The hand motor hotspot for seed-based functional connectivity of hand motor networks at rest

Abstract

In the seed-based method for studying functional connectivity (FC), seed selection is relevant. Here, we propose a new methodological approach for resting-state FC analysis of hand motor networks using the individual hand motor hotspot (hMHS) as seed. Nineteen right-handed healthy volunteers underwent a transcranial magnetic stimulation (TMS) session and resting-state fMRI. For each subject, the hMHS in both hemispheres was identified by TMS with the contralateral abductor pollicis brevis muscle as the target, the site eliciting the highest and most reliable motor-evoked potentials. Seed regions were built on coordinates on the cortex corresponding to the individual left and right hMHSs. For comparison, the left and right Brodmann's area 4 (BA4) masks extracted from a standard atlas were used as seed. The left and right hMHSs showed FC patterns at rest mainly including sensorimotor regions, with a bilateral connectivity only for the left hMHS. The statistical contrast BA4 > hMHS for both hemispheres showed different extension and lateralization of the functionally connected cortical regions. On the contrary, no voxels survived the opposite contrast (hMHS > BA4). This suggests that detection of individual hand motor seeds by TMS allows to identify functionally connected motor networks that are more specific with respect to those obtained starting from the a priori atlas-based identification of the primary motor cortex.

Keywords: hand motor hotspot; hand motor networks; resting-state functional MRI; seed-based functional connectivity (FC); transcranial magnetic stimulation (TMS).

FIGURE 1

Regions of interest selected for seed-based functional connectivity analyses, overlaid on a T1 single-subject template. (A) Left hemisphere, (B) right hemisphere. l-BA4 and r-BA4 are shown in red; the other ROIs are differently colored to represent the individual seed regions based on the hand motor hotspots detected by TMS (l-hMHS and r-hMHS). Please note that some of the ROIs might be overlapped.

FIGURE 2

Group seed-to-whole-brain functional connectivity results using as seeds (A) the l-hMHS, (B) the r-hMHS, (C) the l-BA4, and (D) the r- BA4. Voxels significantly connected with the different seed regions are displayed on a rendered surface (p < 0.05 FWE-corrected, extent of threshold k = 20 voxels, height threshold t = 7.05).

FIGURE 3

Surface rendering showing voxels surviving the statistical contrasts between the group seed-to-whole-brain functional connectivity maps obtained with the different ROIs separately for the two brain hemispheres (p < 0.05 FWE-corrected, extent of threshold k = 20 voxels, height threshold t = 7.05). (A) l-BA4 > l-hMHS and (B) r-BA4 > r-hMHS.