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Random change in cortical load representation suggests distinct control of posture and movement


Accurately maintaining a fixed limb posture and quickly moving between postures underlies both everyday skills, including holding and lifting a cup of coffee, and expert skills, such as an Olympic wrestler's holding and throwing an opponent. A fundamental question in limb motor control is whether the brain manages these contrasting goals of posture and movement through a single, robust control process or whether each engages a specialized control process. We addressed this question by examining how individual neurons in the primary motor cortex of macaque monkeys represent mechanical loads during posture and movement tasks. Notably, approximately half of the neurons that expressed load-related activity did so exclusively during either posture only or movement only. Further, those neurons with load-related activity during both tasks randomly switched their magnitude of response between tasks. These random changes in load representation suggest specialized control processes, one for posture and one for movement.

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Figure 1: Neural activity of three cells during posture and reaching tasks.
Figure 2: Load sensitivity of proximal forelimb muscles and M1 neurons during posture and reaching tasks.
Figure 3: Load-related activity across posture and reaching periods with constant-magnitude load.
Figure 4: Running analysis of load-related activity during posture and movement.


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S.H.S. is supported by Canadian Institutes of Health Research (CIHR) Investigator Award and this work is supported by funds from CIHR (MOP 13462 and 200112NET). We thank K. Moore, J. Scott and J. Swaine for expert technical assistance, and P. Cisek, R. Flanagan, J. Kalaska and D. Munoz for comments on earlier versions of the manuscript.

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Correspondence to Stephen H Scott.

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S.H.S. is the president of BKIN technologies, which commercializes the robotic technology used in this study.

Supplementary information

Supplementary Fig. 1

Reaching accuracy with loads. (PDF 304 kb)

Supplementary Fig. 2

Alternative measures of response gain. (PDF 97 kb)

Supplementary Fig. 3

Intra-task reliability of response gain and response direction measures. (PDF 259 kb)

Supplementary Note (PDF 199 kb)

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Kurtzer, I., Herter, T. & Scott, S. Random change in cortical load representation suggests distinct control of posture and movement. Nat Neurosci 8, 498–504 (2005).

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