Ronald G. Marteniuk

Selective perturbation of visual input during prehension movements

SummaryPrehension 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. Collectively, these results suggest that the two visuomotor channels have different time constraints with the time-constant of the channel activated by the perturbation constraining the timing of the other.

Bimanual movement control: Information processing and interaction effects

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.

Functional relationships between grasp and transport components in a prehension task

Abstract Prehension in adults is a highly developed motor skill that affords the study of how components of a movement are coordinated to produce the near endless variety of acts that serve to acquire objects in near body space. This study used three-dimensional movement analysis to describe the kinematic characteristics and coordination of the transport (the reach) and the manipulation (the grasp) components of prehension. Six subjects reached for and grasped 10 different sized wooden disks. Results indicated that the initial phase of the prehension movement could be considered as structured in advance of the movement in that, over the ten disk sizes, time of limb transport was a constant up to peak deceleration. The time after peak deceleration to object contact, however, increased as disk size decreased. This led to the movement trajectories being uniquely shaped for each disk size and as such did not support a proportional duration based model of motor programming. Other findings indicated that maximum grip aperture was reached progressively sooner as disk size was decreased and that maximum grip aperture was highly related to the size of the to-be-grasped disk. In terms of the coordination between the transport and grasp components, support was not found for a temporal linkage between them nor did their coordination appear to be dependent on a motor program. Rather, from an analysis of the spatial variability of the transport and grasp components, evidence was found for supporting the idea that the coordination between these two components was achieved by a sensorimotor process. Through this process, and given the goal of a prehension movement, it was argued that the two components are linked functionally rather than temporally or spatially. These results are discussed in terms of current sensorimotor models of motor control and prehension.

Three-Dimensional Movement Trajectories in Fitts' Task: Implications for Control:

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.

A Sensorimotor Basis for Motor Learning: Evidence Indicating Specificity of Practice

Our previous work (Proteau, Marteniuk, Girouard, & Dugas, 1987) was concerned with determining whether with relatively extensive practice on a movement aiming task, as the skill theoretically starts becoming open-loop, there would be evidence for a decreasing emphasis on visual feedback for motor control. We eliminated vision of the moving limb after moderate and extensive practice and found that the movement became more dependent on this feedback with greater amounts of practice. In the present study, we wished to test the hypothesis, developed from our previous work, that at the base of movement learning is a sensorimotor representation that consists of integrated information from central processes and sensory feedback derived from previous experiences on the movement task. A strong test of this hypothesis would be the prediction that for an aiming task, the addition of vision, after moderate and relatively extensive practice without vision, would lead to an increasingly large movement decrement, relative to appropriate controls. We found good support for this prediction. From these and previous results, and the idea of the sensorimotor representation underlying learning, we develop the idea that learning is specific to the conditions that prevail during skill acquisition. This has implications for the ideas of generalized motor program and schema theory.

On the type of information used to control and learn an aiming movement after moderate and extensive training

Abstract This experiment was conducted to see if, in an aiming task (MT = 550 msec), where subjects received moderate (200 trials) or extensive practice (2000 trials), performance would benefit from vision of the performing limb and the target to be reached when compared to a situation where only the target to be reached was visually available. As a second goal, a transfer paradigm was used to see to what extent learning was specific to the conditions under which practice occurred. The results indicated that performance was enhanced when subjects were permitted vision of the performing limb. Furthermore, the subjects who benefited from vision of the performing limb in the training period were not able, even after extensive training, to maintain performance in the transfer task (i.e., without vision of the performing limb). These results are consistent with the view that vision of the responding limb is particularly important in learning a perceptual-motor task. Moreover, practice does not decrease the importance of this information for guiding the movement as some of the past literature suggests might happen. The results are seen as supporting the notion that movement learning may involve the development of a complex sensorimotor reference mechanism that acts to control and, when necessary, modify the ongoing movement. Further, this would imply that movement learning is relatively specific to the conditions under which practice occurs.

