The cone/horizontal cell network: a possible site for color constancy

Color vision is spectrally opponent, suggesting that spectrally opponent neurons, such as the horizontal cells in fish and turtle retinae, play a prominent role in color discrimination. In the accompanying paper (Kraaij et al., 1998), it was shown that the output signal of the horizontal cell system to the cones is not at all spectrally opponent. Therefore, a role for the spectrally opponent horizontal cells in color discrimination seems unlikely. In this paper, we propose that the horizontal cells play a prominent role in color constancy and simultaneous color contrast instead of in color discrimination. We have formulated a model of the cone/horizontal cell network based on measurements of the action spectra of the cones and of the feedback signal of the horizontal cell system to the various cone types. The key feature of the model is (1) that feedback is spectrally and spatially very broad and (2) that the gain of the cone synapse strongly depends on the feedback strength. This makes the synaptic gain of the cones strongly dependent on the spectral composition of the surround. Our model, which incorporates many physiological details of the outer retina, displays a behavior that can be interpreted as color constancy and simultaneous color contrast. We propose that the horizontal cell network modulates the cone synaptic gains such that the ratios of the cone outputs become almost invariant with the spectral composition of the global illumination. Therefore, color constancy appears to be coded in the retina.     

Color appearance: neutral surrounds and spatial contrast

The experimental data in this paper show that chromatic bars presented in alternation with equiluminant neutral-appearing bars are seen as more saturated than the same chromaticity presented as a uniform rectangle. This effect was diminished but not eliminated when test and match stimuli were presented within a slightly dimmer neutral surround. The test stimulus was a 2 x 5 degrees rectangle with a 0.7 cpd square wave grating composed of alternating equiluminant chromatic test bars and neutral bars. Asymmetric matching was used to match the test bar appearance to a uniform 2 x 5 degrees comparison rectangle. Test and comparison stimuli were presented to separate eyes in a haploscope and appeared flanking a central fixation target. Data were collected with test and inducing stimuli on the cardinal axes of color space. Test bars separated by neutral bars appeared more saturated than the comparison rectangle for both the l- and s-axis directions. Manipulation of excitation on one cardinal axis did not affect the appearance matches made for the other cardinal axis.     

Time course of inhibition in color-response and word-response versions of the Stroop task.

Translation models of the Stroop effect predict inhibition when the relevant stimulus type does not match the response type, but a lack of inhibition when it matches. All 4 combinations of relevant stimulus type (color or word) and response type (color or word) were evaluated at several stimulus onset asynchronies (SOAs) in a button-pressing version of the Stroop task to assess this prediction. Inhibition was greatest when the relevant stimulus type did not match the response type. However, in contrast to predictions of translation models, color and word responses produced different patterns of inhibition and facilitation over SOA, implying differences in the word-to-color and color-to-word translation mechanisms, and inhibition was obtained in both of the color-response tasks. A modification of the translation model is proposed that incorporates a translation mechanism and accommodates special characteristics of word processing. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Interactions between color and word processing in a flanker task.

To examine interactions between color and word attributes, participants responded, either manually or vocally, to a central target (color patch or word) flanked by a Stroop stimulus. Color and word attributes of the flanker affected both vocal and manual responding to color patches. Color and word flankers also affected manual responding to word targets, but only word flanker affected vocal responding to word targets. These results are not consistent with models (e.g., translational models) of Stroop tasks, which posit that interactions between colors and words occur only when vocal responding is required. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Color constancy under natural and artificial illumination

Color constancy was studied under conditions simulating either natural or extremely artificial illumination. Four test illuminants were used: two broadband phases of daylight (correlated color temperatures 4000 and 25,000 K) and two spectrally impoverished metamers of these lights, each consisting of only two wavelengths. A computer controlled color monitor was used for reproducing the chromaticities and luminances of an array of Munsell color samples rendered under these illuminants. An asymmetric haploscopic matching paradigm was used in which the same stimulus pattern, either illuminated by one of the test illuminants, or by a standard broadband daylight (D65), was alternately presented to the left and right eye. Subjects adjusted the RGB settings of the samples seen under D65 (match condition), to match the appearance of the color samples seen under the test illuminant. The results show the expected failure of color constancy under two-wavelengths illumination, and approximate color constancy under natural illumination. Quantitative predictions of the results were made on the basis of two different models, a computational model for recovering surface reflectance, and a model that assumes the color response to be determined by cone-specific contrast and absolute level of stimulation (Lucassen & Walraven, 1993). The latter model was found to provide somewhat more accurate predictions, under all illuminant conditions.     

