Biomedical subjects

Eli Brenner

Publications and source records attributed to Eli Brenner.

At least 37 records · Page 2Linked to original sources

PubMed2004

Body-centered visuomotor adaptation.

Previous research has shown that humans generalize distortions of visuomotor feedback in terms of egocentric rotations. We examined whether these rotations are linked to the orientation of the eyes or of the shoulder of the arm that was used. Subjects moved a hand-held cube between target locations in a sequence of adaptation and test phases. During adaptation phases, subjects received either veridical or distorted visual feedback about the location of the cube. The distortions were changes in azimuth either relative to the eyes or to the shoulder. During test phases subjects received no visual feedback. Test phases were performed either with the arm that was exposed to the distorted feedback or with the unexposed arm. We compared test movement endpoints after distorted feedback with ones after veridical feedback. For the exposed arm, the spatial layout of the changes in endpoints clearly reflected the small differences between a rotation around the shoulder and around the eyes. For the unexposed arm, the changes in endpoints were smaller for both types of distortions and were less consistent with the distortions. Thus although the adaptation closely matches the imposed distortion, it does not appear to be directly linked to the orientation of the eyes or of the exposed arm.

Adaptation, Physiological↗

PubMed2004

The quantitative use of velocity information in fast interception.

We ask whether a target's velocity is considered when planning a fast interceptive action. Human volunteers hit targets that could move at different velocities from across a tilted screen (the hand starting 40 cm away from the screen). We examined how the direction in which the hand initially moved depended on the target's velocity, using various analyses. For slow targets, the initial movement direction was appropriate for the target's velocity. This is evidence that velocity information was used quantitatively in directing the hand. A model analysis showed though that velocity information is probably not used to predict the future target position. For targets moving at a velocity above average, or above 12 cm/s, the initial movement direction did not depend on the target's velocity. Similar behaviour is also known from pursuit eye movements.

Adult↗

PubMed2004

Quickly tapping targets that are flashed during smooth pursuit reveals perceptual mislocalisations.

In various studies subjects have been shown to misperceive the positions of targets that are flashed during pursuit eye movements. They mislocalise them in the direction of pursuit. Nevertheless, Hansen (1979) found that subjects accurately hit targets that are flashed during pursuit with a quick hammer blow. We examined whether this is because there is a fundamental difference between the information that determines our perceptual judgements of a target's position and the information that is used to guide our hand to a similar target. Subjects were asked to quickly tap targets that were flashed during pursuit with their index finger. They systematically tapped ahead of the position of the flash, in accordance with the above-mentioned perceptual mislocalisations. Thus the lack of systematic errors in Hansen's study is not a general property of fast motor responses.

Fixation, Ocular↗

PubMed2004

On the relation between object shape and grasping kinematics.

Despite the many studies on the visual control of grasping, little is known about how and when small variations in shape affect grasping kinematics. In the present study we asked subjects to grasp elliptical cylinders that were placed 30 and 60 cm in front of them. The cylinders' aspect ratio was varied systematically between 0.4 and 1.6, and their orientation was varied in steps of 30 degrees. Subjects picked up all noncircular cylinders with a hand orientation that approximately coincided with one of the principal axes. The probability of selecting a given principal axis was the highest when its orientation was equal to the preferred orientation for picking up a circular cylinder at the same location. The maximum grip aperture was scaled to the length of the selected principal axis, but the maximum grip aperture was also larger when the length of the axis orthogonal to the grip axis was longer than that of the grip axis. The correlation between the grip aperture--or the hand orientation--at a given instant, and its final value, increased monotonically with the traversed distance. The final hand orientation could already be inferred from its value after 30% of the movement distance with a reliability that explains 50% of the variance. For the final grip aperture, this was only so after 80% of the movement distance. The results indicate that the perceived shape of the cylinder is used for selecting appropriate grasping locations before or early in the movement and that the grip aperture and orientation are gradually attuned to these locations during the movement.

