GRIN REORGANIZATION DURING WRIST TRANSPORT - THE INFLUENCE OF AN ALTERED APERTURE

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 posit ion to one of maximal finger extension, our previous work has shown th at the grasp component appears to reorganize itself during the reach. This reorganization, consisting of a brief closing and reopening of th e grip aperture, only slightly influenced the temporal components of t he wrist transport. The present experiment continues this research the me by examining the characteristics of grip aperture reorganization th rough 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 reorganizat ion is task dependent it should be related to object size. The experim ent found that in the Altered grip condition reorganization did occur, as indicated by a slight closing and reopening of the aperture withou t influencing the transport of the wrist. The amplitude of and the tim e to the observed inflection point in the aperture time course were re lated to object size. The velocity of grip closing for the large objec t showed double peaks, with the first substantially smaller than the s econd. Moreover, for the small object, the velocity of grip aperture c losing also was double peaked, but the difference between peaks was le ss pronounced. These changes in grip velocity suggest that the grip re organization 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 statis tically 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 maxi mum grip aperture was correlated with both time to peak wrist velocity and time peak to wrist deceleration. Thus, as the reach progressed to ward the object, the grip and transport components became more interde pendent. The results are consistent with the notion that, when a well- practiced, coordinated act such as prehension is confronted with an Al tered grip posture at the onset of the reach, the grip can be reorgani zed during the transport to preserve the relative timing between them. Thus these data add to the growing awareness that not only is there t emporal coupling between the reach and grasp components but that these components may be integrated by higher-order control mechanism.