Search

Ambiguity is implicit in neural representations of the physical world. Previous work has examined how the visual system resolves ambiguous neural signals that represent various features, such as the percept resulting from rivalrous chromaticities or forms. Relatively little is known, however, about the contribution of unambiguous neural representations to perceptual resolution of ambiguous ones. This is addressed here by measuring perceptual resolution of ambiguity by grouping, which is operationalized as the tendency for multiple similar ambiguous representations to be seen as identical to each other. Multiple chromatically ambiguous representations were created using interocular switch rivalry and presented together with a nearby but separate unambiguous (non-rivalrous) chromaticity. The magnitude of grouping the chromatic regions was compared when ambiguous regions were seen alone versus with unambiguous regions seen simultaneously. Contrary to prevailing theory that the resolution of the ambiguous percepts would follow the unambiguous ones, the ambiguous chromatic regions consistently appeared identical to each other, but their appearance was not found to be attracted to the unambiguous color percept. This supports the proposition that the ambiguity itself in a neural representation is a linking feature contributing to perceptual disambiguation.

Color is a perceptual construct that arises from neural processing in hierarchically organized cortical visual areas. Previous research, however, often failed to distinguish between neural responses driven by stimulus chromaticity versus perceptual color experience. An unsolved question is whether the neural responses at each stage of cortical processing represent a physical stimulus or a color we see. The present study dissociated the perceptual domain of color experience from the physical domain of chromatic stimulation at each stage of cortical processing by using a switch rivalry paradigm that caused the color percept to vary over time without changing the retinal stimulation. Using functional MRI (fMRI) and a model-based encoding approach, we found that neural representations in higher visual areas, such as V4 and VO1, corresponded to the perceived color, whereas responses in early visual areas V1 and V2 were modulated by the chromatic light stimulus rather than color perception. Our findings support a transition in the ascending human ventral visual pathway, from a representation of the chromatic stimulus at the retina in early visual areas to responses that correspond to perceptually experienced colors in higher visual areas.

Some images evoke bistable percepts: two different visual experiences seen in alternation while continuously viewing an unchanged stimulus. The Necker Cube and Rubin's Vase are classic examples, each of which gives alternating percepts of different shapes. Other bistable percepts are alternating colors or directions of motion. Although stimuli that result in salient bistability are rare and sometimes cleverly constructed to emphasize ambiguity, they have been influential for over 150 years, since the work of von Helmholtz, who considered them to be evidence for perceptual visual processes that interpret retinal stimuli. While bistability in natural viewing is uncommon, the main point of this review is that implicit ambiguity in visual neural representations is pervasive. Resolving ambiguity, therefore, is a fundamental and ubiquitous process of vision that routinely affects what we see, not an oddity arising from cleverly crafted images. This review focuses on the causes of widespread ambiguity, historical perspectives on it, and modern knowledge and theory about resolving it.

Two basic principles of human color vision are (1) color is not in light but instead constructed within the (human) perceiver and (2) in natural viewing, photoreceptor signals fail to determine uniquely the colors we see. The visual system, therefore, must resolve the ambiguity implicit in the neural representation of the stimulus. This paper focuses on a perceptual property used by the visual system to resolve ambiguity when two or more parts of a scene share the same ambiguous chromatic neural representation: the parts are grouped so tend to appear the same color. Chromatic interocular-switch rivalry, a novel paradigm, is described and then used to demonstrate grouping of two, four or 16 parts of an image with the same ambiguous chromatic representation.

The past 20 years have seen a surge of interest in how animals perceive colour, setting the stage for a much more detailed examination of how colour perception differs among species, how a species’ colour perception relates to its environment and how it all fits into the framework of animal communication. We need to address two major questions: first, how do general mechanisms of signal processing work within whole clades of animals, and second, how do these mechanisms modulate differences among related species within clades? The receptor noise-limited (RNL) model ( Vorobyev & Osorio, 1998 ) has made a critical advance in the field. Relevant parameters of the model, including screening pigments in the eye, visual pigments and relative numbers of the different receptor cells, can be measured to predict how species detect objects in their environment, based on wavelength. Details of the opponent channels, however, which compare the outputs of the retinal receptors and can determine the signals sent to the brain, are unknown for most species and are not required by the model. Here, we unpack the RNL model, focusing on experiments in humans and birds, and explore the impact of including specific opponent channels in the model. Incorporating opponent channels into the RNL model could help us better understand the selective forces and coevolutionary processes that shape the visual system and determine visual adaptations. Present evidence shows that the RNL model works as a good first approximation and points to critical parameters we need to measure, such as noise in receptor cells.

