Mapping effective connectivity between the frontal and contralateral primary motor cortex using dual-coil transcranial magnetic stimulation

Abstract

Background: Cytoarchitectonic, anatomical and electrophysiological studies have divided the frontal cortex into distinct anatomical and functional subdivisions. Many of these subdivisions have functional connections with the contralateral primary motor cortex (M1); however, effective neurophysiological connectivity between these regions is not well defined in humans.Objective: We aimed to use dual-coil transcranial magnetic stimulation (TMS) to map, with high spatial resolution, the effective connectivity between different frontal regions of the right hemisphere and contralateral M1 (cM1).Methods: TMS was applied over the left M1 alone (test pulse) or after a conditioning pulse that was applied to different targets over the right frontal cortex, while subjects sat at rest, or during the preparation of index finger abduction or a precision grip.Results: We found four clusters in the precentral gyrus and premotor regions, namely area 6d/FEF, area 43, area 6v, and area 44, which showed significant differential modulations of contralateral MEPs at rest compared to during preparation of index finger abduction and precision grip. Moreover, two clusters in the dorsal lateral prefrontal cortex (8C and 8Av-46-8C) showed differential modulation of contralateral MEPs during preparation of index finger abduction compared to precision grip.Conclusion: We show a gradient of task-related connectivity whereby interactions between caudal premotor regions and cM1 differentiate between rest and movement preparation, but more rostrally, interactions between dorsolateral prefrontal regions and cM1 differentiate between preparation of different types of hand movements. These results thus demonstrate the utility of dual-coil TMS to define fine-grained sub-regions in the human frontal cortex, which are functional and causally involved in hand movements.