Hemisphere Specialization: A Mechanism for Unifying Anterior and Posterior Brain Regions

Abstract

Hemispheric specialization in the human brain was recognized largely through the study of hemispheric contributions to cognitive processes. In each of the research approaches to studying asymmetries of brain function, including studies of unilateral brain damage, commisurotomy studies and work with lateralized stimulus presentation with normals, the emphasis has been on describing hemispheric cognitive functions, rather than emotional aspects of hemispheric specialization. As described by Gainotti (this volume), there is a growing body of evidence which indicates that the hemispheres differ in their contributions to the understanding and expressing of emotion, and furthermore, may differ in their characteristic emotional valences. A fruitful approach to explaining these hemispheric differences in emotional functioning may be to consider how lateralized cognitive processes may afford different advantages in the cognitive handling of emotional experience and behavior (Buck, 1985, Safer and Levanthal, 1977; Tucker, 1981). In the present paper, however, we will take the reverse approach, proposing that hemispheric specialization for elementary emotional and motivational mechanisms has been the primary factor in the evolution of brain lateralization, and that many aspects of asymmetric cognitive function can be explained by these asymmetric emotional and motivational controls. This paper begins by reviewing concepts of lateralized activation and arousal systems proposed by Tucker and Williamson (1984). Next, it then attempts to develop this theory further, in an effort to avoid the simplistic division of brain operations into those which are exclusively motor versus those which are exclusively perceptual. The new theoretical proposal is that hemispheric specialization evolved because it allowed the higher-order elaboration of basic activation and arousal mechanisms, each within its own hemisphere. Thus, new phenomena were created: the activation system modulating not only motor systems, but perceptual mechanisms within the posterior left hemisphere; the arousal system modulating not only perceptual operations but motor organization within the anterior right hemisphere. These complementary aspects of hemispheric specialization afford interesting new theoretical possibilities not only for the attentional control of cognition, but also for the motivational and emotional control of experience and behavior.