The relation between movement parameters and motor learning.

In a recent paper, Flament et al (1999) studied the learning to flex the elbow fast. They concluded from their data that time-related parameters (e.g. movement time) changed faster during learning than magnitude-related parameters (e.g. peak velocity), and discussed this finding in terms of neural substrates responsible for the apparently different learning mechanisms. In this note, I will argue that finding different time constants does not imply different learning mechanisms. I will give a theoretical example of the development of parameters during learning to move faster. Despite the fact that I model only one learning process, various kinematic parameters show different time courses of learning. The differences the model predicts are comparable with the experimental results

A new view on grasping

Reaching out for an object is often described as consisting of two components that are based on different visual information. Information about the object’s position and orientation guides the hand to the object, while information about the object’s shape and size determines how the fingers move relative to the thumb to grasp it. We propose an alternative description, which consists of determining suitable positions on the object — on the basis of its shape, surface roughness, and so on — and then moving one’s thumb and fingers more or less independently to these positions. We modelled this description using a minimum jerk approach, whereby the finger and thumb approach their respective target positions approximately orthogonally to the surface. Our model predicts how experimental variables such as object size, movement speed, fragility, and required accuracy will influence the timing and size of the maximum aperture of the hand. An extensive review of experimental studies on grasping showed that the predicted influences correspond to human behaviour

Visuomotor delays when hitting running spiders.

In general, information about the environment (for instance a target) is not instantaneously available for the nervous system. A minimal delay for visual information to affect the movement of the hand is about 110 ms. However, if the movement of a target is predictable, humans can pursue it with zero delay. To make this prediction, information about the speed of the target is necessary. Our results show that this information is used with a delay of about 200 ms. We discuss that oculomotor efference is a likely source of information for this prediction

Continuously updating one’s predictions underlies successful interception

This paper reviews our understanding of the interception of moving objects. Interception is a demanding task that requires both spatial and temporal precision. The required precision must be achieved on the basis of imprecise and sometimes biased sensory information. We argue that people make precise interceptive movements by continuously adjusting their movements. Initial estimates of how the movement should progress can be quite inaccurate. As the movement evolves, the estimate of how the rest of the movement should progress gradually becomes more reliable as prediction is replaced by sensory information about the progress of the movement. The improvement is particularly important when things do not progress as anticipated. Constantly adjusting one’s estimate of how the movement should progress combines the opportunity to move in a way that one anticipates will best meet the task demands with correcting for any errors in such anticipation. The fact that the ongoing movement might have to be adjusted can be considered when determining how to move, and any systematic anticipation errors can be corrected on the basis of the outcome of earlier actions

The target as an obstacle:Grasping an object at different heights

Humans use a stereotypical movement pattern to grasp a target object. What is the cause of this stereotypical pattern? One of the possible factors is that the target object is considered an obstacle at positions other than the envisioned goal positions for the digits: while each digit aims for a goal position on the target object, they avoid other positions on the target object even if these positions do not obstruct the movement. According to this hypothesis, the maximum grip aperture will be higher if the risk of colliding with the target object is larger. Based on this hypothesis, we made a set of two unique predictions for grasping a vertically oriented cuboid at its sides at different heights. For cuboids of the same height, the maximum grip aperture will be smaller when grasped higher. For cuboids whose height varies with grip height, the maximum grip aperture will be larger when grasped higher. Both predicted relations were experimentally confirmed. This result supports the idea that considering the target object as an obstacle at positions other than the envisioned goal positions for the digits is underlying the stereotypical movement patterns in grasping. The goal positions of the digits thus influence the maximum grip aperture even if the distance between the goal positions on the target object does not change

Potential Systematic Interception Errors are Avoided When Tracking the Target with One's Eyes

Directing our gaze towards a moving target has two known advantages for judging its trajectory: the spatial resolution with which the target is seen is maximized, and signals related to the eyes' movements are combined with retinal cues to better judge the target's motion. We here explore whether tracking a target with one's eyes also prevents factors that are known to give rise to systematic errors in judging retinal speeds from resulting in systematic errors in interception. Subjects intercepted white or patterned disks that moved from left to right across a large screen at various constant velocities while either visually tracking the target or fixating the position at which they were required to intercept the target. We biased retinal motion perception by moving the pattern within the patterned targets. This manipulation led to large systematic errors in interception when subjects were fixating, but not when they were tracking the target. The reduction in the errors did not depend on how smoothly the eyes were tracking the target shortly before intercepting it. We propose that tracking targets with one's eyes when one wants to intercept them makes one less susceptible to biases in judging their motion

Using position dependent damping forces around reaching targets for transporting heavy objects:A Fitts' law approach

Passive assistive devices that compensate gravity can reduce human effort during transportation of heavy objects. The additional reduction of inertial forces, which are still present during deceleration when using gravity compensation, could further increase movement performance in terms of accuracy and duration. This study investigated whether position dependent damping forces (PDD) around targets could assist during planar reaching movements. The PDD damping coefficient value increased linearly from 0 Ns/m to 200 Ns/m over 18 cm (long PDD) or 9 cm (short PDD). Movement performance of reaching with both PDDs was compared against damping free baseline conditions and against constant damping (40 Ns/m). Using a Fitts' like experiment design 18 subjects performed a series of reaching movements with index of difficulty: 3.5, 4.5 and 5.5 bits, and distances 18, 23 and 28 cm for all conditions. Results show that PDD reduced (compared to baseline and constant damping) movement times by more than 30% and reduced the number of target reentries, i.e. increasing reaching accuracy, by a factor of 4. Results were inconclusive about whether the long or short PDD conditions achieved better task performance, although mean human acceleration forces were higher for the short PDD, hinting at marginally faster movements. Overall, PDD is a useful haptic force to get humans to decrease their reaching movement times while increasing their targeting accuracy

How Can You Best Measure Reaction Times?

Comparing many ways of measuring and analyzing reaction times reveals that the chosen method influences both the judged reaction time and, more importantly, conclusions about how the reaction time depends on the circumstances under study. The task was to lift one's finger in response to a tone. The response amplitude was either constrained or not. Constraining the amplitude made the response less vigorous. When the response was less vigorous it took longer to move far enough to release a switch or exceed the elasticity of the finger pulp. Although using a micro-switch would have made the reaction time appear to be longer for the constrained movement, reaction times determined in the most reliable ways were not systematically longer for the constrained movement. The most reliable method is to use extrapolation of the change in the average force that the finger exerts on the surface to estimate the reaction time