Microscale organization and separability of upper extremity representations in the human motor homunculus

Understanding the microscale spatial organization of the human motor homunculus is essential for designing surface-based brain-computer interfaces (BCIs). We investigated these dynamics using the highest-density clinically available subdural microelectrode arrays (1024 channels, 400 micrometers pitch) temporarily implanted in 11 neurosurgical patients undergoing awake surgery. We mapped broadband high gamma activity (>80 Hz) during upper extremity movements across 9 joints and hand gestures (rock, paper, scissors). Gestures produced consistent, localized spatial patterns in M1/S1, revealing shared microscale hand somatotopy across participants. Joint mapping revealed somatotopic representations organized as concentrically larger activation regions from distal to proximal joints. We characterized persistent spatial gradients in high gamma activity and representational overlap at microscale resolution. While previous macroscale studies showed overlapping motor representations, our high-density recordings provided a much finer mapping of this overlap and revealed a relationship between overlap degree and decoding performance. Our findings reveal a previously unobserved microscale mapping of motor commands in M1 and S1 and suggest that finer spatial resolution is necessary to decode complex movements from the brain surface.