (face with eye gaze directed straight, fa

ce with averted gaze). The fixation times were significantly longer for the face with the direct gaze. Further, the number

of orientations was higher with the straight gaze than with the averted gaze.

4-month-olds: direct eye contact enhances the perceptual processing of faces.

High-frequency EEG bursting in the gamma (40 Hz) range correlate with the BOLD response used in fMRI.

[Grossmann et al.] Gamma oscillations varied as a function of gaze direction only in the context of an upright face: dire

ct gaze within an upright face also elicited a late (300 ms) induced gamma burst over right prefrontal channels.

The empirical evidence gathered on the development and neural basis of eye gaze processing in infants is consistent wit

h the interactive specialization view: the same primitive representation of high-contrast elements (Conspec) in newborn

s may also be sufficient to direct them toward faces with direct eye contact. Therefore, the more frequent orienting to th

e direct gaze in newborns could be mediated by the same mechanism that underlies newborns’ tendency to orient to face

s in general. This primitive bias ensures a biased input of human faces with direct gaze to the infant over the first days a

nd weeks of life, a bias that helps to lay a further foundation stone for the emerging social brain.

[Johnson] The eye region of the STS does not develop in isolation, or in a modular fashion, but its functionality emerges

within the context of interacting regions involved in either general face processing or in motion detection. While the 4-

month-old has good face processing and general motion perception, it has not yet integrated these two aspects of percep

tion together into adult eye gaze perception. Making eye contact with an upright face fully engages face processing, whi

ch then facilitates the orienting of attention by lateral motion. At older ages, eye gaze perception becomes a fully integra

ted function where even static presentations of averted eyes are sufficient to facilitate gaze.

Understanding and Predicting the Behavior of Others

STS can be activated by dynamic social stimuli from mid-childhood, becoming increasingly specific in its response pro

perties to biological motion with increasing age.

[Lloyd-Fox et al.] (NIRS) 5-month-old infants: patterns of activation consistent with STS activity when children viewed

dynamic social stimuli as compared to dynamic non-social stimuli (not precise degree of specialization of this respons

e). In both STS and FFA, similar processes of emerging functional specialization may occur.

The perception of others’ actions induces subthreshold motor activity in the adult observer (monkeys and humans): mirr

or neuron system (MNS; Rizzolatti & Craighero), in which the same cells and circuits are involved both in generating a

motor action, and in processing the visual input that arises from watching someone else performing a similar action.

[Southgate et al.] used EEG to measure changes in sensorimotor alpha band activity during the course of observing pred

ictably occurring repeated actions in 9-month-old infants. In adults, sensorimotor activity is modulated both by the exec

ution and observation of goal-directed actions and originates in primary somatosensory cortex; infants exhibited subthre

shold motor activity during action observation that matched directly the neural signal occurring during their own similar

actions. Rather than occurring only in response to an observed action, the motor activation was evident prior to the onse

t of the repeated observed action once infants could anticipate its occurrence (motor activation during action observatio

n may reflect a process of anticipating how an action will unfold).

The developing child needs to develop neural and cognitive mechanisms for understanding the behavior of others in ter

ms of their intentions, goals, and desires (“mentalizing” or “theory of mind”): the Medial Prefrontal Cortex (MPFC) is c

onsistently activated when children are engaged in mentalizing.

In children, there was found a positive correlation between age and fusiform gyrus (FG) activity and a negative correlati

on of age with extent of MPFC activity. [Blakemore et al.] reported that adolescent, when thinking about intentions, sho

wed more extensive activity in MPFC than adults, whereas adults activated parts of the right STS more than adolescent

s. With age the extent of MPFC activation during mentalizing tasks becomes more focal with development, whereas acti

vity in (task-dependent) posterior (temporal) cortical areas sometimes increases.

These developmental patterns generally concord with predictions from the interactive specialization view as they involv

e a diffuse to focal transition in prefrontal cortex functional activity during development. The functional involvement of

posterior (temporal) cortex may show more protracted development than that of frontal cortex.

[Sigman et al.] visual shape discrimination task: extensive learning resulted in increased activity in the posterior visual

cortex and decreased activity in the frontal and parietal cortex (large-scale reorganizations of brain activity in the adult c

ortex as reduced executive control and increased automatisation).

[Kobayashi et al.] compared early and late bilinguals (Japanese who also speak English) of similar proficiency in a ment

alizing task: early bilinguals (children 8–12) showed an overlap in brain activity between the first and second language i

n MPFC during a mentalizing task, whereas late bilinguals (adults 18–40) activate more dorsal MPFC in the first langua

ge mentalizing condition and more ventral MPFC during the second language mentalizing condition.

The Atypical Social Brain

Autism

It was initially assumed that there would be a focal structural deficit somewhere in the brain, and that this hole in the bra

in could be associated with a particular pattern of cognitive deficits. Structural brain imaging and postmortem neuroanat

omical studies have variously implicated the brain stem, cerebellum, limbic system, thalamus, and frontal lobes, enlarge

ment of the ventricles, atrophy in adjacent limbic and frontal structures. However, like several of the brain atypicalities t

hat have been reported in autism, ventricular enlargement is not specific to autism.

Deficit in the cerebellum: is unclear whether this is a postnatal effect or whether it is caused by atypical cell migration b

etween 3 and 5 months of gestation. Some evidence for migrational failures in the cortex has also been observed, thoug

h these do not appear to be restricted to any particular area. It is likely that structures and areas that develop latest are m

ost likely to be affected by earlier deviations from the normal developmental trajectory.

The latest region of the cortex to show structural changes in postnatal development, the frontal cortex, has been a favour

ite region of focus for studies of autism and other developmental disorders such as phenylketonuria.

While the brain damage that gives rise to autism may be diffuse and variable, the resulting cognitive profile presents a c

learer picture. Many social processes observed in early childhood appear to be intact in children with autism.

However, they generally show deficits in aspects of mentalizing or “theory of mind” (ability to comprehend another per

son’s thought processes, such as their feelings, beliefs, and knowledge).

“False-belief” task: “Sally has a marble that she puts in a basket. Sally then goes away for a walk. While Sally is away,

from the basket to a box. The participant viewing this scenario is then asked “Wh

Anne comes in and moves the marble

en Sally returns, where will she look for her marble?” If the participant simply tries to predict Sally’s response on the ba

she will answer “the box”. If, on the other hand, she predicts Sally’s res

sis of their own personal knowledge, then he or

ponse on the basis of Sally’s (false) belief, then the participant will correctly predict that Sally will look in the basket for

the marble.”

Another false-belief scenario involves showing the participant a container that normally holds candy well known to the

child and asking the child what is in the container. The participant will reply with the name of the appropriate candy.

The box is then opened and the child shown that it contains a pencil, and not candy. The child is then told that a friend

will come in a moment and be shown the closed container and asked what is in it. The participant is asked what the frie


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DETTAGLI
Corso di laurea: Corso di laurea magistrale in psicologia dello sviluppo e dell'educazione
SSD:
Università: Padova - Unipd
A.A.: 2014-2015

I contenuti di questa pagina costituiscono rielaborazioni personali del Publisher AlessioBellatoOfficial di informazioni apprese con la frequenza delle lezioni di Developmental Cognitive Neuroscience e studio autonomo di eventuali libri di riferimento in preparazione dell'esame finale o della tesi. Non devono intendersi come materiale ufficiale dell'università Padova - Unipd o del prof Farroni Teresa.

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