Deceleration requirements and the control of pointing movements

When a limb is moved from one position to a target object, the limb and the target frequently collide. Often, the goal of the movement is to strike the target with a particular magnitude of impact. For single-aiming movements, impact forces have been shown to increase systematically with both an increased movement amplitude and a decreased movement time, thus providing deceleration to the moving limb. Models of speed-accuracy trade-off, however, have neglected to account for the contribution of these impact forces in the control of accurate movements. The aim of this experiment was to examine the modifications in the control strategy as a function of the amount of impact force a subject is allowed to use in decelerating his or her limb. Results showed that the structure of the acceleration-time functions was dictated by the amount of impact force subjects were allowed to use in decelerating the limb. Movement endpoint variability decreased as more impact force was used. The experiment suggests that the impact with a target is an important contributor to the deceleration of the moving limb and a critical determinant of movement organization.     

The role of vision in the control of continuous multijoint movements

The authors investigated whether visual fixations during a continuous graphical task were related to arm endpoint kinematics, joint motions, or joint control. The pattern of visual fixations across various shapes and the relationship between temporal and spatial events of the moving limb and visual fixations were assessed. Participants (N=16) performed movements of varying shapes by rotating the shoulder and elbow joints in the transverse plane at a comfortable pace. Across shapes, eye movements consisted of a series of fixations, with the eyes leading the hand. Fixations were spatially related to modulation of joint motion and were temporally related to the portions of the movement where curvature was the highest. Gathering of information related to modulation of interactive torques arising from passive forces from movement of a linked system occurred when the velocity of the movement (a) was the lowest and (b) was ahead of the moving limb, suggesting that that information is used in a feedforward manner.     

Sequential processes for controlling distance in multijoint movements

The purpose of this study was to examine the mechanisms underlying control of distance during multijoint movements in different directions. The findings revealed 2 sequential muscle torque impulses, which correlated with 2 events in the hand acceleration profile. These 2 events occurred prior to peak velocity, characterizing control in the initial acceleration phase of motion. The contribution of shoulder and elbow joint torque to each event varied with movement direction. However, regardless of direction, these 2 torque events appeared to be functionally distinguishable: a preplanned initiation event was responsible for the initial hand acceleration, whereas a 2nd modulation event adjusted acceleration in compensation for variations in acceleration. Thus, the findings support the idea that control of distance during multijoint movement occurs through sequential control mechanisms.     

The temporal organization of hand, eye, and head movements during reaching and pointing

Two experiments are reported that address the issue of coordination of the eyes, head, and hand during reaching and pointing. Movement initiation of the eyes, head, and hand were monitored in order to make inferences about the type of movement control used. In the first experiment, when subjects pointed with the finger to predictable or unpredictable locations marked by the appearance of a light, no differences between head and eye movement initiation were found. In the second experiment, when subjects pointed very fast with the finger, the head started to move before the eyes did. Conversely, when subjects pointed accurately, and thus more slowly, with the finger, the eyes started to move first, followed by the head and finger. When subjects were instructed to point to the same visual target only with their eyes and head, both fast and accurately, however, eye movement always started before head movement, regardless of speed-accuracy instructions. These results indicate that the behavior of the eye and head system can be altered by introducing arm movements. This, along with the variable movement initiation patterns, contradicts the idea that the eye, head, and hand system is controlled by a single motor program. The time of movement termination was also monitored, and across both experiments, the eyes always reached the target first, followed by the finger, and then the head. This finding suggests that movement termination patterns may be a fundamental control variable.     

Posture control and complex arm coordination: analysis of multijoint coordinative movements and stability of stance

The authors addressed the interactions between control of bimanual multijoint coordination tasks and posture. Participants (N = 6) performed 8 coordination patterns that differed in degree of complexity by using their bilateral elbows and wrists under 3 scaled motion speeds while standing on 2 force plates. Results indicated that producing complex bimanual multijoint coordinative tasks affected postural sway, thus resulting in an increase of sway activity. Behavioral as well as mechanical factors accounted for the increased disturbance in postural sway. Those findings suggest that performing complex coordination tasks disrupts postural control in normal young adults.     

Coordinated control of eye and hand movements in dynamic reaching

In the present study, we integrated two recent, at first sight contradictory findings regarding the question whether saccadic eye movements can be generated to a newly presented target during an ongoing hand movement. Saccades were measured during so-called adaptive and sustained pointing conditions. In the adapted pointing condition, subjects had to direct both their gaze and arm movements to a displaced target location. The results showed that the eyes could fixate the new target during pointing. In addition, a temporal coupling of these corrective saccades was found with changes in arm movement trajectories when reaching to the new target. In the sustained pointing condition, however, the same subjects had to point to the initial target, while trying to deviate their gaze to a new target that appeared during pointing. It was found that the eyes could not fixate the new target before the hand reached the initial target location. Together, the results indicate that ocular gaze is always forced to follow the target intended by a manual arm movement. A neural mechanism is proposed that couples ocular gaze to the target of an arm movement. Specifically, the mechanism includes a reach neuron layer besides the well-known saccadic layer in the primate superior colliculus. Such a tight, sub-cortical coupling of ocular gaze to the target of a reaching movement can explain the contrasting behavior of the eyes in dependency of whether the eye and hand share the same target position or attempt to move to different locations.     

