Seeing gray through the ON and OFF pathways

Asymmetries between achromatic increments and decrements: Perceptual scales and discrimination thresholds

The perceptual response to achromatic incremental (A+) and decremental (A-) visual stimuli is known to be asymmetrical, due most likely to differences between ON and OFF channels. In the current study, we further investigated this asymmetry psychophysically. In Experiment 1, maximum likelihood difference scaling (MLDS) was used to estimate separately observers' perceptual scales for A+ and A-. In Experiment 2, observers performed two spatial alternative forced choice (2SAFC) pedestal discrimination on multiple pedestal contrast levels, using all combinations of A+ and A- pedestals and tests. Both experiments showed the well-known asymmetry. The perceptual scale curves of A+ follow a modified Naka-Rushton equation, whereas those of A- follow a cubic function. Correspondingly, the discrimination thresholds for the A+ pedestal increased monotonically with pedestal contrast, whereas the thresholds of the A- pedestal first increased as the pedestal contrast increased, then decreased as the contrast became higher. We propose a model that links the results of the two experiments, in which the pedestal discrimination threshold is inversely related to the derivative of the perceptual scale curve. Our findings generally agree with Whittle's previous findings (Whittle, 1986, 1992), which also included strong asymmetry between A+ and A-. We suggest that the perception of achromatic balanced incremental and decremental (bipolar) stimuli, such as gratings or flicker, might be dominated by one polarity due to this asymmetry under some conditions.

Large enhancement of simultaneous color contrast by white flanking contours

Simultaneous color contrast and assimilation are mutually opposing effects on color appearance, and their magnitude depends on spatial context. The Monnier-Shevell illusion induces a large color shift by a synergy of simultaneous assimilation and contrast using the alternating color of proximal and distant surrounds. The illusion induces a prominent effect along the blue-yellow color axis, but a subtle effect along the orthogonal color axis. In this study, we report an illusion generated by an extremely thin gray line on a cyan background that appears reddish when the line is flanked by thin white contours. We quantified the color appearance of the gray line in a color matching experiment and found that the color shift of the gray line with white contours induced large color shifts. It is also known that luminance contrast between a center and its surrounds affects the magnitude of simultaneous color contrast. However, our color contrast effects were larger for a dark line rather than for a pale line. In contrast, the perceived color shift of the line without the contours increased as the luminance of the gray line increased, supporting the known effect of Kirschmann's third law. These results indicate that Kirschmann's third law fails to explain the perceived color shift of our illusion, even after accounting for optical factors like aberrations. Observed color shifts could be explained by an augmented synergy theory based on intensity space, rather than chromaticity.

Interocular ND filter suppression: Eccentricity and luminance polarity effects

The depth and extent of interocular suppression were measured in binocularly normal observers who unilaterally adapted to neutral density (ND) filters (0, 1.5, 2, and 3 ND). Suppression was measured by dichoptically matching sectors of a ring presented to the adapted eye to a fixed contrast contiguous ring presented to the non-adapted eye. Other rings of alternating polarity were viewed binocularly. Rings were defined by luminance (L), luminance with added dynamic binary luminance noise (LM), and contrast modulating the same noise (CM). Interocular suppression depth increased with increasing ND, nearing significance (p = 0.058) for 1.5 ND. For L and LM stimuli, suppression depth across eccentricity (±12° visual field) differed for luminance increment (white) versus luminance decrement (black) stimuli, potentially confounding eccentricity results. Suppression for increment-only (white) luminance stimuli was steeper centrally and extended across the visual field, but was deeper for L than for LM stimuli. Suppression for decrement-only (black) luminance stimuli revealed only central suppression. Suppression was deeper with CM than LM stimuli, suggesting that CM stimuli are extracted in areas receiving predominantly binocular input which may be more sensitive to binocular disruption. Increment (white) luminance stimuli demonstrate deeper interocular suppression in the periphery than decrement (black) stimuli, so they are more sensitive to changes in peripheral suppression. Asymmetry of suppression in the periphery for opposite polarity luminance stimuli may be due to interocular receptive field size mismatch as a result of dark adaptation separately affecting ON and OFF pathways. Clinically, measurement of suppression with CM stimuli may provide the best information about post-combination binocularity.

