The Contribution of Whole-Limb Kinematics to Proprioceptive Representations in the Central Nervous SystemPublic
We have a remarkable ability to perform complex, coordinated movements without much conscious effort. In addition to the computations required to generate commands for muscles, a key aspect of coordinated motor control is incorporating sensory feedback about the movement. One of the most important feedback routes is through proprioception, the sense of body position, movement, and related forces. This sense is so critical that loss of proprioception typically leaves a person wheelchair-bound, despite the retained ability to activate muscles. The sensory organs for proprioception lie in the muscles, sensing their length and the forces on them. However, classic studies of proprioceptive neurons in primary somatosensory cortex (S1) suggest that activity relates to hand movement, implying that on the way to S1, proprioceptive signals are transformed from representing kinematics in terms of muscle to one in terms of the hand. Surprisingly, another classic study in the dorsal spinocerebellar tract (DSCT) suggested that this transformation may take place as early as the spinal cord. However, these classic studies did not consider how musculoskeletal geometry might contribute to the proprioceptive activity in DSCT and S1. This dissertation outlines my work in examining how proprioceptive information is processed as it travels from muscle receptors to S1. Chapter 2 details a simulation study of DSCT, in which I found that an apparent limb-endpoint-based representation can arise simply from a convergence of muscle inputs. Chapter 3 extends these results to an electrophysiological study in monkeys, where I found that neurons in S1 represent not just the kinematics of the hand, but the whole arm. Taken together, these studies suggest that if proprioceptive signals transform to a hand-based representation in the brain, this transformation likely occurs beyond S1.
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