The underlying processes in movement organization and control were studied by varying the conditions under which arm movements were made. The three-dimensional movement trajectories of the following conditions were contrasted: pointing to a target with the index finger versus grasping a disk the same size as the target, grasping a fragile object versus a soft resilient object, and grasping a disk either to throw into a large box or place into a tight fitting well. Results snowed that the arm trajectories, as represented by the resultant velocity profile of the wrist, varied considerably in their shape with the main factor being when peak velocity was reached as a function of the total duration of the movement. It appeared that when task demands required greater precision, the main deceleration phase of the trajectory was increased in duration. These results do not support a movement production mechanism that has access to an abstract representation of a base velocity profile and that creates trajectories by a simple scaling procedure in the temporal domain. Rather, the results support a view of movement production as relatively specific to the past experience of the performer and the constraints of the task.

Prehension involves processing information in two hypothesized visuomotor channels: one for extrinsic object properties (e.g., the spatial location of objects) and one for intrinsic objects properties (e.g., shape and size). The present study asked how the two motor components that correspond to these channels (transport and grasp, respectively) are related. One way to address this question is to create a situation where unexpected changes occur at the input level of one of the visuomotor channels, and to observe how the movement reorganizes. If transport and grasp are independent components, then changing the object location, for example, should affect only the transport, not the grasp component. Subjects were requested to reach, grasp and lift as accurately as possible one of three dowels using the distal pads of the thumb and index finger. On certain trials, upon movement initiation towards the middle dowel, the dowel was made to instantaneously change its location to one of the two other positions, requiring the subject to reorient the hand to the new dowel location. Results consisted of comparing the movement characteristics of the transport and grasp components of these perturbed movements with appropriate control movements. Kinematics of the wrist trajectory showed fast adjustments, within 100 ms, to the change of dowel position. This duration seems to correspond to the minimum delay required within the visuomotor system for visual and/or proprioceptive reafferents to influence the ongoing movement. In addition, these delays are much shorter than has been found for conditions where object location changes before movement initiation (approximately 300 ms). The faster times may relate to the dynamic character of the deviant limb position signals, with the only constraint being the physiological delays for visual and kinaesthetic signals to influence the movement. A spatiotemporal variability analysis of the movement trajectories for non-perturbed trials showed variability to be greatest during the acceleration part of the movement, interpreted as due to control by a relatively inaccurate directional coding mechanism. Control during the deceleration phase, marked by low trajectory variability, was seen to be due to a sensorimotor process, using motor output signals, and resulting in an optimized trajectory supporting a successful grasp. Analysis of the grasp component of prehension showed that perturbing object location influenced the movement of the fingers suggesting a kinematic coupling of the two components. However, forthcoming work shows that, when object size changes, and location remains constant, there is a clear temporal dissociation of the two components of prehension.(ABSTRACT TRUNCATED AT 400 WORDS)

No abstract

According to Fitts (1954), movement time (MT) is a function of the combined effects of movement amplitude and target width (index of difficulty). Aiming movements with the same index of difficulty and MT may have different planning and control processes depending on the specific combination of movement amplitude and target size. Trajectories were evaluated for a broad range of amplitudes and target sizes. A three-dimensional motion recording system (WATSMART) monitored the position of a stylus during aiming movements. MT results replicated Fitts’ Law. Analysis of the resultant velocity profiles indicated the following significant effects: As amplitude of movement increased, so did the time to peak resultant velocity; peak resultant velocity increased slightly with target size, and to a greater extent with increases in the amplitude of movement; the time after peak resultant velocity was a function of both amplitude and target size. Resultant velocity profiles were normalized in the time domain to look for scalar relation in the trajectory shape. This revealed that: the resultant velocity profiles were not symmetrical; the proportion of time spent prior to and after peak speed was sensitive to target size only, i.e. as target size decreased, the profiles became more skewed to the right, indicating a longer decelerative phase; for a given target size, a family of curves might be defined and scaled on movement amplitude. These results suggest that a generalized program (base trajectory representation) exists for a given target width and is parameterized or scaled according to the amplitude of movement.

