Modulation of somatosensory evoked magnetic fields by intensity of interfering stimuli in human somatosensory cortex: an MEG study.

Somatosensory evoked responses are known to be modulated by previous interfering stimuli. Here, we first investigated the modulatory effects of interfering stimuli with different intensities on somatosensory evoked magnetic field in human primary (S1) and secondary (S2) somatosensory cortices. In the control condition of the study, test stimulus, set to strong intensity, was delivered to the left median nerve. Interfering stimuli with three different levels of intensity from weak (WI) through moderate (MI) and finally to strong (SI) were interspersed to the left median nerve between the test stimuli in each interfering condition. The cortical responses to the test stimulus were modeled with equivalent current dipoles in the contralateral S1 and bilateral S2 cortices from 17 subjects. The amplitude of the N20m deflection from the S1 was not changed by any interfering stimuli, whereas the amplitude of later P35m deflection was reduced by MI stimulus. The amplitude of P60m deflection was reduced by MI and SI stimuli. The extent of amplitude reduction of the bilateral S2 response was markedly increased as intensity of interfering stimuli increased from weak to moderate, but further reduction by the SI stimuli compared to MI stimuli was not observed. Those results indicated that somatosensory cortical activation in the S1 (P35m and P60m) and S2 were modulated by intensity of interfering stimuli. Our findings of a greater gating effect on the bilateral S2 compared to the contralateral S1 indicate that S2 may play an important role in temporal integration of different intensity levels of somatosensory inputs.