Ray Over

Color Adaptation of Spatial Frequency Detectors in the Human Visual System

Observers exposed alternately to a vertical grating of one spatial frequency in red light and a vertical grating of different spatial frequency in green light subsequently report frequency-specific color aftereffects when shown gratings in white light. Aftereffects occur, however, only when inspection gratings differ in spatial frequency by one octave or more and the frequency of at least one grating is above 3 cycles per degree. This spatial selectivity of the aftereffect is considered in terms of a neural adaptation model incorporating evidence on the tuning of spatial frequency detectors in the human visual system.

Motion aftereffect with subjective contours.

Stationary lines appear to move from left to right following exposure to lines moving from right to left. This aftereffect, which normally is generated by exposure to moving edges that are defined in terms of local luminance discontinuity, can also be induced by adaptation to displays containing subjective contours. In both cases, stereodeficient observers demonstrated reduced interocular transfer of the aftereffect relative to stereonormal observers. Since interocular transfer of the motion aftereffect entails binocular function within the visual system, these results suggest that the perception of subjective contours depends on excitation of neural feature detectors rather than simply on cognitive inference.

Colour selectivity in orientation masking and aftereffect.

Abstract Two experiments demonstrated that the extent to which exposure to one grating impaired detection of another grating (masking) or modified its apparent tilt (aftereffect) depended on the relative orientations of the gratings and also whether they were viewed in the same or different coloured light. Colour-specificity in perception was lost when the two gratings differed by more than 15°, or when the inspection grating was viewed by one eye and the test grating by the other eye. These results support those explanations of orientation-selective colour aftereffects (the McCullough effect) that have been offered in terms of adaptation of neural analysers tuned to both orientation and wavelength.

Spatial determinants of the aftereffect of seen motion

Observers were required to track the perceived motion of a stationary grating viewed following exposure to a moving grating. The size of the motion aftereffect was complexly controlled by relationships between the velocity and periodicity of the adaptation grating and the periodicity of the test grating. In addition the motion aftereffect was selective to the direction of motion of the adaptation grating. These results are considered in terms of a model attributing the motion aftereffect to selective postexcitatory suppression in the response of directionally-excited motion detectors in the visual system.

Orientation Masking and the Tilt Illusion with Subjective Contours

Exposure to one edge renders another edge less visible as a function of relative orientation. Experiment 1 showed that orientation-selective masking occurs between phenomenal edges located at sites where the visual display is homogeneous (subjective contours) as well as between edges defined in terms of luminance discontinuity (real edges). In addition, real contours can be masked by subjective contours, and vice versa. In experiment 2 it was found that the tilt illusion (apparent expansion of the angle formed by intersecting lines) can be induced with subjective as well as with real contours. These results suggest that it is inappropriate to attribute the perception of real and subjective contours to fundamentally different processes.

Absence of binocular interaction between spatial and color attributes of visual stimuli

The hypothesis that induction of the McCollough effect (spatially selective color aftereffects) entails adaptation of monocularly driven detectors tuned to both spatial and color attributes of the visual stimulus was examined in four experiments. The McCollough effect could not be generated by displaying contour information to one eye and color information to the other eye during inspection, even in the absence of binocular rivalry. Nor was it possible to induce depth-specific color aftereffects following an inspection period during which random-dot stereograms were viewed, with crossed and uncrossed disparity seen in different colored light. Masking and aftereffect in the perception of stereoscopic depth were also nonselective to color; in both cases, perceptual distortion was controlled by stereospatial variables but not by the color relationship between the inspection and test stimuli. The results suggest that binocularly driven spatial detectors in human vision are insensitive to wavelength.

Stereoscopic depth aftereffects with random-dot patterns

Random-dot stereograms were used to measure aftereffects in the perception of binocular depth. Observers were required to judge the depth of a binocular target following exposure to a stereostimulus in which horizontal disparity was systematically varied. The target appeared nearer after adaptation to uncrossed disparity and farther away after adaptation to crossed disparity. The maximum aftereffect (70″ of disparity) occurred following exposure to 6-8′ of disparity. These results extend the data on stereoscopic aftereffects reported by Blakemore and Julesz, and provide further evidence that the human visual system possesses neural channels that are selectively responsible to binocular disparity.

Color-selectivity in simultaneous motion contrast

Thirty-two Ss were required to estimate the apparent motion of stationary vertical lines viewed against a background of moving vertical lines when both patterns were seen by the same eye (monoptic conditions) or the center pattern was seen by one eye and the surrounds by the other eye (dichoptic conditions). The stationary lines appeared to be moving from right to left as the surrounds moved left to right. The simultaneous motion contrast found under monoptic conditions was maximal when the center pattern and the surrounds were the same color and was reduced when they differed in color. The surrounds had limited influence on the apparent motion of the center section under dichoptic condition, and the color relationship was no longer important. Related color selectivity has been reported for the motion aftereffect (successive motion contrast), and both sets of data can be attributed to inlaibitory interaction (simultaneous in one case and successive in the other) among neural detectors tuned to wavelength as well as the direction of image motion.

Color-selective tilt aftereffects with subjective contours

Displays yielding edges visible at sites where the visual stimulus was homogeneous (subjective contours) as well as with edges defined by spatial discontinuities in luminance (real contours) were used to induce the tilt aftereffect. Under monoptic conditions, the aftereffect was larger when the inspection and test edges were shown in the same colored light than when they were shown in different colored lights. Under dichoptic conditions (display of inspection edges to one eye and test edges to the other eye), the aftereffect was reduced in size and it was no longer selective to the color relationship between the inspection and test stimuli. Similar results were obtained with subjective and real contours. In the recent literature, subjective contours have been treated as products of cognitive and inferential operations, whereas neural edge detectors have been implicated in the perception of real contours. The present data suggest, however, the need for caution in attributing the perception of real and subjective contours to fundamentally different processes.

Curvature-specific color aftereffects

Adaptation to convex and concave arcs in different colored light results in curvature-specific color aftereffects when arcs are later viewed in white light. In three experiments, it was shown that these color aftereffects often are partial (restricted to limited regions of the test arcs) rather than uniform, and in addition that aftereffects induced by exposure to arcs transfer to straight-line displays of particular orientation, and vice versa. These data were interpreted as evidence that arcs are processed in the visual system in relation to the orientations of local straight line approximations instead of on a global basis. In these terms, curvature-specific color aftereffects are merely complex forms of the orientation-specific color aftereffects first described by McCollough (1965).