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Abstract:
Visual assessment of a surface texture prior to actually touching it is crucial when interacting with objects. In particular, perception of surface characteristic is of great importance to refine planning of grasping movements. To date, a number of functional magnetic resonance imaging (fMRI) studies have consistently reported significant activations in human somatosensory cortices during observation of touch actions, i.e. in absence of any haptic sensation. However, it is still debated which brain region is mainly associated with the processing of observed touch: The primary somatosensory cortex (S1) [1], the secondary somatosensory cortex (S2) [2], or the posterior parietal cortex (PPC) [3]. In this study, using whole-brain searchlight multivoxel pattern analyses (MVPA), we searched for brain regions exhibiting neural activity patterns encoding perceived roughness intensities. Fifteen healthy volunteers with no deficits in tactile and visual processing participated in this study. During the experiment, participants first explored a set of differently colored sandpapers of increasing roughness intensity with their right index fingertip outside of the MR room. During functional image acquisition (T2*-weighted a slice-accelerated multiband gradient-echo-based EPI, TR = 1,520 ms, voxel size = 3.0 × 3.0 × 3.0 mm), participants performed 3 fMRI runs with 25 trials. Each trial was consisted of two consecutive periods: A stimulation period of 3 s followed by a fixation resting period of 9 s. During the stimulation period, 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 representation of the roughness intensity of the sandpapers 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 information processing of observed touch.
