Predictive motor learning of temporal delays

Anticipatory responses can minimize the disturbances that result from the a ction of one part of the body on another. Such a predictive response is evi dent in the anticipatory increase in grip force seen when one hand pulls on an object held in the other hand, thereby preventing the object from slipp ing. It is postulated that such a response depends on predicting the conseq uences of the descending motor command, as signaled by efference copy, usin g an internal model of both one's own body and the object. Here we investig ate how the internal model learns the temporal consequences of the motor co mmand. We employed two robots to simulate a virtual object held in one hand and acted on by the other. Delays were introduced between the action of on e hand on the object and the effects of this action on the other hand. An i nitial reactive grip force response to the delayed load decayed with the de velopment of appropriate anticipatory grip force modulation. However, no pr edictive modulation was seen when the object's movement was not generated b y the subject, even when the motion was cued by a tone. These results sugge st that, when an internal model learns new temporal relationships between a ctions and their consequences, this learning involves generating a novel re sponse rather than adapting the original predictive response.