Proprioception: a sense to facilitate action

Abstract Proprioception is commonly described as the sense of body position and movement. However, an underappreciated aspect of this “hidden sixth sense” is how intimately it is tied to the systems that execute movement. Loss of proprioception devastates our ability to move; without the ability to replace it, a motor brain-computer interface cannot be expected to function well. Proprioceptive receptors, located mostly within the muscles and joints, signal limb state that is the consequence of descending motor commands to the muscles and their interaction with the effects of external forces. As proprioceptive afferents ascend the neuroaxis, they contribute to reflexes and muscle coordination before reaching the brainstem and thalamus, subject to descending modulatory influence from both the brainstem and sensorimotor cortex at all levels. Within the somatosensory cortex, area 3a, with its largely muscle-receptor inputs, shares many common features with the adjacent motor cortex and is certainly intimately involved in movement execution. The higher level representation that results from combination with cutaneous inputs in area 2 likely provides critical limb state information to the posterior parietal cortex for use in movement planning. Proprioceptive signals are also involved in internal model formation and error calculation in the cerebellum, where deficits share many features in common with those experienced following deafferentation. The challenge of developing a proprioceptive brain interface is certainly a technical one, but also one of reaching a deeper understanding of all the systems—and their interconnectedness—outlined in this chapter.