Individual variation in brain and behavior in typical and atypical development: Antecedents, consequences, and pathways

This thesis explores individual variation in brain and behavior and its developmental antecedents and consequences. It begins by introducing a theoretical account of the co-development of brain and behavior, whereby extended brain-body-behavior networks mutually shape and constrain one another across timescales. On this view, brain networks both reflect the history of past brain-body-behavior interactions and generate subsequent behavior (in turn modifying brain networks). This process both underlies and creates the developmental pathway taken by each individual, along which variation emerges. Next, empirical studies of individual variation in brain and behavior in typical and atypical development are presented. Four behavioral studies examine incidental learning about multi-digit numbers in young children prior to formal instruction. Together the results suggest that children begin instruction with considerably more multi-digit number knowledge than previously appreciated, but that this knowledge varied profoundly across even the youngest individuals. The next study asks whether this variation in multi-digit number knowledge might relate to functional brain variation, specifically in fusiform specialization during perception of numbers. The results indicate instead that individual differences in early reading, not number, knowledge were associated with fusiform selectivity during multi-digit number perception. Turning to atypical development, the final study examines inter-subject similarity of brain responses in adults with autism and controls as they view semi-naturalistic social interactions. The results demonstrate profound heterogeneity of brain responses within the ASD group, such that brain responses of many individuals were indistinguishable from those of controls, while those of others were profoundly idiosyncratic and unreliable. Heterogeneity in brain responses within the ASD group was not predicted by neuropsychological scores but was linked with specific impairments in social comprehension. The thesis concludes by discussing how reinterpreting these disparate findings by considering individual brain-behavior variation as emergent along the individual developmental pathway provides new insights and suggests future inquiry.