Mechanisms contributing to increment threshold and decrement threshold spectral sensitivities

The shape of the human spectral sensitivity function depends on how it is measured. In the increment threshold (IT) technique, sensitivity is typically measured as the inverse of threshold for detection of increments of monochromatic light presented for relatively long durations on achromatic pedestals. Spectral sensitivity functions derived from IT techniques have long been used to reveal contribution from opponent color channels. Although IT functions have been studied extensively, little attention has been given to functions derived from decrement thresholds (DT), partly due to technical challenges of producing appropriate stimuli. Comparison of IT and DT spectral sensitivities may be of interest because there are known asymmetries in the visual system between on- and off-pathways and between increment and decrement responses within these pathways. Consequently, spectral sensitivity functions obtained using DT measures may reveal a different complement of contributing mechanisms than those that produce IT functions. We report here that IT and DT derived spectral sensitivities were essentially identical over much of the visible spectrum. However, decrement sensitivity was slightly greater than increment sensitivity in the shorter wavelengths at modest light levels. This difference was not present at higher light levels, implicating rod pathways as a possible source of the difference. In sum, it appears that under conditions shown to reveal strong contribution from opponent mechanisms, decrement functions are either 1) determined by a similar complement of spectrally opponent mechanisms as those that define increment spectral sensitivities or 2) that the present conditions are insensitive to underlying asymmetries.

The functional asymmetry of ON and OFF channels in the perception of contrast

To fully understand the relationship between perception and single neural responses, one should take into consideration the early stages of sensory processing. Few studies, however, have directly examined the neural underpinning of visual perception in the lateral geniculate nucleus (LGN), only one synapse away from the retina. In this study we recorded from LGN parvocellular (P) ON-center and OFF-center neurons while monkeys either passively viewed or actively detected a full range of contrasts. We found that OFF neurons were more sensitive in detecting negative contrasts than ON neurons were in detecting positive contrasts. Also, OFF neurons had higher spontaneous activities, higher peak response amplitudes, and were more sustained than ON neurons in their contrast responses. Puzzlingly, OFF neurons failed to show any significant correlations with the monkeys' perceptual choices, despite their greater contrast sensitivities. If, however, choice probabilities were calculated from interspike intervals instead of spike counts (thus taking into account the higher firing rates of OFF neurons), OFF neurons but not ON neurons were significantly correlated with behavioral choices. Taken together, these results demonstrate in awake, behaving animals that: 1) the ON and OFF pathways do not simply mirror each other in their functionality but instead carry qualitatively different types of information, and 2) the responses of ON and OFF neurons can be correlated with perceptual choices even in the absence of physical stimuli and interneuronal correlations.

Black-white asymmetry in visual perception

With eleven different types of stimuli that exercise a wide gamut of spatial and temporal visual processes, negative perturbations from mean luminance are found to be typically 25% more effective visually than positive perturbations of the same magnitude (range 8-67%). In Experiment 12, the magnitude of the black-white asymmetry is shown to be a saturating function of stimulus contrast. Experiment 13 shows black-white asymmetry primarily involves a nonlinearity in the visual representation of decrements. Black-white asymmetry in early visual processing produces even-harmonic distortion frequencies in all ordinary stimuli and in illusions such as the perceived asymmetry of optically perfect sine wave gratings. In stimuli intended to stimulate exclusively second-order processing in which motion or shape are defined not by luminance differences but by differences in texture contrast, the black-white asymmetry typically generates artifactual luminance (first-order) motion and shape components. Because black-white asymmetry pervades psychophysical and neurophysiological procedures that utilize spatial or temporal variations of luminance, it frequently needs to be considered in the design and evaluation of experiments that involve visual stimuli. Simple procedures to compensate for black-white asymmetry are proposed.

