Functional Topography of Cortical Thumb Movement Representations in Human Primary Motor Cortex

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Intracortical microstimulation and single cell recordings in non-human primates showed that both, muscles and movements are represented in primary motor cortex (M1). This was also suggested in humans using electrophysiological and neuroimaging techniques. Transcranial magnetic stimulation (TMS) thus far was used to study motor cortical muscle representations, but data on movement representations in man are scarce. Therefore, we used TMS of M1 to evoke directional (flexion, extension) thumb movements and generated topographic movement maps (TMS-mov-map) in fourteen healthy individuals. TMS-mov-maps were related to the individual voluntary motor output (Vol-mov, directional thumb acceleration). Conventional TMS-motor evoked potentials (TMS-MEP) were simultaneously recorded from the prime movers (flexor and extensor pollicis brevis) and also compared to the voluntary motor output. Remarkably stable topographic maps were generated with both, thumb flexion and extension being multiply represented, overlapped and interspersed. Thumb Flexion was usually more robust than extension. TMS-mov-maps rather than TMS-MEP-maps seemed to better reflect the individual voluntary motor output. The findings emphasize existing models of multiple, overlapping finger movement representations in human M1, and indicate that this model also adapts to antagonistic thumb movements. The results suggest that investigating topographic movement maps may be an interesting adjunct in studying human motor cortical topography and its relevance for motor control.