P 8. Bringing TMS-based corticomotor mapping into shape: Shape-based TMS mapping reveals motor finger somatotopy in M1-HAND

Abstract

Movements of the fingers involve overlapping neural representations in the hand region of the primary motor cortex (M1 HAND ). Despite of these overlapping representations, somatotopic gradients of cortical motor representations exist in M1 HAND , but cortical motor somatotopy is difficult to capture with conventional transcranial magnetic stimulation (TMS) mapping techniques. To develop a novel TMS mapping approach, which takes into account the sulcal shape to map motor finger somatotopy in human M1 HAND . In nine healthy young subjects, neuronavigated single-pulse TMS was given to the left M1 HAND via a small figure-of-eight coil to map muscle-specific corticomotor representations. We compared three different mapping approaches which differed in terms of target grids and coil orientations: (1) Line-45°condition: Six targets located on a straight medio-to-lateral line which corresponded to the overall orientation of the central sulcus using a fixed coil orientation that induced a posterior-to-anterior current having a direction of 45° relative to the mid-sagittal line, (2) Shape-45°condition: Seven targets which followed the bending of the central sulcus with a fixed coil orientation as in the first condition, and (3) Shape-90°condition: Seven targets which were individually positioned following the bending of the central sulcus as in condition 2 but with the coil orientation adjusted to produce a current direction perpendicular to the central sulcus. We compared cortico-motor representations of the abductor digiti minimi (ADM) and the first dorsal interosseus (FDI) muscle at rest, during isometric contraction of either the FDI or ADM muscles or during co-contraction of both muscles. Based on the amplitudes of the motor evoked potential (MEP) at each target site, we generated muscle excitability profiles, the position of the weighted mean MEP amplitude. In contrast to the other mapping conditions, the shape-90°condition demonstrated clear somatotopy of the FDI and ADM muscle representations in M1 HAND and yielded state-dependant shifts in the weighted mean positions of the curves when switching from rest to active conditions. Within-hand motor somatotopy in M1 HAND can be readily studied with neuronavigated TMS that follows the sulcal shape and creates a tissue current perpendicular to the central sulcus at all mapping sites.