Kinematic markers of distance-specific control in linear hand movements

In this article, the authors analyze kinematic characteristics of reaching movements to memorized visual target locations. An increase in target distance was associated with a decrease in correlation between peak acceleration and movement distance and with a simultaneous increase in correlation between peak acceleration and movement time. According to the previous work on motor control in isometric force responses and in reaching movements these results seem to indicate a continuous transition from a rather preplanned to a more corrective mode of movement control, which may be associated with an adaptive mechanism serving to counteract an increase in signal-dependent noise of the motor system.     

Inhibition of return and manual pointing movements

To examine whether the motor inhibition of return (IOR) postulated by Taylor and Klein (1998, 2000) generalizes to manual guided movements or is restricted to saccadic responses, the following three experiments were conducted. The first experiment combined peripheral cues (which generate IOR) with four types of manual responses made to central targets (central arrow indicating the response location). The responses were made on a touch-screen and were the equivalent of either a detection keypress, a choice keypress, a detection-guided pointing movement, or a choice-guided pointing movement. No IOR was found for any of the responses. The second experiment replicated the main result under eye fixation control. In Experiment 3, peripheral cues and peripheral targets were used, and IOR was present in all responses. Overall, these finding suggest that motor-based IOR is restricted to the oculomotor system. Implications for motor-based IOR and attention-based IOR are discussed.     

Visual perception of writing and pointing movements

Studies of movement production have shown that the relationship between the amplitude of a movement and its duration varies according to the type of gesture. In the case of pointing movements the duration increases as a function of distance and width of the target (Fitts' law), whereas for writing movements the duration tends to remain constant across changes in trajectory length (isochrony principle). We compared the visual perception of these two categories of movement. The participants judged the speed of a light spot that portrayed the motion of the end-point of a hand-held pen (pointing or writing). For the two types of gesture we used 8 stimulus sizes (from 2.5 cm to 20 cm) and 32 durations (from 0.2 s to 1.75 s). Viewing each combination of size and duration, participants had to indicate whether the movement speed seemed "fast", "slow", or "correct". Results showed that the participants' perceptual preferences were in agreement with the rules of movement production. The stimulus size was more influential in the pointing condition than in the writing condition. We consider that this finding reflects the influence of common representational resources for perceptual judgment and movement production.     

Attentional strategies and the visual control of discrete movements

The relationship between the availability of visual information and attention demands during the production of a discrete motor act was examined. Subjects were required to move a linear slide a distance of 15 cm in both 150 and 600 msec conditions under three light manipulations, viz., light always on; light always terminated as movement began; or, subject was unsure as to whether light would stay on or terminate. A simple key press by the index finger of the opposite hand to a tone was used as a secondary task and a measure of attention demands. The light manipulation influenced attention demands on the rapid 150 msec movement such that more attention was demanded when the subject knew the light would terminate. No such attentional strategy differences were found with the slow 600 msec movement. These findings suggest that task constraints in the form of kinematic criteria, together with the perceived availability of visual information, contribute to determining attentional strategy in movement production.     

Predictive pointing movements and saccades toward a moving target

The authors investigated whether and, if so, how velocity information is used to control predictive manual pointing movements and saccades. Participants (N = 6) intercepted an occluded moving target as if it were still visible. They kept their eyes fixated while the target moved. The target traveled over a fixed distance and changed its velocity on the way. The presentation time of the final velocity was varied. Both the eye and the hand overshot the slow target and undershot the fast target, particularly when the duration of the final velocity was short. Thus, responses were biased in the direction of the target's initial velocity. The error seemed to arise because participants did not take their latency into account when aiming at the target. Instead, they strategically aimed farther ahead when the target was fast. Amplitude was also more related to the position of velocity change than to final velocity duration. Both findings suggest that target velocity is not extrapolated but that individuals add an increment to the position of velocity change.     

The visual control of aimed hand movements to stationary and moving targets.

