Mental object rotation and the planning of hand movements

Recently, we showed that the simultaneous execution of rotational hand movements interferes with mental object rotation, provided that the axes of rotation coincide in space. We hypothesized that mental object rotation and the programming of rotational hand movements share a common process presumably involved in action planning. Two experiments are reported here that show that the mere planning of a rotational hand movement is sufficient to cause interference with mental object rotation. Subjects had to plan different spatially directed hand movements that they were asked to execute only after they had solved a mental object rotation task. Experiment 1 showed that mental object rotation was slower if hand movements were planned in a direction opposite to the presumed mental rotation direction, but only if the axes of hand rotation and mental object rotation were parallel in space. Experiment 2 showed that this interference occurred independent of the preparatory hand movements observed in Experiment 1. Thus, it is the planning of hand movements and not their preparation or execution that interferes with mental object rotation. This finding underlines the idea that mental object rotation is an imagined (covert) action, rather than a pure visual-spatial imagery task, and that the interference between mental object rotation and rotational hand movements is an interference between goals of actions.     

Mental object rotation and the planning of hand movements.

Recently, we showed that the simultaneous execution of rotational hand movements interferes with mental object rotation, provided that the axes of rotation coincide in space. We hypothesized that mental object rotation and the programming of rotational hand movements share a common process presumably involved in action planning. Two experiments are reported here that show that the mere planning of a rotational hand movement is sufficient to cause interference with mental object rotation. Subjects had to plan different spatially directed hand movements that they were asked to execute only after they had solved a mental object rotation task. Experiment 1 showed that mental object rotation was slower if hand movements were planned in a direction opposite to the presumed mental rotation direction, but only if the axes of hand rotation and mental object rotation were parallel in space. Experiment 2 showed that this interference occurred independent of the preparatory hand movements observed in Experiment 1. Thus, it is the planning of hand movements and not their preparation or execution that interferes with mental object rotation. This finding underlines the idea that mental object rotation is an imagined (covert) action, rather than a pure visual-spatial imagery task, and that the interference between mental object rotation and rotational hand movements is an interference between goals of actions.     

Visual monitoring of goal-directed aiming movements

Goal-directed movements are subject to intrinsic planning and execution variability, which requires that the central nervous system closely monitor our movements to ensure endpoint accuracy. In the present study, we sought to determine how closely the visual system monitored goal-directed aiming movements. We used a cursor-jump paradigm in which a cursor was unexpectedly translated soon after movement initiation. Some of the trials included a second cursor jump, and the cursor remained visible for different durations. The results indicate that seeing the cursor for only 16?ms after the second cursor jump was sufficient to influence the movement endpoint, which suggests that the visual system continuously monitored goal-directed movements. The results also suggest that the perceived position/trajectory of the effector was likely to have been averaged over a period of approximately 70?ms.     

Vector coding in slow goal-directed arm movements

It has been found that the estimate of relative target direction is consistently biased. Relative target direction refers to the direction in which a target is located relative to another location in space (e.g., a starting position in the case of goal-directed movements). In this study, we have tested two models that could underlie this biased estimate. The first proposed model is based on a distorted internal representation of locations (i.e., we perceive a target at the “wrong” location). We call this thedistorted location model. The second model is based on the idea that the derivation of target direction from spatial information about starting and target position is biased. We call this thebiased direction model. These two models lead to different predictions of the deviations that occur when the distance between the starting position and the target position is increased. Since we know from previous studies that the initial direction of slow arm movements reflects the target direction estimate, we tested the two models by analyzing the initial direction of slow arm movements. The results show that the biased direction model can account for the biases we find in the target direction estimate for various target distances, whereas the distorted location model cannot. In two additional experiments, we explored this model further. The results show that the biases depend only on the orientation of the line through starting position and target position relative to the plane through longitudinal head or body axis and starting position. We conclude that the initial part of (slow) goal-directed arm movements is planned on the basis of a (biased) target direction estimate and not on the basis of a wrong internal representation of target location. This supports the hypothesis that we code displacements of our limbs in space as a vector.     

