AN ANALYSIS OF SPATIOTEMPORAL VARIABILITY DURING PREHENSION MOVEMENTS- EFFECTS OF OBJECT SIZE AND DISTANCE

Human prehension movements have been studied with regard to the parall el processing of motor control and sensorimotor coordination. Temporal aspects of the movement (e.g., onset time and duration) have been stu died extensively, while spatial aspects have not been studied systemat ically. Thus, the purpose of this study was to examine spatiotemporal variability of the transport (wrist trajectory) and grasp (grip apertu re between the index finger and the thumb) components. In this experim ent, the extrinsic (e.g., distance) and intrinsic object properties (e .g., object size) were manipulated. Subjects were required to pick up an aluminum cylinder as quickly and accurately as possible using the i ndex finger and the thumb. It was found that object size significantly affected both transport and grasp components. Distance mainly affecte d the transport component. These kinematic results were consistent wit h the findings of earlier studies. Furthermore, the distribution of me an within-subject variability across normalized movement time for the transport component was not the same as that of the grasp component, s uggesting that the different motor control processes exist. The peak a mplitudes in variability of the wrist trajectory and the grip aperture were obtained at similar points throughout movement time. Furthermore , the peak of wrist variability depended on distance not object size, while that of aperture variability depended on both distance and objec t size. These results strongly support the hypothesis that the grasp c omponent is adjusted using dynamic information provided from the trans port component as the wrist moves toward the object. We also found tha t wrist variability converged to the target point, while aperture vari ability was biphasic: it converged, at least, around the point of maxi mum aperture in the first phase and then remained constant in the seco nd phase. This result suggests that the two components are under diffe rent control processes. We hypothesize that the transport component ca n be modeled as a single feedforward system, while the grasp component can be divided into two separate mechanisms.