Generalization properties of a "saccadic-like" hand-reaching adaptation along a single degree of freedom

Visuomotor-adaptation experiments devoted to the study of plasticity are also used to indirectly test hypotheses about how the brain encodes the spatio-temporal characteristics of arm movement directed at a visual target. A current major theory, the vectorial coding hypothesis, postulates that arm movements are processed differentially for direction and amplitude. This approach, at first developed in an extrinsic Cartesian frame of references, has been also adopted in an intrinsic joint space. In the present paper, we report an experiment that corroborates this last point of view. Subjects performed pointing movements in a one degree of freedom condition, while systematic self-attributed endpoint errors were introduced. Through an observation of motor behavior in a battery of pre- and post-tests, we suggested that adaptation consisted in an increase in the motor gain in the adapted direction, with a perfect transfer to all starting points in the experimental reaching space. We explained the results by the absence of intersensory conflict and of correlative sensory adaptive component. As this paradigm was adapted from the saccadic adaptation paradigm, we eventually compared the two paradigms and highlighted that both induced mostly motor effects.