Co-activation of the secondary somatosensory and auditory cortices facilitates frequency discrimination of vibrotactile stimuli

The contribution of the auditory cortex to tactile information processing was studied by measuring somatosensory evoked magnetic fields (SEFs). Three kinds of vibrotactile stimuli with frequencies of 180, 280 and 380 Hz were randomly delivered on the right index finger with a probability of 40, 20 and 40%, respectively. Twenty normal subjects participated in four kinds of tasks: a control condition to ignore these stimuli, a simple task to discriminate the 280-Hz stimulus from the other two stimuli (discrimination task for the vibrotactile stimuli, Ts task), a feedback task modified from the Ts task by adding acoustic feedback of the vibratory frequency at 1300 ms poststimulus (tactile discrimination with auditory clues, TA), and an easy version of the TA task (TA-easy) to discriminate the 280-Hz stimulus (20% target) from the 180- or 380-Hz stimuli (80% nontarget). The Ts and TA tasks required accurate perception of the vibrotactile frequencies to discriminate among the three kinds of stimuli. Under such a task demand, the post hoc auditory feedback in the TA task was expected to induce acoustic imagery for the tactile sensation. The SEFs for the nontarget stimuli were analyzed. A middle-latency component (M150/200) was specifically evoked by the three discrimination tasks. In the Ts and TA-easy tasks, the M150/200 source indicated inferior parietal cortical activities (SII area). In the TA task, 11 subjects showed activity in both the SII area and the superior temporal auditory region and increased accuracy of discrimination compared with the Ts task, in contrast with other subjects who showed activity only in the SII area and small changes in task accuracy between the Ts and TA tasks. Asynchronous auditory feedback for the vibrotactile sensation induced the auditory cortex activity in the SEFs in relation to the progress in tactile discrimination, which suggested an induction of acoustic imagery to complement the tactile information processing.