REACH TO GRASP - THE NATURAL RESPONSE TO PERTURBATION OF OBJECT SIZE

This study assessed the reach to grasp movement and its adaptive respo nse 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 g iven. Subjects thus naturally used a precision grip (PG) for the small cylinder and whole hand prehension (WHP) for the large cylinder. Unde r condition 2, subjects were instructed to utilize a PG for grasps of both the large and small cylinders. For condition 3, the instruction w as 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 show ed a temporal arrangement of both components (transport and manipulati on) which differed from that of conditions 2 and 3. In perturbed trial s, 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 gene rally achieved by an earlier temporal setting of peak wrist decelerati on. For conditions 2 and 3, where a change of aperture was required, m ovement 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 prox imal component. Assessment of grasps constrained by instructions may l ead to interpretations of central control of the reach to grasp moveme nt which differ from those obtained by assessing more natural prehensi le patterns.