Visual Assessment of Tactile Roughness Intensity

A number of neuroimaging studies have consistently reported significant activations in human somatosensory cortices during observation of touch actions. However, it is still debated which brain region is mainly associated with the processing of observed touch (e.g. primary somatosensory cortex; S1, secondary somatosensory cortex; S2, posterior parietal cortex; PPC). In this fMRI study, we searched for brain regions exhibiting neural activity patterns encoding visually evoked roughness intensities. Fifteen healthy volunteers with no deficits in tactile and visual processing participated. They first explored a set of differently colored sandpapers with their right index fingertip outside of the MR room. During the fMRI experiment, video clips of tactile explorations of the sandpaper set were presented and the participants were asked to recall the perceived roughness intensity as vividly as possible. The neural representations of the roughness intensities could be successfully decoded from the brain signals elicited by the video clips in the absence of any intrinsic tactile content. In particular, a random-effects group analysis revealed that four brain regions encoded the different roughness intensities distinctively: The bilateral PPC, the primary visual cortex (V1), and the ipsilateral S1. Although we found brain activations in ipsilateral S1, we cannot confirm the S1 engagement because the majority of previous studies have reported brain activations in contralateral S1. Significant decoding accuracies in V1 may be attributed to differences of visual contents in the presented video clips. Therefore, among the three brain regions mentioned above, our findings supported the hypothesis that especially the PPC plays an important role in the processing of observed touch