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PERSPECTIVES IN BIOLOGY AND MEDICINE Volume 23 ¦ Number 2, Part 1 ¦ Winter 1980 REFLECTIONS ON MOTRICITY RAGNAR GRANIT* My intention here is to make some reflections on what Sherrington once termed "motricity," enumerate some difference between it and the sensory sphere, and consider prospects of advance in studies of motor behavior. Time, Training, and Timing The act of training for a movement introduces a highly pertinent domain of differences between the motor and sensory fields. There is no equivalent role for training in the realm of perception. We may speak of a trained observer, but by this expression we mean a person who from experience knows what to go for. Training in motor performance signifies adjustment of entries and exits of different muscles, of forces applied and their proper duration, all in the service of perfect execution of a motor act of greater or lesser complexity. Correct timing of agonists and antagonists is a major item in the operations subject to training. Consider, for instance, organ playing. The score is read visually, and its instructions are transmitted immediately to arms, hands, fingers, and feet. An incredible number of motoneurons along the spinal cord are being mobilized to act in strictly timed sequences of excitation and inhibition , variable recruitment orders, and firing rates. Anyone who has seen an organist in action must have noticed that the rest of his body is also engaged in balancing out movements of arms and legs, often moving in different directions. All this could not be done by detailed instructions from the brain unless there were mechanisms serving swift exchanges of muscles at the *The Nobel Institute for Neurophysiology, Karolinska Institutet, S-104 01 Stockholm 60, Sweden.© 1980 by The University of Chicago. 0031-5982/80/2302-01 15$01.00 Perspectives in Biology and Medicine · Winter 1980 \ 171 spinal level near the sources of power that have to be handled. The recent careful studies of reciprocal innervation by the Gothenburg workers (Lundberg, Hultborn, Jankowska), as summarized by Lundberg [1], have shown that such switches really exist and are supraspinally modifiable with incredible finesse. The emphasis on timing in movement should not be taken to mean that time is negligible in perception. In the visual field, it is rendered into a perception of velocity; in acoustics, time differences in the stimulation of the two ears are translated into ajudgment on sound localization, and the tonal world of perception is built upon an analysis of periodical time differences. However, this role of"time" in perception differs from that in motricity in characteristic respects; thus, sensory time is not closely tied to a change of purpose as are timed movements of, for instance, hands and fingers which immediately alter their pattern of execution in response to commands from above. It is not possible to produce an entirely new tone or colour, never heard of or seen before, by time changes alone, whereas wholly new movements can be created instantaneously as a matter of course by retiming the entries and exits of agonists and antagonists around a joint. Time pervades the whole domain of biology, but the emphasis on behaviorally significant mechanisms for fast timing is a special feature of motricity. We have, in fact, made striking advances in motor physiology by following up problems of timing in motor circuits, whereas progress in sensory physiology has taken place along different lines as an analysis of building blocks for perception. Invariance and Variability An important role of the sense organs is to create a measure of predictable invariance and to explore deviations from this norm. This becomes strikingly obvious if we allow ourselves to imagine that evolution at one time in the past created a lens focusing light on a sensitized surface. This invention was made at the tremendous cost of distorting the environment, in particular by making the size of the image depend upon the distance of the object from the receptors. For survival, this fault had to be corrected at the neural level, and this was engineered by placing an analyzing piece of the brain itself (the retina) in the eye and creating a supraretinal apparatus for interpreting the information in order to reestablish the lost invariance of...


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