Interaction of tactile input in the human primary and secondary somatosensory cortex - A magnetoencephalographic study

Interaction of simultaneous tactile input at two finger sites in primary (S I) and secondary somatosensory cortex (SII) was studied by whole-head magne toencephalography. Short pressure pulses were delivered to fingers of the r ight and left hand at an interstimulus interval of 1.6 s. The first phalanx of the left digit 1 and four other sites were stimulated either separately or simultaneously. We compared four sites with increasing distance: the se cond phalanx of left digit 1, left digit 5, and digits 1 and 5 of the right hand. The temporal evolution of source activity in the contralateral SI an d bilateral SII was calculated using spatiotemporal source analysis. Intera ction was assessed by comparing the source activity during simultaneous sti mulation with the sum of the source activities elicited by separate stimula tion. Significant suppressive interaction was observed in contralateral SI only for stimuli at the same hand, decreasing with distance. In SII, all di gits of the same and the opposite hand interacted significantly with left d igit 1. When stimulating bilaterally, SII source waveforms closely resemble d the time course of the response to separate stimulation of the opposite h and. Thus, in bilateral simultaneous stimulation, the contralateral input a rriving first in SII appeared to inhibit the later ipsilateral input. Simil arly, the separate response to input at two unilateral finger sites which a rrived slightly earlier in SII dominated the simultaneous response. Our res ults confirm previous findings of considerable overlap in the cortical hand representation in SII and illustrate hemispheric specialization to contral ateral input when simultaneous stimuli occur bilaterally. (C) 2001 Academic Press.