Intellectual disability is a neurodevelopmental disorder that continues throughout the life span of the affected person. It is essential to understand how persons with intellectual disability progress throughout their life span from infancy to old age. The maturation of the brain, their environmental experiences, and the mastery of developmental challenges and tasks must all be considered. A focus on brain development is in keeping with neuroscience research indicating that progressive brain maturation is accompanied by successive synaptic reorganization as one moves from one developmental stage to the next. Anatomical Magnetic Resonance Imaging Studies are playing a major role in understanding the developmental trajectories of normal brain development (Durston et al., 2001; Giedd et al., 1999). Understanding the developmental trajectories of normal brain development is crucial to the interpretation of brain development in neurodevelopmental disabilities. During normal development, white matter volume increases with age, and although gray matter volumes increase during childhood, they decrease before adulthood. These changes in the brain are accompanied by changes in cognitive processing; for example, executive functioning shows a progressive emergence from the preschool years (Espy et al., 1999) into the adolescent years. Working memory and inhibitory processes may be measured during the preschool years. By adolescence, abstract reasoning, anticipatory planning, and mental judgment have emerged and may be measured. Cognitive abilities in adolescence are qualitatively different from those of young children as a result of the reorganization of the prefrontal cortex during maturation. How genetic background and environment interact in producing these changes is the object of ongoing study, yet investigators are beginning to understand how physiological processes of synaptic development, circuits, and neuronal network formation relate to processes of cognitive development (Fossella et al., 2003). The development of persons with intellectual disability is now being evaluated systematically, and developmental trajectories are being established for known neurogenetic syndromes. These studies are making up for a surprising lack of application of a developmental perspective to persons with intellectual disability. Developmental theorists have, for the most part, monitored and measured development in normally intelligent persons in establishing developmental landmarks.
Dynamic N6-methyladenosine RNA methylation in brain and diseases