Aging of human short-wave cone pathways

The retinal image is sampled concurrently, and largely independently, by three physiologically and anatomically distinct pathways, each with separate ON and OFF subdivisions. The retinal circuitry giving rise to an ON pathway receiving input from the short-wave-sensitive (S) cones is well understood, but the S-cone OFF circuitry is more controversial. Here, we characterize the temporal properties of putative S-cone ON and OFF pathways in younger and older observers by measuring thresholds for stimuli that produce increases or decreases in S-cone stimulation, while the middle- and long-wave-sensitive cones are unmodulated. We characterize the data in terms of an impulse response function, the theoretical response to a flash of infinitely short duration, from which the response to any temporally varying stimulus may be predicted. Results show that the S-cone response to increments is faster than to decrements, but this difference is significantly greater for older individuals. The impulse response function amplitudes for increment and decrement responses are highly correlated across individuals, whereas the timing is not. This strongly suggests that the amplitude is controlled by neural circuitry that is common to S-cone ON and OFF responses (photoreceptors), whereas the timing is controlled by separate postreceptoral pathways. The slower response of the putative OFF pathway is ascribed to different retinal circuitry, possibly attributable to a sign-inverting amacrine cell not present in the ON pathway. It is significant that this pathway is affected selectively in the elderly by becoming slower, whereas the temporal properties of the S-cone ON response are stable across the life span of an individual.

Age-related changes in contrast gain related to the M and P pathways

Neural contributions to the age-related reduction in spatial vision are incontrovertible. Whether there are differential age-related changes in the magnocellular (M) and parvocellular (P) pathways across the life span has not been tested extensively. We studied psychophysically the contrast gain signature of the M and P pathways for 13 younger and 13 older observers. Two separate paradigms thought to separate the M and P pathways based on their contrast gain (J. Pokorny & V. C. Smith, 1997) signature were used. A four-square array was presented as an increment or decrement on a background of 115 Td for 35 ms, with one test square presented at a slightly higher or lower retinal illumination. Using a four-alternative forced-choice procedure, the observer's task was to choose the unique square. The two paradigms differed only in the pretrial adaptation and inter-stimulus array. Data were fitted with models of contrast discrimination derived from the unique contrast gain signatures. The fitted models indicate a change in the discrimination functions with age for both the M and P pathways, revealing a shift in the contrast gain slope. Results indicate that both M and P pathways undergo age-related changes, but functional losses appear greater for the P pathway under the conditions tested.

Evaluation of early state of cyanopsia with subjective color settings immediately after cataract removal surgery

We evaluated cyanopsia by means of achromatic-point settings at several time points started from the day before intraocular lens (IOL) implantation for cataract removal surgery. We intensively measured the initial drift in color appearance; we started the measurement less than 30 min after eyepatch removal, and the measurement continued for several weeks. The shifts were mainly observed in the direction of color space that selectively varies short-wavelength-sensitive cone (S-cone) responses. The time constant of shifts in color appearance was estimated at the initial stage of cyanopsia by fitting exponential curves. The result of fitting suggests that color appearance is recalibrated during cyanopsia by some neural mechanisms with a time constant of several hours. It also became clear that the migration of the achromatic point becomes slower within approximately 12 h after eyepatch removal.

The ON–OFF dichotomy in visual processing: From receptors to perception

Vision scientists long ago pointed to black and white as separate sensations and saw confirmation in the fact that in the absence of light, one perceives the visual field as gray against which the negative after-image of a bright light appeared blacker. The first recordings from optic nerve fibers in vertebrates revealed ON and OFF signals, later associated with separate streams, arising already at the synapse between receptors and bipolar cells. These can be identified anatomically and physiologically and remain distinct all the way to the lateral geniculate nucleus, whose fibers form the input to the primary visual cortex. The dichotomy has been probed by electroretinography and analyzed by means of pharmacological agents and dysfunction due to genetic causes. The bi- rather than a unidirectional nature of the retinal output has advantages in allowing small signals to remain prominent over a greater dynamic range. The two streams innervate cortical neurons in a push-pull manner, generating receptive fields with spatial sensitivity profiles featuring ON and OFF subregions. Manifestations of the dichotomy appear in a variety of simple visual discriminations where there are often profound threshold differences in patterns with same polarity as compared with mixed contrast-polarity components. But even at levels in which the spatial, contrast and color attributes have already been securely established and black and white elements participate equally, a categorical difference between blackness and whiteness of a percept persists. It is an opponency, akin to the ones in the color domain, derived from the original ON and OFF signals and subsequently bound with the other attributes to yield a feature's unitary percept.

