Different laterality between the thumb and index finger in human SII activities.

We compared the ipsilateral and contralateral evoked activities in the secondary somatosensory cortex (SII) of four human subjects. Magnetic cortical fields, evoked by electrical stimulation of the distal interphalangeal joint of either the thumb or the index finger, were recorded by an 80-channel whole head type magnetoencephalography system (MEG). We calculated the ratio of the equivalent current dipole (ECD) moment of the ipsilateral area to that of the contralateral area. The mean ratio for the thumb across the subjects was 0.43 +/- 0.13, while that for the index finger was 0.77 +/- 0.20 (mean +/- s.d). The ratio for the index finger was significantly higher than that for the thumb (p = 0.039, two tailed t-test). These results show that the cortical index finger area is activated more bilaterally than the thumb area.