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The Journal of Neurophysiology Vol. 84 No. 1 July 2000, pp. 334-343
Copyright ©2000 by the American Physiological Society
Sobell Department of Neurophysiology, Institute of Neurology, University College London, London WC1N 3BG, United Kingdom
Witney, Alice G.,
Susan J. Goodbody, and
Daniel M. Wolpert.
Learning and Decay of Prediction in Object Manipulation. J. Neurophysiol. 84: 334-343, 2000. Anticipating the consequences of our own actions is a fundamental
component of normal sensorimotor control and is seen, for example,
during the manipulation of objects. When one hand pulls on an object
held in the other hand, there is an anticipatory increase in grip force
in the restraining hand that prevents the object from slipping. This
anticipation is thought to rely on a forward internal model of the
manipulated object and motor system, enabling the prediction of the
consequences of our motor commands. Here we investigate the development
of such a predictive response. Each hand held an object that was
attached to its own torque motor. On each trial the subject was
required to pull on the object held in the left hand and to maintain
the position of the object held in the right hand. The torque motors
were computer controlled so that the objects could be either
"linked" so that the forces on the objects were equal and opposite,
acting as though they were a single object, or "unlinked," so that
they acted as two independent objects. A predictive response in the
restraining hand is only necessary when the objects are linked and is
unnecessary in the unlinked condition where there is no risk of the
object slipping. To examine the learning and decay of predictive
responses, we measured the grip force responses during unlinked trials
that followed a linked trial. After a single linked trial, anticipatory grip force was quick to develop, but decayed slowly over the following unlinked trials. Varying the time between trials showed that the rate
of decay depended on the number of trials since the last linked trial
rather than time. Increasing the frequency of linked trials showed an
increased level of subsequent grip force modulation, but did not alter
the decay rate. When the torque motors simulated a linked object that
did not have normal physical properties, prediction was reduced. These
results show that the use of predictive responses has a different time
course for learning and decay, and the response depends on experience
and the physical properties of the objects.
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