Project Summary/Abstract: The larger goal of this study is to gain foundational knowledge and demonstrate the feasibility of restoring percepts of arm muscle force using intracortical microstimulation (ICMS) in cortical area 3a. Restoring this specific sensory percept is particularly important to paralyzed individuals that use brain-controlled functional electrical stimulation (FES) of their peripheral nerves to restore arm movements by thought after spinal cord injury. Appropriate balancing of antagonist muscle forces in this population is critical for maintaining limb stability and minimizing muscle fatigue and battery drain. In Aim 1, we will use primate imaging and surgical planning software to implant custom chronic microwire electrode arrays deep into the elbow region of the central sulcus of two Rhesus macaques (four hemispheres total) to target the difficult-to-reach somatosensory area 3a. Area 3a receives a range of proprioceptive signals from Golgi tendon organs (conveys muscle force) and muscle spindle fibers (conveys muscle length and change in length) and should provide muscle-specific percepts most directly relevant to optimizing FES arm movement restoration. However, most of what is known about 3a proprioceptive encoding has been collected from acute recordings. In this study, we will use chronic microelectrode recordings and a range of active and passive sensorimotor tasks across days to more fully characterize 3a encoding. These data will then be used to quantify the relative function and spatial distribution of these different sensory encoding characteristics on the spatial scale relevant to restoring these sensations via ICMS. In the second aim, we will use a novel force tracking game to test the animals' ability to perceive ICMS of force-dominant electrode locations as an increase in muscle force. This game is designed to prevent the animals from just learning to respond to some unnatural `sensation' for a reward and will verify that the subjects are actually utilizing the force percepts in their motor planning. Finally, we will conduct these tests using a range of stimulus parameters to quantify the magnitude of the force percepts as a function of stimulation parameters.