The effects of response priming on the planning and execution of goal-directed movements in the presence of a distracting stimulus

Two models of selective reaching have been proposed to account for deviations in movement trajectories in cluttered environments. The response vector model predicts movement trajectories should deviate towards or away from the location a distractor of little or large salience, respectively. In contrast, the response activation model predicts that a distractor with large salience should cause movement deviations towards it whereas a distractor with little salience should not influence the movement. The precuing technique was combined with the distractor interference paradigm to test these predictions. Results indicate that when the target was presented at the precued (salient) location, movements were unaffected by a distractor. Conversely, when the distractor was presented at the precued location while the target was presented at an uncued (non-salient) location, participants demonstrated increased reaction times and trajectory deviations towards the location of the distractor. These findings are consistent with the model of response activation.     

Manual asymmetries in the preparation and control of goal-directed movements

The primary purpose of this experiment was to determine if left hand reaction time advantages in manual aiming result from a right hemisphere attentional advantage or an early right hemisphere role in movement preparation. Right-handed participants were required to either make rapid goal-directed movements to small targets or simply lift their hand upon target illumination. The amount of advance information about the target for a particular trial was manipulated by precuing a subset of potential targets prior to the reaction time interval. When participants were required to make aiming movements to targets in left space, the left hand enjoyed a reaction advantage that was not present for aiming in right space or simple finger lifts. This advantage was independent of the amount or type of advance information provided by the precue. This finding supports the movement planning hypothesis. With respect to movement execution, participants completed their aiming movements more quickly when aiming with their right hand, particularly in right space. This right hand advantage in right space was due to the time required to decelerate the movement and to make feedback-based adjustments late in the movement trajectory.     

Stiffness control after fast goal-directed arm movements

Mechanical parameters of the effector system directly after the termination of fast goal-directed arm movements were studied.Subjects were asked to move their hand as fast as possible to a target the instant the target was presented. Only movements of the subjects' forearms were allowed. They were also instructed not to react actively to forces applied suddenly to their forearm after the movement. As a result of such a force pulse the arm moved to a new position. The apparent stiffness, i.e. the quotient of the applied force and the resultant change of position, was measured. This stiffness is a measure for the resistance of the forearm to externally applied mechanical disturbances.It was found that after the arm has reached the target the apparent stiffness decreases as a function of time. This is an agreement with the declining amplitude of the electromyographic activity of the muscles that effect the movement.Arguments are given to support the hypothesis that this apparent stiffness control is part of the motor programme for movements of the forearm, i.e. the stiffness is planned together with the movement.     

Simultaneous processing of visual information and planning of hand movements in a visuo-manual search task

When searching for a target with eye movements, saccades are planned and initiated while the visual information is still being processed. If hand movements are needed to perform a search task, can they too be planned while visual information from the current position is still being processed? To find out we studied a visual search task in which participants had to move their hand to shift a window through which they could see the items. The task was to find an O in a circle of Cs. The size of the window and the sizes of the gaps in the Cs were varied. Participants made fast, smooth arm movements between items and adjusted their movements, when on the items, to the window size. On many trials the window passed the target and returned, indicating that the next movement had been planned before identifying the item that was in view.     

Mechanisms underlying accuracy in fast goal-directed arm movements in man

This study investigated how accuracy is attained in fast goal-directed arm movements. Subjects were instructed to make arm extension movements over three different distances in random order, with and without visual feedback. Target width was varied proportionally with distance. Movement time was kept as short as possible, but there were well-defined limits with respect to accuracy. There appeared to be a large relative variability (variation coefficient [VC]) in the initial acceleration. The VC in the distance the hand moved during the acceleration phase was much smaller. This reduction was accompanied by a strong negative correlation between the initial acceleration and the duration of the acceleration phase. Further, the VC in the total distance moved was less than the VC in the distance moved during acceleration. This result indicates asymmetry between the acceleration and the deceleration phase. This is confirmed by the negative correlation between the distance the hand moved during acceleration and the distance it moved during deceleration. Withdrawal of visual feedback had a significant effect on movement accuracy. No differences were found in the parameters of the acceleration phase in the two feedback conditions, however. our results point to the existence of a powerful variability compensating mechanism within the acceleration phase. This mechanism seems to be independent of visual feedback; this suggests that efferent information (efference copies) and/or proprioceptive information is/are responsible for the timing of agonist and antagonist activation. The asymmetry between the acceleration and deceleration phase contributes to a reduction in the relative variability in the total distance moved. The fact that the withdrawal of visual feedback affected movement variability only during the deceleration phase indicates that visual information is used in the adjustment of antagonist activity.     