A long-standing question in the field of vision research is whether scalp-recorded EEG activity contains sufficient information to identify stimulus chromaticity. Recent multivariate work suggests that it is possible to decode which chromaticity an observer is viewing from the multielectrode pattern of EEG activity. There is debate, however, about whether the claimed effects of stimulus chromaticity on visual evoked potentials (VEPs) are instead caused by unequal stimulus luminances, which are achromatic differences. Here, we tested whether stimulus chromaticity could be decoded when potential confounds with luminance were minimized by (1) equating chromatic stimuli in luminance using heterochromatic flicker photometry for each observer and (2) independently varying the chromaticity and luminance of target stimuli, enabling us to test whether the pattern for a given chromaticity generalized across wide variations in luminance. We also tested whether luminance variations can be decoded from the topography of voltage across the scalp. In Experiment 1, we presented two chromaticities (appearing red and green) at three luminance levels during separate trials. In Experiment 2, we presented four chromaticities (appearing red, orange, yellow, and green) at two luminance levels. Using a pattern classifier and the multielectrode pattern of EEG activity, we were able to accurately decode the chromaticity and luminance level of each stimulus. Furthermore, we were able to decode stimulus chromaticity when we trained the classifier on chromaticities presented at one luminance level and tested at a different luminance level. Thus, EEG topography contains robust information regarding stimulus chromaticity, despite large variations in stimulus luminance.

Ambiguity resolution, perceptual grouping, and feature integration all occur seamlessly and subconsciously. When multiple regions of an image share ambiguous features, perceptual grouping can yield an integrated object percept rather than one of multiple objects, each with its individual features. Here, perceptual resolution and grouping of chromatically rivalrous Necker cubes were investigated in three experiments to determine the principles that underlie these coherent percepts. The first experiment showed perceptual grouping beyond independent resolution of each cube’s color and orientation, but the second experiment did not show grouping greater than expected from separate color- and orientation-grouping processes. The third experiment found no reliable difference in grouping when two features (color and orientation) were part of the same object versus when they were distributed across separate objects. These findings fail to support a role for feature conjunctions in grouping objects with multiple ambiguous features.

Competing rivalrous neural representations can be resolved at several levels of the visual system. Sustained percepts during interocular-switch rivalry (ISR), in which rivalrous left- and right-eye stimuli swap between eyes several times a second, often are attributed to competing binocularly driven neural representations of each rivalrous stimulus. An alternative view posits monocular neural competition together with a switch in eye dominance at the moment of each stimulus swap between eyes. Here, a range of experimental conditions was tested that would change the colors seen if mediated by eye dominance but not if by competition between binocularly driven responses. Observers viewed multiple chromatically rivalrous discs in various temporal and spatial patterns, and reported when all discs in view appeared the same color. Unlike typical ISR paradigms that swap the complete stimulus in each eye, some of the rivalrous discs were swapped at a different time, or faster frequency, than other discs. Monocular dominance of one eye at a time implies that all discs will rarely be seen as identical in color when some discs swap at a different frequency than others. On the other hand, competing binocularly driven neural responses are not affected by asynchronous swap timing among the individual discs. Results for every observer are in accord with competing responses at the level of binocularly driven, chromatically tuned neurons. Although an account based on eye dominance can be constructed using many small retinotopic zones that have independent timing for the moment of switching the dominant eye, competing binocularly driven responses are a more parsimonious explanation.

