Observation-related activity in the human motor cortex increases with effector anthropomorphicity

Neurons in the motor cortex can be engaged not only in motor execution but also during observation of movements performed by other anthropomorphic agents (i.e., humans or monkeys). However, it is unknown how motor cortical neurons respond during observation of the range of assistive or prosthetic devices controlled by people using intracortical brain-computer interfaces (iBCIs). We recorded single-unit activity in the precentral gyrus while iBCI users viewed grasp-like movements performed by a spectrum of virtual effectors that included human, robotic, and hand-like dot stimuli. We found a relationship between neural modulation and effector anthropomorphicity (i.e., human-likeness) that existed on an ensemble-wide and individual-neuron level, suggesting that human motor cortex activity incrementally increases in response to the visually observed agent's human-likeness. Both solicited and spontaneous feedback from the participant indicated a relationship between neural activity and subjective assessments of anthropomorphicity, revealing a powerful contribution of context on observation-induced activity in the motor cortex. The activity of the motor cortex remained similar during attempted hand movements while different effectors were being observed, suggesting that intuitive external device control via iBCIs may not be overtly affected by the anthropomorphicity of the effector.