Asymmetry of parietal interhemispheric connections in humans

Visuospatial abilities arepreferentially mediatedbythe right hemisphere. Although this asymmetryoffunctionisthoughttobeduetoan unbalanced interaction between cerebral hemispheres, the underlying neurophysiological substrate isstill largely unknown. Here, using a methodof trifocal transcranial magnetic stimulation,weshow that the right, but not left, human posterior parietal cortex exertsastrong inhibitory activity over the contralateral homologous area byashort-latency connection. Wealso clarify, using diffusion-tensor magnetic resonance imaging, that such an interaction is mediated by direct transcallosal projections located in the posterior corpus callosum. We argue that this anatomo-functional network may represent a possible neurophysiological basis for the ongoing functional asymmetry between parietal cortices, and that its damage could contribute to the clinical manifestations of neglect.