Psychology and Aging 1995. Vol. 10. No. 2, 204-214 Copyright 1995 by the American Psychological Association, Inc. 0882-7974/95/S3.00 Reorganization of Prehension Components Following Perturbation of Object Size Keree M. B. Bennett European Medical Center Umberto Castiello Monash University and European Medical Center This study provides a kinematic comparison of the response of 12 older persons (ages 60-70) and 12 young persons (ages 20-26) to a perturbation of object size during a reach-to-grasp movement. For 80 out of 100 trials, each participant reached to grasp an illuminated cylinder of either small or large diameter (0.7 and 8.0 cm, respectively). For 20 out of 100trials, a visual perturbation occurred at movement onset. This perturbation consisted of a switch of illumination from 1 to the other cylinder. For the switch from large to small cylinder, participants changed the distal grasp from whole- hand prehension to precision grip. For the opposite switch, participants changed from precision grip to whole-hand prehension. The older participants successfully adapted to these perturbations but showed a more conservative approach. Generally the approach time as the hand neared the object was prolonged, and the coordination between transport and manipulation components was main- tained when comparing perturbed with nonperturbed trials, \bung participants showed a more flex- ible pattern with a decrease or loss of temporal coupling between the components. It is hypothesized that the more rigid movement pattern of older participants to unanticipated motor requirements could be a contributory factor to the higher incidence of accidents for this group. Recent studies have used the perturbation paradigm to eluci- date the sensorimotor neural processes of human movement. In such paradigms, the participant is unexpectedly presented with the requirement to change motor task. For example, an initially presented stimulus can suddenly be replaced by an al- ternative stimulus, and the participant must adapt motor out- put accordingly. Such a visual perturbation can be presented at different stages of the premovement and movement periods. With kinematic studies of the reach-to-grasp movement, visual perturbation is often given at movement onset. Thus, just as the participant begins to reach for an object at a particular location, an object at a different location is displayed. The participant must then quickly change the length (Gentilucci, Chieffi, Scarpa, & Castiello, 1992; Scarpa & Castiello, 1994) or direc- tion (Castiello, Paulignan, & Jeannerod, 1991; Paulignan, Mac- Kenzie, Marteniuk, & Jeannerod, 1991) of the reaching move- ment to grasp this latter object. Object size can also be per- turbed. For example, the participant can begin a reaching movement for a small target but then have to quickly change for the grasp of a larger object (Castiello, Bennett, & Paulignan, 1992; Castiello, Bennett, & Stelmach, 1993; Castiello & Jeannerod, 1991; Paulignan, Jeannerod, MacKenzie, & Mar- teniuk, 1991). This requires a change from the initially planned Keree M. B. Bennett, European Medical Center, Bologna, Italy; Um- berto Castiello, Department of Psychology, Monash University, Victo- ria, Australia, and European Medical Center, Bologna, Italy. We would like to thank Professor Razzaboni and Doctor Colonna for allowing us to use the excellent facilities at the European Medical Cen- ter. We thank BTS for providing excellent technical support and Ms. Uguzzoni for her generous assistance with the participants. Correspondence concerning this article should be addressed to Um- berto Castiello, Department of Psychology, Gippsland Campus, Monash University, Churchill, Victoria 3842, Australia. precision grip to a grasp such as whole-hand prehension (Castiello etal., 1992,1993). The aim of such double-step perturbation paradigms is to determine the central nervous system response to an unex- pected motor requirement. In the temporal domain, such ex- periments can assist in determining how quickly the system adapts to the perturbation. How soon is the initial output sup- pressed and the second output initiated? With regards to quality of movement, kinematic analyses show how the movement has been reorganized. Thus, previous studies of young participants (ages 18-35) demonstrated a rapid and smooth change from one to another grasp type when object size was perturbed at movement onset (Castiello etal., 1991, 1992, 1993). The ability of older participants to respond to a visual pertur- bation has been little investigated. With nonperturbed move- ments, older participants generally show longer reaction times and movement durations than younger participants (Stelmach & Goggin, 1989; Stelmach & Nahom, 1992; Welford, 1977, 1984). From such results, one could tentatively suppose that this generalized slowness would also be evident when older par- ticipants must unexpectedly change motor output. Stelmach, Goggin, and Amrhein (1988) used a precue paradigm to assess the time taken to restructure a planned motor response. For most trials (75%), the premovement cue corresponded to the subsequent extent, direction, and limb used in the aiming task. However, for some trials (20% ), the cue was invalid; this obliged the participant to unexpectedly restructure one or more move- ment parameters. Both older and younger participants showed an increase of reaction time for these latter invalid precue trials. However, the reaction time for the older participants was greater. This suggested that the older participants needed more time to restructure a planned motor response. The current study gives a kinematic assessment of the ability of older participants to respond to a visual perturbation. At the 204 This document is copyrighted by the American Psychological Association or one of its allied publishers. This article is intended solely for the personal use of the individual user and is not to be disseminated broadly.