On the reorganization of sensory hand areas after mono-hemispheric lesion: a functional (MEG)/anatomical (MRI) integrative study

Abstract

The topography of primary sensory cortical hand area following a monohemispheric lesion (sudden=stroke; progressive=neoplasm) was investigated in relationship with clinical recovery of sensorimotor deficits. Twenty seven patients with monohemispheric lesions were studied in a clinically stabilized condition. Functional informations from magnetoencephalography (MEG) were integrated with anatomical data from magnetic resonance imaging (MRI). MEG localizations of the neurons firing at early latencies in primary sensory cortex after separate stimulation of median nerve, thumb and little fingers of each hand were carried out. Characteristics of cerebral equivalent current dipoles (ECDs) activated by each contralateral stimulation, the `hand extension' (i.e., the distance in millimetres between ECDs of first and fifth digits), as well as interhemispheric differences of the tested parameters were investigated. Finally, ECDs' locations were integrated with MRI. Lesions involving cortical (C) or subcortical (SC) areas receiving sensory input from the hand were often combined to increase interhemispheric asymmetry of the tested parameters (22% for C and 49% for SC lesions). This might be due to an activation of neuronal districts which in the affected hemisphere (AH) differ from those normally activated in the unaffected hemisphere (UH) and in the control population. Moreover, the `hand extension' was enlarged on the AH—more frequently after a SC lesion—mainly due to a medial shift of the little finger ECD, combined to a tendency of both finger ECDs to shift frontally. After a C lesion, responses from the AH were often stronger than normal. Spatial reorganizations were also seen in the UH (7% of C and 14% of SC lesions). `Hand extension' in the UH was selectively enlarged for the P30m only when combined with a similar enlargement in the AH. Significant interhemispheric asymmetries due to neuronal reorganization in the AH were associated with worse clinical outcomes compared to patients without asymmetries.