The topography of visually-guided grasping in the premotor cortex: a dense-transcranial magnetic stimulation (TMS) mapping study

Visuo-motor transformations at the cortical level occur along a network where posterior parietal regions are connected to homologous premotor regions. Grasping-related activity is represented in a diffuse, ventral and dorsal system in the posterior parietal regions, but no systematic causal description of a premotor counterpart of a similar diffuse grasping representation is available. To fill this gap, we measured the kinematics of right finger movements in 17 male and female human participants during grasping of 3 objects of different sizes. Single-pulse transcranial magnetic stimulation (spTMS) was applied 100 ms after visual presentation of the object over a regular grid of 8 spots covering the left premotor cortex (PMC) and 2 Sham stimulations. Maximum finger aperture during reach was used as the feature to classify object size in different types of classifiers. Classification accuracy was taken as a measure of the efficiency of visuo-motor transformations for grasping. Results showed that TMS reduced classification accuracy compared to Sham stimulation when it was applied to two spots in the ventral PMC and 1 spot in the medial PMC, corresponding approximately to the ventral premotor cortex and the dorsal portion of the supplementary motor area. Our results indicate a multifocal representation of object geometry for grasping in the PM that matches the known multifocal parietal maps of grasping representations. Additionally, we confirm that by applying a uniform spatial sampling procedure TMS can produce cortical functional maps independent of a priori spatial assumptions.SIGNIFICANCE STATEMENTVisually-guided actions activate a large frontoparietal network. Here, we used a dense grid of TMS spots covering the whole premotor cortex (PMC), to identify with accurate spatial mapping the functional specialization of the human PMC during grasping movement. Results corroborate previous findings about the role of the ventral PMC in pre-shaping the fingers according to the size of the target. Crucially, we found that the medial part of PMC, putatively covering the supplementary motor area, plays a direct role in object grasping. In concert with findings in non-human primates, these results indicate a multifocal representation of object geometry for grasping in the PMC and expand our understanding of how our brain integrates visual and motor information to perform visually-guided actions.