Biomechanical Phenomena↗

PubMed2004

No evidence for sequential effects of the interaction of stereo and motion cues in judgements of perceived shape.

The interaction of the depth cues of binocular disparity and motion parallax could potentially be used by the visual system to recover an estimate of the viewing distance. The present study investigated whether an interaction of stereo and motion has effects that persist over time to influence the perception of shape from stereo when the motion information is removed. Static stereoscopic ellipsoids were presented following the presentation of rotating stereoscopic ellipsoids, which were located either at the same or a different viewing distance. It was predicted that shape judgements for static stimuli would be better after presentation of a rotating stimulus at the same viewing distance, than after presentation of one at a different viewing distance. No such difference was found. It was concluded that an interaction between stereo and motion depth cues does not influence the perception of subsequently presented static objects.

Cues↗

PubMed2004

Components of motion perception revealed: two different after-effects from a single moving object.

If motion that one has been looking at for some time suddenly stops, or if one shifts one's gaze to a static object, one will see motion in the opposite direction: the motion after-effect. If two transparent surfaces move with different speeds in different directions, then the direction of the motion after-effect will depend on the test pattern. For such transparent surfaces both the local motion and the global percept have two components. When looking at a normal moving object, there is only one perceived global motion. However, we know that locally there can be considerable ambiguity (the aperture problem). Does one adapt to all the local components, including those that one does not perceive, or only to the perceived global motion? We designed a stimulus that is perceived to be a fast rotating object, but also has a slow local radial component of motion. By selecting an appropriate test pattern we could either get a radial or a rotating motion after-effect. Thus we show that adaptation to motion must (also) occur at a stage at which local motions have not yet been integrated to give a unified percept.

Adaptation, Physiological↗

PubMed2004

Combining cues while avoiding perceptual conflicts.

A common assumption in cue combination models is that small discrepancies between cues are due to the limited resolution of the individual cues. Whenever this assumption holds, information from the separate cues can best be combined to give a single, more accurate estimate of the property of interest. We examined whether information about the discrepancy itself is lost when this is done. In our experiments, subjects were required to combine cues to match certain properties while avoiding perceptual conflicts. In part 1, they combined expansion and change in disparity to estimate motion in depth; and in part 2, they combined perspective and binocular disparities to estimate slant. We compared the pattern in the way that subjects set the two cues with the patterns predicted by models of cue combination with and without a loss of information about the discrepancy. From this comparison we conclude that little information about the discrepancies between cues is lost when the cues are combined.

Cues↗

PubMed2003

Illusions as a tool to study the coding of pointing movements.

Pictorial illusions bias our judgments about certain visual attributes. Such illusions are therefore only expected to influence a task if these attributes are used to perform the task. When pointing to a position, different visual attributes could be used to guide the hand: direction and distance (or length) of the required displacement (vector coding) or the final position (position coding). In this study we used the Brentano illusion (an illusion of length) to determine which attributes are used in pointing. Several conditions were tested in which the visibility of the hand and the stimulus were varied. The illusion influenced movements between two points along the shaft of the figure, but not movements perpendicular to the shaft. When the hand and/or target were invisible during the movement, the influence of the illusion increased. Pointing movements under different visual conditions were based on different relative contributions of position and vector coding. The contribution of vector coding was always rather modest.

Analysis of Variance↗

PubMed2003

Perceptual requirements for fast manual responses.

The on-line visual control of human movements can be exceptionally fast. Whether it is fast depends on the kind of visual information that is involved. In the present study we examine whether fast on-line control is specific to the magnocellular visual pathway. Fast manual responses become evident when an ongoing movement has to be adjusted, for instance because the target is displaced. We examined whether the response to such perturbations is faster for stimuli that only activate the magnocellular pathway than for equally conspicuous stimuli that only activate the parvocellular pathway. The response was indeed about 35 ms faster for stimuli that activate the magnocellular pathway. However, we argue that the slower response to stimuli that only stimulate the parvocellular pathway is due to the properties of the neurones involved and the less direct connection to the motor areas, rather than to fast reactions being driven exclusively by magnocellular input.

