Frame of reference and adaptation to directional bias in a video-controlled reaching task.

The present study (N=56) investigated spatio-temporal accuracy of horizontal reaching movements controlled visually through a vertical video monitor. Direct vision of the hand was precluded and the direction of hand trajectory, as perceived on the video screen, was varied by changing the angle of the camera. The orientation of the visual scene displayed on the fronto-parallel plane was thus congruent (0 degrees condition) or non-congruent (directional bias of 15 degrees, 30 degrees or 45 degrees counterclockwise) according to the horizontal working space. The goal of this study was to determine whether local learning of a directional bias can be transferred to other locations in the working space, but taking into account the magnitude of the directional bias (15 degrees, 30 degrees or 45 degrees ), and the position of the successive objectives (targets at different distances (TDD) or different azimuths (TDA)). Analysis of the spatial accuracy of pointing movements showed that when introducing a directional bias, terminal angular error was linearly related to the amount of angular perturbation (around 30%). Seven trials were, on average, necessary to eliminate this terminal error, whatever the magnitude of the directional bias and the position of the successive targets. When changing the location of the spatial objective, transfer of adaptation was achieved in the TDD condition but remained partial in the TDA condition. Furthermore, initial orientation of the trajectory suggested that some participants used a hand-centred frame of reference whereas others used an external one to specify movement vector. The adaptation process differed as a function of the frame of reference used, but only in the TDA condition. Adaptation for participants using a hand-centred frame of reference was more concerned with changes in the shape of the trajectory, whereas participants using an external frame of reference adapted their movement by up-dating the initial direction of hand trajectory. As a whole, these findings suggest that the processes involved in remote visual control of hand movement are complex with the result that tasks requiring video-controlled manipulation like video-controlled surgery require specific spatial abilities in actors and consequential plasticity of their visuo-motor system, in particular concerning the selection of the frame of reference for action.