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IMAGING TECHNIQUES
Since the advent of imaging techniques which allow us to see structural images of the brain (magnetic resonance imaging [MRI]), to measure brain activity (functional MRI [fMRI]) in living people and more recently to detect changes in white matter microstructure (diffusion tensor imaging [DTI]), numerous studies have been conducted to explore cerebral anatomical changes and to try to relate them to behavioural changes. Because they are non-invasive, these techniques can be used to study the development and the effects of experience on the brain.
DEVELOPMENT
Recent evidence in young children indicates that total brain volume increases 101% in the first year, followed by a 15% increase in the second year. In the first year, the major growth is that of grey matter (149%), white matter increase is less important (11%). The cerebellar volume increases 240% in the first year, whereas cerebral hemispheres increase by 90%. From ages 3 to 30, white matter volumes increase, while grey matter volumes rise and then fall, peaking at a characteristic time specific for each brain region during childhood and adolescence. Concurrently, connectivity between brain areas increases both structurally and functionally and the balance between limbic /subcortical and frontal lobe functions changes until young adulthood. Moreover, studies using genomic imaging indicate that genes are involved in shaping the brain. Twin studies carried out in adults as well as in children and adolescents show the high heritability of volume measured in different regions of the grey matter.
EARLY LIFE STRESS
Early life stress may also affect the brain volume. Animal models have shown that the amygdala, the prefrontal cortex and the hippocampus undergo stress-induced structural remodelling, which alters behavioural and physiological responses, including anxiety, aggression, mental flexibility, memory and other cognitive processes. Research in humans increasingly suggests that severe early life stressors (e.g., trauma, maltreatment, neglect) may result in decreased brain volumes. However, numerous scientific studies support the conclusion that providing supportive, responsive relationships as early in life as possible can prevent or reverse the damaging effects of toxic stress.
ATTENTION
Recording the electrical activity of the brain is a less recent method than imaging techniques; however it allows researchers to obtain event-related potentials (ERPs) which are electrical potentials in the brain in response to specific stimuli. Attention ERPs studies conducted in infants reveal a Negative central (Nc) component which is greater in amplitude when heart rate indicates attention.
VISION
In the early months of life, the visual system is still developing. At birth, the infant vision is mainly controlled at a subcortical level, with the cortex starting to mature at about 2 months postnatally. Due to immaturities of the eye components, the newborn is moderately farsighted. Visual attention and visual searching begin by 3 months; the infant begins to associate visual stimuli with an event (e.g., the bottle and feeding). Using variants of the simple visual orienting task known as the Gap task, studies indicate that the disengage operation becomes operative between 3 and 4 months of age. Prior to age 4 months, infants are able to selectively focus their attention, but once engaged on a particular stimulus, they have difficulty disengaging and moving their attention elsewhere. Rather, they tend to fixate for prolonged periods.
AUDITION
Auditory cortex shows a very prolonged developmental trajectory, with completely mature responses to simple sounds not achieved until about 18 years of age. At the same time, the brain’s responses to occasional changes in a repeating auditory stimulus can be measured in 2-month old infants.
MEMORY
Dramatic changes in the brain areas implicated in memory occur in the first two years of life. To assess declarative memory (“remembering”) in preverbal children, researchers have used elicited imitation (infants are shown an action (i.e., ringing a bell) and given opportunities to imitate the modelled action). Improvements in memory with age are consistent with brain development.
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