Psychophysical investigations into cortical encoding of vibrotactile stimuli.

Neurons in primary somatosensory cortex (S1) respond to vibrotactile stimuli by firing in phase with each cycle of the vibration. We have investigated how neural activity in S1 might contribute to people's perception of vibration frequency. The contribution of S1 was confirmed using transcranial magnetic stimulation (TMS): accuracy in comparing the frequency of two sequential vibrations was reduced by a single TMS pulse delivered to S1 in the interval between the two vibrations. More recent experiments have revealed that participants use the velocity (or energy) of the stimulus when judging its frequency. This is consistent with a contribution from S1: electrophysiological recording in S1 cortex of rats shows that neurons in S1 do not explicitly code vibration frequency, but instead code the product of frequency and amplitude (proportional to the mean velocity or energy of the vibration). Further, frequency discrimination is reduced by the addition of even very small amounts of noise to the temporal structure of the vibrations (making them irregular). However, noise has no effect if the two vibrations are presented on opposite fingertips (i.e. beyond the range of receptive field sizes of neurons in S1), or if there is no difference in their velocity. Therefore, when judging vibration frequency, humans utilize information about stimulus velocity as coded by neurons in S1, but this coding is dependent on the temporally regular input of the vibration.