Auditory cortex involvement in processing inaudible vibrotactile inputs: Differences between tactile hypo- and hyper-sensitive groups: 39th Annual Meeting of the Society for Neuroscience

Given that many activities that produce tactile sensations also produce sound, it is not surprising that a growing body of evidence suggests that tactile stimulation can activate auditory cortex even when the stimulus is inaudible. Recently, it has been suggested that coactivation of somatosensory and auditory cortex by stimuli that are felt and heard improves vibrotactile frequency discrimination in healthy individuals (Iguchi et al 2007). We used functional magnetic resonance imaging (fMRI) to investigate the possible relationship between somatosensory and auditory cortices in tactile processing in two groups with aberrant sensitivities to tactile stimulation compared to controls: 13 individuals with temporomandibular disorders (hyposensitive) and 6 individuals with autism (hypersensitive). A series of 4 second pulses of inaudible vibrotactile stimuli (26 Hz, 400 μm peak-to-peak) were applied to the pad of the right index finger, and subjects were instructed to keep their eyes closed. Because the extent to which tactile input accesses auditory cortex has been shown to be dependent on the extent of spatial smoothing applied to fMRI images (Schurmann et al, 2006), we employed minimal smoothing (less than the extent of 2 voxels in any direction). Separate control groups were used for comparison with the two patient groups to ease IQ-matching with the autism group. Preliminary results indicate that the tactile stimuli were highly effective in evoking responses in contralateral primary and secondary somatosensory cortex (SI and SII, respectively) in all groups, although some dissociation between groups within these regions was observed. In addition, our TMD group demonstrated greater activation bilaterally in primary auditory cortex than controls, and decreasing peak percent signal change in area 1 of contralateral SI predicted increasing peak percent signal change in contralateral primary auditory cortex in the TMD group. While activity was evoked in a small region of auditory cortex in our autism group, there was no significant difference in this area between our autism and control groups, suggesting that auditory cortex is differentially involved in the modulation of tactile sensation in autism and TMD.