The symptomatology of aphasia after left-hemisphere damage should not distract us from the effects of right-hemisphere damage on the verbal performances of individuals who suffer it. These effects are subtle and are not manifested in a single subsystem like language, but affect the patient's ability to represent complex structures such as narratives and other forms of discourse. By considering complex performances (narratives, living-space descriptions, and descriptions of networks), plus tracking the nonverbal components of the performance (the speaker's gestures that accompany speech), we can study the deficits due to right-hemisphere damage in a clear and rigorous way. Our hypothesis is that normal speaking -- with a dual presentation of verbal and imagistic forms of representations - - involves the cooperative interaction of both hemispheres, each making its own specific contribution. Our hypothesis recognizes that speech and gesture constitute, at some level, a single system. Spatial representations thus can condition linguistic choices and vice versa. Patients with right hemisphere damage syndrome have seemingly unimpaired speech, but manifest poor story structure and gesture. There can be dissociation such that the strictly linguistic contributions to discourse structure occur in the absence of the appropriate imagery. This produces an 'empty' linguistic performance, not backed by the visuospatial representations that are part of normal metanarrative structure. In contrast, in left hemisphere patients, there are intact visuospatial representations, however imperfectly they may be expressed in linguistic terms, and these show themselves in both gesture and speech. Thus, according to our hypothesis, the narrative performance of left hemisphere patients may be underestimated by a too-narrow focus on the linguistic details of narrative speech and the performance of right hemisphere patients, due to the same narrow focus, may be overestimated . The relevance of this research for health lies in the understanding it will offer the contribution of the right and left hemispheres to language, gesture and discourse; of the role it tests of spatial imagistic representations in the formation of narrative and other discourse beyond specific linguistic choices; and of possible new approaches to therapy and evaluation of hemisphere damaged patients.
