Adaptation of reach-to-grasp movement in response to force perturbations

Abstract

This study examined how reach-to-grasp movements are modified during adaptation to external force perturbations applied on the arm during reach. Specifically, we examined whether the organization of these movements was dependent upon the condition under which the perturbation was applied. In response to an auditory signal, all subjects were asked to reach for a vertical dowel, grasp it between the index finger and thumb, and lift it a short distance off the table. The subjects were instructed to do the task as fast as possible. The perturbation was an elastic load acting on the wrist at an angle of 105 deg lateral to the reaching direction. The condition was modified by changing the predictability with which the perturbation was applied in a given trial. After recording unperturbed control trials, perturbations were applied first on successive trials (predictable perturbations) and then were applied randomly (unpredictable perturbations). In the early predictable perturbation trials, reach path length became longer and reaching duration increased. As more predictable perturbations were applied, the reach path length gradually decreased and became similar to that of control trials. Reaching duration also decreased gradually as the subjects adapted by exerting force against the perturbation. In addition, the amplitude of peak grip aperture during arm transport initially increased in response to repeated perturbations. During the course of learning, it reached its maximum and thereafter slightly decreased. However, it did not return to the normal level. The subjects also adapted to the unpredictable perturbations through changes in both arm transport and grasping components, indicating that they can compensate even when the occurrence of the perturbation cannot be predicted during the inter-trial interval. Throughout random perturbation trials, large grip aperture values were observed, suggesting that a conservative aperture level is set regardless of whether the reaching arm is perturbed or not. In addition, the results of the predictable perturbations showed that the time from movement onset to the onset of grip aperture closure changed as adaptation occurred. However, the spatial location where the onset of finger closure occurred showed minimum changes with perturbation. These data suggest that the onset of finger closure is dependent upon distance to target rather than the temporal relationship of the grasp relative to the transport phase of the movement.

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

“…The closure distance was approximately constant across all transport distance variations within a certain transport-speed condition (Fig. 4, Table 1), which is consistent to previous studies (Alberts et al 2000(Alberts et al ,2002Rand and Stelmach 2005;Rand et al 2004;Stelmach 1998,2001). This suggests that spatial information on the distance from the hand to the object plays a dominant role in triggering the aperture closure.…”

“…Therefore, each subject produced a relatively constant closure distance across trials in relation to the total transport distance for each speed condition. Such stability in aperture closure distance in comparison to the aperture opening distance was in agreement with other previous studies (Alberts et al 2000(Alberts et al , 2002Rand and Stelmach 2005;Rand et al 2004;Stelmach 1998, 2001). However, across the range of speed conditions the closure distance changed (Fig.…”

“…It may be possible that the aperture closure distance could be altered, if a much greater range of transport speed were examined. Similar to our previous studies (Alberts et al 2000(Alberts et al , 2002Rand et al 2004;Saling et al 1998;Stelmach 1998, 2001), the current study examines whether transport-grasp coordination is better formulated in terms of spatial or in terms of temporal characteristics of the movement by comparing the extent of invariance in those characteristics with regard to transport speed and distance. For this purpose, in this study we expanded the range of speed manipulations from that previously tested and examined how the aperture closure distance varied across that range.…”

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

“…Based on the results from their mechanical perturbation study (1991 (1995), they suggested that the formation of grip aperture is adjusted to maintain the spatial relationship between the grasp and transport components. Similar implications were made in recent studies with neurologically healthy subjects (Alberts et al 2002;Rand and Stelmach 2005;Rand et al 2004;Stelmach 1998, 2001), and PD patients (Alberts et al 2000;Wang and Stelmach 2000).…”

“…This instruction was used to impose the maximum stress on the motor control system for examining the subject's capacity to coordinate the grasp and transport components under movement perturbations. Fast reach-to-grasp movements were also examined for similar reasons in some previous studies (Paulignan et al 1991a, b;Desmurget et al 1996;Rand et al 2004;Rand and Stelmach 2005). Before the perturbation set, the subjects were told that the perturbation would occur randomly across trials, but that they were to continue reaching for and grasping the target as quickly as possible in every trial.…”