Riassunto esame Developmental Cognitive Neuroscience, prof. Farroni, libro consigliato Developmental cognitive neuroscience, Johnson

Developmental Cognitive Neuroscience

Cap. 1 - The biology of change

Viewpoints on development

From birth to adolescence, there is a four-fold increase in the volume of the brain, with changes in behavior, thought, and emotion. Developmental cognitive neuroscience has emerged at the interface between two of the most fundamental questions that challenged humankind:

• Relation between mind and body, between the physical substance of the brain and the mental processes it supports (cognitive neuroscience).
• Origin of organized biological structures, such as the highly complex structure of the adult human brain (study of development → phylogeny: Darwin and the origin of species) vs ontogeny (individual development within a life span).

Once a particular set of genes have been selected by evolution, ontogeny is simply a process of executing the “instructions” coded for by those genes: the ontogenetic question essentially reduces to phylogeny. Johnson argues that ontogenetic development is an active process through which biological structure is constructed afresh in each individual by means of complex and variable interactions between genes and their environments. The information is not in the genes, but emerges from the constructive interaction between genes and their environment.

Nature–nurture issue

At one extreme, most of the information necessary to build a human brain, and the mind it supports, is latent within the genes of the individual (most of the information is common to the species, each individual has some specific information that will make them differ from others): development as a process of unfolding or triggering the expression of information within the genes.

At the opposing extreme, most of the information that shapes the human mind comes from the structure of the external world (some facets of the environment will be common throughout the species, while other aspects of the environment will be specific to the individual).

Both of these views assume that the information for the structure of an organism exists (either in the genes or in the external world) prior to its construction. In contrast to this, the biological structure emerges anew within each individual’s development from constrained dynamic interactions between genes and various levels of environment, and is not easily reducible to simple genetic and experiential components. Mental abilities of adults seem the result of complex interactions between genes and environment.

Historical perspectives on the nature–nurture debate

17th century debate in biology:

• Ontogenetic change was driven by “vital” life forces.
• Vitalists: a complete human being was contained in either the male sperm (“spermists”) or the female egg (“ovists”). There was a simple and direct mapping between the seed of the organism and its end state (simultaneous growth of all the body parts). There is a fixed mapping between a pre-existing set of coded instructions and the final form.
• A child’s psychological abilities could be entirely shaped by its early environment; recently some developmental psychologists have suggested that the infant’s mind is shaped largely by the statistical regularities latent in the external environment (statistical learning).
• Constructivism: biological structures are viewed as an emergent property of complex interactions between genes and environment. [Jean Piaget]: the relationship between the initial state and the final product can only be understood by considering the progressive construction of information, a dynamic process to which multiple factors contribute. Genes and environment combine in a constructive manner such that each developmental step will be greater than the sum of the factors that contributed to it.

Analyzing development

Innate vs Acquired

“Innate” is simply no longer useful since it has become evident that genes interact with their environment at many levels, including the molecular. There's the need to describe the interaction between factors intrinsic to the developing child and features of the external environment. [Johnson and Morton] Various levels of interaction between genes and their environment:

• Molecular internal environment
• Cellular internal environment (innate: level of the interaction between genes and environment, not the source of the information)
• Organism-external environment species-typical environment (primal): common aspects of the external environment
• Individual-specific environment (learning): unique aspects of the environment

[Greenough, Black, Wallace]

• “Experience-expectant” information storage (=species-typical): changes induced by aspects of the environment that are common to all members of a species → selective synaptic loss.
• “Experience-dependent” information storage (=individual-specific): interactions with the environment that are specific to an individual → generation of new synaptic connections.

Different aspects of brain structure and function are probably differentially sensitive to the effects of postnatal experience. The human brain is composed of very complex neural circuits bathed in a variety of chemicals that can regulate and modulate function. Connectionist neural network models involve nodes (simplified neurons) and links that can vary in strength (simplified synapses and dendrites). Learning in such networks takes place by varying the strength or extent of connections between nodes according to learning rules, some of which approximate those thought to be used in real brains.

There are several ways it could be sensitive to training:

• The basic architecture of the network could alter as a result of experience (change in the number of nodes, learning rule, extent to which the nodes are interconnected).
• While the basic architecture of the network is fixed, the strength of the connections between the nodes varies according to a weight-adjustment learning rule (this is the way that most connectionist neural networks encode information): different representations may emerge as a result of experience → details of microcircuits and synaptic efficacy.
• Innate architecture: the representations that emerge as a result of training are constrained by the architecture of the network.
• Both the basic architecture of the network and the patterns and strengths of links between nodes are innate, and thus insensitive to external input (innate representations: there is little evidence that the human neocortex possesses them).
• Both the architecture and the detailed pattern and strength of links are malleable as a result of training (this is possible only under extremely atypical environmental conditions or in cases of genetic atypicality).