Information processing in the primate retina: circuitry and coding

The function of any neural circuit is governed by connectivity of neurons in the circuit and the computations performed by the neurons. Recent research on retinal function has substantially advanced understanding in both areas. First, visual information is transmitted to the brain by at least 17 distinct retinal ganglion cell types defined by characteristic morphology, light response properties, and central projections. These findings provide a much more accurate view of the parallel visual pathways emanating from the retina than do previous models, and they highlight the importance of identifying distinct cell types and their connectivity in other neural circuits. Second, encoding of visual information involves significant temporal structure and interactions in the spike trains of retinal neurons. The functional importance of this structure is revealed by computational analysis of encoding and decoding, an approach that may be applicable to understanding the function of other neural circuits.

Spatial and temporal properties of cone signals in alert macaque primary visual cortex

Neurons in the lateral geniculate nucleus cannot perform the spatial color calculations necessary for color contrast and color constancy. Under neutral-adapting conditions, we mapped the cone inputs (L, M, and S) to 83 cone-opponent cells representing the central visual field of the next stage of visual processing, primary visual cortex (V1), to determine how the color signals are spatially transformed. Cone-opponent cells, constituting approximately 10% of V1 cells, formed two populations, red-green (L vs M; 66 of 83) and blue-yellow (S vs L+M; 17 of 83). Many cone-opponent cells (48 of 83) were double-opponent, with circular receptive-field centers and crescent-shaped surrounds (0.63 degree offset) that had opposite chromatic tuning to the centers and a time-to-peak 11 ms later than the centers. The remaining cone-opponent cells were either spatially opponent in only one cone system (20 of 83) or lacked spatial opponency (15 of 83). Cells lacking spatial opponency had smaller receptive fields (0.5-0.7 degrees) than spatial-opponent cell centers (approximately 1 degree). We found that red-green cells received S-cone input, which aligned with M input, and, unlike blue-yellow cells, red-green cells gave push-pull responses: receptive-field centers of red-ON cells were excited by both L increments (bright red) and M decrements (dark red) and were suppressed by both L decrements (dark green) and M increments (bright green). Excitatory responses to decrements were slightly larger than to increments, which may account for the lower detection and discrimination thresholds of decrements shown psychophysically. By virtue of their specialized receptive fields, the neurons described here spatially transform the cone signals and represent the first stage in the visual system at which spatially opponent color calculations are made.

Bayesian model of human color constancy

Vision is difficult because images are ambiguous about the structure of the world. For object color, the ambiguity arises because the same object reflects a different spectrum to the eye under different illuminations. Human vision typically does a good job of resolving this ambiguity-an ability known as color constancy. The past 20 years have seen an explosion of work on color constancy, with advances in both experimental methods and computational algorithms. Here, we connect these two lines of research by developing a quantitative model of human color constancy. The model includes an explicit link between psychophysical data and illuminant estimates obtained via a Bayesian algorithm. The model is fit to the data through a parameterization of the prior distribution of illuminant spectral properties. The fit to the data is good, and the derived prior provides a succinct description of human performance.

The loci of achromatic points in a real environment under various illuminant chromaticities

Under colored illumination, the achromatic point (the point in the chromaticity diagram seen as colorless) shifts toward the chromaticity of the illuminant. This investigation measured the loci of achromatic points for various intensities of a test field presented in a real rather than a simulated environment, lit by illuminants of various chromaticities. The achromatic point varied markedly with the intensity level of the test field: for dim test fields it was close to the surround chromaticity, but for high luminance test fields it was almost invariant with the surround chromaticity. The varying achromatic settings imply a variation in the relative effectiveness of the different cone types, but this variation originates in the postreceptoral system rather than at the photoreceptors themselves: flicker photometric sensitivity was almost independent of the illuminant in all cases. Nor does the variation take the simple form of a sensitivity-scaling coefficient; such a model can not predict the observed dependence of the achromatic setting on test intensity. The data could, however, be modeled with a scheme in which the log of the relative cone weight implicit in the achromatic setting depends almost linearly on (1) the log of the relative cone excitation by the illuminant and (2) the log of the test field intensity.

