Biomedical subjects

Volker H Franz

Publications and source records attributed to Volker H Franz.

6 recordsLinked to original sources

PubMed2006

The influence of the Brentano illusion on eye and hand movements.

When making an eye movement and a hand movement toward a visual target, the movements could be guided by visual judgments of direction and distance (or length) of the required displacement (vector coding), estimates of the final position (position coding), or both. Using the same information for the eyes and the hand is efficient; however, if this information contains an error, this causes both the eye and the hand to be incorrect. In this study, we tried to find out whether saccades and pointing movements use the same source of information when eye and hand movements are performed either concurrently or separately. Four experiments have been performed using the Brentano illusion, which primarily influences judgments of length but not those of position. This illusion only influences movements if the illusory length is relevant for the task, demonstrating that vector coding is involved. Subjects made saccades, pointing movements, or both between vertices of the Brentano illusion. The illusion influenced saccades and pointing movements when these movements were performed concurrently and separately, showing that the eye and the hand use vector coding. However, depending on the task, eye and hand movements were influenced to a different extent. This favors the interpretation that the eyes and the hand use a common motor command but each with a different relative contribution of vector coding.

Fixation, Ocular↗

PubMed2005

Is prior knowledge of object geometry used in visually guided reaching?

We investigated whether humans use prior knowledge of the geometry of faces in visually guided reaching. When viewing the inside of a mask of a face, the mask is often perceived as being a normal (convex) face, instead of the veridical, hollow (concave) shape. In this "hollow-face illusion," prior knowledge of the shape of faces dominates perception, even when in conflict with information from binocular disparity. Computer images of normal and hollow faces were presented, such that depth information from binocular disparity was consistent or in conflict with prior knowledge of the geometry. Participants reached to touch either the nose or cheek of the faces or gave verbal estimates of the corresponding distances. We found that reaching to touch was dominated by prior knowledge of face geometry. However, hollow faces were estimated to be flatter than normal faces. This suggests that the visual system combines binocular disparity and prior assumptions, rather than completely discounting one or the other. When comparing the magnitude of the hollow-face illusion in reaching and verbal tasks, we found that the flattening effect of the illusion was similar for verbal and reaching tasks.

Cognition↗

PubMed2005

Effects of rearranged vision on event-related lateralizations of the EEG during pointing.

We used event-related lateralizations of the EEG (ERLs) and reversed vision to study visuomotor processing with conflicting proprioceptive and visual information during pointing. Reversed vision decreased arm-related lateralization, probably reflecting the simultaneous activity of left and right arm specific neurons: neurons in the hemisphere contralateral to the observed action were probably activated by visual feedback, neurons in the hemisphere contralateral to the response side by the somatomotor feedback. Lateralization related to the target in parietal cortex increased, indicating that visual to motor transformation in parietal cortex required additional time and resources with reversed vision. A short period of adaptation to an additional lateral displacement of the visual field increased arm-contralateral activity in parietal cortex during the movement. This is in agreement with the, which showed that adaptation to a lateral displacement of the visual field is reflected in increased parietal involvement during pointing.

Adult↗

PubMed2004

Representational momentum in perception and grasping: translating versus transforming objects.

Representational momentum is the tendency to misremember the stopping point of a moving object as further forward in the direction of movement. Results of several studies suggest that this effect is typical for changes in position (e.g., translation) and not for changes in object shape (transformation). Additionally, the effect seems to be stronger in motor tasks than in perceptual tasks. Here, participants judged the final distance between two spheres after this distance had been increasing or decreasing. The spheres were two separately translating objects or were connected to form a single transforming object (a dumbbell). Participants also performed a motor task in which they grasped virtual versions of the final objects. We found representational momentum for the visual judgment task for both stimulus types. As predicted, it was stronger for the spheres than for the dumbbells. In contrast, for grasping, only the dumbbells produced representational momentum (larger maximum grip aperture when the dumbbells had been growing compared to when they had been shrinking). Because type of stimulus change had these different effects on representational momentum for perception and action, we conclude that different sources of information are used in the two tasks or that they are governed by different mechanisms.

Field Dependence-Independence↗

PubMed2003

Planning versus online control: dynamic illusion effects in grasping?

The planning/control model of action assumes that grasping is sensitive to the context of an object only in early stages of the movement (planning), but not in later stages (control). In consequence, the effects of context-induced illusions (such as the Ebbinghaus/Titchener illusion) should decrease during a grasping movement. Here, we tested this claim by reanalysing a large data set (N = 26) on grasping in the Ebbinghaus illusion. Contrary to the predictions of the planning/control model, we found that the effects of the illusion did not decrease over time. Instead, the illusion effects stayed remarkably constant.

Adult↗

PubMed2002

Effects of pointing direction and direction predictability on event-related lateralizations of the EEG.

In two experiments, we investigated hemispheric electroencephalography (EEG) differences in 9(12) healthy volunteers during pointing to lateral and central targets. The questions addressed were whether horizontal pointing direction and the predictability of pointing direction modulated hemispheric differences (event-related lateralizations of the EEG = ERLs). To vary pointing direction predictability, targets were displayed either randomly at one of nine different positions on a screen (random) or at the same horizontal position in five subsequent trials (sequenced) while vertical positions varied randomly. Event-related lateralizations (ERLs) varied with pointing direction. This was true across changes in target eccentricity and pointing distance. Foci of the ERLs were in premotor and posterior parietal cortex, which might reflect the critical involvement of these areas in the control of visually guided reaching. Direction predictability reduced the parietal and premotor ERL before pointing onset, probably reflecting a lesser effort in visuomotor transformation. Predictability also added an overlying N2pc component to the early ERL after target onset and increased direction effects during movement.

Adolescent↗