The coupling of arm and finger movements during prehension

The experiments reported here were aimed at testing the degree of coupling of motor components during the act of prehension. Hand movements were recorded bidimensionnally by a Selspot system which monitored the displacement of IREDS placed at the thumb and index finger tips, at the metacarpophalangeal joint of the index and at the radial styloid. Targets were three-dimensional trnaslucent dowels placed concentrically at 30 cm from the subject. The dowels were 10° apart from each other. In blocked and control trials, one dowel was illuminated and served as a target for the movement. In the perturbed trials (20% of cases) one dowel was illuminated first and the light was unexpectedly shifted to another dowel at the onset of the subjects movements. Kinematic analysis of the movement revealed the following: 1. In blocked and control trials, the wrist moved with a single acceleration to the target dowel. Meanwhile, the finger grip (computed as the distance between thumb and index IREDS) increased up to a maximum size, located in time at about 60% of movement time and then decreased until contact with the dowel. 2. In perturbed trials the initial wrist acceration was aborted. A new acceleration started about 180 ms after the first, in order to reorient the hand to the new target. Similarly, the initial grip aperture also aborted and reincreased in synchrony with the second wrist acceleration. 3. Perturbations increased movement time by only 95 ms on average. The first peak in acceleration indicating abortion of the initial movement occured 100 ms after the movement onset, i.e., 30 ms earlier than in non perturbed trials. These data revealed very fast alterations in movements kinematics in response to perturbations at the visual input level, which preserved accuracy of the movements. In addition, they showed temporary coupling of the finger grip with acceleration of the wrist.

Retention Characteristics of Motor Short-Term Memory Cues

The retention characteristics of several cues thought to underlie movement reproduction ability were examined and the results were discussed in terms of two models of motor short-term memory (Laabs, 1973; Pepper & Herman, 1970). Trace decay was indexed by constant error and not variable error. It appeared that the movement cues studied all had access to the central processing capacity in that forgetting did not occur until rehearsal was blocked by the introduction of a secondary task. However, there was some evidence to indicate that different cues are centrally represented in varying degrees of exactness. In this respect reliance on active movement cues and location cues produced better reproduction than passive movement and distance cues, respectively. The existence of an adaptation level established from the range of movement utilized was supported, and short movements were more dependent on central processing capacity than were long movements.

Achieving Coordination in Prehension: Joint Freezing and Postural Contributions.

The focus of the present study was on the intersegmental relationships that emerge when both task and oganismic constraints are imposed upon the coordination system. Seven right-handed subjects were required to reach and grasp a cup (hand transport phase) and place it on a designated target (cup transport phase), using either their preferred or nonpreferred hand. The kinematics of the movement were examined as a function of task (grasping a full cup versus grasping an empty one) and organismic (preferred or nonpreferred hand) constraints. During the hand transport phase, a task constraint effect was revealed through an increase in the low-velocity phase for the full cup condition. This constraint coexisted with a decrease in angular motion of the shoulder and elbow joints, indicating subjects reduced the number of variables to be independently controlled in the final homing-in stage of the movement. Accompanying this decrease in angular change was an increase in the displacement of the trunk. During the cup transport phase, the trunk was shown to contribute significantly more to the movement in the full cup condition and for the left hand movements, thereby increasing the stability of the movement system. These findings are in agreement with Bernsteins (1967) notion of fixating parts of the body as an initial solution to a movement problem, and they lend support to the concept of a proximodistal organization of coordination.

The effects of object weight on the kinematics of prehension

The purpose of these experiments was to determine the effects of object weight and condition of weight presentation on the kinematics of human prehension. Subjects performed reaching and grasping movements to metal dowels whose visible characteristics were similar but whose weight varied (20, 55, 150, 410 g). Movements were performed under two conditions of weight presentation, random (weight unknown) and blocked (weight known). Three-dimensional movements of the thumb, index finger, and wrist were recorded, using a WATSMART system to obtain information regarding the grasp and transport components. The results of the first experiment indicated that object weight and condition of presentation affected the temporal and kinematic measures for both the grasp and transport components. In conjunction with the results of a second experiment, in which time in contact with the dowel was measured, it was shown that the free-motion phase of prehension (i.e., up to object contact) was invariant over the different conditions, however. The changes were observed in the finger-object interaction phase (when subjects applied forces after contact with the dowel), prior to lift-off. These results were interpreted as indicating (a) object weight does not influence the planning and execution of the free-motion phase of prehension and (b) there are at least two motor control phases involved in prehension, one for making contact with the object and the other for finger-object interaction. The changing contributions of visual, kinesthetic, and haptic information during these two phases is discussed.