The peculiar nature of simultaneous colour contrast in uniform surrounds

We present evidence from asymmetric colour matching experiments which strongly suggests that uniform surrounds evoke induction effects of a very peculiar nature, not representative of colour induction effects in variegated surrounds. Given the widespread use of uniform surrounds in studies of colour vision, this finding is of interest in relation to a number of current research issues, such as contrast coding of colour, functionally equivalent surrounds and colour constancy. A framework that systematises the seemingly complex colour appearance changes induced by uniform surrounds is presented and its implications are discussed.

Macaque retina contains an S-cone OFF midget pathway

Psychophysical results suggest that the primate visual system is equally sensitive to both the onset and offset of short-wavelength light and that these responses are carried by separate pathways. However, physiological studies of cells in the retina and lateral geniculate nucleus find far fewer OFF-center than ON-center cells whose receptive-field centers are driven by short-wavelength-sensitive (S) cones. To determine whether S cones contact ON and OFF midget bipolar cells as well as (ON) "blue-cone bipolar" cells (Mariani, 1984), we examined 118 contiguous cone terminals and their bipolar cells in electron micrographs of serial sections from macaque foveal retina. Five widely spaced cone terminals do not contact ON midget bipolar cells. These five cone terminals contact the dendrites of "blue-cone bipolar" cells instead, showing that they are the terminals of S cones. These S-cone terminals are smaller and contain more synaptic ribbons than other terminals. Like neighboring cones, each S cone contacts its own OFF midget bipolar cell via triad-associated (flat) synaptic contacts. Moreover, each S-cone OFF midget bipolar cell has a synaptic terminal in the outer half of the inner plexiform layer, where it contacts an OFF midget ganglion cell.

Color vision: how the cortex represents color

Our understanding of how we see color has benefited from the long tradition of visual psychophysics. More recently, models and methods from psychophysics are guiding modern neuroimaging experiments on color vision. Combining the two techniques can lead to discoveries that neither can make alone.

Chromatic light adaptation measured using functional magnetic resonance imaging

Sensitivity changes, beginning at the first stages of visual transduction, permit neurons with modest dynamic range to respond to contrast variations across an enormous range of mean illumination. We have used functional magnetic resonance imaging (fMRI) to investigate how these sensitivity changes are controlled within the visual pathways. We measured responses in human visual area V1 to a constant-amplitude, contrast-reversing probe presented on a range of mean backgrounds. We found that signals from probes initiated in the L and M cones were affected by backgrounds that changed the mean absorption rates in the L and M cones, but not by background changes seen only by the S cones. Similarly, signals from S cone-initiated probes were altered by background changes in the S cones, but not by background changes in the L and M cones. Performance in psychophysical tests under similar conditions closely mirrored the changes in V1 fMRI signals. We compare our data with simulations of the visual pathway from photon catch rates to cortical blood-oxygen level-dependent signals and show that the quantitative fMRI signals are consistent with a simple model of mean-field adaptation based on Naka-Rushton (Naka and Rushton, 1966) adaptation mechanisms within cone photoreceptor classes.

Saccadic eye movements modulate visual responses in the lateral geniculate nucleus

We studied the effects of saccadic eye movements on visual signaling in the primate lateral geniculate nucleus (LGN), the earliest stage of central visual processing. Visual responses were probed with spatially uniform flickering stimuli, so that retinal processing was uninfluenced by eye movements. Nonetheless, saccades had diverse effects, altering not only response strength but also the temporal and chromatic properties of the receptive field. Of these changes, the most prominent was a biphasic modulation of response strength, weak suppression followed by strong enhancement. Saccadic modulation was widespread, and affected both of the major processing streams in the LGN. Our results demonstrate that during natural viewing, thalamic response properties can vary dramatically, even over the course of a single fixation.

