Constraints on visuo-motor adaptation depend on the type of visual feedback during practice.

Adaptation to a novel visuo-motor gain has been found to generalize across target directions, whereas simultaneous adaptation to different direction-related visuo-motor gains turned out to be impossible. We ask whether this is a rigid constraint on human adaptability or a soft constraint that can be overcome by optimized conditions of practice. In particular, we compared practice with continuous visual feedback, as used in previous studies, to practice with terminal visual feedback. With terminal visual feedback only the final positions of the movements are shown. Both kinds of visual feedback in principle can serve the acquisition of an internal model of direction-related visuo-motor gains, but with continuous feedback on-line visual closed-loop control permits accurate movements without access to an internal model. Whereas we found no indication of visuo-motor adaptation after continuous-feedback practice, there was adaptation after terminal-feedback practice. This was revealed both by (direction-related) adaptive shifts of movement amplitudes in an open-loop test with cued visuo-motor transformation and by (direction-related) aftereffects in an open-loop test with absence of the visuo-motor transformation being cued. None of the two groups gave evidence of explicit knowledge of the direction-related visuo-motor gains. These findings show that constraints on human adaptability can depend on the kind of experience with visuo-motor transformations, in particular on the kind of feedback during practice.