DOES THE BRAIN USE SLIDING VARIABLES FOR THE CONTROL OF MOVEMENTS

Delays in the transmission of sensory and motor information prevent er rors from being instantaneously available to the central nervous syste m (CNS) and can reduce the stability of a closed-loop control strategy . On the other hand, the use of a pure feedforward control (inverse dy namics) requires a perfect knowledge of the dynamic behavior of the bo dy and of manipulated objects. Sensory feedback is essential both to a ccommodate unexpected errors and events and to compensate for uncertai nties about the dynamics of the body, Experimental observations concer ning the control of posture, gaze and limbs have shown that the CNS ce rtainly uses a combination of closed-loop and open-loop control. Feedf orward components of movement, such as eye saccades, occur intermitten tly and present a stereotyped kinematic profile, In visuo-manual track ing tasks, hand movements exhibit velocity peaks that occur intermitte ntly. When a delay or a slow dynamics are inserted in the visuo-manual control loop, intermittent step-and-hold movements appear clearly in the hand trajectory. In this study, we investigated strategies used by human subjects involved in the control of a particular dynamic system . We found strong evidence for substantial nonlinearities in the comma nds produced. The presence of step-and-hold movements seemed to be the major source of nonlinearities in the control loop. Furthermore, the stereotyped ballistic-like kinematics of these rapid and corrective mo vements suggests that they were produced in an open-loop way by the CN S. We analyzed the generation of ballistic movements in the light of s liding control theory assuming that they occurred when a sliding varia ble exceeded a constant threshold. In this framework, a sliding variab le is defined as a composite variable (a combination of the instantane ous tracking error and its temporal derivatives) that fulfills a speci fic stability criterion. Based on this hypothesis and on the assumptio n of a constant reaction time, the tracking error and its derivatives should be correlated at a particular time lag before movement onset. A peak of correlation was found for a physiologically plausible reactio n time, corresponding to a stable composite variable. The direction an d amplitude of the ongoing stereotyped movements seemed also be adjust ed in order to minimize this variable. These findings suggest that, du ring visually guided movements, human subjects attempt to minimize suc h a composite variable and not the instantaneous error. This minimizat ion seems to be obtained by the execution of stereotyped corrective mo vements.