Color appearance of spatial pattern: the role of increments and decrements

In a number of recent adaptational studies evidence for a different processing of incremental and decremental cone signals has been reported. The present study examined whether such asymmetries occur in spatial pattern as well. Subjects set color matches between a uniform, 2 degrees matching box and bars within squarewave patterns. The squarewaves varied in spatial frequency, color direction, and contrast. For all three cone signals the asymmetric matches showed clear evidence for increment-decrement asymmetries: Although both incremental and decremental matches scaled roughly linearly with pattern contrast, in general, the scalings for the two types of color signals differed. This difference in scaling increased with spatial frequency, thus leading to an increase in the size of the increment-decrement asymmetry with spatial frequency. The matches were well described by means of two-stage models, consisting of a color transformation in the first stage and a pattern-dependent scaling in the second stage. Analyses based on these pattern-color separable models suggest that the asymmetries are mediated mainly through a white-black mechanism and much less, if at all, through a red-green and yellow-blue mechanism.

Second-order statistics of colour codes modulate transformations that effectuate varying degrees of scene invariance and illumination invariance

We argue, from an ethology-inspired perspective, that the internal concepts 'surface colours' and 'illumination colours' are part of the data format of two different representational primitives. Thus, the internal concept of 'colour' is not a unitary one but rather refers to two different types of 'data structure', each with its own proprietary types of parameters and relations. The relation of these representational structures is modulated by a class of parameterised transformations whose effects are mirrored in the idealised computational achievements of illumination invariance of colour codes, on the one hand, and scene invariance, on the other hand. Because the same characteristics of a light array reaching the eye can be physically produced in many different ways, the visual system, then, has to make an 'inference' whether a chromatic deviation of the space-averaged colour codes from the neutral point is due to a 'non-normal', ie chromatic, illumination or due to an imbalanced spectral reflectance composition. We provide evidence that the visual system uses second-order statistics of chromatic codes of a single view of a scene in order to modulate corresponding transformations. In our experiments we used centre surround configurations with inhomogeneous surrounds given by a random structure of overlapping circles, referred to as Seurat configurations. Each family of surrounds has a fixed space-average of colour codes, but differs with respect to the covariance matrix of colour codes of pixels that defines the chromatic variance along some chromatic axis and the covariance between luminance and chromatic channels. We found that dominant wavelengths of red-green equilibrium settings of the infield exhibited a stable and strong dependence on the chromatic variance of the surround. High variances resulted in a tendency towards 'scene invariance', low variances in a tendency towards 'illumination invariance' of the infield.

Surface-illuminant ambiguity and color constancy: effects of scene complexity and depth cues

Two experiments were conducted to study how scene complexity and cues to depth affect human color constancy. Specifically, two levels of scene complexity were compared. The low-complexity scene contained two walls with the same surface reflectance and a test patch which provided no information about the illuminant. In addition to the surfaces visible in the low-complexity scene, the high-complexity scene contained two rectangular solid objects and 24 paper samples with diverse surface reflectances. Observers viewed illuminated objects in an experimental chamber and adjusted the test patch until it appeared achromatic. Achromatic settings made tinder two different illuminants were used to compute an index that quantified the degree of constancy. Two experiments were conducted: one in which observers viewed the stimuli directly, and one in which they viewed the scenes through an optical system that reduced cues to depth. In each experiment, constancy was assessed for two conditions. In the valid-cue condition, many cues provided valid information about the illuminant change. In the invalid-cue condition, some image cues provided invalid information. Four broad conclusions are drawn from the data: (a) constancy is generally better in the valid-cue condition than in the invalid-cue condition: (b) for the stimulus configuration used, increasing image complexity has little effect in the valid-cue condition but leads to increased constancy in the invalid-cue condition; (c) for the stimulus configuration used, reducing cues to depth has little effect for either constancy condition: and (d) there is moderate individual variation in the degree of constancy exhibited, particularly in the degree to which the complexity manipulation affects performance.