Why take a cognitive neuroscience approach to development?

Until the past decade, the majority of theories of perceptual and cognitive development were generated without recourse to evidence from the brain. Some authors argued strongly for the independence of cognitive-level theorizing from considerations of the neural substrate. Our understanding of brain function has improved significantly over the past twenty years or so.

Cognitive neuroscience: evidence about brain development (neuroanatomy, brain imaging, behavioral or cognitive effects of brain lesions) but also evidence from ethology (study of a whole organism within its natural environment). Powerful new methods and tools (biological basis of cognitive and perceptual development), theories which incorporate and reveal relationships between brain structures and cognitive functions (effects of early brain injury or genetic disorder on cognitive development), evidence derived from infants with congenital and acquired brain damage (theories about functional specification, critical periods, and plasticity in the brain).

Why take a developmental approach to cognitive neuroscience?

Ontogenetic development: constructive process by which genes interact with their environment at various levels to yield complex organic structures such as the brain and the cognitive processes it supports. The study of development must be multidisciplinary.

The cause of developmental change

Predetermined epigenesis: unidirectional causal path from genes to structural brain changes to brain function and experience (maturational view of developmental psychology → infants have reduced versions of the adult mind which increase by steps as particular brain pathways or structures mature).

Probabilistic epigenesis: bidirectional interactions between genes, structural brain changes, and function (constructivist view of development → dynamic relations between intrinsic and extrinsic structure progressively restrict the phenotypes that can emerge). The infant mind is viewed as being comparable to adults with focal brain injury (specific cognitive mechanisms are either present or absent at a given age; circuits that support components of the adult system are assumed to come “online” at various ages).

Probabilistic epigenesis approach to biological development: development involves the progressive restriction of fate. Early in development, a system has a range of possible developmental paths and end states, dependent on the particular sets of constraints that operate.

[Waddington → influenced Piaget] There are developmental pathways, or necessary epigenetic routes, the “chreods” (valleys in an epigenetic landscape). Self-regulatory processes ensure that the organism (a ball rolling down the landscape) returns to its channel following small perturbations. Large perturbations (e.g.: live in darkness) can result in a quite different valley route being taken, especially when these occur near a decision point (regions of the epigenetic landscape where a small perturbation can lead to a different route being taken). While for the typically developing child the same endpoint will be reached despite the small perturbations that arise from slightly different rearing environments, a deviation from the normal path early in development, at a decision point, or a major perturbation later in development, may cause the child to take a different developmental path and reach one of a discrete set of possible alternative end states (phenotypes).

Developmental disorders are possible developmental trajectories that are responses to different sets of constraints: from the moment when the developmental trajectory deviates from the normal one, a variety of new factors and adaptations will come into play (reorganization of brain functioning).

Three viewpoints on human functional brain development

Remarkable changes in motor, perceptual, and cognitive abilities during the first decade or so of human life are related to neuroanatomical changes during the development of the brain.

1. Maturational perspective

The goal is to relate the “maturation” of particular regions of the brain cortex to newly emerging sensory, motor, and cognitive functions. Evidence concerning the differential neuroanatomical development of brain regions is used to determine an age when a particular region is likely to become functional (e.g.: success in a new behavioral task at this age may then be attributed to the maturation of this “new” brain region). Functional brain development is the reverse of adult neuropsychology, with the difference that specific brain regions are added-in instead of being damaged.

Limits: it does not successfully explain some major aspects of human functional brain development: associations between neural and cognitive changes based on age of onset can be weak due to the great variety of neuroanatomical and neurochemical measures that change at different times in different regions of the brain.

2. Interactive specialization

Postnatal functional brain development, at least within the cerebral cortex, involves a process of organizing patterns of inter-regional interactions [Johnson]; the response properties of a specific region are partly determined by its patterns of connectivity to other regions, and their patterns of activity. During postnatal development, changes in the response properties of cortical regions occur as they interact and compete with each other to acquire their role in new computational abilities. Some cortical regions may begin with poorly defined functions, and consequently are partially activated in a wide range of different contexts and tasks. During development, activity-dependent interactions between regions improve the functions of regions such that their activity becomes restricted to a narrower set of circumstances (e.g., a region originally activated by a wide variety of visual objects may come to confine its response to upright human faces). The onset of new behavioral competencies during infancy will be associated with changes in activity over several regions, and not just with the onset of activity in one or more additional region(s).