A coherent percept of our visual world is important for functioning. Ambiguities, however, are implicit in visual neural representations and must be resolved for stable perception of objects and scenes. Grouping processes can link multiple neurally ambiguous fragments across the visual field. Experiments here determined how multiple visual features of each fragment contribute to perceptual resolution of ambiguity by grouping. Chromatic interocular-switch rivalry, a technique for presenting competing dichoptic images, was used to induce ambiguous neural representations for equiluminant chromatic discs and gratings. Two dichoptic stimuli were presented simultaneously to measure the amount of time they both appeared the same in at least one feature domain. The two stimuli were grouped when they appeared to share ambiguous features such as color, orientation, and spatial frequency more often than chance. Experiments here tested whether unshared and unambiguous features impeded grouping of the ambiguous components. Overall, the results show that grouping can be driven by neural ambiguity that is common for fragments across the visual field, even when the fragments also have other unshared, unambiguous features.

Perceptual grouping contributes to the resolution of visual ambiguity of multiple spatially separate regions in view by enhancing their perceptual similarity. Here, the same ambiguous neural representations are shown also to enhance perceived dissimilarity among the regions. Two separated equiluminant gratings were made ambiguous by introducing rivalry for one of two of their features: orientation or chromaticity. Observers perceived two gratings (above and below fixation) to be different in both color and orientation more often than chance. Overall, a disambiguating process was found to select often for maximal perceived dissimilarity between two objects.

Multiple regions, each with the same ambiguous chromatic neural representation, are resolved to have the identical perceived color more often than chance [Proc. Natl. Acad. Sci. USA 93, 15508 (1996)PNASA60027-842410.1073/pnas.93.26.15508; J. Opt. Soc. Am. A 35, B85 (2018)JOAOD60740-323210.1364/JOSAA.35.000B85]. This reveals that the regions are grouped, but it is unclear whether they are grouped because each one has the identical competing representations of the same stimuli (that is, the same chromaticities) or, alternatively, identical competing representations of the same colors one sees. The current study uses chromatic induction, as in Nat. Neurosci. 6, 801 (2003)NANEFN1097-625610.1038/nn1099, to disentangle whether grouping depends on identical (though ambiguous) stimulus chromaticities or on perceived colors, by (1) inducing one chromaticity to appear in two different colors or (2) inducing two different chromaticities to appear in the same color. All stimuli were equiluminant gratings with chromatic inducing and test fields. Three observers were tested, first completing color matches to measure induced color-appearance shifts and second completing grouping measurements using interocular-switch rivalry, a method with rivalrous dichoptic images swapped between the eyes at 3.75 Hz [J. Vis. 17, 9 (2017)1534-736210.1167/17.5.9]. Each of two separate areas, one above and one below fixation, had dichoptic rivalry. The two sets of regions had either identical or different chromaticities that could appear either as the same color or not. Observers reported their percepts when both areas above and below fixation were grouped by color or by chromaticity (or neither in an additional experimental condition). All conditions showed significant groupings for every observer, including when neither color nor chromaticity was identical in a “group.” Moreover, there was never a significant effect of chromaticity versus color for any observer. This is the result expected if neither color nor chromaticity must match between two regions in order for them to be grouped and suggests that, instead, some other feature drives grouping.

The color appearance of a light depends on variation in the complete visual field over both space and time. In the spatial domain, a chromatic stimulus within a patterned chromatic surround can appear a different hue than the same stimulus within a uniform surround. In the temporal domain, a stimulus presented as an element of a continuously changing chromaticity can appear a different color compared to the identical stimulus, presented simultaneously but viewed alone. This is the flash-lag effect for color, which has an analog in the domain of motion: a pulsed object seen alone can appear to lag behind an identical pulsed object that is an element of a motion sequence. Studies of the flash-lag effect for motion have considered whether it is mediated by a neural representation for the moving physical stimulus or, alternatively, for the perceived motion. The current study addresses this question for the flash-lag effect for color by testing whether the color flash lag depends on a representation of only the changing chromatic stimulus or, alternatively, its color percept, which can be altered by chromatic induction. Methods baseline measurements for spatial chromatic induction determined the chromaticity of a flashed ring within a uniform surround that matched a flashed ring within a patterned surround. Baseline measurements for the color flash-lag effect determined the chromaticity of a pulsed ring presented alone (within a uniform surround) that matched a pulsed ring presented in a sequence of changing chromaticity over time (also within a uniform surround). Finally, the main experiments combined chromatic induction from a patterned surround and the flash-lag effect, in three conditions: (1) both the changing and pulsed rings were within a patterned chromatic surround; (2) the changing ring was within a patterned surround and the pulsed ring within a uniform surround; and (3) the changing ring was within a uniform surround and the pulsed ring within a patterned surround. Results the flash-lag measurements for a changing chromaticity were affected by perceptual changes induced by the surrounding chromatic pattern. Thus, the color shifts induced by a chromatic surround are incorporated in the neural representation mediating the flash-lag effect for color.

