Tactile interference to the face affects magnetic responses elicited by electric thumb stimulation

Objective: We examined the effect of tactile interference to the face on somatosensory evoked magnetic fields (SEFs) following electric thumb stimulation. Methods: SEFs were elicited by electric stimulation of the right thumb in a control and two interference conditions. In the interference conditions, continuous tactile stimuli were delivered to the skin surface over the right upper face or the right thumb. Results: The face interference significantly attenuated N20m and enhanced P30m. The amplitudes of N20m in the face and thumb interference conditions were 90.3 and 70.3% of the value in the control condition, respectively, while those of P30m were 120.2 and 74.4%. Conclusions: In human somatosensory area 3b, the representation of the thumb is immediately adjacent to that of the face although the thumb and face are physically distant. We suggest, therefore, that the effect of tactile interference on N20m depends on a cortical distance between electrically and mechanically activated 3b areas, rather than a physical distance between the body parts to which these two stimuli were administered. Although it is unclear why the face interference specifically enhanced the P30m, it is suggested that the generating mechanism of the interference effect on P30m may be different from that on N20m. Significance: The tactile interference effect on N20m does not depend on the physical distance between electrically and mechanically activated skin areas, but on the distance of the 3b cortex receiving these two inputs.