Deafferentation and pointing with visual double-step perturbations

The capability of reprogramming movement responses following changes in the visual goal has been studied through the double-step paradigm. These studi es have shown that: (a) continuous internal feedback-loops correct unconsci ously the dynamic errors throughout the movement; (b) proprioceptive inform ation and/or the efference copy have a privileged status among central proc esses, insuring on-line regulation of the initial motor commands; and (c) g eneration of the motor program starts after target presentation, and is con tinuously updated in the direction of the current internal representation o f the target, at least until the onset of hand movement. This main correcti ve process of the initial program appears to be basically independent of vi sual reafference from the moving hand, However, the agreement with the poss ibility of a visuomotor loop, based on the comparison of the new updated re presentation of the target position and on the information from the moving hand, has not determined whether the correcting process is proprioceptive f eedback dependent, or whether internal feedback-loops (efferent copies) are responsible for quick corrections of unfolding motor responses. To answer this question, the present experiment investigated the pointing behavior of a deafferented subject, using a double-step paradigm under various conditi ons of visual feedback and movement initiation. Overall, the present study (a) clearly showed the capacity of the motor system to modify and correct e rroneous trajectories on the mere basis of internal feedback-loops and (b) emphasized the crucial role played by the target jump/arm triggering delay and the importance of the eye efferent copy for providing information about the spatial goal of the movement.