Within-limb somatotopy in primary motor cortex--revealed using fMRI.

INTRODUCTION

Controversy exists regarding within-limb somatotopy, i.e., organization of representations of within-limb segments (e.g., elbow, fingers), in the primary motor cortex (M1). While some believe that within-limb representations overlap, others postulate somatotopic distinction. The purpose of our study was to aid the resolution of this controversy by exploring a) overlap between finger and elbow representations using high-resolution functional magnetic resonance imaging (fMRI), b) somatotopic differences between the two representations with and without the overlap, c) different location markers--center of mass (COM) and peak of activation (POA) and d) intensity differences between the two representations with and without the overlap.

METHODS

Twenty-four subjects underwent fMRI during finger and elbow tracking for definition of activation-based finger and elbow representations and their overlapping parts. Differential analysis was used to generate parts of finger and elbow representations that did not overlap. Location measures, COM and POA and fMRI signal intensity for all parts of finger and elbow representations were recorded.

RESULTS

Finger and elbow representations overlap but still possess distinct somatotopic centers, i.e., finger is lateral to overlap, which is lateral to the elbow. When overlap is excluded, locations of finger and elbow representations become more distinct. COM shows distinction along x, y and z-axes, but POA only shows distinction along the x-axis. Overlap has the highest intensity, while non-overlapping finger and elbow representations have the lowest intensity during finger and elbow tracking, respectively.

CONCLUSIONS

Somatotopic gradients exist in M1 despite the overlap. This finding supports 'functional somatotopy', i.e., within-limb representations overlap for multi-joint coordination, yet possess discrete centers for individuated control. Such a flexible somatotopy might allow representational reorganization. COM could be marker for 'location' and POA within in a high-intensity zone could emphasize 'reorganization' following learning/disease. Also, fMRI intensity could be a marker of change of overlap or somatotopic distinction.