On-line trajectory modifications of planar,goal-directed arm movements

Sometimes a goal-directed arm movement has to be modified en route due to an unforeseen perturbation such as a target displacement or a hand displacement by an external force. In this paper several aspects of that modification process are addressed. Subjects had to perform a point-to-point movement task on a computer screen using a mouse-coupled pointer as the representation of the hand position. Trajectory modifications were imposed by unexpectedly changing the position of the target or by changing the relation between mouse and screen pointer.In the first series of experiments, we examined how often a trajectory is updated. Here, trajectory modifications were imposed by unexpectedly changing the normal relation between mouse and pointer to a shear-like relation, where a percentage of the forward/backward position of the hand was added to the pointer position in the left/right direction. Withdrawal of visual feedback during the movement revealed that trajectories were updated at interval times shorter than 200 ms. From the similarity with experiments where the original relation between mouse and pointer was restored during the movements, we conclude that motor plans are updated on-line to move the hand from its current perceived position to the target.In a second series of experiments, we studied whether a continuous change in target position yields similar trajectory modifications as a continuous hand displacement. To mimic the latter perturbation, we used the above-mentioned distortion of the mouse-pointer relation. We found that the resulting hand paths did not differ for the two visual perturbations and conclude that the perturbed, goal-directed movements are modified in a consistent way, irrespective of whether the position of the target or hand was perturbed. Simulations of the experimental data with a kinematic reaching model support this conclusion.     

Relative damping improves linear mass-spring models of goal-directed movements

A limitation of a simple linear mass-spring model in describing goal directed movements is that it generates rather slow movements when the parameters are kept within a realistic range. Does this imply that the control of fast movements cannot be approximated by a linear system? In servo-control theory, it has been proposed that an optimal controller should control movement velocity in addition to position. Instead of explicitly controlling the velocity, we propose to modify a simple linear mass-spring model. We replaced the damping relative to the environment (absolute damping) with damping with respect to the velocity of the equilibrium point (relative damping). This gives the limb a tendency to move as fast as the equilibrium point. We show that such extremely simple models can generate rapid single-joint movements. The resulting maximal movement velocities were almost equal to those of the equilibrium point, which provides a simple mechanism for the control of movement speed. We further show that peculiar experimental results, such as an 'N-shaped' equilibrium trajectory and the difficulties to measure damping in dynamic conditions, may result from fitting a model with absolute damping where one with relative damping would be more appropriate. Finally, we show that the model with relative damping can be used to model subtle differences between multi-joint interceptions. The model with relative damping fits the data much better than a version of the model with absolute damping.     

Learning rhythmic hand movements

Twenty-seven subjects tracked targets moving up and down in a straight-line path formed from the sum of three sine waves. There were nine such tracks varying in average frequency and length of repeating subunit. The latter variable had little effect on performance, but low frequency targets were tracked better than high frequency targets. Only the component frequencies present in the tracks were reproduced to any extent in the subjects' performance. For components in different tracks the highest frequencies were reproduced with the lowest amplitude. However, for components within the track the highest frequency component tended to be reproduced with the highest amplitude. In addition the greatest amount of learning appeared in the highest frequency component within a track.     

Modification of muscle activation patterns during fast goal-directed arm movements

The motor programming of fast goal-directed arm movements was studied in a tracking task. A target jumped once or twice randomly to the left or right direction with an interstimulus interval (ISI) in a range between 50 and 125 msec. Double step stimuli were either two steps in the same direction (C-trial) or in opposite direction (R-trial). Tracking results show that at the beginning average EMG-activity is the same for responses to single step trials, R-trials and C-trials. Differences set in after some time equal to or somewhat shorter than ISI. It was concluded that muscle activation patterns of fast goal-directed movements are not preprogrammed but that they can be modified during the movement. The time interval between second target step and the moment when EMG activity of the double step response deviates from the EMG activity of a single step (RT2) could be smaller than the time interval between first target displacement and EMG onset. (RT1). If modification of the muscle activation pattern required a longer or larger activation of the active muscle, RT2 tended to be smaller than RT1, whereas RT2 was about equal to RT1 if the new muscle activation required a termination of the ongoing muscle activation pattern and the activation of another muscle.     

Object-based eye movements: The eyes prefer to stay within the same object

The present study addressed the question of whether we prefer to make eye movements within or between objects. More specifically, when fixating one end of an object, are we more likely to make the next saccade within that same object or to another object? Observers had to discriminate small letters placed on rectangles similar to those used by Egly, Driver, and Rafal (1994). Following an exogenous cue, observers made a saccade to one end of one of the rectangles. The small target letter, which could be discriminated only after it had been fixated, could appear either within the same or at a different object. Consistent with object-based attention, we show that observers prefer to make an eye movement to the other end of the fixated same object, rather than to the equidistant end of a different object. It is concluded that there is a preference to make eye shifts within the same object, rather than between objects.     

