Abstract:
An understanding of the physiology of vision and language production is essential for an evaluation of some of the theories put forward to account for laterality differences. Before reviewing the literature on laterality differences, a brief account is given below of the cerebral structures involved. This is based largely on an article by Geschwind (1972).
Figure I shows the visual pathways and the centres in the brain involved in the production of language. Almost everything we know about the organization of language in the human brain has been derived from some sort of "abnormal" intervention, be this in the form of surgical intervention, electrical stimulation of brains exposed during surgery, brain damage or the effect of drugs such as sodium amytal on speech. The most fruitful source of information has been the study of language disorders followed by postmortem analysis of the brain in patients who have suffered brain damage.
The most important areas in the brain for the production of language are Broca's area, Wernicke's area and the angular gyrus. Damage to Broca's area results in slow and laboured speech but comprehension remains intact while damage to Wernicke's area results in speech being fluent but impoverished in content and comprehension is usually lost. The angular gyrus acts as a way station between the auditory and visual regions so that damage to it separates the visual and auditory language areas. Although words are seen correctly, they are meaningless to the subject as they have to be converted into an auditory form to be comprehended. Similarly, damage to the angular gyrus means that words received in an auditory form cannot be transformed into a visual pattern and subsequently be spelled out by the subject.
