Using monkey hand exoskeleton to explore finger passive joint movement response in primary motor cortex

While neurons in primary motor cortex (M1) have been shown to respond to sensory stimuli, exploration of this phenomenon has proven challenging. Accurate and repeatable presentation of sensory inputs is difficult. Here, we describe a novel paradigm to study response to joint motion and fingertip force. We employed a custom exoskeleton to drive index finger metacarpophalangeal joint (MCP) of a macaque to follow sinusoid trajectories at 4 different frequencies (0.2, 0.5, 1, 2Hz) and 2 movement ranges (68.4, 34.2 degrees). We highlight results of a specific M1 unit that displayed sensitivity to direction (more active during flexion than extension), frequency (greater firing rate at higher frequencies), and movement amplitude (higher rate at larger amplitude). Joint movement trajectories were accurately reconstructed from this single unit with mean R
²
=0.64 ± 0.13. The exoskeleton holds promise for examination of sensory feedback. In addition, it can be used as an external device controlled by a brain-machine interface (BMI) system. The proprioceptive related units in M1 may contribute to improving BMI control performance.