Infants' imitation of goal-directed actions: the role of movements and action effects

This paper reviews studies on infants' imitation of goal-directed actions in the first two years of life. Special emphasis is given to the role of the two observable components of an action, that is, the movement and the action effects, on infants' replication of target actions. The reviewed studies provide evidence that infants benefit most from a full demonstration of both movements and effects. If movements are demonstrated in isolation, infants may encode this information, but they preferentially reproduce actions that lead to salient effects. If action effects are presented in isolation, infants younger than 19 months usually fail to emulate the unseen movements that would be necessary to produce these effects. Infants' ability to predict action effects or to infer unseen movements from incomplete demonstrations improves substantially at the end of the second year of life. It is concluded that the capability to learn relations between movements and action effects by observation, and the knowledge about movement-effect relations acquired so far, may be important factors underlying the developmental changes in infants' imitation of goal-directed actions.     

On the hand transport component of prehensile movements

Jeannerod (1981) proposed that prehensile movements involve two independent visuomotor channels that are responsible for hand transport and hand aperture. In many studies, the movement of a marker placed on the wrist has been used as an index of hand transport because wrist movement is unaffected by the movements of the digits responsible for hand aperture. In the present study, the spatial paths of the wrist, index finger, and thumb of 5 adults, each performing 50 reaching movements, were measured with a WATSMART movement tracking system, and their variability was analyzed. The measures of movement variability suggest that the motor system is more concerned with thumb position than with wrist position during hand transport. Although the wrist is a technically convenient index of hand transport, the thumb may be a more appropriate index from the point of view of motor control     

The planning of precision movements

Two experiments are reported in which subjects were required to make rapid aiming movements to targets of various sizes. Probe reaction time (RT) procedures were used to investigate the preparation of the response to the target. It was proposed that if the precision of movement was planned in advance, this would be reflected by the lengthening of RTs to probes presented during the latency phase of the response. The more precise the movement (to smaller targets) the longer will be the delays to the probes. The results generally supported the prediction and the probe RTs were correlated with target size. There was also some evidence that the probe was lengthening during the movement and in the region of the target.     

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.     

Kinematics properties of rapid aimed hand movements

Generalized motor program theory and the models of Schmidt, Zelaznik, and Frank (1978), and Meyer, Smith, and Wright (1982) of speed-accuracy relationships in aimed hand movements require that the underlying acceleration-time patterns exhibit time rescalability, in which all acceleration-time functions in an aimed hand movement are generated from one rescalable pattern. We examined this property as a function of movement time in Experiment 1, and as a function of movement time and movement distance in Experiment 2. Both experiments failed to demonstrate strict time rescalability in acceleration-time patterns, with the time to peak positive acceleration being invariant across movement time. This suggests that time rescalability is not a necessary condition for the linear relation between speed and spatial variability. A second major finding was that the variability in distance traveled at the end of positive acceleration was independent of movement time, contrary to the symmetric-impulse-variability model of Meyer et al. (1982). The findings of both experiments suggest that the processes involved in decelerating the limb play an important, but yet to be understood, role in determining the linear speed-accuracy trade-off. Finally, these results suggest that generalized motor programs are not based on simple, time-rescalable acceleration patterns.     

Movement structure in young and elderly adults during goal-directed movements of the left and right arm

Elderly adults often exhibit performance deficits during goal-directed movements of the dominant arm compared with young adults. Recent studies involving hemispheric lateralization have provided evidence that the dominant and non-dominant hemisphere-arm systems are specialized for controlling different movement parameters and that hemispheric specialization may be reduced during normal aging. The purpose was to examine age-related differences in the movement structure for the dominant (right) and non-dominant (left) during goal-directed movements. Young and elderly adults performed 72 aiming movements as fast and as accurately as possible to visual targets with both arms. The findings suggest that previous research utilizing the dominant arm can be generalized to the non-dominant arm because performance was similar for the two arms. However, as expected, the elderly adults showed shorter relative primary submovement lengths and longer relative primary submovement durations, reaction times, movement durations, and normalized jerk scores compared to the young adults.