Cognitive Development In Children`s Brains.

The extent to which structural changes influence children's cognitive ability will be examined by looking at the key concepts of brain development. Many physiological processes take place during early brain development and these will be discussed as they provide vital insights into how structure could influence functional change. Because of the significance of the prefrontal cortex with regards to cognitive ability (Milner, 1982; Goldman-Rakic, 1987; Fuster, 1989), its structural development and how it might relate to functional change, will be reviewed by looking at two competing views. Research evidence in favour and against the idea that innate brain structures relate to specific functions will also be examined. The important domain of language will then be looked at in order to examine this relationship between brain structure and function further.

Early brain development provides clues as to its changing functions, and how these functions relate to cognitive development. During early postnatal development there is a dramatic increase in the number of neural connections in the brain (Huttenlocher, 1990). The three stages of the development of neurons (the basic building blocks of the brain) in the prenatal brain, can be summarized as follows. First, neurons are born (along the inner surface of the neural tube which will form the brain) through call division. Second, they migrate from their place of birth to their final location within the brain. Third, they differentiate, or take up their final form. The communication points between neurons are called synapses and there is a rapid increase in extra connections just before and after birth with some decrease between 1 year and maturity as these develop into fully functional synapses. Myelination (a fatty sheath that forms around neurons that helps them to transmit signals more quickly) begins to form before birth and continues to do so for many years. Specificity in the brain is achieved by means of selective "pruning", where useful connections remain and surplus ones are eliminated. A possible explanation for this "pruning" could be through the process of "selectionism" where the decrease in synaptic connectivity could be viewed as a kind of Darwinian ("survival of the fittest") selection (Changeux, 1985). This means that neural pathways that are used more frequently are preserved, and those that are activated less frequently are weakened. Interaction with the environment (which will activate neural circuits) preserve the neurons themselves and therefore selectionism could be viewed as a more constructivist process of development.

Cells might take on any number of forms and functions which means that when one area is damaged, other regions of the brain can take over the processing from the damaged region. This is referred to as plasticity. The brain is less adaptable in adult life, and brain damage would be harder to overcome. As brain development continues. Cells assume their mature form and connectivity whereby they achieve functional specialization. Plasticity is therefore the state of not yet having achieved functional specialization.

Developmental plasticity suggests that the initial products of the genes (cells and tissues) can themselves combine so as to have "self-organising" properties. This means that they can to some extent direct their own development. Self-organization is the process where structure forms as a result of a system's dynamic interactions with an environment. The child interacts with its environment and as a result, particular neural structures and modules emerge. Self-organization is regarded by many researchers as a fundamental characteristic or the brain (Changeux et al., 1984; von der Malsberg, 1995).

As the cerebral cortex differentiate into different structural areas, it results in a brain that may have different functional units. These units specialise in processing particular types of information, in specific ways. These functional units are called "modules". It is important to note that these cognitive modules are hypothetical constructs and that neural modules are real, structural units of the brain. As these cognitive modules aim to clarify the relationship between the brain`s structure and its function, it is important to look at how these modules develop.

There is debate between two theorists - Fodor and Karmiloff-Smith - even though they agree that the brain does function in a modular way. They disagree as to how these modules develop. Fodor (1983) is the key advocate of innately specified modularity and argues from a natavist standpoint. He believed that the capacity of humans to develop information processing systems that helps them to make sense of the world, is innate and something that they are born with. He argued that the structure and function of modules were influenced by the environment over a long period and over generations (phylogenetic). However, he believed that the environment did not play a crucial role in the development of modules in one life time (ontogenetic). Karmiloff-Smith (1992) questions whether such modules are innately pre-specified. She argues that the more highly pre-specified a cognitive system is in its infancy, the less creative and flexible it can become as it develops. Human minds are remarkably creative and flexible, which makes a high level of innate specify in brain function unlikely. She states that modules are the product of development (constructivist) and propose that the brain is a self-organising system.

According to Mareschal et al. (2004, p129), most of the evidence (O`Leary, 1989; Elman et al., 1996; Katz and Shatz, 1996; Johnson, 1997) suggests that an epigenetic system is responsible for the differentiation of function in most areas of the cortex. This means that both molecular and genetic factors on the one hand and environmental factors on the other, influence functional differentiation. Neural activity itself is the product of both biological and environmental factors. Neural activity takes place through mostly genetic determinism before birth, even though the womb already provides an environment in which the baby can hear. After birth, however, neural activity is more influenced by both genes and the environment (through sensory and motor experience).

Neville`s (1991) study showed the effects of experience on cortical structure and function. This study used scalp recorded event-related potentials, ERPs, which measures the electrical activity of the brain as groups of neutrons fire at the same time. She found that with congenitally deaf participants, areas of the temporal lobe, which in typical development respond to auditory, or multi-modal input, had become dominated by responses to visual input. This highlights the influence of the environment (sensory experience) on function of these regions.

