An evolutionary perspective on hemispheric asymmetries

The evolution of hemispheric specialization of function has obvious benefit s in terms of overall processing capacity, but it may also have associated costs to each hemisphere in isolation. These costs are not apparent in an i ntact brain since information can be readily transferred between the hemisp heres via the cortical commissures. We suggest that the evolution of langua ge in the left hemisphere may have occurred at the expense of some visuospa tial functions. Because the light hemisphere was still capable of performin g those functions, the relative left-hemisphere impairment for visuospatial processing would be invisible. If the cortical commissures are severed, us in callosotomy patients, the costs of specialization may become more appar ent. We report data supporting the idea that the left hemisphere may have l ost visuospatial abilities that it once possessed, and suggest that this pr ocess may also result in right hemisphere deficits in abilities related to linguistic processing. (C) 2000 Academic Press.