Inter- and intra-limb generalization of adaptation during catching.

We have previously shown that healthy adults require a few trials to adapt to a changed ball weight during catching. It is not known whether this adaptation generalizes to the opposite arm or to different configurations of the same arm. We tested healthy adult subjects catching balls of different weight while maintaining the hand within a vertical spatial "window." In experiment 1, subjects caught a series of light and heavy balls, first with one hand and then with the other. In experiment 2, subjects caught a series of light and heavy balls, first with the catching arm in either a "bent" or a "straight" configuration and then with the same arm in the other configuration. A percentage transfer value was calculated to determine the degree to which previous experience with a given ball weight in one context affected performance of the same task in a new context (i.e., different arm or different arm configuration). Results showed that generalization occurred both between arms and within an arm. However, the subjects who switched arms showed less generalization than those who switched arm positions. Specifically, the percentage transfer value for subjects who switched arms was 58%, while the percentage transfer for those who switched arm positions was 100%. These results support the idea that the motor system is able to generalize adaptive control of ball catching to the contralateral arm and to different arm configurations. Our findings are also in agreement with the recent notion that multiple internal representations of a task may exist in the CNS. Because there was partial generalization between the two arms, we conclude that there must be a representation stored and used for catching that is not effector specific, but rather can be utilized by brain regions controlling either arm. However, because generalization was only complete within an arm, we conclude that another sensorimotor representation exists, which might only be stored in brain regions specific to a single arm.