In a classic study, Kovacs et al. [Proc. Natl. Acad. Sci. USA93, 15508 (1996)PNASA60027-842410.1073/pnas.93.26.15508] used an array of many disks presented dichoptically with half of the disks in one eye "red" and the other half "green;" disk chromaticities in the fellow eye were reversed, resulting in binocular color rivalry for every disk, thus creating color ambiguity. Surprisingly, the binocularly fused percept sometimes was all disks of the same color (red or green), which showed that perceptual resolution of the many ambiguous neural representations did not rely completely on monocular dominance or on independent resolution for each disk. The present study replicates and expands on the original with the aim to isolate binocularly driven neural mechanisms of perceptual resolution without contamination from monocular dominance. Observers viewed a color-rivalrous array with 16 disks presented either steadily to each eye, as in Kovacs et al., or with chromatic interocular-switch rivalry (CISR), which swaps the two images between the eyes every 133 ms. The total proportion of viewing time when the 16 disks were perceived to be all red or all green was measured. For three observers, the disks all appeared the same color more often with CISR than with steady rivalrous presentation, suggesting that monocular dominance interferes with grouped perceptual resolution of ambiguous stimuli in the Kovacs paradigm. This conclusion was supported by an additional condition using CISR, but with every disk the same color in one eye at each instant (e.g., all "red" disks in one eye and all "green" in the other). This condition was never significantly different from the original CISR condition, as expected if CISR reveals only binocularly mediated perceptual resolution of the disks' color, irrespective of monocular neural representations. In conclusion, chromatically tuned binocularly driven neurons account for perceptual resolution of CISR.

The visual system must transform a point-by-point biological representation from the photoreceptors into neural representations of separate objects. Even a uniform circular patch of light that slowly modulates in luminance can be segmented into separate central and surrounding areas merely by introducing black lines to outline a central square. The black lines cause brightness induction in the center even though the light inside and outside the square is always identical, as predicted by spatial antagonism between the square central area and its surround. Importantly, illusory Kanizsa lines forming the square are as effective for this brightness induction as real black lines, suggesting a 'form-cue invariant' cortical neural representation that does not distinguish between a central region set off by real or illusory edges. An open question is whether separate subsystems generate objects defined by real versus illusory edges, each providing the same form-cue invariant neural representation of an object, or whether form-cue invariance extends to integrating component pieces that together define an object. Experiments here show object segmentation when subparts of a square are defined by a mixture of real and illusory edges. Subjects adjusted the Michelson contrast of a separate patch to match the perceived modulation depth within the central region of a circular field that slowly oscillated in luminance. A closed, four-sided figure, no matter how constructed, reduced the perceived modulation depth within the central region. This shows that both real and illusory subparts can be integrated to segment center from surround. It supports a strong version of form-cue invariance in which neural mechanisms responsible for object segmentation are impartial to the piecemeal cues that are integrated to define an object.

The percept of a time-varying light depends on the temporal properties of light within the surrounding area. The locus of the neural mechanism mediating this lateral interaction is controversial; neural mechanisms have been posited at the LGN (Kremers et al., 2004) or cortical level (D’Antona & Shevell, 2007). To determine the neural locus, changes in perceived temporal variation were compared with ipsilateral versus contralateral surrounding context. In both cases, a temporally varying central field was viewed within a temporally varying surround; relative phase between center and surround was varied. Perceived modulation depth in the central field depended strongly on the relative phase between center and surround, in both the ipsilateral and contralateral conditions. The results revealed lateral interactions arising from both a weak monocular (plausibly LGN) and a stronger central (cortical) mechanism. The monocular contribution was similar over the range of temporal frequencies tested (approx. 3–12Hz), while the central component showed low-pass temporal-frequency selectivity.

