Concurrent Adaptation of Reactive Saccades and Hand Pointing Movements to Equal and to Opposite Changes of Target Direction

Eye as well as hand movements can adapt to double-step target displacements, but it is still controversial whether both motor systems use common or distinct adaptive mechanisms. Here, we posit that analyses of the concurrent adaptation of both motor systems to equal versus different double-steps may provide more conclusive evidence than previous work about the transfer of adaptation from one motor system to the other. Forty subjects adapted to double-steps which called for a change of response direction. The same (group S) or the opposite change (group O) was required for eyes and hand. Group ON equaled O, except that no visual feedback of the hand was provided. Groups E and H served as controls for eyes-only and hand-only adaptation, respectively. We found no differences between groups or motor systems when comparing S, E and H. Adaptation was faster in O than in S, E and H, and faster still in ON. However, the magnitude of eye adaptation was much smaller in O and ON than in S, E and H. We conclude that concurrent adaptation of eye and hand directions to opposite double-steps attenuates recalibration which, at least for the hand, is largely replaced by workaround strategies. The mechanisms for eye and hand adaptation therefore seem to be coupled, in a way that hinders divergent recalibration of both motor systems. The possible neuronal substrate for our findings is discussed.