Somatosensory signals and sensorimotor transformations in reactive control of grasp

Summary

While restraining an object subjected to unpredictable destabilizing load forces, subjects use sensory information related to load changes to automatically generate grip responses via a task-specific ‘load-to-grip’ sensorimotor transformation. Tactile receptors of the skin in contact with the object are the only type of mechanoreceptors capable of triggering and scaling appropriate grip responses, but muscle and joint afferents may provide information related to the reactive forces produced by the subject. The employed sensorimotor transformation appears to exploit short term predictions of the rate of load changes. Furthermore, the friction in the object-digit interface modifies the gain of the entire load-to-grip force transformation based on frictional information obtained during initial skin-object contact. The initiation, as well as the regulation of the motor responses, seem to depend on supraspinal control mechanisms. Evidence is provided that the human motor cortex is involved in the regulation of already initiated grip responses, but that their initiation most likely depends on brain stem networks. Afferent inflow may also contribute in the regulation of already initiated responses via fast spinal networks.