Handedness, Dexterity, and Motor Cortical Representations

Motor system organization varies with handedness. However, previous work has focused almost exclusively on direction of handedness (right or left) as opposed to degree of handedness (strength). In the present study, we determined whether measures of interhemispheric interactions and degree of handedness are related to contra- and ipsilateral motor cortical representations. Participants completed a battery of handedness assessments including both handedness preference measures and behavioral measures of intermanual differences in dexterity, a computerized version of the Poffenberger paradigm (PP) to estimate interhemispheric transfer time (IHTT), and they underwent transcranial magnetic stimulation (TMS) mapping of both motor cortices while we recorded muscle activity from the first dorsal interosseous muscle bilaterally. A greater number of ipsilateral motor evoked potentials (iMEPs) were elicited in less lateralized individuals with the number of iMEPs correlated with IHTT. There were no relationships between handedness or lateralization of dexterity and symmetry of contralateral motor representations, although this symmetry was related to IHTT. Finally, IHTT was positively correlated with multiple measures of laterality and handedness. These findings demonstrate that degree of laterality of dexterity is related to the propensity for exhibiting iMEPs and the speed of interhemispheric interactions. However, it is not clear whether iMEPs are directly mediated via ipsilateral corticospinal projections or are transcallosally transmitted.

Interhemispheric Transmission of Visuomotor Information in Humans: fMRI Evidence

Normal human subjects underwent functional magnetic resonance imaging (fMRI) while performing a simple visual manual reaction-time (RT) task with lateralized brief stimuli, the so-called Poffenberger's paradigm. This paradigm was employed to measure interhemispheric transmission (IT) time by subtracting mean RT for the uncrossed hemifield-hand conditions, that is, those conditions not requiring an IT, from the crossed hemifield-hand conditions, that is, those conditions requiring an IT to relay visual information from the hemisphere of entry to the hemisphere subserving the response. The obtained difference is widely believed to reflect callosal conduction time, but so far there is no direct physiological evidence in humans. The aim of our experiment was twofold: first, to test the hypothesis that IT of visuomotor information requires the corpus callosum and to identify the cortical areas specifically activated during IT. Second, we sought to discover whether IT occurs mainly at premotor or perceptual stages of information processing. We found significant activations in a number of frontal, parietal, and temporal cortical areas and in the genu of the corpus callosum. These activations were present only in the crossed conditions and therefore were specifically related to IT. No selective activation was present in the uncrossed conditions. The location of the activated callosal and cortical areas suggests that IT occurs mainly, but not exclusively, at premotor level. These results provide clear cut evidence in favor of the hypothesis that the crossed-uncrossed difference in the Poffenberger paradigm depends on IT rather than on a differential hemispheric activation.