Biomedical subjects

Gerben Rotman

Publications and source records attributed to Gerben Rotman.

6 recordsLinked to original sources

PubMed2006

Eye movements when observing predictable and unpredictable actions.

We previously showed that, when observers watch an actor performing a predictable block-stacking task, the coordination between the observer's gaze and the actor's hand is similar to the coordination between the actor's gaze and hand. Both the observer and the actor direct gaze to forthcoming grasp and block landing sites and shift their gaze to the next grasp or landing site at around the time the hand contacts the block or the block contacts the landing site. Here we compare observers' gaze behavior in a block manipulation task when the observers did and when they did not know, in advance, which of two blocks the actor would pick up first. In both cases, observers managed to fixate the target ahead of the actor's hand and showed proactive gaze behavior. However, these target fixations occurred later, relative to the actor's movement, when observers did not know the target block in advance. In perceptual tests, in which observers watched animations of the actor reaching partway to the target and had to guess which block was the target, we found that the time at which observers were able to correctly do so was very similar to the time at which they would make saccades to the target block. Overall, our results indicate that observers use gaze in a fashion that is appropriate for hand movement planning and control. This in turn suggests that they implement representations of the manual actions required in the task and representations that direct task-specific eye movements.

Adult↗

PubMed2006

The role of uncertainty in the systematic spatial mislocalization of moving objects.

It only makes sense to talk about the position of a moving object if one specifies the time at which its position is of interest. The authors here show that when a flash or tone specifies the moment of interest, subjects estimate the moving object to be closer to where it passes the fixation point and further in its direction of motion than it really is. The authors propose that these biases arise from a combination of a large temporal uncertainty, a temporal asymmetry related to sampling the moving object's position, and a bias toward believing that one is looking at what one sees.

Adult↗

PubMed2005

Flashes are localised as if they were moving with the eyes.

Targets that are flashed during smooth pursuit are mislocalised in the direction of the pursuit. It has been suggested that a similar mislocalisation of moving targets could help to overcome processing delays when hitting moving objects. But are moving targets really mislocalised in the way that flashed ones are? To find out we asked people to indicate where targets that were visible for different periods of time had appeared. The targets appeared while the subjects' eyes were moving, and were either moving with the eyes or static. For flashed targets we found the usual systematic mislocalisation. For targets that moved with the eyes the mislocalisation was at least as large, irrespective of the presentation time. For static targets the mislocalisation decreased with increasing presentation time, so that by the time the presentations reached about 200 ms the targets were not mislocalised at all. A simple model that combines smooth retinal motion with information about the velocity of smooth pursuit could account for the measured tapping errors. These findings support the notion that the systematic mislocalisation of flashed targets is related to the way in which people intercept moving objects.

Humans↗

PubMed2004

Mislocalization of targets flashed during smooth pursuit depends on the change in gaze direction after the flash.

Subjects mislocalize the position of a target that is flashed while they are making pursuit eye movements. This mislocalization is in the direction of pursuit. However, it is not clear whether it is the movement of the eyes or the movement of the pursuit target that matters. Neither is it clear whether it is the movement after the flash or the movement before the flash that matters. To resolve these issues, we asked subjects to pursue a disk that regularly changed its movement direction. Each change was followed by a change in the direction of gaze movement. Subjects were asked to tap targets that were flashed close to the moment at which the pursuit disk changed direction. We measured the movements of the eyes, head, and index finger. Subjects did not make saccades to the position they tapped but kept pursuing the disk. We compared the direction of the mislocalization with the changes in gaze and in target position during different intervals relative to the flash. We found that the mislocalization is related to the change in gaze after the flash.

Cues↗

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↗

PubMed2002

Spatial but not temporal cueing influences the mislocalisation of a target flashed during smooth pursuit.

Human subjects misjudge the position of a target that is flashed during a pursuit eye movement. Their judgments are biased in the direction in which the eyes are moving. We investigated whether this bias can be reduced by making the appearance of the flash more predictable. In the normal condition, subjects pursued a moving target that flashed somewhere along its trajectory. After the presentation, they indicated where they had seen the flash. The mislocalisations in this condition were compared to mislocalisations in conditions in which the subjects were given information about when or where the flash would come. This information consisted of giving two warning flashes spaced at equal intervals before the target flash, of giving two warning beeps spaced at equal intervals before the target flash, or of showing the same stimulus twice. Showing the same stimulus twice significantly reduced the mislocalisation. The other conditions did not. We interpret this as indicating that it is not predictability as such that influences the performance, but the fact that the target appears at a spatially cued position. This was supported by a second experiment, in which we examined whether subjects make smaller misjudgments when they have to determine the distance between a target flashed during pursuit and a reference seen previously, than when they have to determine the distance between the flashed target and a reference seen afterwards. This was indeed the case, presumably because the reference provided a spatial cue for the flash when it was presented first. We conclude that a spatial cue reduces the mislocalisation of targets that are flashed during pursuit eye movements. The cue does not have to be exactly at the same position as the flash.

Cues↗