Evolutionary advantages of human hemispheric asymmetries

Abstract

Hemispheric asymmetries are a basic principle of human brain organization. Once thought to be unique to humans, hemispheric asymmetries have meanwhile been documented in a wide range of species, suggesting they contain an evolutionary advantage. However, there are a few theories as to why asymmetry confers such an advantage and, moreover, there is a paucity of empirical work which is chiefly limited to a small number of animal studies. The present thesis is concerned with directly testing theories about potential evolutionary advantages in humans. Because it is widely believed that hemispheric asymmetries generally enhance cognitive processing, the first study Investigated the general relationship between functional lateralization and cognitive performance using two visual half-field paradigms. The second study employed the same paradigms to test the notion that hemispheric asymmetries specifically enhance parallel processing. The final study tested the notion that high degrees of lateralization (determined with a dichotic listening test) are associated with enhanced left-right discrimination. It was hypothesized that in all studies highly lateralized participants would outperform less lateralized participants. In contrast to our hypotheses however, highly lateralized participants were consistently outperformed by less lateralized participants. Less lateralized participants showed higher cognitive performance and excelled at parallel processing and left-right discrimination. The results of the present thesis thus challenge a) the general notion that high degrees of lateralization are associated with enhanced cognitive processing, b) the specific notions that lateralization enhances parallel processing and left-right discrimination and c) the idea that hemispheric asymmetries are advantageous for cognitive processing per se. Taken together with previous studies, it is argued that advantages of hemispheric asymmetries depend on the degree of lateralization and situational requirements. That is, high, low and intermediate degrees of lateralization of the brain are each associated with distinct advantages (and disadvantages), depending on the demands placed upon it.