Showing papers by "Clifton M. Schor published in 2005"

Stereo-slant adaptation is high level and does not involve disparity coding.

Abstract: We have investigated the potential stages of visual processing at which adaptation may occur to a slanted surface produced by horizontal magnification. Predictions of three hypotheses were tested utilizing a property of depth from binocular disparity, namely that slant scales with distance. If adaptation occurs at the disparity level, then the after-effect expressed in units of horizontal magnification will be independent of the test distance. If adaptation occurs at either a perceived slant or mapping level, then the after-effect, expressed in units of slant, will be independent of the test distance. If adaptation is contingent on distance, then the after-effect will not transfer over distance. Subjects adapted to a stereo-defined slanted surface at a distance of 57 cm. The after-effect was measured with a test stimulus at a distance of 28, 57, 85, or 114 cm by means of a nulling method. When the after-effect was expressed in units of slant, we found that it was larger at the adapting distance than other test distances, and that the after-effect was constant at test distances different from the adaptation distance. These results suggest that two types of adaptation occurred, namely adaptation on a mapping/perception level and adaptation contingent on distance.

A pulse-step model of accommodation dynamics in the aging eye

Abstract: We have developed a dynamic model of accommodation that combines independent phasic-velocity and tonic-position neural signals to control position, velocity and acceleration properties of accommodative step responses. Phasic and tonic signals were obtained from neural integration of a fixed-height acceleration-pulse and variable-height velocity-step respectively to control independent acceleration and velocity properties of the step response. Duration and amplitude of the acceleration-pulse are increased with age to compensate for age-related increases of visco-elastic properties of the lens to maintain youthful velocity. The model illustrates a neural control strategy that is similar to the classical neural control model of step changes by the saccadic and vergence systems.

Adaptation to the induced effect stimulus normalizes surface slant perception and recalibrates eye position signals for azimuth.

Abstract: A frontoparallel plane viewed with unequal vertical magnification of the two ocular images appears rotated about a vertical axis (i.e., induced effect; Ogle, 1938). Several experiments were conducted to investigate changes in the visual system that occurred after adapting to the induced effect. Adaptation at 57 cm was tested using tall stimuli at various viewing distances to test for the adaptation of vertical size ratio (VSR) information and normalization of the slant percept. When aftereffects were expressed in units of slant, they were larger at 57 cm than other test distances and were not significantly different from each other at other distances. Short stimuli were used to test adaptation of eye position signals for azimuth. The aftereffects were in the opposite direction to those measured with tall stimuli. The combined results suggest that the visual system normalizes slant percepts based on the surface slant of the adaptation stimulus and when there is a conflict between VSR signals and eye position cues for azimuth that the primary eye position signal for azimuth is recalibrated toward the direction indicated by the binocular differential vertical magnification in the adaptation stimulus.

Effects of partial occlusion on perceived slant difference.

Abstract: When two slanted surfaces are placed in proximity, the perceived slant difference between them is exaggerated. This effect has been called slant contrast. When a partial occluder is presented in front of the gap between them, the perceived slant difference between the surfaces is reduced. We refer to this reduction in perceived slant difference as stereo-slant assimilation. We investigated two properties of the occluder that might affect perceived stereo-slant difference. Three vertically aligned random-dot patterns were presented either with a partial occluder over the gaps between them or without it. Observers judged the perceived slant difference between the center pattern and two surround random-dot patterns that had the same slant. The perceived slant difference was reduced when the partial occluder was present. We varied stereo-depth and slant of the occluder and found that the decreased perceived slant difference was not due to either of these. Note that the surfaces were all simulated and presented on a computer screen and the results may not apply to real surfaces. The effect of the occluder on perceived slant differences could have resulted from either a reduction of slant contrast or an increase of slant assimilation.

Spatio-temporal interactions that promote the smoothness constraint for binocular matches

Abstract: Early in his career, Bela Julesz introduced the stereo matching problem while working at Bell Labs on an encryption project. The common belief at that time was based on Wheatstone’s proposal that 2-D space perception of form preceded coding of disparity for 3-D space perception. However, with the random-dot stereogram, Julesz demonstrated that stereoscopic depth could be perceived in the absence of any identifiable objects or perspective cues available to either eye alone. This work inspired many algorithms for binocular matching including the smoothness constraint. Wheatstone’s and Julesz’s proposals as to whether binocular matches are solved at a low level, prior to form perception, or after form is perceived are still debated. We have examined spatio-temporal interactions that promote binocular matches and yield percepts of smooth surfaces in depth. We identified low-level processes for estimating depth differences between surface patches that require their proximity in both time and space, and a high level process that minimizes their depth differences when surface texture of adjacent patches appears to belong to the same surface. This suggests that the stereo-matching solution is influenced by a priori assumptions about the surface configuration of the scene and by monocular and binocular spatial cues.