The present study attempted to determine if during short-duration movements visual feedback can be processed in order to make adjustments to changes in the environment. The effect that varying the importance of monitoring target position has on the relative importance of vision of hand and vision of target (Carlton 1981a; Whiting and Cockerill 1974) was also examined. Subjects performed short- (150 ms) and longer-duration (330 ms) aimed hand movements under four visual feedback conditions (lights-on/lights-off by target-on/target-off) to stationary and moving targets. For the lights-off and target-off conditions, the lights and target, respectively, were extinguished 50 ms after movement initiation. For all moving-target conditions, the target started to move as the movement was initiated. Subjects were able to process visual information in 165 ms, as movement endpoints were biased in the direction of target motion for movements of this duration. Removing visual feedback 50 ms after movement initiation did not alter this finding. Subjects performed equally well with target and lights on or off, independent of whether the target remained stationary or moved. Presumably, during the first 50 ms of the movement subjects received sufficient visual information to aid in movement control.     

Distraction and body-focused hand movements.

The hypothesis that apparently irrelevant self- or object-manipulatory hand movements may act as a means of coping with distraction was tested by experimentally manipulating the amount and type of distraction experienced by 10-year-old children while they engaged in the Stroop colour-confusion and colour-naming tasks. If the hypothesis was correct, then increases in distraction were expected to be associated with increases in the frequency of these body-focused movements. The external distractions consisted of either the occurrence of a light signalling the need to perform a reaction time task or listening to distracting sounds through headphones. None of the hand movements increased in frequency with increases in secondary distraction, whether the secondary distractor was visual or auditory.     

The use of prescanning in the parameterization of sequential pointing and reaching movements

The accuracy of reaching movements improves when active gaze can be used to fixate on targets. The advantage of free gaze has been attributed to the use of ocular proprioception or efference signals for online control. The time course of this process, however, is not established, and it is unclear how far in advance gaze can move and still be used to parameterize subsequent movements. In this experiment, the authors considered the advantage of prescanning targets for both pointing and reaching movements. The authors manipulated the visual information and examined the extent to which prescanning of targets could compensate for a reduction in online visual feedback. In comparison with a conventional reaching/pointing condition, the error in pointing was reduced, the eye-hand lead decreased, and both the hand-closure time and the size of the maximum grip aperture in reaching were modulated when prescanning was allowed. These results indicate that briefly prescanning multiple targets just prior to the movement allows the refinement of subsequent hand movements that yields an improvement in accuracy. This study therefore provides additional evidence that the coordinate information arising from efference or ocular-proprioceptive signals can, for a limited period, be buffered and later used to generate a sequence of movements.     

Contribution of visual information to feedforward and feedback processes in rapid pointing movements

The purpose of this study was to investigate what types of visual cues may contribute to improving movement accuracy in a pointing task, and to determine in what kind of control processes these cues are involved. During the experiment, subjects had to point their finger at visual targets as accurately as possible making rapid movements. Subjects were required to perform a movement with an amplitude of 40 cm within a series of times ranging from 110 to 270 msec. Five visual feedback conditions were applied: no feedback (NF), dynamic ongoing feedback on the complete hand trajectory (CF), static error feedback on the movement end-point (EF), and two partial feedback conditions in which dynamic feedback was available from either the initial (IF) or the terminal (TF) part of the trajectory. The results showed that under the NF and IF conditions accuracy was lowest; constant error was not speed-dependent, whereas dispersion increased with movement speed. Accuracy was highest under the CF and TF conditions and was speed-dependent, as shown by both constant error and dispersion. Under the EF condition, the accuracy level was intermediate, and was also speed-dependent. The time course of performance during the series was analyzed by comparing the mean error of the first and the last five-trial blocks in the series under the three feedback conditions resulting in accuracy improvement and speed-dependence (CF, TF and EF). The block effect was significant overall, with the last block showing greatest accuracy. The block effect was found to be significant for rapid movements only under the CF and TF conditions (with a Block × Speed interaction under the CF condition), and for all movement speeds under the EF condition. But the feedback and speed effects turned out to be significant for each block. The results are discussed in terms of the interchange between ‘corrected’ ongoing responses vs ‘amended’ delayed responses within the motor regulatory processes, the preponderance of one or the other type of response being dependent on feedback availability and movement speed.     

Impairment in the control of coupled cyclic movements of ipsilateral hand and foot after total callosotomy

Integrity of both cerebral hemispheres is required to control in-phase or anti-phase coupling of ipsilateral hand and foot oscillations, as shown by the impairment of these tasks when performed on the healthy side of hemiplegic patients. On this basis, coupling of hand–foot movements was analysed in a right-handed subject (ME) who underwent a total resection of the corpus callosum. Oscillations of the prone hand and foot, paced by a metronome at different frequencies, as well as EMG activity in extensor carpi radialis (ECR) and tibialis anterior (TA) muscles were analysed by measuring the average phase difference between the hand and foot movements and EMG cycles.

