Motor learning with fading and growing haptic guidance.

Haptic guidance has been shown to have both facilitatory and interfering effects on motor learning. Interfering effects have been hypothesized to result from the particular dynamic environment, which supports a passive role of the learner, and they should be attenuated by fading guidance. Facilitatory effects, in particular for dynamic movement characteristics, have been hypothesized to result from the high-quality information provided by haptic demonstration. If haptic demonstration provides particularly precise information about target movements, the motor system's need for such information should more likely increase in the course of motor learning, in which case growing guidance should be more beneficial for learning. We contrasted fading and growing guidance in the course of learning a spatio-temporal motor pattern. To stimulate an active role of the learner, practice trials consisted of three phases, a visual demonstration of the target movement, a guided reproduction, and a reproduction without haptic guidance. Performance was assessed in terms of variable duration errors, relative-timing errors, variable path-length errors, and shape errors. Motor learning with growing and fading guidance turned out to be largely equivalent, so that the notion of an increasing optimal precision of haptic demonstrations, which matches a demand of increasingly precise information on the target movement, found no support. Duration errors declined only with fading, but not with growing guidance. Relative timing revealed a benefit of immediately preceding haptic demonstration, but learning was not different between the two practice protocols. This contrast between absolute and relative timing adds to other evidence according to which acquisition of these two aspects of motor timing involves different learning mechanisms. Whereas relative timing gained from immediately preceding haptic demonstration, but revealed no practice-related improvement in the presence of haptic guidance, the opposite pattern of results was found for the shape error. This finding is consistent with the claim that haptic demonstration is particularly efficient with respect to relative timing, but not with respect to spatial movement characteristics.