Adaptation to temporal contrast in primate and salamander retina

Visual adaptation to temporal contrast (intensity modulation of a spatially uniform, randomly flickering stimulus) was examined in simultaneously recorded ensembles of retinal ganglion cells (RGCs) in tiger salamander and macaque monkey retina. Slow contrast adaptation similar to that recently discovered in salamander and rabbit retina was observed in monkey retina. A novel method was developed to quantify the effect of temporal contrast on steady-state sensitivity and kinetics of light responses, separately from nonlinearities that would otherwise significantly contaminate estimates of sensitivity. Increases in stimulus contrast progressively and reversibly attenuated and sped light responses in both salamander and monkey RGCs, indicating that a portion of the contrast adaptation observed in visual cortex originates in the retina. The effect of adaptation on sensitivity and kinetics differed in simultaneously recorded populations of ON and OFF cells. In salamander, adaptation affected the sensitivity of OFF cells more than ON cells. In monkey, adaptation affected the sensitivity of ON cells more than OFF cells. In both species, adaptation sped the light responses of OFF cells more than ON cells. Functionally defined subclasses of ON and OFF cells also exhibited asymmetric adaptation. These findings indicate that contrast adaptation differs in parallel retinal circuits that convey distinct visual signals to the brain.

Increments and decrements in color constancy

The present study examines whether increment-decrement asymmetries reported in a number of recent center-surround situations occur in more complex images as well. Subjects saw the CRT simulation of a whole uniformly illuminated array of foreground surfaces presented against a large background surface and, for a number of different viewing contexts, made achromatic settings over a wide range of luminance values. Three results emerged. First, subjects' achromatic loci did not fall on a single straight line in color space but rather fell on two separate lines intersecting at some point in this space. Second, the intersection points were not identical to but dependent largely on background color and showed only small effects of foreground colors. Third, cone signals that were decremental relative to the intersection point were more responsive to illuminant changes than cone signals that were incremental, the latter additionally showing some variation with foreground colors. The results are interpreted in terms of increment-decrement asymmetries. They suggest that these asymmetries occur in more complex images as well.

Control of chromatic adaptation: signals from separate cone classes interact

Match stimuli presented on one side of a contextual image were adjusted to have the same appearance as test stimuli presented on the other side. Both full color and isochromatic contextual images were used. Contextual image pairs were constructed that had identical S-cone image planes, while their L- and M-cone image planes differed. The data show that the S-cone component of the matches depends on the L- and M-cone planes of the contextual image. This dependence means that matches obtained using isochromatic stimuli (lightness matches) may not be used directly to predict full color matches.

Predicting color from gray: the relationship between achromatic adjustment and asymmetric matching

Achromatic adjustment has been used widely to study color context effects. In the achromatic adjustment procedure, an observer adjusts a test stimulus until it appears black, gray, or white. By its nature, achromatic adjustment directly measures the effect of context only for stimuli that appear gray. We present achromatic loci measured in two contexts and asymmetric color matches measured across the same two contexts. The results indicate that achromatic adjustments, together with a gain-control model, may be used to make accurate predictions of the chromaticity of asymmetric matches. Thus measurements of the effect of context for test stimuli that appear gray may be used to predict the effect of context for stimuli that appear colored. The experiments also indicate that accurate prediction depends on ensuring that observers use similar fixational strategies for the two judgments.

Trichromatic opponent color classification

Stimuli varying in intensity and chromaticity, presented on numerous backgrounds, were classified into red/green, blue/yellow and white/black opponent color categories. These measurements revealed the shapes of the boundaries that separate opponent colors in three-dimensional color space. Opponent color classification boundaries were generally not planar, but their shapes could be summarized by a piecewise linear model in which increment and decrement color signals are combined with different weights at two stages to produce opponent color sensations. The effect of background light on classification was largely explained by separate gain changes in increment and decrement cone signals.

Mechanisms of color constancy under nearly natural viewing

Color constancy is our ability to perceive constant surface colors despite changes in illumination. Although color constancy has been studied extensively, its mechanisms are still largely unknown. Three classic hypotheses are that constancy is mediated by local adaptation, by adaptation to the spatial mean of the image, or by adaptation to the most intense image region. We measure color constancy under nearly natural viewing conditions, by using a design that allows us to test these three hypotheses directly. By suitable stimulus manipulation, we are able to titrate the degree of constancy between 11% and 83%, indicating that we have achieved good laboratory control. Our results rule out all three classic hypotheses and thus suggest that there is more to constancy than can be easily explained by the action of simple visual mechanisms.