ME performed in-phase movements (right-hand extension coupled to right-foot dorsal flexion) at frequencies up to 3 Hz, though the hand cycle progressively lagged the foot cycle as the frequency increased. At 3 Hz the hand lag reached −142° (as compared to about 25° in healthy subjects). The lag increased even further after application of an inertial load to the hand, reaching 180° at 1.8 Hz (about 50° in healthy subjects). ME's hand lag is caused by the lack of any anticipatory reaction in hand movers. In contrast to healthy subjects, which activate the ECR earlier than the TA when the frequency increases, ME activated the ECR later than TA at all frequencies higher than 0.9 Hz.

Anti-phase movements (hand extension coupled to foot plantar flexion) were performed only upto 1 Hz in unloaded conditions. At 0.6 Hz, movements were in tight phase-opposition (3°), but at 1 Hz, the hand lag reached −34° because of a delayed ECR activation. After hand loading ME was unable to couple movements in anti-phase. In contrast, normal subjects maintain a tight anti-phase coupling up to 2.0 Hz, both with an unloaded or loaded hand. Similar deficits were observed by ME when performing in-phase and anti-phase coupling on the left side, as well as when he was blindfolded.

In normal subjects, an anticipated muscular activation of hand movers compensates for hand loading. Since this compensation must depend on monitoring the hand delay induced by loading, the absence in ME of such compensatory reaction suggests that callosal division had apparently compromised the mechanisms sustaining feedback compensation for differences in the biomechanical limb properties. They also confirm and reinforce the idea that elaboration of the afferent message, aiming at controlling the phase of the movement association, needs the co-operation of both cerebral hemispheres.     

Directional variability of the isometric force vector produced by the human hand in multijoint planar tasks

Numerous studies have examined control of force magnitude, but relatively little research has considered force direction control. The subjects applied isometric forces to a handle and the authors compared within-trial variability when force is produced in different directions. The standard deviation of the force parallel to the prescribed direction of force production increased linearly with the targeted force level, as did the standard deviation of the force perpendicular to the instructed direction. In contrast, the standard deviation of the angle of force production decreased with increased force level. In the 4 (of 8) instructed force directions where the endpoint force was generated due to a joint torque in only 1 joint (either the shoulder or elbow) the principal component axes in force space were well aligned with the prescribed direction of force production. In the other directions, the variance was approximately equal along the 2 force axes. The variance explained by the first principal component was significantly larger in torque space compared to the force space, and mostly corresponded to positive correlation between the joint torques. Such coordinated changes suggest that the torque variability was mainly due to the variability of the common drive to the muscles serving 2 joints, although this statement needs to be supported by direct studies of muscle activation in the future.     

The gap effect for eye and hand movements

A temporal gap between fixation point offset and stimulus onset typically yields shorter saccadic latencies to the stimulus than if the fixation stimulus remained on. Several researchers have explored the extent to which this gap also reduces latencies of other responses but have failed to find a gap effect isolated from general warning effects. Experiment 1, however, showed a robust gap effect for aimed hand movements (which required determination of a precise spatial location), regardless of whether the hand moved alone or was accompanied by a saccadic eye movement. Experiment 2 replicated this aimed hand gap effect and also showed a smaller effect for choice manual keypress responses (which required determination of the direction of response only). Experiment 3 showed no gap effect for simple manual keypress responses (which required no spatial determination). The results are consistent with an interpretation of the gap effect in terms of facilitation of spatially oriented responses.     

The influence of eye-gaze and arrow pointing distractor cues on voluntary eye movements

We investigated Ricciardelli et al.'s (2002) claim, that the tendency for gaze direction to elicit automatic attentional following is unique to biologically significant information. Participants made voluntary saccades to targets on the left or the right of a display, which were either congruent or incongruent with a centrally presented distractor (eye-gaze or arrow). Contrary to Ricciardelli et al., for both distractor types, saccade latencies were slower, and participants made more directional errors, on incongruent than on congruent trials. Moreover, a cost-benefit analysis showed no difference between the two distractor types. However, latencies for erroneous saccades were faster than correctly directed saccades for the eye-gaze distractors, but not for the arrow distractors.     

To point a finger: attentional and motor consequences of observing pointing movements

Recent studies showed that action observation activates neural circuits used in performing the same action and facilitates execution of a similar motor program. This system for direct mapping of observed actions onto observer’s own motor representation is considered critical for human imitation capabilities. The present study shows that observing a pointing action activates a representation of that action in anatomical space, irrespectively of whether the action is shown in allocentric or egocentric perspective. This finding is at odds with the studies on imitation which showed that humans tend to imitate in a spatially compatible (specular) way, as if looking in a mirror. Our results suggest that shared representations for actions are organized in the same spatial coordinates; however, a transformation of this representation might be required for imitation tasks in order to accommodate the goals of imitative action.