Cerebral Dominance and Auditory Perceptual Asymmetries in Normal and Dyslexic Children

Abstract

The cerebral cortex, like many other organs of the human body, is a "paired" organ — for just as there are two arms, two legs, two lungs, and two kidneys, there are in essence two cerebral hemispheres. Shaped like half circles, these cerebral hemispheres are connected by a series of transverse neural pathways called the corpus callosum. The cerebral cortex, composed of the left and right hemispheres and connecting neural fibers, is the seat of higher cognitive function. A difference in the function of the two hemispheres was recognized in the mid-1800s by neurologists working with patients suffering from damage to one side of the brain as a result of penetrating brain wounds or occlusion of blood vessels. In 1836, a French general practitioner, Marc Dax, first proposed the idea that the two cerebral hemispheres were not functionally equivalent. In a short address to the Congress Meridional, he suggested that left — but not right — hemisphere damage would result in deficits in language function (Critchley, 1962). Dax's work received no attention at that time and remains largely unknown. Instead, Paul Broca is usually credited with the first publication relating disruption of language function (aphasia) to localized left hemispheric lesions. He specifically implicated the left frontal area in aphasia with his famous pronouncement: "We speak with the left hemisphere" (Broca, 1865). This statement soon became the accepted belief of the neurologic community and innumerable studies subsequently related loss of language function to left hemisphere damage (Geschwind, 1970; Brown, 1976).