Distributed digit somatotopy in primary somatosensory cortex

We obtained high-resolution somatotopic maps of the human digits using 4.0 T functional magnetic resonance imaging (fMRI). In separate experiments, the volar surface of either the right thumb, index, or ring finger was stimulated in a sliding-window fashion in both distal-to-proximal and proximal-to-distal directions using a custom-built pneumatic apparatus. Analysis of the functional images was restricted to Brodmann's areas 3b and 1 and control areas 4 and 3a, as well as a randomized simulation of the functional data in each of these areas. Using in-house algorithms, we detected discrete regions of cortical activation showing phase reversal coinciding with alternation in stimulation direction. Most stimulation-related phase maps of the digits were obtained in areas 3b and 1, rather than areas 3a or 4, despite the somatic input to the latter two areas. The area 3b and 1 representations thus appear to be relatively discrete and somatotopic compared to other somatic processing regions. Our results within areas 3b and 1 confirm the nonlinear mapping of the body surface suggested by recordings in nonhuman primates in terms of phase band topography, scaling, and frequency relative to the actual digit surfaces. The scaling and frequency nonlinearities were more evident within area 3b than area 1, suggesting a functional differentiation of these regions as has previously been observed only in more invasive recordings. Specifically, the area 1 representations were larger overall than those observed in area 3b, and the frequencies of area 3b phase bands and voxels were related disproportionately to thumb and index finger stimulation and to particular areas on the digit surface, suggesting a weighting based in part on receptor distribution.