Very-long-term (VLT) chromatic adaptation results from exposure to an altered chromatic environment for days or weeks. Color shifts from VLT adaptation are observed hours or days after leaving the altered environment. Short-term chromatic adaptation, on the other hand, results from exposure for a few minutes or less, with color shifts measured within seconds or a few minutes after the adapting light is extinguished; recovery to the pre-adapted state is complete in less than an hour. Here, both types of adaptation were combined. All adaptation was to reddish-appearing long-wavelength light. Shifts in unique yellow were measured following adaptation. Previous studies demonstrate shifts in unique yellow due to VLT chromatic adaptation, but shifts from short-term chromatic adaptation to comparable adapting light can be far greater than from VLT adaptation. The question considered here is whether the color shifts from VLT adaptation are cumulative with large shifts from short-term adaptation or, alternatively, does simultaneous short-term adaptation eliminate color shifts caused by VLT adaptation. The results show the color shifts from VLT and short-term adaptation together are cumulative, which indicates that both short-term and very-long-term chromatic adaptation affect color perception during natural viewing.

This research had two goals. First, a new method of very-long-term chromatic adaptation was compared to an older method of long-wavelength ambient illumination. In the new method, the observer viewed for 1 h per day for 12 or 14 days a CRT screen composed of oriented lines that appeared red. One observer also replicated a previous procedure (Neitz et al., 2002) in which she was exposed to long-wavelength room illumination for 4 h per day for 14 days. For both methods, equilibrium yellow was measured each day about 20 h after the end of the adaptation period. Both methods of very-long-term chromatic adaptation gave similar results. Second, shifts in equilibrium yellow were measured over a 30:1 range of light levels to determine if changes in color percepts were explained solely by a gain change in cone sensitivities (von Kries coefficient law). The magnitude of shift of equilibrium yellow depended on the level of the test light, which was not consistent with a gain theory of very-long-term chromatic adaptation.

Under dichoptic viewing conditions, rivalrous gratings that differ in both color and form can give the percept of the color from one eye in part of the form in the other eye. This study examined the afterimage following such misbinding of color to form. The first experiment established that afterimages of the misbound percept were seen. Two possible mechanisms for the misbound afterimage are (1) persisting retinal representations that are rivalrous and subsequently resolved to give misbinding, as during rivalrous viewing, and (2) a persisting response from a central neural representation of the misbound percept with the form from one eye and color from the other eye. The results support afterimage formation from a central representation of the misbound percept, not from resolution of rivalrous monocular representations.

The appearance of an object or surface depends strongly on the light from other objects and surfaces in view. This review focuses on color in complex scenes, which have regions of different colors in view simultaneously and/or successively, as in natural viewing. Two fundamental properties distinguish the chromatic representation evoked by a complex scene from the representation for an isolated patch of light. First, in complex scenes, the color of an object is not fully determined by the light from that object reaching the eye. Second, the chromatic representation of a complex scene contributes not only to hue, saturation, and brightness, but also to other percepts such as shape, texture, and object segmentation. These two properties are cornerstones of this review, which examines color perception with context that varies over space or time, including color constancy, and chromatic contributions to such percepts as orientation, contour, depth, and motion.

Flicker adaptation reduces subsequent temporal contrast sensitivity. Recent studies show that this adaptation likely results from neural changes in the magnocellular visual pathway, but whether this adaptation occurs at a monocular or a binocular level, or both, is unclear. Here, two experiments address this question. The first experiment exploits the observation that flicker adaptation is stronger at higher than lower temporal frequencies. Observers' two eyes adapted to 3Hz flicker with an incremental pulse at 1/4 duty cycle, either in-phase or out-of-phase in the two eyes. At the binocular level, the flicker rate was 6Hz in the out-of-phase condition if the two eyes' pulse trains sum. Similar sensitivity reduction was found in both phase conditions, as expected for independent monocular adapting mechanisms. The second experiment tested for interocular transfer of adaptation between eyes. Results showed that (1) flicker adaptation was strongest with adapting and test fields in only the same eye, (2) adaptation can be partially transferred interocularly with adaptation in only the opposite eye, and (3) adaptation was weakened when both eyes were adapted simultaneously at different contrasts, compared to test-eye adaptation alone. Taken together, the findings are consistent with mechanisms of flicker adaptation at both the monocular and binocular level.

