Selective perturbation of visual input during prehension movements

Abstract

1. Subjects were instructed to reach and grasp cylindrical objects, using a precision grip. The objects were two concentric dowels made of translucent material placed at 35 cm from the subject. The inner ("small") dowel was 10 cm high and 1.5 cm in diameter. The outer ("large") dowel was 6 cm high and 6 cm in diameter. Prehension movements were monitored using a Selspot system. The displacement of a marker placed at the wrist level was used as an index for the transport of the hand at the location of the object. Markers placed at the tips of the thumb and the index finger were used for measuring the size of aperture of the finger grip. 2. Kinematics of transport and grasp components were computed from the filtered displacement signals. Movement time (MT), time to peak velocity (TPV) and time to peak deceleration (TPD) of the wrist, time to peak velocity of grip aperture (TGV), time to maximum grip aperture (TGA) were the main parameters used for comparing the movements in different conditions. Spatial paths of the wrist, thumb and index markers were reconstructed in two dimensions. Variability of the spatial paths over repeated trials was computed as the surface of the ellipses defined by X and Y standard deviations from the mean path. 3. Computer controlled illumination of one of the dowels was the signal for reaching toward that dowel. Blocks of trials were made to the small dowel and to the large dowel. Mean MT during blocked trials was 550 ms. The acceleration phase of the movements (measured by parameter TPV) represented 33% of MT. About half of MT (52%) was spent after TPD in a low velocity phase while the hand was approaching the object. This kinematic pattern was not influenced by whether movements were directed at small or large dowels. 4. Grip aperture progressively increased during transport of the hand. TGA corresponded to about 60% of MT, that is, maximum grip aperture was reached during the low velocity phase of transport. Following TGA, fingers closed around the object until contact was made. This pattern of grip formation differed whether the movement was directed at the large or the small dowel: TGA occurred often earlier for the small dowel, and the size of the maximum grip aperture was larger for the large dowel. Variability of both the wrist and finger spatial paths was larger during the first half of MT, and tended to become very low as the hand approached the dowels.(ABSTRACT TRUNCATED AT 400 WORDS)

Effect of speed manipulation on the control of aperture closure during reach-to-grasp movements

“…These distances were calculated as a cumulative resultant trajectory length between two positions of the wrist IRED. It should be noted that the end of the arm transport was simultaneous to the target touch in this study, which is slightly earlier than the time defined in many other studies (the end of grasp, Alberts et al 2002;Castiello et al 2000;Paulignan et al 1991a), because the fingers continued to close after the target touch. Forward displacement of the wrist IRED was observed from the target touch to the completion of the grasp, most often for the faster speed conditions.…”

“…When reach-to-grasp movements were performed under various transport durations or speeds, the maximum aperture occurred at a relatively fixed timing in relation to overall movement duration (Paulignan et al 1991a;Marteniuk et al 1990;Rand et al 2000;Smeets and Brenner 1999;Wallace and Weeks 1988;Wallace et al 1990). However, some other studies demonstrated that the duration for aperture closure movement was unaffected by different task conditions (Bootsma and van Wieringen 1992;Gentilucci et al 1992;Paulignan et al 1991b;Watson and Jakobson 1997;Zaal et al 1998). These observations led to a hypothesis that grasp formation and kinematics of the arm transport are temporally linked to coordinate the transport and grasp components (Hoff and Arbib 1993;Jeannerod et al 1995;Jeannerod 1999).…”

Prehension is really reaching and grasping

“…Furthermore, we know from previous studies that hand aperture adapts in a functional way to object-size perturbations (e.g., Castiello et al 1993;Gentilucci et al 1992;Paulignan et al 1991a). Therefore, we conclude that Jeannerod was right after all, and that prehension should be seen as the act of coordinated reaching and grasping.…”

“…In this sense, the two components (i.e., the two visuo-motor channels) were hypothesized to be independent. This is why many studies designed to test the independence of the two components of prehension involved perturbations of intrinsic object properties, such as size (e.g., Castiello et al 1993;Paulignan et al 1991a) or of extrinsic object properties, such as location (e.g., Gentilucci et al 1992;Paulignan et al 1991b), to see if the perturbations would have an eVect on the component that should not be dependent on information of either object property. Object-size perturbations, for instance, should only aVect the grasping component in Jeannerod's model.…”

Control of aperture closure during reach-to-grasp movements in parkinson’s disease

“…The perturbation consisted of a shift of the illuminated LED during movement from the center target to either the left or right target, which remained lit until the end of the trial. Perturbations were applied at two different times: (1) 50 ms after release of the start button (early perturbation); and (2) at the time when the subject's hand broke the light beams 17.5 cm away from the start button (late perturbation) [Note that in similar studies the onset of perturbation occurred simultaneously with the release of the start button (Castiello et al 1999;Gentilucci et al 1992;Paulignan et al 1991a). In the present study, the early perturbation timing was set 50 ms after the start button release because our preliminary study on healthy subjects showed that this timing produced a more distinct and consistent modification of hand transport movement in response to the perturbation.]…”