Hierarchical and context-dependent encoding of actions in human posterior parietal and motor cortex.

Action understanding requires internal models that link visual input to motor goals. In monkeys, mirror neurons have been proposed to support this process through motor resonance during observation, yet single-unit evidence in humans remains scarce. We recorded neural activity from the motor cortex (MC) and the superior parietal lobule (SPL) in two tetraplegic participants implanted with Utah arrays while they intended or observed hand actions. MC strongly encoded intention but showed weak responses during observation, evident primarily at the population level. In contrast, SPL supported shared representations across intended and observed actions at both the single-unit and population levels. When incongruent instructed and observed actions occurred simultaneously, SPL encoded the observed actions only when they were behaviorally relevant, whereas MC remained intention dominant. Our results identify a context-dependent gating mechanism in SPL and suggest a hierarchical organization in which MC maintains intention-specific codes while SPL flexibly links observed actions with internal goals.