Effect of acceleration of auditory inputs on the primary somatosensory cortex in humans.

Cross-modal interaction occurs during the early stages of processing in the sensory cortex; however, its effect on neuronal activity speed remains unclear. We used magnetoencephalography to investigate whether auditory stimulation influences the initial cortical activity in the primary somatosensory cortex. A 25-ms pure tone was randomly presented to the left or right side of healthy volunteers at 1000 ms when electrical pulses were applied to the left or right median nerve at 20 Hz for 1500 ms because we did not observe any cross-modal effect elicited by a single pulse. The latency of N20 m originating from Brodmann's area 3b was measured for each pulse. The auditory stimulation significantly shortened the N20 m latency at 1050 and 1100 ms. This reduction in N20 m latency was identical for the ipsilateral and contralateral sounds for both latency points. Therefore, somatosensory-auditory interaction, such as input to the area 3b from the thalamus, occurred during the early stages of synaptic transmission. Auditory information that converged on the somatosensory system was considered to have arisen from the early stages of the feedforward pathway. Acceleration of information processing through the cross-modal interaction seemed to be partly due to faster processing in the sensory cortex.