Hand↗

PubMed2003

Hitting moving targets: a dissociation between the use of the target's speed and direction of motion.

Previous work has indicated that people do not use their judgment of a target's speed to determine where to hit it. Instead, they use their judgment of the target's changing position and an expected speed (based on the speed of previous targets). In the present study we investigate whether people also ignore the target's apparent direction of motion, and use the target's changing position and an expected direction of motion instead. Subjects hit targets that moved in slightly different directions across a screen. Sometimes the targets disappeared after 150 ms, long before the subjects could reach the screen. This prevented subjects from using the target's changing position to adjust their movements, making it possible to evaluate whether subjects were relying on the perceived or an expected (average) direction to guide their movements. The background moved perpendicular to the average direction of motion in some trials. This influences the target's perceived direction of motion while leaving its perceived position unaffected. When the background was stationary, subjects hit disappearing targets along their trajectory, just as they hit ones that remained visible. Moving the background affected the direction in which subjects started to move their hand, in accordance with the illusory change in direction of target motion. If the target disappeared, this resulted in a hit that was systematically off the target's trajectory. If the target remained visible, subjects corrected their initial error. Presumably they did so on the basis of information about the target's changing position, because if the target disappeared they did not correct the error. We conclude that people do use the target's perceived direction of motion to determine where to hit it. Thus the perceived direction of motion is treated differently than the perceived speed. This suggests that the motion of an object is not broken down into speed components in different directions, but that speed and direction are perceived and used separately.

Humans↗

PubMed2003

Similar effects of a motion-in-depth illusion on manual tracking and perceptual judgements.

We previously demonstrated that changing the apparent extent of a target's apparent motion-in-depth, by manipulating pictorial depth cues in the surrounding, affected perceptual judgements and manual pursuit to the same extent. Here, we investigated whether a different manipulation of the extent of motion (expanding and contracting the object itself) also has the same effect on both tasks. Objects were presented that changed in size as they moved on an elliptical path. The size was related to the object's position in the sagittal plane, suggesting additional motion in depth; therefore the illusion was expected to affect sagittal measures for both perception and action. We measured manual tracking and perceptual judgements of the lateral and sagittal extents of the object's elliptical trajectories. A significant correlation was found across subjects between the effect of the illusion on the perceptual and the motor task. As expected, the illusion only had a significant influence on the sagittal dimension. The size of this illusory effect was equal for perception and action.

Cues↗

PubMed2003

The influence of obstacles on the speed of grasping.

The movement time of a reach-to-grasp movement increases when obstacles are placed close to the target object. We investigated whether this increase can best be explained by limits on the grip aperture or by limits on the paths of the individual digits. In our experiment subjects were instructed to pick up an object with their index finger and thumb. There was an obstacle at either side of the object. The increase in movement time when either obstacle was placed closer to the object was best described by a model in which the movement amplitude and the distance between each obstacle and the target object are independent factors. We conclude that the way that obstacles influence the movement time in reach-to-grasp movements is determined by the extent to which they limit the digits' paths.

Efficiency↗

PubMed2003

Chromatic induction and the layout of colours within a complex scene.

A target's apparent colour is influenced by the colours in its surrounding. If the surrounding consists of a single coloured surface, the influence is a shift 'away' from the surface's colour. If the surface is more than 1 degrees from the target area the shift is very small. If there are many surfaces, then not only the average luminance and chromaticity of the surfaces matters, but also the chromatic variability. It is not yet clear whether it makes any difference where the chromatic variability is within the scene, so we constructed stimuli in which the chromatic variability was restricted to certain regions. We found that it made very little difference where the chromatic variability was located. The extent to which the average colour of nearby surfaces influences the apparent colour of the target seems to depend on the average chromatic variability of the whole scene.

Color Perception↗

PubMed2003

Impact forces cannot explain the one-target advantage in rapid aimed hand movements.

