The contribution of color to depth perceived from motion parallax

Luminance and Color Contrast Sensitivity and Vep Latency in Subjects with Normal and Defective Binocularity

Luminance contrast sinusoidal gratings (spatial frequencies 1, 2 and 4 cycles/degree) were compared with the corresponding color contrasting patterns (along the protan, deutan and tritan axes) to see whether they demonstrated normal binocular function in humans, and distinguished between normals and persons with defective binocularity. Contrast sensitivity and transient pattern VEP latency (on-responses) were measured in normals (n = 11, median age 36, range 12-46 years) and subjects with no stereopsis (n = 6, median age 13, range 8-38 years). The normal group had significantly higher contrast sensitivity with binocular stimulation for all patterns except tritan contrast gratings of 2 and 4 c/deg. The stereo-deficient group showed no higher binocular contrast sensitivity for any pattern. Differences between groups were significant with all gratings of 4 c/deg, and also with protan and deutan contrast gratings of 2 c/deg. In the normal group, binocular VEP latency was significantly shorter than the monocular with protan contrast gratings of 2 c/deg and tritan contrast gratings of 1 and 2 c/deg. Differences between the normal and the stereo-deficient groups were significant for all color contrast patterns of 2 c/deg; and tritan contrast gratings of 1 c/deg. We conclude that color contrast sensitivity and VEP measurements are potentially useful for demonstrating binocular function, and for separating normals from stereo-blind subjects. Color contrast patterns however are less effective than the corresponding luminance contrast patterns in evoking cortical potentials.

Pink Cricket Balls Through Rose-Tinted Glasses: Enhancing Interceptive Timing

Cricket is a popular but potentially dangerous sport. It is played with a hard ball that can travel at great speeds. Serious injuries, including fatalities, have occurred when balls have struck participants. The game is traditionally played during daylight with a dark red ball, but recent games have been played during the day and at night using a ‘pink’ ball. We have reported data that seemed to justify concerns raised regarding the visibility of these new pink balls, as they were revealed to have a very low luminance contrast against pertinent backgrounds during twilight. Here, we report on the findings of a psychophysical experiment, wherein we mimicked twilight lighting conditions in an interceptive timing experiment using a pink moving disc as an analogue for pink cricket balls. We show that interceptive timing performance is diminished in conditions that mimic twilight. More importantly, we show that wearing glasses with a rose-tinted filter can alleviate this adverse impact by enhancing the luminance contrast of the pink ‘ball’ relative to pertinent backgrounds.

Effects of Colored Light, Color of Comparison Stimulus, and Illumination on Error in Perceived Depth with Binocular and Monocular Viewing

Two experiments assessed the effects of colored light, color of a comparison stimulus, and illumination on error in perceived depth with binocular and monocular vision. Exp. 1 assessed effects of colored light, color of comparison stimulus, and source of depth cues on error in perceived depth. A total of 29 women and 19 men, Taiwanese college or graduate students ages 20 to 30 years (M=24.0, SD= 2.5) participated; they were randomly divided into five groups, each being assigned to one of five possible colored light conditions. Analyses showed color of the comparison stimulus significantly affected the error in perceived depth, as this error was significantly greater for a red comparison stimulus than for blue and yellow comparison stimuli. Colored light significantly affected error in perceived depth since error under white and yellow light was significantly less than that under green light. Moreover, error in perceived depth under white light was significantly less than that under blue light but not sensitive to white, yellow, and red light. Error in perceived depth for binocular viewing was significantly less than that for monocular viewing but not sex. In Exp. 2, the effect of illumination on error in perceived depth was explored with 21 women and 15 men, Taiwanese college students with a mean age of 19.8 yr. (SD= 1.1). Analysis indicated that illumination significantly affected error in perceived depth, as error for a 40-W condition was significantly greater than under 20- and 60-W conditions, although the latter were not different.

The effect of overall stimulus velocity on motion parallax

This study examined the effectiveness with which motion parallax information can be utilized by rhesus monkeys for depth perception. A visual display comprised of random-dots that mimicked a rigid, three-dimensional object rocking back and forth was used. Differential depth was produced by presenting sub-regions of the dots moving at different velocities from the rest of dots in the display. The tasks for the monkeys were to detect or discriminate a target region that was protruding the furthest from the background plane. To understand the role of stimulus movement, we examined the accuracy and the rapidity of the saccadic responses as a function of rocking velocity of the entire three-dimensional object. The results showed that performance accuracy improved and reaction times decreased with increasing rocking velocities. The monkeys can process the motion parallax information with remarkable rapidity such that the average reaction time ranged between 212 and 246 milliseconds. The data collected suggest that the successive activation of just two sets of cones is sufficient to perform the task.