The color appearance of a physically steady central region can appear to vary over time if a surrounding chromatic light varies in time. The induced temporal variation, however, is strongly attenuated at surround temporal frequencies above ∼3 Hz. At these higher temporal frequencies, the central region appears steady (De Valois et al., 1986). The posited explanation is a cortical low-pass temporal filter. Here, we investigate whether higher temporal-frequency surrounds induce color shifts in the steady appearance of the central test. Surrounds modulated in time along the l or s chromatic direction of MacLeod-Boynton color space were symmetric around equal-energy white (EEW). The temporal frequency of the surround was varied. If observers perceived the central test to be temporally modulating between two points in time, they set two separate matches to the extreme points of this modulation. If the central test appeared steady in time, then color matches were made to this steady appearance. Corroborating previous reports, measurements showed that surround temporal frequencies below ∼3 Hz induced temporal modulation. At higher temporal frequencies, however, the surround induced steady color shifts, compared to a steady surround at its time average (EEW). The measurements imply that a nonlinear neural process affects chromatic induction from time-varying context.

Temporally varying chromatic-inducing light was used to infer receptive-field organization. Time-varying shifts in color appearance within a test field were induced by a surrounding chromatic pattern; the shifts were then nulled by adding a time-varying stimulus to the test area so the observer perceived a steady test. This method measured chromatic induction without requiring an observer to judge the color appearance of the test. The induced color shifts were consistent with a +s/−s spatially antagonistic neural receptive field, which also accounts for color shifts induced by static chromatic patterns (Monnier & Shevell, 2003, Monnier & Shevell, 2004). The response of this type of receptive-field, which is found only in the visual cortex, increases with S-cone stimulation at its center and decreases with S-cone stimulation within its surround. The measurements also showed a negligible influence of temporal inducing frequency in the range 0.5–4 Hz.

Chromatic assimilation is the shift in color appearance toward nearby light. Assimilation was measured using nearby light with time-varying chromaticity. This light induced time-varying assimilation within the test area. Assimilation was quantified by the amplitude of temporally varying test-area light – in counter-phase to the induced assimilation – required to null the assimilation. Unlike previous studies of assimilation, observers here judged only the steadiness of the test area, not its color. The inducing light was varied in luminance, temporal frequency and chromaticity. The measured assimilation could not be explained by only optical factors affecting receptoral quantal absorption. This implies a neural process contributes to assimilation. The nulling measurements showed also that assimilation was not induced independently within the L/M- and S-cone pathways. � 2005 Elsevier Ltd. All rights reserved.

Chromatic assimilation is the shift in color appearance of a test field toward the appearance of nearby light. Possible explanations of chromatic assimilation include wavelength independent spread light, wavelength-dependent chromatic aberration and neural summation. This study evaluated these explanations by measuring chromatic assimilation from a concentric-ring pattern into an equal-energy-white background, as a function of the inducing rings width, separation, chromaticity and luminance. The measurements showed, in the s direction, that assimilation was observed with different inducing-ring widths and separations when the inducing luminance was lower or higher than the test luminance. In general, the thinner the inducing rings and the smaller their separation, the stronger the assimilation in s. In the l direction, either assimilation or contrast was observed, depending on the ring width, separation and luminance. Overall, the measured assimilation could not be accounted for by the joint contributions from wavelength-independent spread light and wavelength-dependent chromatic aberration. Spatial averaging of neural signals explained the assimilation in s reasonably well, but there were clear deviations from neural spatial averaging for the l direction. � 2004 Elsevier Ltd. All rights reserved.

Patterned backgrounds that selectively stimulate the S-cones cause conspicuous color shifts. These shifts are accounted for by an S-cone antagonistic (+S/-S) center-surround receptive field [Monnier, P., & Shevell, S. K. (2004). Chromatic induction from S-cone patterns. Vision Research, 44, 849-856]. The present study tested two additional implications of the S-cone receptive field for color shifts: (1) proportionality of the shifts with respect to S-cone contrast within the inducing pattern and (2) bandpass selectivity of the shifts with respect to the spatial frequency of the inducing pattern. Measurements showed that the magnitude of the color shift was linear with S-cone contrast and that the largest color shift was observed with inducing patterns at an intermediate spatial frequency. These results further support an S-cone spatially antagonistic receptive field as the neural substrate mediating the large color shifts from S-cone patterns.

