In right-handed individuals, aphasia is almost invariably related to a left cerebral lesion; aphasia in such individuals as a result of purely right cerebral lesions (“crossed aphasia”) is very rare, occurring in only 1 percent of cases (Joanette et al). Cerebral language dominance in ambidextrous and left-handed persons is not nearly so uniform. In a large series of left-handed patients with acquired aphasia, 60 percent had lesions confined to the left cerebral hemisphere (Goodglass and Quadfasel). Furthermore, in the relatively rare case of aphasia caused by a right cerebral lesion, the patient is nearly always left handed and the language disorder is less severe and less enduring than in right-handed patients with comparable lesions in the left hemisphere (Gloning; Subirana). Taken together, these findings suggest a bilateral—albeit unequal—representation of language functions in non-right-handed patients. This has been affirmed by the Wada test; Milner and colleagues found evidence of bilateral speech representation in 32 (about 15 percent) of 212 consecutive left-handed patients.
The undoubted language capacities of the nondominant hemisphere have been documented by lesional neurology. In cases of congenital absence (or surgical section) of the corpus callosum, which permits the testing of each hemisphere, there has been virtually no demonstrable language function of the right hemisphere. However, Levine and Mohr found that the nondominant hemisphere retains a limited capacity to produce oral speech after extensive damage to the dominant hemisphere; their patient recovered the ability to sing, recite, curse, and utter oneor two-word phrases, all of which were completely abolished by a subsequent right hemisphere infarction. The fact that varying amounts of language function may remain after dominant hemispherectomy in adults with glioma also suggests a definite though limited capacity of the adult nondominant hemisphere for language production. Kinsbourne’s observations of the effect of sodium amytal injections into the right-hemispheral arteries of patients who are aphasic from left-sided lesions make the same point.
Despite its minimal contribution to the purely linguistic or propositional aspects of language, the right hemisphere does have a role in the implicit communication of emotion through the subtleties of propositional language. These modulative aspects of language are subsumed under the term prosody, by which is meant the melody of speech, its intonation, inflection, and pauses, all of which have emotional overtones. The prosodic components of speech and the gestures that accompany them enhance the meaning of the spoken word and endow speech with its richness and vitality. The related issue of an individual’s accent, which carries such a strong regional identity and is acquired early in life, may also have an anatomic basis, but one that remains obscure (see later comments on the “Foreign Accent Syndrome”).
Many diseases and focal cerebral lesions mute or reduce the prosody of speech, the most dramatic examples being the hypophonic monotone of Parkinson disease and the effortful utterances of Broca’s aphasia. Largely through the work of Ross, it has become apparent that prosody is also greatly disordered in patients with strokes involving portions of the nondominant hemisphere that mirror the language areas of the left hemisphere. In these cases, there is impairment both of comprehension and of production of the emotional content of speech and its accompanying gestures. A prospective study of middle cerebral artery infarctions by Darby corroborated this view: aprosodia, as it has come to be called, was present only in those patients with lesions in the territory of the inferior division of the right middle cerebral artery. The deficit was most prominent soon after the stroke and was not found with lacunar lesions. In our patients, using bedside tests, we have had difficulty in appreciating aprosodia as a result solely of right perisylvian lesions, and in most cases, the damage has been more widespread.