Dissociation between "where" and "how" judgements of one's own motor performance in a video-controlled reaching task

The aim of the present study is to show that the sensorimotor system makes a differential use of visual and internal (proprioception and efferent copy) signals when evaluating either the spatial or the dynamical components of our own motor response carried out under a remote visual feedback. Subjects were required to monitor target-directed pointings from the images furnished by a video camera overhanging the workspace. By rotating the camera, the orientation of the movement perceived on the screen was either changed by 45 degrees (visual bias) or maintained in conformity with the actual trajectory (0 degrees ). In either condition, after completing twenty pointings, participants had to evaluate their visuomotor performance in two non visual testing: They were both asked to reach the target in a single movement (evaluation of "how to reach the target"), and to evaluate the mapping of the spatial layout where they acted (evaluations of "where the starting position was and, what movement direction was"). Results revealed that though motor performance in the 45 degrees conditions was adapted to the visuomotor conflict, participants' evaluation of the spatial aspect of the performance was affected by the biased visual information. A different pattern was revealed for the evaluation of "how" the target was reached which was not affected by the visual bias. Thus, it is suggested that segregated processing of visual and kinesthetic information occurs depending upon the dimension of the performance that is judged. Visual information prevails when identifying the spatial context of a motor act whereas proprioception and/or efferent copy related signals are privileged when evaluating the dynamical component of the response.