Evidence against a single coordinate system representation in the motor cortex
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Understanding the coordinate systems in which the motor cortical cells encode movement parameters such as direction is a fundamental yet unresolved issue. Although many studies have assumed that motor cortex encodes direction in an extrinsic, Cartesian (CA) coordinate system, other studies have provided evidence for encoding in intermediate coordinate systems such as a shoulder-centered (SC) or in a purely intrinsic, joint-angle-based (JA) coordinate frame. By simultaneously recording from multiple single units in primary motor cortex, we examined movement direction encoding under each of these three coordinate systems. We directly compared the degree of directional tuning invariance over multiple sub-regions in the workspace. We also compared the mutual information between neuronal firing rate and movement direction in the three systems. We observed a broad range of directional invariance in all three coordinate systems with no strong dominance of any single coordinate system. The mutual information analyses corroborated this observation. However, we found a small but significant bias toward the SC coordinate frame, which was also supported by population vector decoding. Similar results were found when we compared hand/torque force direction encoding in all three coordinate systems. These results suggest that the motor cortex employs a coordinate system that is yet to be discovered or perhaps that the motor cortex should not be viewed as a substrate for any coordinate system representation.
KeywordsMutual Information Motor Cortex Firing Rate Joint Angle Directional Tuning
We thank Greg Ojakangas for rederiving and implementing the equations of motion of the exoskeletal robot together with the animal’s arm. We also thank S. Francis, D. Paulsen and J. Reimer for training the monkeys and collecting the data. Finally, we thank Z. Haga for helpful discussions regarding the data analyses. This work was supported by a grant from the Whitehall Foundation and grants N01-NS-2-2345 and R01 NS45853-01A2 from the NINDS. N. Hatsopoulos has stock ownership in a company, Cyberkinetics, Inc., that is commercializing neural prostheses for severely motor disabled people.