Representational overlap of adjacent fingers in multiple areas of human primary somatosensory cortex depends on electrical stimulus intensity: an fMRI study

Functional magnetic resonance imaging (fMRI) was used to examine the influe nce of non-painful electrical stimulus intensity on the BOLD response in hu man primary somatosensory cortex (SI). In ten healthy subjects, index and m iddle finger of the right hand were stimulated separately at two different stimulus intensities. The activated volume of single finger representations as well as the volume of representational overlap of the two activations i ncreased following an increase in stimulus intensity. This effect was seen in two different subdivisions of SI, one in the depth of the central sulcus , presumably corresponding to Brodmann area (BA) 3b, and one on the crown o f the postcentral gyrus, presumably corresponding to BA 1/2. Relative overl ap (ratio of overlap volume to volume of individual finger representation) was larger in BA 1/2 than in BA 3b. Additionally, in both areas relative ov erlap increased significantly from low to high stimulus intensity. Relative overlap did not change when different correlation thresholds were employed arguing against an unspecific 'spillover effect'. Analysis of signal inten sity time courses indicated that the response difference to high versus low stimulus strength was not present during the initial seconds of stimulatio n, during which both led to a similar signal intensity increase. Only durin g the following maintenance level of the response did the response to high stimulus intensity reach a significantly higher plateau level than the one due to low intensity stimulation, an effect which was present in both areas , BA 3b and BA 1/2, respectively. (C) 2001 Elsevier Science B.V. All rights reserved.