EVOLUTION OF HEMISPHERIC ASYMMETRIES IN PERCEPTION

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It is commonly noted that the left hemisphere is specialized for language and related cognitive processes while the right hemisphere is specialized for visuospatial processes. In this proposal, we investigate the idea that specialization of function has an associated cost. As each hemisphere becomes increasingly specialized for particular functions, we hypothesize that cortical regions are co-opted for these burgeoning functions. Since these co-opted cortical regions were previously performing some other function, that function will necessarily be adversely affected by the loss of cortex devoted to it. We hypothesize that this cost will be in higher-level perceptual functions, because lower-level perceptual functions are required by both hemispheres, and should be bilaterally represented. We hypothesize that the costs associated with hemispheric specialization are predictable. It has been hypothesized that the development of language in the left hemisphere is predicated on its relative superiority for temporal and sequential processing. We argue that this asymmetry "snowballed" over the course of human evolution, resulting in increasing specialization for timing and sequencing in the left hemisphere. This, in turn, resulted in a loss of visuospatial grain in the left hemisphere as tissue previously devoted to this function was co-opted for language. Relieved of the necessity of making such discriminations, we hypothesize that cortical resources previously dedicated to temporal processing in the right hemisphere were co-opted for fine-grained perceptual processing, effectively trading off temporal for spatial resolution. We propose to investigate the pattern of hemispheric specializations for perceptual abilities and their associated processing costs. Callosotomy ("split-brain") patients provide a unique opportunity to investigate the perceptual processes of each hemisphere in isolation. Although the notion of hemispheric specialization is not a new one, the idea that specialization has associated costs has implications for theories of brain function and for evolutionary theories of brain development.