Bi-articular muscles and the accuracy of motor control

In a model study, the behaviour of two sets of muscles in controlling multi-joint arm movements is compared. Both the sensory and the motor accuracy of the set containing bi-articular muscles were in general better than those of the set containing only mono-articular muscles. Accuracy considerations can explain differences in strategies for the control of redundant muscle sets between situations which do not differ biomechanically from each other. Furthermore, the role of bi-articular muscles for the robustness of motor programming is discussed

Early components of the human vestibulo-ocular response to head rotation: latency and gain

To characterize vestibulo-ocular reflex (VOR) properties in the time window in which contributions by other systems are minimal, eye movements during the first 50-100 ms after the start of transient angular head accelerations ( approximately 1000 degrees /s(2)) imposed by a torque helmet were analyzed in normal human subjects. Orientations of the head and both eyes were recorded with magnetic search coils (resolution, approximately 1 min arc; 1000 samples/s). Typically, the first response to a head perturbation was an anti-compensatory eye movement with zero latency, peak-velocity of several degrees per second, and peak excursion of several tenths of a degree. This was interpreted as a passive mechanical response to linear acceleration of the orbital tissues caused by eccentric rotation of the eye. The response was modeled as a damped oscillation (approximately 13 Hz) of the orbital contents, approaching a constant eye deviation for a sustained linear acceleration. The subsequent compensatory eye movements showed (like the head movements) a linear increase in velocity, which allowed estimates of latency and gain with linear regressions. After appropriate accounting for the preceding passive eye movements, average VOR latency (for pooled eyes, directions, and subjects) was calculated as 8.6 ms. Paired comparisons between the two eyes revealed that the latency for the eye contralateral to the direction of head rotation was, on average, 1.3 ms shorter than for the ipsilateral eye. This highly significant average inter-ocular difference was attributed to the additional internuclear abducens neuron in the pathway to the ipsilateral eye. Average acceleration gain (ratio between slopes of eye and head velocities) over the first 40-50 ms was approximately 1.1. Instantaneous velocity gain, calculated as Veye(t)/Vhead(t-latency), showed a gradual build-up converging toward unity (often after a slight overshoot). Instantaneous acceleration gain also converged toward unity but showed a much steeper build-up and larger oscillations. This behavior of acceleration and velocity gain could be accounted for by modeling the eye movements as the sum of the passive response to the linear acceleration and the active rotational VOR. Due to the latency and the anticompensatory component, gaze stabilization was never complete. The influence of visual targets was limited. The initial VOR was identical with a distant target (continuously visible or interrupted) and in complete darkness. A near visual target caused VOR gain to rise to a higher level, but the time after which the difference between far and near targets emerged varied between individuals

Prediction of a moving target's position in fast goal-directed action

Subjects made fast goal-directed arm movements towards moving targets. In some cases, the perceived direction of target motion was manipulated by moving the background. By comparing the trajectories towards moving targets with those towards static targets, we determined the position towards which subjects were aiming at movement onset. We showed that this position was an extrapolation in the target's perceived direction from its position at that moment using its perceived direction of motion. If subjects were to continue to extrapolate in the perceived direction of target motion from the position at which they perceive the target at each instant, the error would decrease during the movements. By analysing the differences between subjects' arm movements towards targets moving in different (apparent) directions with a linear second-order model, we show that the reduction in the error that this predicts is not enough to explain how subjects compensate for their initial misjudgements

Independent control of acceleration and direction of the hand when hitting moving targets

Human subjects were asked to hit moving targets as quickly as they could. Nevertheless the speed with which the subjects moved toward identical stimuli differed between trials. We examined whether the subjects compensated for a lower initial acceleration by aiming further ahead of the target. We found that the initial acceleration of the hand and its initial direction were hardly correlated. Thus subjects did not aim further ahead when they hit more slowly. This supports our earlier suggestion that the acceleration of the hand and the direction in which it moves are controlled separately

