Carl Wernicke more than any other person must be credited with the anatomic–psychologic scheme upon which many contemporary ideas of aphasia rest. Earlier, Paul Broca (1865), and, even before him, Dax (1836), made the fundamental observations that a lesion of the insula and the overlying operculum deprived a person of speech and such lesions were always in the left hemisphere. Wernicke’s thesis was that there were two major anatomic loci for language: (1) an anterior locus, in the posterior part of the inferior frontal lobe (Broca area), in which were contained the “memory images” of speech movements, and (2) the insular region and adjoining parts of the posterior perisylvian cortex, in which were contained the images of sounds. Meynert had already shown that aphasia could occur with lesions in the temporal lobe, the Broca area being intact. Wernicke believed that the fibers between these regions ran in the insula and mediated the reflex arc between the heard and spoken word. Later, Wernicke came to accept von Monakow’s view that the connecting fibers ran around the posterior end of the sylvian fissure, in the arcuate fasciculus. Wernicke gave a comprehensive description of the receptive, or sensory, aphasia that now bears his name. The four main features he pointed out were (1) a disturbance of comprehension of spoken language and (2) of written language (alexia), (3) agraphia, and (4) fluent paraphasic speech. In Broca aphasia, by contrast, comprehension was intact, but the patient was mute or employed only a few simple words.
Wernicke also theorized that a lesion interrupting the connecting fibers between the two cortical speech areas would leave the patient’s comprehension undisturbed but would prevent the intact sound images from exerting an influence on the choice of words. Wernicke proposed that this variety of aphasia be called Leitungsaphasie, or conduction aphasia (called central aphasia by Kurt Goldstein and deep aphasia by Martin and Saffran). Careful case analyses since the time of Broca and Wernicke have borne out these associations between a receptive (Wernicke) type of aphasia and lesions in the posterior perisylvian region, and between a predominantly (Broca) motor aphasia and lesions in the posterior part of the inferior frontal lobe and the adjacent insular and opercular regions of the frontal cortex. We have certainly encountered cases that conform to the Wernicke model of conduction aphasia; the lesion in these cases may lie in the parietal operculum, involving the white matter deep to the supramarginal gyrus, where it presumably interrupts the arcuate fasciculus and posterior insular subcortex (the issue of conduction aphasia is discussed further on).
How these regions of the brain are organized into separable but interactive modules, resulting in the complex behavior of which we make daily use in interpersonal communication, is still being studied by linguists and cognitive neuropsychologists. They have dissected language into its most basic elements—phonemes (the smallest units of sound recognizable as language), morphemes (the smallest meaningful units of a word), graphemes, lexical and semantic elements (words and their meanings), and syntax (sentence structure). In general, as a restatement of the Wernicke-Broca scheme, phonologic speech output difficulties are derived from left frontal lesions; semantic–comprehension difficulties are the result of left temporal lesions; and alexia and agraphia are associated with inferior parietal lesions. These “modules” of language have been diagrammed by psycholinguists as a series of boxes and are connected to one another by arrows to indicate the flow of information and the manner in which they influence the spoken output of language. “Boxologies,” as they are called, are consistent with current cognitive theory, which views language functions as the result of synchronized activity in vast neuronal networks made up of many cerebrocortical regions and their interconnecting pathways (Damasio and Damasio, 1989).