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Development of an afferent neural interface designed to mimic natural proprioception

Brain Machine Interfaces (BMIs) that use recordings from motor areas of the brain to effect movement of a robotic limb or even a patient's paralyzed limb have progressed tremendously in the past decade. However, a major issue to be addressed is the need to provide proprioceptive feedback through an afferent neural interface. Loss of proprioception largely eliminates the ability to plan movement dynamics or to make rapid corrections to limb perturbations even in the presence of vision. The representation of proprioceptive signals within the cortex has been far less studied than has touch, and while some progress has been made toward restoring touch through intracortical micro-stimulation of somatosensory cortex (S1), there has been as yet, very little corresponding success for proprioception. We have completed a series experiments designed to study the way limb movements are encoded by neurons in area 2 of S1. These neurons signal limb movement, whether generated actively by the monkey or as the result of a passive limb displacement. The discharge of most neurons is tuned to the direction of hand movement and can be summarized reasonably accurately by a sinusoidal tuning curve with a single “preferred direction” (PD). There is even evidence of an efference copy component of S1 activity that precedes the onset of active movement and is well aligned spatially with the afferent component. The representation of different movement directions by populations of S1 neurons is linearly separable, as is the brain state representing active and passive movements. The latter is likely due to the interaction of kinematic and force representation by individual neurons. We have now begun a new series of experiments, the goal of which is to evoke a sensation of directed limb movement by stimulating electrodes within S1 to recreate these natural patterns of cortical activity. By stimulating small groups of electrodes with similar PDs, we have succeeded in inducing perceptions of limb motion that appear to be similar to those caused by actual movement. We are working to develop a neuroprosthesis based on continuously varying stimulation of many electrodes, in order to restore proprioceptive feedback to patients with high-level spinal cord injury.