Biomedical subjects

L Kehrer

Publications and source records attributed to L Kehrer.

4 recordsLinked to original sources

PubMed2003

A space-variant filter model of texture segregation: parameter adjustment guided by psychophysical data.

This article presents a space-variant version of a standard spatial filter model of texture segregation of the "back-pocket" type (i.e., two filter layers with an intermediate pointwise nonlinearity). The model was tested with psychophysical data from experiments with line textures in which target lines differed in orientation from background lines. The textures were presented briefly and then masked. Segregation performance was evaluated along the horizontal meridian up to retinal eccentricities of about 10 deg. Data are reported from two experiments with different line densities (Kehrer 1989) and two experiments with different orientation contrasts between target lines and background lines (Kehrer 1990). Segregation performance proved to depend strongly on these texture variations, and it peaked several degrees from fixation in all cases. The filter model provided satisfactory predictions of experimental data when model parameters were adjusted appropriately. It is concluded (1) that filter models defined in strictly spatial terms (i.e., without temporal properties) offer a sufficient framework to account for the psychophysical data and (2) that the particular course of the performance curve (i.e., the performance peak outside the central region) must be attributed to the characteristics of second-layer filters.

Humans↗

PubMed1994

Peripheral and foveal segmentation of angle textures.

Studies of the effects of retinal eccentricity on the visual segmentation of textures are presented. The textures used in these studies were composed of angle elements. These were presented tachistoscopically to college students in three different experiments. Results showed that there were different relationships between segmentation performance and eccentricity, depending on the width of the angles used in the background and target texture. One major difference was that peak performance was found in the fovea in some conditions, and in peripheral areas in other conditions. Performance in the fovea and the periphery seemed to be determined by qualitatively different features. It was assumed that an appropriate explanation is that the system-internal representation of a specific stimulus within the early visual system differs as a function of the retinal location at which it is projected. Thus, the critical features discriminating between target and background texture have to be sought in the system-internal representation of the stimulus instead of in the stimulus itself. The data show that a relatively exact system-internal representation of the stimulus is present in the fovea, where performance is determined by angle width. In the periphery, in contrast, angles seem to be represented as "blobs," and performance is determined by the orientation of the blobs' main axes.

Depth Perception↗

PubMed1989

Central performance drop on perceptual segregation tasks.

The main concern of this study is the range of the central (foveal) performance drop in perceptual segregation tasks reported by Kehrer (Spatial Vision 2, 247-261, 1987). This effect suggests that parafoveal areas of the retina make a significant contribution to the perceptual segregation of textures. Results showed that: (1) the central performance drop was not due to retinal criterion shifts in the sense of signal detection theory; (2) the central performance drop was not restricted to very short presentation times but could also be observed in 80- or 120-ms presentation times; (3) the retinal area in which maximal segmentation performance was found could be shifted by manipulating the spacing between the elements of the stimulus. This finding suggests that each different area of the retina is linked to the processing of a specific, limited area of the spatial frequency band.

Female↗

PubMed1987

Perceptual segregation and retinal position.

This study is concerned with the dependence of perceptual segregation performance on the retinal position at which the performance is evaluated. Segregation performance consisted in detecting a target texture composed of line elements of constant length and orientation embedded in a background texture. The background texture was made up of the same line elements as the target texture but the background line elements were set at 90 deg to the target elements. Results showed that, at least for 40- and 50-ms presentation times, this task could be much more effectively completed outside the fovea centralis than within this area. These findings indicate that extrafoveal areas of the retina may make a significant and previously underestimated contribution to perceptual segregation.

Female↗