The past two decades of neuroimaging research on developmental disabilities appear to reflect the development of the field of neuroimaging itself. The vast majority of magnetic resonance imaging (MRI) studies on developmental disabilities have come from modular perspectives of brain function – where specific areas are associated with specific mental operations, developmental patterns, and clinical symptoms. More recently, it has become clear that, although critical, modular perspectives ought not to obscure the importance of distributed perspectives, where cognitive functions are implemented via complex interactions between multiple brain areas.
This chapter reviews MRI findings on developmental disabilities that arise from both modular and distributed perspectives on brain function, although the former clearly represent the majority of existing data. The chapter will focus exclusively on autism-spectrum disorders (ASD, including the diagnoses of autism, Asperger's Syndrome, and Pervasive Developmental Disorder – Not Otherwise Specified). Some of the most recent and exciting developments in the neurobiology of ASD relate to abnormal functional and structural connectivity – in particular, findings of decreased long-range connectivity between brain areas. We conclude the chapter by discussing important futures avenues, with particular emphasis on study designs and techniques that are capable of integrating data from multiple sources.
The autism spectrum
The classic “triad” of autism includes (1) delayed or impaired communication abilities, (2) diminished social skills and social isolation, and (3) repetitive behaviors or restricted interest repertoires. These symptoms typically emerge early in development (noticeable by age 2 or 3) and are generally lifelong (Volkmar et al.,2005).