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

The special role of distant structures in perceived object velocity

AbstractHow do we judge an object's velocity when we ourselves are moving? Subjects compared the velocity of a moving object before and during simulated ego-motion. The simulation consisted of moving the visible environment relative to the subject's eye in precisely the way that a static environment would move relative to the eye if the subject had moved. The ensuing motion of the background on the screen influenced the perceived target velocity. We found that the motion of the “most distant structure” largely determined the influence of the moving background. Relying on retinal motion relative to that of distant structures is usually a reliable method for accounting for rotations of the eye. It provides an estimate of the object's movement, relative to the observer. This strategy for judging object motion has the advantage that it does not require metric information on depth or detailed knowledge of one's own motion. Copyright © 1996 Elsevier Science Lt

Humans combine the optic flow with static depth cues for robust perception of heading

The retinal flow during normal locomotion contains components due to rotation and translation of the observer. The translatory part of the flow-pattern is informative of heading, because it radiates outward from the direction of heading. However, it is not directly accessible from the retinal flow. Nevertheless, humans can perceive their direction of heading from the compound retinal flow without need for extra-retinal signals that indicate the rotation. Two classes of models have been proposed to explain the visual decomposition of the retinal flow into its constituent parts. One type relies on local operations to remove the rotational part of the flow field. The other type explicitly determines the direction and magnitude of the rotation from the global retinal flow, for subsequent removal. According to the former model, nearby points are most reliable for estimating one's heading. In the latter type of model the quality of the heading estimate depends on the accuracy with which the ego-rotation is determined and is therefore most reliable when based on the most distant points. We report that subjects underestimate the eccentricity of heading, relative to the fixated point in the ground plane, when the visible range of the ground plane is reduced. Moreover we find that in perception of heading, humans can tolerate more noise than the optimal observer (in the least squares sense) would do if only using optic flow. The latter finding argues against both schemes because ultimately both classes of model are limited in their noise tolerance to that of the optimal observer, which uses all information available in the optic flow. Apparently humans use more information than is present in the optic flow. Both aspects of human performance are consistent with the use of static depth information in addition to the optic flow to select the most distant points. Processing of the flow of these selected points provides the most reliable estimate of the ego-rotation. Subsequent estimates of the heading direction, obtained from the translatory component of the flow, are robust with respect to noise. In such a scheme heading estimates are subject to systematic errors, similar to those reported, if the most distant points are not much further away than the fixation point, because the ego-rotation is underestimated

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

Modifying one’s hand’s trajectory when a moving target’s orientation changes

The path that the hand takes to intercept an elongated moving target depends on the target’s orientation. How quickly do people respond to changes in the moving target’s orientation? In the present study, participants were asked to intercept moving targets that sometimes abruptly changed orientation shortly after they started moving. It took the participants slightly more than 150 ms to adjust their hands’ paths to a change in target orientation. This is about 50 ms longer than it took them to respond to a 5-mm jump in the moving target’s position. It is only slightly shorter than it took them to initiate the movement. We propose that responses to changes in visually perceived orientation are not exceptionally fast, because there is no relationship between target orientation and direction of hand movement that is sufficiently general in everyday life for one to risk making an inappropriate response in order to respond faster

Serial search for fingers of the same hand but not for fingers of different hands

In most haptic search tasks, tactile stimuli are presented to the fingers of both hands. In such tasks, the search pattern for some object features, such as the shape of raised line symbols, has been found to be serial. The question is whether this search is serial over all fingers irrespective of the hand, or whether it is serial over the fingers of each hand and parallel over the two hands. To investigate this issue, we determined the speed of static haptic search when two items are presented to two fingers of the same hand and when two items are presented to two fingers of different hands. We compared the results with predictions for parallel and serial search based on the results of a previous study using the same items and a similar task. The results indicate that two fingers of the same hand process information in a serial manner, while two fingers of two different hands process information in parallel. Thus, considering the individual fingers as independent units in haptic search may not be justified, because the hand that they belong to matters. © 2009 Springer-Verlag

Avoiding moving obstacles

To successfully move our hand to a target, we must consider how to get there without hitting surrounding objects. In a dynamic environment this involves being able to respond quickly when our relationship with surrounding objects changes. People adjust their hand movements with a latency of about 120 ms when the visually perceived position of their hand or of the target suddenly changes. It is not known whether people can react as quickly when the position of an obstacle changes. Here we show that quick responses of the hand to changes in obstacle position are possible, but that these responses are direct reactions to the motion in the surrounding. True adjustments to the changed position of the obstacle appeared at much longer latencies (about 200 ms). This is even so when the possible change is predictable. Apparently, our brain uses certain information exceptionally quickly for guiding our movements, at the expense of not always responding adequately. For reaching a target that changes position, one must at some time move in the same direction as the target did. For avoiding obstacles that change position, moving in the same direction as the obstacle is not always an adequate response, not only because it may be easier to avoid the obstacle by moving the other way, but also because one wants to hit the target after passing the obstacle. Perhaps subjects nevertheless quickly respond in the direction of motion because this helps avoid collisions when pressed for time. © 2008 Springer-Verlag