Another argument in favour of Karmiloff-Smith`s argument of an epigenetic view of brain development and that modularisation happens as a product of development, is that the cerebral cortex alone contains some 100,000,000,000,000 synapses. The genes can most probably no (in any direct way) encode the full information necessary to generate this level of complexity according to a predetermined plan (Elman et al., 1996).

One way of exploring the relationship between structural developments and cognitive ability within the prefrontal cortex, is to look at advances in cognitive ability at a given age, and to relate these to observed changes in the prefrontal cortex. It is important to note that a correlation does not necessarily mean causation, but rather a relationship that is worthy of further investigation. There are two competing views of how structural developments in the prefrontal cortex might relate to functional changes in cognitive ability. The first states that various aspects of cognitive development of infants could be related to structural changes in the prefrontal cortex (Diamond and Goldman-rakic, 1989; Fox and Bell, 1990). This means that there is an increased involvement of the prefrontal cortex with certain cognitive tasks, such as ones which require the maintenance of spatial or object information, or information about objects, over short periods of time. The second view holds that the prefrontal cortex is specifically associated with the early stages of the development of a new skill, and its involvement decreases with the development of that skill (Johnson and colleagues, 1998; Csibra et al., 1998). This could mean that cortical regions that are crucial for a particular ability change with the stage of acquisition of that ability. It could also mean that in adulthood the skills that have been learned have "moved" to a different, more specialised area of the cortex.

Language is an important domain in which to examine the relationship between brain structure and function as it is regarded as one of the most "biological special" human abilities (Johnson, 1997, p.137). Pinker (1994) in his book "The language instinct", made a strong naturist case for the innateness of language. His argument consisted of four basic components namely: the existence of pidgins and creoles, the observations regarding the so-called "poverty of input", the commonality of certain grammatical formulations across different languages, and evidence about brain structure and function. The first part of his argument stated that migrant workers that collected in Hawaii at the beginning of the twentieth century developed a pidgin to communicate whilst working on booming sugar plantations. Pidgin is a non-grammatical form of communication that helps adult speakers, who share no common language, to still communicate. However, the children within this community who were only exposed to pidgin, started speaking fully fledged creole (a fully grammatical language developed out of pidgin). Pinker argues that the children reinvented language anew as they use it and that this is concrete evidence for a language instinct.

The conflict between an innate language "seat" and a more constructivist approach can also be framed in terms of the concept of "equipotentiality". This points to the proposition that both the left and right hemispheres of the brain have "equal potential" for developing language. Evidence in favour of equipotentiality would seriously weaken the nativist argument for innate language-specific areas of cortex. The majority of language functions in adults are normally seated within the left hemisphere. However, early brain plasticity supports the case that language processing capabilities are not necessarily compelled to a specific area. If it becomes necessary, it seems that other brain structures within the brain can process language (as a result childhood injury or illness to the areas that would typically come to carry language functions). "There are constraints on the organisation of the neural systems that mediate formal language ... however, it is clear that the nature and timing of sensory and language experience simnifically affect the development of the language system of the brain" (Neville and Bavelier, 2001, p.283).

The above epigenetic view is further supported by Neville et al`s. (1998) study and used functional magnetic resonance imaging (fMRI), to examine the brain regions that are involved in language processing in deaf and hearing participants. Both the hearing and deaf participants showed activation in language areas of the left hemispheres when processing language stimuli in their native language. The level of right hemisphere activation was higher for the deaf participants than for the hearing participants. This shows that the left hemisphere typically process language, but also that other areas of the right hemisphere are capable of supporting language-related information processing.

Connectionist models (neural networks) can further clarify the nature-nurture debate between structure and function. Connectionist models are dynamic computer simulations that consist of "nodes" and "connections" that are similar in structure and function to neurons with their synaptic connections. These computer simulations have shown that less information has to be genetically pre-specified within the brain than might have been thought to be the case. This is clear from Elman et al`s (1996, p.99) statement: "One of the exciting lessons from the connectionist work is that relatively simple learning algorithms are capable of learning complex things".

Structural changes in children's brains influence functional changes in their cognitive ability to a great extent. This is evident in the modular way that the brain organizes itself and how these modules map onto specific functional units that accounts for cognitive ability. However, because of the brain's plasticity and the self-organising properties of cells and tissues in early development, the capacity of the brain to adapt and change in response to the environment (or injury for example) is equally profound. The relationship between structure and function is therefore not a clear one and is complex. Brain structures have genetic constraints on what they can and cannot do, even though cortical regions interact with the environment to develop their ideal cognitive function. This means that genetic determinism would inadequately explain the relationship between structure and function in the developing brain. Brain structure and cognitive function develop to epigenetic principles, with the genes and environment unavoidably linked. There is still much to learn about how structures relate to cognitive functions and about how cognitive functions influence the development of structures.