The trajectory of adaptation to the visuo-motor transformation of virtual and real sliding levers.

We studied adaptation to the visuo-motor transformation of a virtual and a real two-sided sliding lever. In a previous study (Sülzenbrück and Heuer in Exp Brain Res 195:153-165, 2009) we had found essentially no differences. However, adaptation had been restricted to a simplified symmetry approximation of the transformation. In the present study practice conditions were designed to facilitate adaptation (e.g., terminal rather than continuous visual feedback). In visual open-loop tests, differences between the effects of practice with a virtual and a real lever were found for curvature of hand movements, whereas movement end positions were not different. Curved hand movements induced by the use of the real lever persisted in subsequent open-loop tests with the virtual lever. Early in practice end-position errors were strongly biased toward the simplified symmetry approximation, but this bias was reduced later on. Thus, the symmetry approximation is a transient state in the trajectory of adaptation that is reached quickly and from which there is a slow and gradual transition to an accurate internal representation of the visuo-motor transformation of the sliding lever.