Functional overlap of finger representations in human SI and SII cortices

We aimed to find out to what extent functional representations of different fingers of the two hands overlap at the human primary and secondary somato sensory cortices SI and SII. Somatosensory evoked fields (SEFs) were record ed with a 306-channel neuromagnetometer from 8 subjects. Tactile stimuli, p roduced by diaphragms driven by compressed air, were delivered to the finge rtips in three different conditions. First, the right index finger was stim ulated once every 2 s. Then two other stimuli were interspersed, in differe nt sessions, to right- or left-hand fingers (thumb, middle finger, or ring finger) between the successive right index finger stimuli. Strengths of the responses to right index finger stimuli were evaluated in each condition. Responses to right index finger stimuli were modeled by three current dipol es, located at the contralateral SI and the SII cortices of both hemisphere s. The earliest SI responses, peaking around 65 ms, were suppressed by 18% (P<0.05) when the intervening stimuli were presented to the same hand; inte rvening stimuli to the other hand had no effect. The SII responses were bil aterally suppressed by intervening stimuli presented to either hand: in the left SII, the suppression was 39 and 42% (P<0.01) and in the right SII 67 and 72% (P<0.001) during left- and right-sided intervening stimuli, respect ively. Left- and right-sided intervening stimuli affected similarly the SII responses and had no effect on the response latencies. The results indicat e a strong and symmetric overlap of finger representations for both hands i n the human SII cortices, and a weaker functional overlap for fingers of th e same hand in the SI cortex.