These sensory and motor language areas are intricately connected with one another by a rich network of nerve fibers, one large bundle of which, the arcuate fasciculus, passes through the isthmus of the temporal lobe and around the posterior end of the sylvian fissure; other connections may traverse the external and extreme capsules of the lenticular nuclei (subcortical white matter of the insula). Many additional corticocortical connections lead into the perisylvian zones and project from them to other parts of the brain. Of special importance for the production of speech are the short association fibers that join the Broca area with the lower rolandic cortex (precentral gyrus), which, as mentioned previously, innervates the muscles of the lips, tongue, pharynx, and larynx. The perisylvian language areas are also connected with the striatum and thalamus and with corresponding areas in the nondominant cerebral hemisphere through the corpus callosum and anterior commissure (see Fig. 23-6).
Having indicated the main regions involved with language, there remains considerable difference of opinion concerning the status of cortical language areas, and objection has been made to calling them “centers,” for they do not represent histologically or circumscribed structures of constant function. Moreover, a neuroanatomist would not be able to distinguish the cortical language areas microscopically from the cerebral cortex that surrounds them.
Knowledge of the anatomy of language has come almost exclusively from the postmortem study of humans with focal brain diseases. Two major theories have emerged from these studies. One has subdivided the language zone into separate afferent (auditory and visual) receptive parts, connected by identifiable tracts to the executive (efferent–expressive) centers. Depending on the exact location of the lesions, a number of special syndromes are elicited.
The other broad theory, advanced originally by Marie (he later claims to have changed his mind) and supported by Head, Wilson, Brain, and Goldstein, favored the idea of a single language mechanism roughly localized in the opercular, or perisylvian region of the dominant cerebral hemisphere. The aphasia in any particular case was presumed to be a result of the summation of damage to input or output modalities relative to this central language zone. Undeniably, there is recognizable afferent and efferent localization within the perisylvian language area, as discussed previously, but there is also an undifferentiated central integrative mass action, in which the degree of deficit is to a considerable extent influenced by the size of the lesion. In addition, a strict division of aphasias into expressive and receptive, while still a strong practical concept, is not fully borne out by clinical observation. Nevertheless, there are several localizable language functions in the perisylvian cortex.