Reach-relevant somatosensory signals modulate activity in the tactile suppression network

Somatosensory signals on a moving limb are typically suppressed. This results mainly from a predictive mechanism that generates an efference copy, and attenuates sensory feedback signals that match the predicted sensory consequences. Sensory feedback is, however, important for movement control. Behavioral studies show that the strength of suppression depends on the relevancy of feedback signals for the movement. It is still unknown how this interplay of predictive and feedback signals is neurally implemented. In this fMRI study, we manipulated the relevancy of somatosensory signals for reaching by asking participants to reach to a somatosensory (static finger) or an external target (touchscreen) without vision. To probe suppression, participants detected brief vibrotactile stimuli on their moving finger prior to reach. As expected, probes were suppressed during reaching compared to baseline (i.e. rest), but suppression was stronger during somatosensory than external reaching. BOLD activation associated with suppression was modulated by the reach goal: processing of probes during somatosensory reaching led to distinct BOLD deactivations in fronto-parietal regions compared to baseline and external reaching. In line with the behavioral results, these deactivations were more pronounced during somatosensory than external reaching in regions linked with somatosensory processing and integration (postcentral gyrus, anterior insula, supramarginal gyrus (SMG)), and sensorimotor prediction (supplementary motor area,). Somatosensory reaching was also linked with increased coupling of the left SMG with the right parietal operculum and the right anterior insula. We suggest that movement-related somatosensory processing is modulated by the relevance of somatosensory feedback signals for reaching by down-regulating activation in regions associated with predictive and feedback processing