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 influence of non-painful electrical stimulus intensity on the BOLD response in human primary somatosensory cortex (SI). In ten healthy subjects, index and middle 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 increased 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 of the postcentral gyrus, presumably corresponding to BA 1/2. Relative overlap (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 overlap 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 intensity time courses indicated that the response difference to high versus low stimulus strength was not present during the initial seconds of stimulation, during which both led to a similar signal intensity increase. Only during 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.

Somatotopic organization of human secondary somatosensory cortex.

This fMRI study investigated the human somatosensory system, especially the secondary somatosensory cortex (SII), with respect to its potential somatotopic organization. Eight subjects received electrical stimulation on their right second finger, fifth finger and hallux. Within SII, the typical finding for both fingers was a representation site within the contralateral parietal operculum roughly halfway between the lip of the lateral sulcus and its fundus, whereas the representation site of the hallux was found more medially to this position at the fundus of the lateral sulcus, near the posterior pole of the insula. Somatotopy in SII seems to be less fine-grained than in primary somatosensory cortex (SI), as, in contrast to SI, no separate representations of the two fingers in SII were observed. A similar somatotopic representation pattern between fingers and the hallux was also observed within ipsilateral SII, indicating somatotopy of contra- as well as ipsilateral SII using unilateral stimulation. Further areas exhibiting activation were found in the superior and inferior parietal lobule, in the supplementary and cingulate motor area, and in the insula.

fMRI shows multiple somatotopic digit representations in human primary somatosensory cortex.

Using electrical finger nerve stimulation in normal human subjects, fMRI detected separate representations for all 5 fingers in the primary somatosensory cortex. Responses were located in the posterior wall of the deep central sulcus (most likely corresponding to Brodmann Area (BA) 3b), and the anterior (BA 1) or posterior crown of the postcentral gyrus (BA 2) with rare activations in BA 3a and 4. In BA 3b we found a regular somatotopic mediolateral digit arrangement for fingers 5 to 1 with a mean Euclidean distance of 16 mm between fingers 1 and 5. In contrast BA 1/2 showed a greater number of adjacent activation foci with significantly more overlap and partly even reversed ordering of neighbouring fingers.

fMRI assessment of somatotopy in human Brodmann area 3b by electrical finger stimulation.

Functional magnetic resonance imaging (fMRI) is capable of detecting focal brain activation induced by electrical stimulation of single fingers in human subjects. In eight subjects somatotopic arrangement of the second and fifth finger was found in Brodmann area 3b of the primary somatosensory cortex. In four subjects the representation area of the second finger was located lateral and inferior to the fifth finger; in one subject the somatotopy was reversed. In three subjects representation areas of the two fingers in Brodmann area 3b were found overlapping. Additional activated areas were found on the crown of ipsilateral and contralateral postcentral gyrus (Brodmann areas 1 and 2) and posterior parietal cortex.