Complex spatiotemporal tuning in human upper-limb muscles.

Correlations between neural activity in primary motor cortex (M1) and arm kinematics have recently been shown to be temporally extensive and spatially complex. These results provide a sophisticated account of M1 processing and suggest that M1 neurons encode high-level movement trajectories, termed "pathlets." However, interpreting pathlets is difficult because the mapping between M1 activity and arm kinematics is indirect: M1 activity can generate movement only via spinal circuitry and the substantial complexities of the musculoskeletal system. We hypothesized that filter-like complexities of the musculoskeletal system are sufficient to generate temporally extensive and spatially complex correlations between motor commands and arm kinematics. To test this hypothesis, we extended the computational and experimental method proposed for extracting pathlets from M1 activity to extract pathlets from muscle activity. Unlike M1 activity, it is clear that muscle activity does not encode arm kinematics. Accordingly, any spatiotemporal correlations in muscle pathlets can be attributed to musculoskeletal complexities rather than explicit higher-order representations. Our results demonstrate that extracting muscle pathlets is a robust and repeatable process. Pathlets extracted from the same muscle but different subjects or from the same muscle on different days were remarkably similar and roughly appropriate for that muscle's mechanical action. Critically, muscle pathlets included extensive spatiotemporal complexity, including kinematic features before and after the present muscle activity, similar to that reported for M1 neurons. These results suggest the possibility that M1 pathlets at least partly reflect the filter-like complexities of the periphery rather than high-level representations.