This study assessed the reach to grasp movement and its adaptive response to a perturbation of object size. In blocked trials, subjects (n = 12) were instructed to reach 35 cm to grasp and lift a small- (0.7 cm) or large-diameter (8 cm) cylinder. Under an unconstrained condition (condition 1), no instructions as to the type of grasp to adopt were given. Subjects thus naturally used a precision grip (PG) for the small cylinder and whole hand prehension (WHP) for the large cylinder. Under condition 2, subjects were instructed to utilize a PG for grasps of both the large and small cylinders. For condition 3, the instruction was to use WHP irrespective of object size. Kinematic organization was determined with analysis of the recordings of active markers placed on the wrist, thumb, and three fingers. For condition 1 the results showed a temporal arrangement of both components (transport and manipulation) which differed from that of conditions 2 and 3. In perturbed trials, illumination shifted from the small to large cylinder or vice versa. With condition 1, subjects automatically switched from one grasp to another with no or little increase of movement duration. This was generally achieved by an earlier temporal setting of peak wrist deceleration. For conditions 2 and 3, where a change of aperture was required, movement duration was prolonged without adaptation of earlier transport component parameters. It is concluded that the adaptive responses to a change of distal patterning also affect the organization of the proximal component. Assessment of grasps constrained by instructions may lead to interpretations of central control of the reach to grasp movement which differ from those obtained by assessing more natural prehensile patterns.

Pointing accuracy of six human subjects was measured in two blocked conditions where the hand was either never visible (T: target only) or only visible in static position prior to movement onset (H+T: hand+target). It was shown in condition H+T that, viewing the hand prior to movement greatly decreased end-point variability compared with condition T. This effect was associated with a significant modification of the movement kinematics: the H+T condition induced a shortened acceleration phase with a corresponding lengthened deceleration phase, compared with the T condition. These results led us to the hypothesis that viewing the hand prior to movement onset allowed a decrease of pointing variability through a feedback process. This hypothesis was further tested by turning the target off during the deceleration phase of the movement at half peak velocity. It was shown that turning the target off had no effect upon the T condition but induced a significant increase of pointing variability in the H+T condition. This result suggests that vision of the static hand enhances the proprioceptive localization of the limb and allows for a better visual to kinesthesic feedback.

Past studies have examined the coupling of reach and grasp components during prehensile movements. Many of these studies have supported the view that these components reflect the output of two parallel, though temporally coupled, motor programs. When the grip aperture is Altered prior to the onset of prehension from its usual, normally flexed position to one of maximal finger extension, our previous work has shown that the grasp component appears to reorganize itself during the reach. This reorganization, consisting of a brief closing and reopening of the grip aperture, only slightly influenced the temporal components of the wrist transport. The present experiment continues this research theme by examining the characteristics of grip aperture reorganization through the comparison of the kinematics of prehension components during movements to two different size objects under normal and Altered grip aperture conditions. It was hypothesized that if the grip reorganization is task dependent it should be related to object size. The experiment found that in the Altered grip condition reorganization did occur, as indicated by a slight closing and reopening of the aperture without influencing the transport of the wrist. The amplitude of and the time to the observed inflection point in the aperture time course were related to object size. The velocity of grip closing for the large object showed double peaks, with the first substantially smaller than the second. Moreover, for the small object, the velocity of grip aperture closing also was double peaked, but the difference between peaks was less pronounced. These changes in grip velocity suggest that the grip reorganization is related to object size. No effect of Altered aperture was observed on the transport component. For both object sizes in the Altered condition, the final peak velocity of grip aperture was statistically significantly correlated with transport time and time to peak deceleration. In contrast, such correlations were not observed for the initial peak velocity of the grip aperture. Furthermore, time to maximum grip aperture was correlated with both time to peak wrist velocity and time peak to wrist deceleration. Thus, as the reach progressed toward the object, the grip and transport components became more interdependent. The results are consistent with the notion that, when a well-practiced, coordinated act such as prehension is confronted with an Altered grip posture at the onset of the reach, the grip can be reorganized during the transport to preserve the relative timing between them. Thus these data add to the growing awareness that not only is there temporal coupling between the reach and grasp components but that these components may be integrated by higher-order control mechanism.

To understand the internal representations used by the nervous system to coordinate multijoint movements, we examined the coordination among the body segments during reach-to-grasp movements which involve grasping by the hand and reaching by the arm and trunk. Subjects were asked to reach and grasp an object using the arm only, the trunk only, and some combinations of both arm and trunk. Results showed that kinematic parameters related to the transport component of the arm and the trunk, such as peak velocity and time to peak velocity, varied across conditions and that the coordination pattern between the arm and trunk was different across conditions. However, parameters related to the grasp component, such as peak aperture, time to peak aperture, and closing distance, were invariant, regardless of whether the hand was delivered to the target by the arm only, the trunk only, or both. We hypothesize that a hierarchy of motor control processes exists, in which the reach and grasp components are governed by independent neuromotor synergies, which in turn are coordinated temporally and spatially by a higher-level synergy.

No abstract