Biomedical subjects

T Quenzer

Publications and source records attributed to T Quenzer.

4 recordsLinked to original sources

PubMed2003

Feature-specific electrophysiological correlates of texture segregation.

Discrimination between a figure and its surround is an important first step of pattern recognition. This discrimination usually relies, as a first step, on the detection of borders between a figure and its surround, for example based on spatial gradients in luminance, colour, or texture. There is evidence that neurones in the visual cortex are specifically activated by segregation between textures, but the relation between segregation based on different types of features such as colour, luminance, and motion is unclear. Evoked EEG potentials specific to texture segregation were investigated in 17 observers in two separate experiments and by means of functional magnetic resonance imaging in a separate study (Fahle et al., in preparation). Differences in either luminance, colour, line orientation, motion, or stereoscopic depth defined a checkerboard pattern. Patterns defined by each of these features elicited segregation-specific potentials. In contrast to earlier reports (Vision Research 37 (1997) 1409), however, we find pronounced differences between the segregation-specific potentials evoked through different features, especially regarding their peak latencies. The topographical distribution of the activity evoked reveals different polarities and partly specific locations for different stimulus features, indicating the existence of different processors for texture segregation based on different features.

Adult↗

PubMed2001

Perceptual deformation induced by visual motion.

The perceived position of a moving object can be misleading because the object has advanced while its previous retinal image has been transmitted through the visual stream, leading to a mismatch between actual location and its neural representation. It has been suggested that the human visual system compensates for neural processing delays to retrieve instantaneous position. However, such a mechanism would require a precise measure of the actual delay in order to provide a reliable position estimate. A novel illusory deformation of moving contours demonstrates that humans misjudge the spatial relationship between parts of coherently moving targets, and therefore do not perfectly account for neural delays. The size of this deformation increases with growing speed. In some subjects this illusion can be reversed by varying the luminance of individual dots; a manipulation that affects the neural delays. Our experiments agree with other evidence that the capacity of the visual system to compensate for processing delays is limited.

Decision Making↗

PubMed2000

Summation of texture segregation across orientation and spatial frequency: electrophysiological and psychophysical findings.

Objects are usually segregated from ground by several visual dimensions. We studied texture segregation in checkerboards defined by gradients in spatial frequency, orientation or both frequency and orientation, using Gabor-filtered noise patterns. Saliency was measured electrophysiologically using the visual evoked potential (VEP) associated with texture segregation ('tsVEP') (an associated component in the visual evoked potential), and psychophysically by a 2AFC task. Spatial frequency and orientation stimuli evoked percepts of texture segregation and tsVEPs in all 11 subjects. The tsVEPs to combined stimuli were larger than those to each dimension alone, but smaller (74%) than the algebraic sum of tsVEPs to both individual dimensions. Psychophysical detection rates differed significantly between all conditions (P < 0.001), with highest rates for the combined stimuli. The findings suggest that segregation based on a combination of 'orientation' and 'spatial frequency' is more salient than that based on either of these alone. The significant deviation from full additivity in the tsVEPs suggests that simultaneous contrasts in spatial frequency and orientation have a common processing stage.

Evoked Potentials, Visual↗

PubMed1999

How to tell circles from ellipses: perceiving the regularity of simple shapes.

Human observers achieve a surprising precision in many visual judgements, such as estimating relative position, colinearity and the regularity of shape. We measured the sensitivity in detecting shape deformations by presenting a square simultaneously with a rectangle of variable aspect ratio, or a circle with an ellipsoid. Weber fractions approach 3-5% and improve to approximately 1% when subjects are asked to tell which of the two objects was 'oriented more vertically', instead of identifying the square or circle. Contour position can be judged with a precision of 10-20 arc s, clearly in the hyperacuity range and also beyond the thresholds known for detecting differences in the curvature of comparable line segments. Our results suggest that detecting deformation in rectangles seems to rely on aspect ratio, whereas performance is improved for ellipsoids by a high sensitivity for changes in local curvature.

Adult↗