Curvature in hand movements as a result of visual misjudgements of direction

The path that our hand takes when moving from one position to another is often slightly curved. Part of this curvature is caused by perceptual errors. We examine here whether this is so for the influence that a surface's orientation has on the approaching hand's path. When moving our hand towards a point on a surface we tend to follow a path that makes the final approach more orthogonal to the surface at that point. Doing so makes us less sensitive to imperfection in controlling our movements. Here we show that this tendency is also present when moving towards a point along an edge of a drawing of an oriented bar. The influence of the bar's orientation is no smaller when people are explicitly asked to move as straight as possible, than when they are instructed to move as fast as possible. The bar's orientation also influences perceptual judgements of a straight path, but this influence is only as large as it is on the curvature of the hand's path for judgements of the direction from the hand's initial position to the target. We conclude that the influence of the bar's orientation on the curvature of the hand's path is caused by a misperception of the initial direction in which the hand has to move to reach the target

Slow corrections to arm movements for target perturbations in depth

Reliable depth perception is essential for successfully guiding ones hand toward objects. Binocular disparities are often considered to be the primary source of metric information about depth. Moreover, as the hand approaches the object, errors in initial depth judgments give rise to relative disparities that could be used to correct the movement. In the present study we examine whether people can quickly adjust their movements on the basis of information from binocular disparities. It only takes 110-150 ms to correct a hand's movement if the object toward which the hand is moving is suddenly displaced. It takes about the same time to adjust the way one moves a computer mouse, and thereby correct the cursor's movement, if the target is displaced on the screen. If arm movements are to be adjusted on the basis of relative disparities it is vital that the response to such information is also this fast, because the information itself only becomes available when the hand comes near the object. We examined cursor movements, rather than hand movements, because this makes it easier to control the visual information that the subjects can use. Subjects sat 80 cm from a computer screen and quickly moved a cursor to a target by moving the mouse. The cursor moved in a horizontal plane at eye level, in the same direction as the mouse. Shutter spectacles were used to present the cursor and target at the desired depths. Once the cursor reached the target the latter disappeared and a new one appeared elsewhere. On some trials the target jumped 15 cm in depth while the cursor was moving toward it. It took subjects more than 200 ms to respond to this displacement. When the experiment was repeated with the targets 8cm below eye height, so that a movement in depth gave rise to a small vertical movement on the screen, the response was much faster. Thus binocular disparities are unlikely to be important for guiding the hand toward objects unless we move rather slowly

Luminance-color correlation is not used to estimate the color of the illumination

Humans can identify the colors of objects fairly consistently, despite considerable variations in the spectral composition of the illumination. It has been suggested that the correlation between luminance and color within a scene helps to disentangle the influences of illumination and reflectance, because the surfaces that reflect the light of the illuminant well will normally be bright. Because the reliability of the luminance-color correlation as an indicator of the chromaticity of the illuminant depends on the number of surfaces that are considered, we expected the correlation to be determined across large parts of the scene. To examine whether this is so, we compared different scenes with matched luminance and chromaticity, but in which the correlation between luminance and chromaticity was manipulated locally. Our results confirm that there is a bias in perceived color away from the chromaticity of bright surfaces. However, the results show that only the correlation within about 1° of the target is relevant. Thus, it is unlikely that the visual system uses the correlation between luminance and color to explicity determine the chromaticity of the illuminant. Instead, this correlation is presumably implicitly considered in the way that the color contrast at borders is determined

Distorted shape perception impairs grasping of real objects

It is known that visual shape perception is distorted in depth, but it is unclear how this influences grasping. We measured how subjects grasp real cylinders that are placed at eye height under normal lighting conditions. The cylinders were 10cm tall with an elliptical base. One principle axis was always 5cm, whereas the other was varied between 2cm and 8cm in steps of 1cm. The cylinders were placed directly in front of the subject, at a distance of either 15cm or 45cm. Their orientation was varied from 0 to 150 in steps of 30 . Grasping performance was compared with that in a previous study in which the cylinders were placed well below eye height so that subjects had no difficulty judging their shape. We found that subjects are less accurate at matching the orientation of their hand to that of the cylinders when the targets are at eye height. This often led to unstable grasps. Moreover the maximum grip aperture was about 2cm larger when the cylinders were placed at eye height. Nonetheless, the correlation between the hand orientation halfway through the movement and the final hand orientation was already about 70%. The correlation between the grip aperture during the movement and the final grip aperture was only 30% when half the distance was traversed, but it was still only 60% when reaching the object. These results indicate that the grasping movement uses incorrectly specified pick-up locations on the cylinder's surface. Uncertainty makes the subjects increase their grip aperture, but we did not find evidence for increased online control