Interhemispheric asymmetries in the perception of unimanual and bimanual cutaneous stimuli - A study using transcranial magnetic stimulation

Previous studies have shown that transcranial magnetic stimulation (TMS) of the sensorimotor cortex can induce a suppression of cutaneous perception f rom the fingers of the contralateral hand. In this work, 17 normal subjects were submitted to focal TMS of frontal and parietal scalp sites of each he misphere. TMS was delivered at two interstimulus intervals (20 and 40 ms) f ollowing a cutaneous electrical stimulation of the first, third and fifth d igits of either hand or both hands near the subjective threshold of percept ion. The aim of our study was to investigate whether TMS could detect an as ymmetrical hemispheric specialization in the sensory perception of unimanua l and bimanual, ipsilateral and contralateral sensory stimuli. At each inte rpulse interval, the right parietal cortex was significantly more sensitive to TMS interference with stimulus detection for both contralateral and ips ilateral stimuli compared with the left parietal cortex. These effects were mainly evident during bimanual discrimination tasks. Our results are indic ative of an interhemispheric difference in the detection of cutaneous sensa tion, showing right hemispheric prevalence in the perception of contralater al as well as of ipsilateral stimuli. They provide neurophysiological suppo rt in normal humans to the clinical evidence which indicates that right hem isphere lesions can indeed produce deficits in the perception of ipsilatera l sensory stimuli.