manual activity are restricted to milder cases and exclude global aphasia to which ideomotor apraxia is often associated, there is no opportunity to demonstrate a dissociation between the two kinds of movements. On the other hand, possible impairment of the spontaneous gestural activity without apraxia in cases of right hemispheric, frontal, or subcortical damage has not yet been documented. But, contemporary perspectives on motor control (e.g., Paillard, 1982) do not facilitate amalgamating the different manifestations in unitary explanations. We hope that showing lateral differences in gesture production may help us to establish the required behavioral taxonomy and to distinguish levels of processing in previously undifferentiated phenomena.
Analysis of lateral differences in gesture production and observation of manual activity during speaking in aphasic subjects lead to an apparent paradox: on the one hand, right-hand preference is assumed to reflect a predominant left-hemisphere control for these movements, but, on the other hand, aphasia originating from left-hemisphere lesions does not disrupt gestural activity. As a matter of fact, the seemingly contradictory findings are consistent with the hypothesis according to which right-hand gestures reflect the left hemisphere load in speech processing.
The difficulty with such an explanation is that it does not account for the patterning of the movements, i.e., their morphological complexity and possible representational value. Not enough data have been gathered on these issues. The extent to which and the conditions in which gestures can convey precise meanings are still unknown. In normal subjects, analysis of the processes in gesture interpretation is greatly underdeveloped relative to the numerous studies analyzing the decoding of facial expressions or visual behavior. The temporal association of gestures and speech is also a poorly documented phenomenon, especially in aphasic subjects. In addition to replications of important pilot studies on normal gestural activity, we need more detailed descriptions of the conditions in which lateral differences appear. From such a perspective, new insights may be gained by bringing together separate fields: the information processing approach to lateral differences, aphasiology, and the experimental analysis of gesture production. Indeed, the position we have adopted in this paper is that explanations of the observed behavioral asymmetries depend on the knowledge of how manual activity is controlled and of how it interacts with verbal expression. A clear interpretation of the plain fact of a right-hand advantage would be premature until we analyze the processes of translating thoughts into words and movements.