1. Subjects were instructed to reach and grasp cylindrical objects, using a precision grip. The objects were two concentric dowels made of translucent material placed at 35 cm from the subject. The inner ("small") dowel was 10 cm high and 1.5 cm in diameter. The outer ("large") dowel was 6 cm high and 6 cm in diameter. Prehension movements were monitored using a Selspot system. The displacement of a marker placed at the wrist level was used as an index for the transport of the hand at the location of the object. Markers placed at the tips of the thumb and the index finger were used for measuring the size of aperture of the finger grip. 2. Kinematics of transport and grasp components were computed from the filtered displacement signals. Movement time (MT), time to peak velocity (TPV) and time to peak deceleration (TPD) of the wrist, time to peak velocity of grip aperture (TGV), time to maximum grip aperture (TGA) were the main parameters used for comparing the movements in different conditions. Spatial paths of the wrist, thumb and index markers were reconstructed in two dimensions. Variability of the spatial paths over repeated trials was computed as the surface of the ellipses defined by X and Y standard deviations from the mean path. 3. Computer controlled illumination of one of the dowels was the signal for reaching toward that dowel. Blocks of trials were made to the small dowel and to the large dowel. Mean MT during blocked trials was 550 ms. The acceleration phase of the movements (measured by parameter TPV) represented 33% of MT. About half of MT (52%) was spent after TPD in a low velocity phase while the hand was approaching the object. This kinematic pattern was not influenced by whether movements were directed at small or large dowels. 4. Grip aperture progressively increased during transport of the hand. TGA corresponded to about 60% of MT, that is, maximum grip aperture was reached during the low velocity phase of transport. Following TGA, fingers closed around the object until contact was made. This pattern of grip formation differed whether the movement was directed at the large or the small dowel: TGA occurred often earlier for the small dowel, and the size of the maximum grip aperture was larger for the large dowel. Variability of both the wrist and finger spatial paths was larger during the first half of MT, and tended to become very low as the hand approached the dowels.(ABSTRACT TRUNCATED AT 400 WORDS)

Three experiments were designed to investigate the underlying processes in bimanual control. With one hand alone, or with both simultaneously, subjects moved styli from the midline of the body to lateral targets as quickly and accurately as possible. The distance moved and the weight of the styli were varied. Results of reaction time, movement time, and kinematic trajectory analyses question the conclusions of Kelso, Southard and Goodman (1979) regarding the synchronicity of movement of the two limbs. Temporal parameters for the two limbs indicated marked departures from synchronicity, and there was evidence for a left-right asymmetry. The dependent variables of movement time and constant error indicated that there was interaction between the two limbs. The results are discussed in terms of three postulated processes underlying bimanual movement: limb selection (one or two), specification of movement locations and the specification of movement intensities.

Earlier studies have suggested that the size of an object to be grasped influences the time taken to complete a prehensile movement. However, the use of cylindrical objects in those studies confounded the effects of object size-extent orthogonal to the reach axis-and object width-extent along the reach axis. In separating these effects, the present study demonstrates that movement time is not affected by manipulation of object size, as long as the latter does not approach the maximal object size that can be grasped. Object width, on the other hand, is shown to exert a systematic influence on movement time: Smaller object widths give rise to longer movement times through a lengthening of the deceleration phase of the movement, thus reproducing the effect of target width on the kinematics of aiming movements. As in aiming, movement amplitude also affects the movement time in prehension, influencing primarily the acceleration phase (i.e. peak velocity attained). The effects of object width and movement amplitude were found to combine in a way predicted by Fitts' law, allowing a generalisation of the latter to the transport component in prehensile actions. With respect to the grasp component, both object size and object width are shown to affect peak hand aperture. Increasing object width thus lowers the spatial accuracy demands on the transport component, permitting a faster movement to emerge. At the same time, the hand opens to a larger grip in order to compensate for eventual directional errors that result.(ABSTRACT TRUNCATED AT 250 WORDS)

The goal of the present study was to investigate how grip forces are applied when transferring stable control of an object from one person to another. We asked how grip forces would be modified by the passer to (1) control for inertial forces as the object was transported toward the receiver and (2) control for the impending perturbation when the receiver made contact with the object. Twelve volunteers worked in pairs during this experiment. One partner, playing the role of passer, transported an object with embedded load cells forward or held the object at an interception location. The second partner, playing the role of receiver, waited at an interception location or reached toward the passed object. Kinematic results indicated that while passers performed a stereotypical movement, receivers were sensitive to the motion of the object as they reached to make contact. Grip force results indicated that passers' grip forces and grip/load force ratios were variable on a trial-to-trial basis, suggesting that a refined internal model of the passing task was not achieved within the timeframe of the experiment. Furthermore, a decoupling of the temporal and magnitude characteristics of the grip and inertial forces was noted in conditions where passers transported the object toward the receiver. During object transfer, it was noted that passers used visual feedback-based anticipatory control to precisely time initial grip force release, while somatosensory control was used by both the passer and receiver to precisely coordinate transfer rate.