Chromatic assimilation is a shift toward the color of nearby light. Several studies conclude that a neural process contributes to assimilation but the neural locus remains in question. Some studies posit a peripheral process, such as retinal receptive-field organization, while others claim the neural mechanism follows depth perception, figure/ground segregation, or perceptual grouping. The experiments here tested whether assimilation depends on a neural process that follows stereoscopic depth perception. By introducing binocular disparity, the test field judged in color was made to appear in a different depth plane than the light that induced assimilation. The chromaticity and spatial frequency of the inducing light, and the chromaticity of the test light, were varied. Chromatic assimilation was found with all inducing-light sizes and chromaticities, but the magnitude of assimilation did not depend on the perceived relative depth planes of the test and inducing fields. We found no evidence to support the view that chromatic assimilation depends on a neural process that follows binocular combination of the two eyes' signals.

Intense scrutiny has been focused on whether chromatic stimuli contribute to motion perception. The present study considers a related but different question: how does motion affect chromatic detection? Detection thresholds were measured for a disk that underwent a brief (13.3 ms) chromatic change in the L/(L+M) chromatic direction. The disk's presentation sequence and speed (0–16 deg/s) were manipulated. In the coherent presentation sequence, the disk moved smoothly along a circular path centered on the fixation point. In the random presentation sequence, the disk appeared randomly at positions along the circular path. In both types of sequences, the disk underwent a brief chromatic change midway through the temporal presentation sequence. Threshold was elevated in the coherent condition compared to the random condition, and threshold decreased with an increase in speed. The threshold elevation observed in the coherent presentation sequence can be accounted for by temporal integration. The decrease in threshold with an increase in speed can be accounted for by spatial integration. The results, therefore, can be explained by spatiotemporal integration, without invoking a neural mechanism specialized for motion.

Theories of induction propose that the brightness of a test patch within a complex surround is explained by local contrast or by integrating contrasts from various regions within the surround, weighted inversely with the distance from the test. Results here corroborate that brightness induction from a patterned background depends on both contiguous and non-contiguous surrounding light, but the measurements were inconsistent with any linear integration of contrast at edges within the scene. In some conditions, assimilation rather than contrast to contiguous surrounding light was observed, depending on the luminance of the light in non-contiguous regions. This finding implies that brightness induction from patterned backgrounds depends on neural processes that can cause contrast and/or assimilation, depending on the luminance relation between contiguous and non-contiguous regions. A model in the literature postulating that the influence of a non-contiguous edge is regulated by the amount of contrast at the contiguous edge can accommodate brightness induced by these patterned backgrounds.

Binocular disparity is an aspect of natural viewing. This research investigates whether disparity affects surface color perception. Achromatic settings were obtained and compared for two stereograms of a scene with specular reflections, one stereogram with binocular disparity and one without it (cyclopean view). Binocular disparity was found to improve color constancy. Next, the geometry of specular highlights, which is distorted without binocular disparity, was specifically examined. Measurements compared color constancy with specular reflections that were either normal (with stereo disparity) or distorted (cyclopean view of the specularities). No significant change in constancy was found due to the geometrical distortion of specular highlights that occurs without stereo disparity, suggesting that constancy depends on other features of the percept affected by disparity. The results are discussed in terms of illuminant estimation in surface color perception.

Chromatic induction is the change in appearance of one light caused by a second, nearby light. We measured chromatic induction in a central test viewed within an inducing field that was varied in only short-wavelength-sensitive (S) cone stimulation. The observer matched the appearance of the central test by adjusting the chromaticity of a haploscopically presented comparison field, seen by the other eye on a dark background. When the central test weakly stimulated S cones, the S-cone level in the surround caused little change in the color appearance of the test. When the central test substantially stimulated S cones, on the other hand, the appearance of the center showed S-cone contrast: raising the level of S in the surround reduced the level of S set to match the central test. Further, a surround that weakly stimulated S cones raised the matching S-cone level above that required without a surround (dark-adapted condition). These results cannot be explained by S-cone sensitivity loss or by a two-process model of adaptation. A cortical mechanism is proposed to mediate S-cone antagonism.