3. Skill learning

The brain regions active in infants during the onset of new perceptual or motor abilities are similar, or identical, to those involved in complex skill acquisition in adults. It is not necessarily incompatible with interactive specialization, and sometimes the two viewpoints make similar predictions.

Cap. 2 - Methods and populations

Behavioral and cognitive tasks

“Preferential looking”: visual stimuli, and recording the time that the infants choose to look at each.

“Habituation”: showing the same stimulus repeatedly until the infant shows a clear decrease in the time she spends looking at it. When a certain criterion for the looking decrement is reached, a novel stimulus is presented and the increase or recovery in looking time is recorded. If there is significant recovery of looking time, we may infer that the infant can discriminate between the two stimuli. If there is little or no recovery, we may infer that the infant is unable to discriminate between them.

“Rate of sucking”:

“Eye tracker” & “Heart rate measures”

“Marker task”: use of specific behavioral tasks which have been related to one or more brain regions in adult humans and non-human primates by neurophysiological and brain imaging studies. By studying the development of performance on the task at different ages and in different contexts, the researcher can gather evidence about how the observed behavioral change is accounted for by known patterns of brain development. Limit: findings from one specific task sometimes do not generalize to others that seem closely related, and it can be difficult to directly compare results from groups of participants that differ significantly; the design of the task may be sufficiently limited in its demands as to give interpretable results with infants or young children, and yet sufficiently demanding to call upon “interesting” cognitive capacities.

Assessing brain function in development

“High-density event-related potentials (HD-ERP)”: method of recording the electrical activity of the brain by means of sensitive electrodes that gently rest on the surface of the scalp. The sensors detect tiny changes in electrical voltage at the scalp surface caused by groups of neurons within the brain firing together. These recordings can either be of the spontaneous natural electrical rhythms of the brain (electroencephalography [EEG]), or the electrical activity evoked by a stimulus presentation or action (event-related potentials [ERPs]).

When studying ERPs, the data from many trials is averaged so that the spontaneous EEG unrelated to the stimulus presentation averages out to zero.

Event-related oscillations [EROs]: rapid bursts of high-frequency EEG, related to stages of information processing in the brain. With a high density of sensors placed on the scalp, algorithms can be employed which infer the position and orientation of the brain sources of electrical activity for the particular pattern of scalp surface electrical activity. HD-ERP and related methods are an excellent way to study brain functions even in very young babies. However, while they offer excellent time resolution (milliseconds), it is difficult to obtain anything other than coarse spatial resolution.

“fMRI”: method that has far greater spatial resolution, albeit at the expense of temporal resolution. As different regions of the brain are activated, the cells in that area require oxygen delivered through networks of tiny blood vessels. Oxygen is transported in the blood by a molecule called hemoglobin, and when a brain region is active it calls for more oxygen resulting in a localized increase in oxygenated hemoglobin and a decrease in deoxygenated hemoglobin. The change in the blood oxygen level dependent (BOLD) response is detected by MRI, thus allowing the non-invasive measurement of cerebral blood oxygen levels in the different parts of the brain with a spatial resolution on the order of millimeters and a time resolution of several seconds. This technique is now routinely being applied to children from 6 or 7 years old in several laboratories. However, it remains technically challenging to use this method with children younger than this age (it is possible with babies, sleeping).

“Near Infra-Red Spectroscopy (NIRS)”: a form of optical imaging that depends on measuring minute changes in the absorption and scatter or bending of weak light beams as they pass through the skull and brain. Tiny light emitters and detectors are embedded within a cap and carefully placed on the child’s head. Like fMRI, changes in blood oxygenation due to brain activity can be detected with this method, but NIRS is less sensitive to motion artifacts and does not require confinement in a scanner. Therefore, NIRS potentially provides an excellent alternative to fMRI for use with infants and toddlers. Indeed, the relatively thin skull of very young children means that light passes through more easily, and thus better optical signals are usually obtained.

Observing brain structure in development

The traditional neuroanatomical methods applied to post-mortem human or animal brains have provided valuable insights into the structural development of the brain. However, these methods are limited in their ability to inform about the dynamic and functional aspects of brain development in living humans, especially in early childhood when rapid changes occur.