Profiles of Development and Plasticity in Human Neurocognition

We describe changes in neural organization and related aspects of processing after naturally occurring alterations in auditory, visual, and language experience. The results highlight the considerable differences in the degree and timeperiods of neuroplasticity displayed by different subsystems within vision, hearing, language and attention. We also describe results showing the two sides of neuroplasticity i.e. the capability for enhancement and the vulnerability to deficit. Finally we describe several intervention studies in which we have targeted systems that display more neuroplasticity and show significant improvements in cognitive function and related aspects of brain organization. Extensive research on animals has elucidated both genetic and environmental factors that constrain and shape neuroplasticity (Hunt et al., 2005; Garel et al., 2003; Bishop et al., 1999; Bishop, 2003). Such research together with non-invasive neuroimaging and genetic sequencing techniques have guided a burgeoning literature characterizing the nature, timecourse, and mechanisms of neuroplasticity in humans (Pascual-Leone et al., 2005; Bavelier & Neville, 2002; Movshon & Blakemore, 1974). Electron microscopic studies of synapses and neuroimaging studies of metabolism and of gray and white matter development in the human brain reveal a generally prolonged postnatal development that nonetheless displays considerable regional variability in timecourse. (Chugani et al., 1987; Huttenlocher & Dabholkar, 1997; Neville, 1998; Webb et al., 2001). In general, development across brain regions follows a hierarchical progression in which primary sensory areas mature before parietal, prefrontal, and association regions important for higher order cognition (Giedd et al., 1999; Gogtay et al., 2004). Within each region there is a pattern of prominent overproduction of synapses, dendrites, and grey matter that is subsequently pruned back to about 50% of the maximum value, which is reached at different ages in different regions. The prolonged developmental timecourse and considerable pruning of connections are considered major forces that permit and constrain human neuroplasticity. Recently an additional factor has been identified which appears to be important. The occurrence of polymorphisms in some genes is widespread in humans and rhesus monkeys but apparently not in other primate species. Polymorphisms provide the capability for environmental modification of the effects of gene expression (gene x environment interactions) and such effects have been observed in rhesus monkeys and humans (Suomi, 2003; Suomi, 2004; Suomi, 2006; Sheese et al., 2007; Bakermans-Kranenberg et al., 2008). For several years we have employed psychophysics, electrophysiological (ERP), and magnetic resonance imaging (MRI) techniques to study the development and plasticity of the human brain. We have studied deaf and blind individuals, people who learned their first or second spoken or signed language at different ages, and children of different ages and of different cognitive capabilities. As detailed in the sections that follow, in each of the brain systems examined in this researchincluding those important in vision, audition, language, and attentionwe observe the following characteristics: • Different brain systems and subsystems and related sensory and cognitive abilities display different degrees and time periods (“profiles”) of neuroplasticity. These may depend on the variable