A pointing movement is executed faster when a subject is allowed to stop at the first target than when the subject has to proceed to a second target ("one-target advantage"). Our hypothesis was that this is because the impact at the target helps to stop the finger when the finger does not have to proceed to a second target. This hypothesis would predict that the horizontal force at contact with the first target should be larger when there is only one-target. Modelling smooth movements with larger forces at contact using a minimum-jerk model, shows that the peak velocity is slightly higher and it occurs later during the movement when there is only one target. Although the one-target advantage was present in our experiment, the horizontal force at contact in the one-target condition was not larger than in the two-target condition. The time of the maximum velocity did not differ, but the maximum velocity was higher in the one-target condition. Thus our hypothesis is rejected, favouring a non-mechanical explanation of the one-target advantage.

Biomechanical Phenomena↗

PubMed2003

Comparing the sensitivity of manual pursuit and perceptual judgments to pictorial depth effects.

We examined whether a pictorial depth illusion influences the manual pursuit of a moving dot to the same extent that it influences the dot's apparent displacement. Fourteen subjects performed two tasks. In one case, they used their unseen hand to track a dot that moved on an elliptical path. In the other, they first watched the dot move on the same path, and then set an ellipse to match the shape of the dot's path. The illusion influenced the two tasks to the same extent, suggesting that the visual information processing is the same for the two tasks.

Attention↗

PubMed2003

When is behavioral data evidence for a control theory? Tau-coupling revisited.

Before an aspect of a movement that is predicted by a control theory can be considered as evidence for that theory, it should be clear that this aspect is not the result of some other property of the movement. We investigate whether this condition is met in studies that claim to provide evidence for the tau-coupling theory. This theory proposes that moving targets are intercepted at a specified goal zone by maintaining a constant ratio between the tau (time to closure) of the gap between the hand and the goal zone and the tau of the gap between the hand and the moving target. In line with the theory, previous research has found a linear relationship between these two decreasing taus during the last part of such a movement. To investigate whether this linear relationship was a side-effect of smooth successful movements, we modeled smooth ballistic hand movements that were independent of the target's movement but led to successful interception. We found that the resulting taus of decreasing gaps were also related linearly. We conclude that this relationship cannot be considered as evidence for the tau-coupling theory.

Behavior↗

PubMed2003

Modeling the time-dependent effect of the Ebbinghaus illusion on grasping.

Various authors have reported a small but consistent effect of the Ebbinghaus illusion on the maximum opening of the hand during prehension. This effect has been interpreted in various ways. In the present study, we focus on the time-course of the effect of contextual elements on grasping. The analysis presented here is based on a model for the control of the digits that uses two movement parameters (the approach parameter and the intended contact positions). These two parameters are based on different spatial attributes (flanker-target distance and target-edge position). As we assume that the perception of both attributes is veridical, there is no need for on-line corrections in the model. We show that this model predicts all time-dependent effects of the Ebbinghaus display on grasping. Human behavior can show a reduction in context effects over time without assuming an underlying shift from illusory towards veridical size information.

Hand Strength↗

PubMed2003

Fast corrections of movements with a computer mouse.

When we reach out for an object with our hand, we transform visual information about the object's position into muscle contractions that will bring our digits to that position. If we reach out with a tool the transformation is different, because the muscle contractions must bring the critical part of the tool to the object, rather than the digits. The difference between the motion of the hand and that of the tool can be quite large, as when moving a computer mouse across a table to bring a cursor to a position on a screen. We examined the responses to unpredictable visual perturbations during such movements. People responded about as quickly to changes in the position of the target when pointing with the mouse as when doing so with their hand. They also responded about as quickly when the cursor was displaced as when the target was displaced. We show that this is not because the visually perceived separation between target and cursor is transformed into a desired displacement of the hand. Our conclusion is that our actions are controlled by the judged positions of the end-effector and the target, even when the former is quite detached from the muscles and joints that are involved in the action.

Computers↗