The Somatosensory Cortex and Body Representation: Updating the Motor System during a Visuoproprioceptive Cue Conflict.

The brain's representation of hand position is critical for voluntary movement. Representation is multisensory, combining visual and proprioceptive cues. When these cues conflict, the brain recalibrates its unimodal estimates, shifting them closer together to compensate. Research suggests such updates to body representation are communicated to the motor system to keep hand movements accurate. The neural mechanism is unclear and may depend on how the brain integrates and recalibrates visuoproprioceptive signals: models ranging from hierarchical convergence after unisensory processing to more distributed frameworks have been proposed. We hypothesized that the primary somatosensory cortex (S1) is crucial in this updating process due to its role in proprioception and connections with both primary motor cortex (M1) and multisensory regions of the posterior parietal cortex (PPC). In human participants of both sexes, Experiment 1 showed that short-latency afferent inhibition, a measure of somatosensory-motor integration, changed with proprioceptive recalibration and only in the presence of a cue conflict. This indicates that S1 activity reflects the results of multisensory computations and is inconsistent with a pure hierarchical convergence model of visuoproprioceptive integration. Experiment 2 found that modulating S1, but not M1, with repetitive transcranial magnetic stimulation increased proprioceptive variance and recalibration. This is consistent with the idea that motor effects of proprioceptive recalibration are mediated by the S1→M1 pathway, although not directly controlled by M1 itself. The specificity of our findings to proprioceptive-not visual-recalibration argues against a fully distributed framework, but our findings support a model of multisensory integration with reciprocal interactions between S1 and PPC.