Cerebral Asymmetry and Dominance in Man

Abstract

The two human cerebral hemispheres are not mirror images of each other. They show anatomical and functional differences that are termed as cerebral asymmetries. The most striking differences in right-handed people are Broca's motor areas of speech in the left inferior frontal gyrus and Wernicke's area for language comprehension and speech production in the left posterior temporal and inferior parietal lobules. These examples have lead to the overall concept of dominant left and subordinate right cerebral hemispheres. Other structural asymmetries are revealed in the perisylvian area, surface area around the central fissure, cortical thickness, cerebral rotation, the volume of the lateral ventricle, lateral geniculate body, posterior cerebral and circle of Willis arteries, deep medullary veins, and type of nervous system of individuals. Functional cerebral asymmetries are also reported regarding verbal and linguistic functions, mathematical skills, analytical thinking, functions of corticobulbar, corticospinal and corticobasal pathways, functional organization of the prefrontal cortex (PFC), orbitofrontal cortex (OFC) and ventromedial temporal cortex (fusiform gyrus, FG), as well as amyloid-β deposition and metabolism. Cerebral asymmetries are thought to play an essential role in the pathogenesis of clinical disorders as aphasia, unilateral left brain lesion, adiadochokinesa, autism spectrum disorder (ASD), corticobasal syndrome, Parkinson's disease (PD), Alzheimer's disease, and adolescents with idiopathic scoliosis (AIS). Age and sex related asymmetries have been sometimes postulated. Genetic, environmental and callosal factors are mentioned to underlie the etiology of cerebral asymmetries. Comparatively, functional, and neuroanatomical asymmetries do exist in nonhuman primates.