Biomedical subjects

G Loffler

Publications and source records attributed to G Loffler.

3 recordsLinked to original sources

PubMed2001

Detecting shape deformation of moving patterns.

This study measured thresholds for the discrimination of rigidly and nonridgidly rotating patterns in two dimensions. The stimuli employed were closed contours created by the sum of two 'radial frequency' components and sensitivity to their deformation was measured as a function of the difference in the angular velocities of the components. Results show that thresholds do not depend on the specific shape of the pattern. To quantify the influence of local computations versus global pooling, thresholds were measured with parts of the pattern covered by (invisible) pie-shaped apertures. One finds thresholds are not simply a function of the total amount of pattern visible but exhibit a dependence on the number of apertures. Moreover, sensitivity to deformation could neither be fully explained on the basis of local computations nor by linear global summation. A simultaneous masking paradigm was employed to elucidate potential mechanisms involved in the computation of deformation. While 1D masks (horizontal gratings and translating random dots) only marginally elevate thresholds, rotating and expanding motion significantly impairs sensitivity. This indicates that detectors tuned to radial and circular motion are involved in the computation of shape deformation.

Discrimination, Psychological↗

PubMed2001

An inverse oblique effect in human vision.

In the classic oblique effect contrast detection thresholds, orientation discrimination thresholds, and other psychophysical measures are found to be smallest for vertical or horizontal stimuli and significantly higher for stimuli near the +/-45 degrees obliques. Here we report a novel inverse oblique effect in which thresholds for detecting translational structure in random dot patterns [Glass, L. (1969). Moiré effect from random dots. Nature, 223, 578-580] are lowest for obliquely oriented structure and higher for either horizontal or vertical structure. Area summation experiments provide evidence that this results from larger pooling areas for oblique orientations in these patterns. The results can be explained quantitatively by a model for complex cells in which the final filtering stage in a filter-rectify-filter sequence is of significantly larger area for oblique orientations.

Contrast Sensitivity↗

PubMed2001

Anisotropy in judging the absolute direction of motion.

The angular dependence of precision measurements is well established as the oblique effect in motion perception. Recently, it has been shown that the visual system also exhibits anisotropic behaviour with respect to accuracy of the absolute direction of motion of random dot fields. This study aimed to investigate whether this angular dependent, directional bias is a general phenomenon of motion perception. Our results demonstrate, for single translating tilted lines viewed foveally, an extraordinary illusion with perceptual deviations of up to 35 degrees from veridical. Not only is the magnitude of these deviations substantially larger than that for random dots, but the general pattern of the illusion is also different from that found for dot fields. Significant differences in the bias, as a function of line tilt and line length, suggest that the illusion does not result from fixed inaccuracies of the visual system in the computation of direction of motion. Potential sources for these large biases are motion integration mechanisms. These were also found to be anisotropic. The anisotropic nature and the surprisingly large magnitude of the effect make it a necessary consideration in analyses of motion experiments and in modelling studies.

Anisotropy↗