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Neurobiology

Making smooth moves

Nature volume 394, pages 725–726 (1998)Cite this article

Whether reaching, throwing, running or dancing, our natural tendency is to make smooth and precise movements. Out of the infinite number of ways that we could have made a particular movement, we generally pick the one that is the smoothest. The current thinking in the field of motor control is that the smooth, stereotyped trajectories made by our motor system are specially chosen to minimize jerkiness1,2 and to maximize efficiency3. Or could it be that smoothness is a by-product of a more fundamental computational goal of the motor system, a goal that only makes us look graceful by accomplishing something else?

On page 780 of this issue4, Harris and Wolpert propose an alternative to the principle of maximum efficiency: the principle of maximum precision. On the face of it, making a precise movement does not seem to imply smoothness. Imagine that the goal is to touch an object as precisely as possible in a fixed amount of time. Getting to the spot as quickly as possible and making a careful landing might be a better way to ensure precision rather than making the arm movement as smoothly as possible. What this high-acceleration strategy does not take into account, however, is that the motor neurons that control the arm are noisy and cannot be counted on to get the arm to the same place for the same command. In particular, giving your muscles strong commands is exactly the wrong thing to do because the variability in the muscle output increases with the strength of the command. When they take activity-dependent motor neuron noise into account, Harris and Wolpert find that optimizing the precision of the endpoint of a movement produces smooth movements with exactly the properties of those that we tend to make.

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Figure 1: Hand trajectories for reaching before and after rotation of the body, showing that smooth movements are made even after adaptation in an altered environment.

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Authors and Affiliations

  1. Howard Hughes Medical Institute, The Salk Institute for Biological Studies, 10010 North Torrey Pines Road, La Jolla, 92037, California, USA

    Terrence J. Sejnowski

  2. University of California at San Diego, 9500 Gilman Drive, La Jolla, 92093, California, USA

    Terrence J. Sejnowski

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  1. Terrence J. Sejnowski
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Sejnowski, T. Making smooth moves. Nature 394, 725–726 (1998). https://doi.org/10.1038/29406

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