The perception of motion in chromatic stimuli

The issue of whether there is a motion mechanism sensitive to purely chromatic stimuli has been pertinent for the past 30 or more years. The aim of this review is to examine why such different conclusions have been drawn in the literature and to reach some reconciliation. The review critically examines the behavioral evidence and concludes that there is a purely chromatic motion mechanism but that it is limited to the fovea. Examination of motion performance for chromatic and luminance stimuli provides convincing evidence that there are at least two different mechanisms for the two kinds of stimuli. The authors further argue that the chromatic mechanism may be at a particular disadvantage when the integration of multiple local motion signals is required. Finally, the authors present a descriptive model that may go some way toward explaining the reasons for the differences in collected data outlined in this article.

Ideal cue combination for localizing texture-defined edges

Many visual tasks can be carried out by using several sources of information. The most accurate estimates of scene properties require the observer to utilize all available information and to combine the information sources in an optimal manner. Two experiments are described that required the observers to judge the relative locations of two texture-defined edges (a vernier task). The edges were signaled by a change across the edge of two texture properties [either frequency and orientation (Experiment 1) or contrast and orientation (Experiment 2)]. The reliability of each cue was controlled by varying the distance over which the change (in frequency, orientation, or contrast) occurred-a kind of "texture blur." In some conditions, the position of the edge signaled by one cue was shifted relative to the other ("perturbation analysis"). An ideal-observer model, previously used in studies of depth perception and color constancy, was fitted to the data. Although the fit can be rejected relative to some more elaborate models, especially given the large quantity of data, this model does account for most trends in the data. A second, suboptimal model that switches between the available cues from trial to trial does a poor job of accounting for the data.

Does segregation by colour/luminance facilitate the detection of structure-from-motion in noise?

The aim in the experiments was to examine whether the detection of structure-from-motion (SFM) in noise was facilitated when target and noise were segregated by colour and/or luminance polarity. The SFM target was a rotating 'V-shape' structure simulated with limited-lifetime Gaussian micropatterns and embedded in random-motion noise. Threshold levels of V-shape slant were measured for stimuli in which target and noise were segregated or unsegregated by colour/luminance, and under two conditions, with and without static form cues to the SFM target. The presence or absence of static form cues to the SFM target was manipulated by varying the relative numbers of micropatterns in target and noise. In the absence of static form cues, segregation of target and noise by colour and/or luminance polarity did not facilitate target detection, even when subjects knew which micropatterns belonged to the target. On the other hand, when static form cues were present, segregation improved performance. These results imply that SFM processing is 'form-cue invariant' except when the target form is immediately identifiable in the static view of the stimulus. The significance of the results for understanding the role of colour vision in breaking camouflage and in 'grouping' is discussed.

Isoluminance and chromatic motion perception throughout the visual field

Isoluminance and chromatic motion perception for red/green gratings were measured throughout an 80 deg visual field. Generally, the red/green isoluminance values changed with increasing eccentricity, i.e., observers increased the red luminance contrast for a fixed green luminance contrast. Enlarging the target size (to compensate for the cone density changes with eccentricity) and decreasing the spatial frequency (to compensate for receptive field property changes with eccentricity) did not change the isoluminance values within the central 20 deg, but the isoluminance ratios decreased beyond 20 deg. Our manipulations did not entirely compensate for a given eccentricity, which implies the need for a post-receptoral scaling function for the perception of drifting chromatic stimuli. Further, the results for isoluminance show heterogeneity between the visual field meridians where the red to green luminance ratio tends to be greater in the superior visual field. In our present conditions, chromatic motion was always perceived (up to 40 deg of eccentricity), but sensitivity generally decreased with increasing eccentricity. The inferior visual field was found to be the most sensitive to chromatic motion. We propose that the lower visual field and not the superior visual field is specialized for colour motion information.

Colour and luminance interact to improve pattern recognition

The authors have previously hypothesised that colour vision has evolved not only to encode colour per se but also, perhaps principally, to enhance luminance-based visual processing so that for colour information to be fully effective, luminance as well as chromatic variations should be present in visual targets. Results of previous experiments, testing detection of spatial gratings and detection and perceived brightness of Mach bands support the hypothesis. Further experiments are reported in which the hypothesis was tested by using a higher-level task of pattern recognition. Subjects had to discriminate between luminance (isochromatic), isoluminant (chromatic), or combined colour/luminance ellipses and circles. It was found that the ability to discriminate between a circle and an ellipse was greatly enhanced when both colour and luminance variations were present as compared with the pure luminance or colour presentations. Summation-square analysis shows linear colour-luminance summation which can be modeled by a single-analyser model.