Differential role of left and right hemispheres for recovery from aphasia: Evidence from SPECT studies

Language organization and functional neuroimaging was discussed with special interest in the roles of the left and right hemispheres in recovery from aphasia. Two mechanisms have been postulated for the process of language restitution following aphasia; 1) partial recovery of left-hemisphere language-related areas, and 2) activation in their homologous counterparts in the right hemisphere. In fact, functional reorganization of the language network may involve both hemispheres. In this review the results of our two experiments, prospective and retrospective (Mimura et al., 1998), are presented, suggesting a chronologically dissociable contribution of the left and right hemispheres for recovery from aphasia. The complementary results of both experiments suggest that the initial language recovery within the first year post onset may be linked primarily to functional recovery in the dominant hemisphere, while subsequent language recovery and long-term recovery from aphasia may be related to slow and gradual compensatory functional activation in the contralateral hemisphere. Recent activation studies using PET and functional MRI, in which activated or compensated areas are directly visible in aphasic patients, have also suggested a time dissociation in the role of left and right hemispheres. SPECT is clinically available in many institutions and suitable for carrying out serial studies. In addition, SPECT data are now analyzable using SPM (Statistical Parametric Mapping). For these reasons, we recommend SPECT for investigating longitudinal clinical issues affecting brain-damaged patients, such as the recovery process from aphasia.