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Overlap of internal models in motor cortex for mechanical loads during reaching


A hallmark of the human motor system is its ability to adapt motor patterns for different environmental conditions, such as when a skilled ice-hockey player accurately shoots a puck with or without protective equipment. Each object (stick, shoulder pad, elbow pad) imparts a distinct load upon the limb, and a key problem in motor neuroscience is to understand how the brain controls movement for different mechanical contexts1,2. We addressed this issue by training non-human primates to make reaching movements with and without viscous loads applied to the shoulder and/or elbow joints, and then examined neural representations in primary motor cortex (MI) for each load condition. Even though the shoulder and elbow loads are mechanically independent, we found that some neurons responded to both of these single-joint loads. Furthermore, changes in activity of individual neurons during multi-joint loads could be predicted from their response to subordinate single-joint loads. These findings suggest that neural representations of different mechanical contexts in MI are organized in a highly structured manner that may provide a neural basis for how complex motor behaviour is learned from simpler motor tasks.

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Figure 1: Two hypotheses about the neural control of different mechanical loads. A single-controller that encapsulates all load contexts (a), or multiple controllers, each of which represent individual loads (b).
Figure 2: Neural activity of two cells during movement in different dynamic loads.
Figure 3: Changes in cell activity across three load conditions.
Figure 4: Neural representation of mechanically dependent loads.


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We thank K. Moore for technical assistance, and D.W. Cabel and S. Chan who assisted in some of the training and neuronal recording sessions. We thank D. Munoz, M. Pare and K. Rose for comments on this manuscript. This work was supported by a CIHR grant and scholarship (S.H.S.) and a CIHR postdoctoral fellowship (P.L.G.).

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Correspondence to Stephen H. Scott.

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S.H.S. holds a US patent for the robotic device used in these experiments.

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Gribble, P., Scott, S. Overlap of internal models in motor cortex for mechanical loads during reaching. Nature 417, 938–941 (2002).

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