Lower-level visual processing and models of light adaptation

Before there was a formal discipline of psychology, there were attempts to understand the relationship between visual perception and retinal physiology. Today, there is still uncertainty about the extent to which even very basic behavioral data (called here candidates for lower-level processing) can be predicted based upon retinal processing. Here, a general framework is proposed for developing models of lower-level processing. It is argued that our knowledge of ganglion cell function and retinal mechanisms has advanced to the point where a model of lower-level processing should include a testable model of ganglion cell function. This model of ganglion cell function, combined with minimal assumptions about the role of the visual cortex, forms a model of lower-level processing. Basic behavioral and physiological descriptions of light adaptation are reviewed, and recent attempts to model lower-level processing are discussed.

Color constancy in the nearly natural image. 2. Achromatic loci

Most empirical work on color constancy is based on simple laboratory models of natural viewing conditions. These typically consist of spots seen against uniform backgrounds or computer simulations of flat surfaces seen under spatially uniform illumination. In this study measurements were made under more natural viewing conditions. Observers used a projection colorimeter to adjust the appearance of a test patch until it appeared achromatic. Observers made such achromatic settings under a variety of illuminants and when the test surface was viewed against a number of different backgrounds. An analysis of the achromatic settings reveals that observers show good color constancy when the illumination is varied. Changing the background surface against which the test patch is seen, on the other hand, has a relatively small effect on the achromatic loci. The results thus indicate that constancy is not achieved by a simple comparison between the test surface and its local surround.

Color constancy in the nearly natural image. I. Asymmetric matches

Most empirical work on color constancy is based on simple laboratory models of natural viewing conditions. These typically consist of spots seen against uniform backgrounds or computer simulations of flat surfaces seen under spatially uniform illumination. We report measurements made under more natural viewing conditions. The experiments were conducted in a room where the illumination was under computer control. Observers used a projection colorimeter to set asymmetric color matches across a spatial illumination gradient. Observers' matches can be described by either of two simple models. One model posits gain control in one-specific pathways. This diagonal model may be linked to ideas about the action of early visual mechanisms. The other model posits that the observer estimates and corrects for changes in illumination but does so imperfectly. This equivalent illuminant model provides a link between human performance and computational models of color constancy.

Dynamics of adaptation at high luminances: adaptation is faster after luminance decrements than after luminance increments

As is well known, dark adaptation in the human visual system is much slower than is recovery from darkness. We show that at high photopic luminances the situation is exactly opposite. First, we study detection thresholds for a small light flash, at various delays from decrement and increment steps in background luminance. Light adaptation is nearly complete within 100 ms after luminance decrements but takes much longer after luminance increments. Second, we compare sensitivity after equally visible pulses or steps in the adaptation luminance and find that detectability is initially the same but recovers much faster for pulses than for increment steps. This suggests that, whereas any residual threshold elevation after a step shows the incomplete luminance adaptation, the initial threshold elevation is caused by the temporal contrast of the background steps and pulses. This hypothesis is further substantiated in a third experiment, whereby we show that manipulating the contrast of a transition between luminances affects only the initial part of the threshold curve, and not later stages.

Color appearance of mixture gratings

We have examined how color appearance varies with spatial pattern. Subjects set color-matches between a uniform, 2 deg matching field and bars within squarewave patterns (1,2 and 4 c/deg) or the superposition of these squarewaves. The matches were set using squarewaves and squarewave mixtures with many different colors and contrasts. The color-matches satisfied the basic properties of a linear system to within a tolerance of twice the precision of repeated matches. Matches satisfied contrast-homogeneity: the contrast of the matching field was proportional to the contrast of the squarewave pattern or the mixture of squarewave patterns. Matches also satisfied pattern-superposition: if a bar in one squarewave matched one uniform field, and a bar in a second squarewave matched a second uniform field, the superposition of the two squarewave bars matched the superposition of the uniform matching fields. Matches are predicted by a model in which the color at a location is predicted by the responses of three linear, pattern-color separable mechanisms. As the individual mechanisms are pattern-color separable, meaningful pattern and color-responsivity functions can be estimated for each of the mechanisms. The estimated color-responsivity functions, based only on asymmetric color-matches, have an opponent-colors organization.