The guidance of eye movements during active visual search

An analysis of monkey eye movements in classic conjunction and feature search tasks was made. The task was to find and fixate a target in an array of stimuli. Saccades targeted stimuli accurately (red and green bars, 1.25 x 0.25 degrees), landing most of the time within 1.0 degree of the stimulus center and rarely in blank areas far from any stimulus. Monkeys used target color, but not orientation, to selectively guide search. Saccades moved the point of fixation on the average just beyond the area that could be examined by focal attentive mechanisms during the current fixation, as described in a previous paper (Motter BC, Belky EJ. The zone of focal attention during active visual search. Vis Res 1998;38:1007-22). This distance scales with the density of relevant stimuli in the scene. The saccade targeting data suggest that the locations of items of a particular color, but apparently not of a particular orientation, are available outside the region of focal attention. Color feature selection can apparently block the distracting effects of color unique distractors during search.     

Metacontrol of hemispheric function in human split-brain patients

Four commissurotomy patients were tested for ability to match tachistoscopically presented stimuli with pictures in free vision, according to either structural appearance or functional/conceptual category. Patients were given ambiguous, structural, or functional instructions on any given run to trials with simultaneous double stimulus input to the two cerebral hemispheres. With ambiguous instructions, appearance and function matches were performed by the right and left hemispheres, respectively. When instructions were specific, appearance instructions tended to elicit appearance matches and right-hemisphere control. When function instructions were given, left-hemisphere control and function matches tended to be elicited. In three of the four patients, however, there was a significant number of dissociations between controlling hemisphere and strategy of matching.     

The role of attention in feature detection and conjunction discrimination: an electrophysiological analysis

An electrophysiological probe technique was used to ascertain whether the same attentional mechanisms are employed for both the detection of simple visual features and the discrimination of conjunctions of features. Visual search arrays containing 14 grey items and 2 colored items were presented; one color was designated relevant for each trial block. Subjects were required to report the presence or absence of the relevant color (feature detection condition) or the shape of the relevantly-colored item (conjunction discrimination condition). Shortly after the onset of the search array, a task-irrelevant probe stimulus was flashed at the location of the relevant or irrelevant color and the event-related potential (ERP) produced by this stimulus was used to assess sensory processing at the probed location. Probes presented at the location of the relevant color were found to elicit enhanced ERP components and probes presented on the opposite side of the display from the relevant color elicited suppressed components. These effects were observed in both the detection and discrimination conditions, indicating that spatially restricted attentional processes are used for both the detection of simple features and the discrimination of conjunctions. However, one ERP component (the PI wave) exhibited these effects in the discrimination condition but not in the detection condition, indicating that conjunction discrimination utilizes additional attentional processes beyond those required for feature detection.     

The Simon effect occurs relative to the direction of an attention shift.

We investigated whether the Simon effect depends on the orienting of attention. In Experiment 1, participants were required to execute left-right discriminative responses to 2 patterns that were presented to the left or right of fixation. The 2 patterns were similar, and the discrimination was difficult. A letter at fixation signaled whether the current trial was a catch trial. The results showed a reversal of the Simon effect. That is, spatially noncorresponding responses were faster than spatially corresponding responses. In Experiment 2, the discrimination of the relevant stimulus attribute was easy. In Experiment 3, the discrimination of the relevant stimulus attribute was difficult, but the stimulus exposure time was long. In either experiment, the regular Simon effect was reinstated. In Experiment 4, the letter that signaled a catch trial appeared to the left or right of the imperative stimulus. The Simon effect occurred relative to the position of the letter. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

The spatial region of integration for visual symmetry detection

Symmetry is a complex image property that is exploited by a sufficiently wide range of species to indicate that it is detected using simple visual mechanisms. These mechanisms rely on measurements made close to the axis of symmetry. We investigated the size and shape of this integration region (IR) by measuring human detection of spatially band-pass symmetrical patches embedded in noise. Resistance to disruption of symmetry (in the form of random phase noise) improves with increasing patch size, and then asymptotes when the embedded region fills the IR. The size of the IR is shown to vary in inverse proportion to spatial frequency; i.e. symmetry detection exhibits scale invariance. The IR is shown to have rigid dimensions, elongated in the direction of the axis of symmetry, with an aspect ratio of ca. 2:1. These results are consistent with a central role for spatial filtering in symmetry detection.     

Lightness, brightness, and brightness contrast: 1. Illuminance variation

Changes of annulus luminance in traditional disk-and-annulus patterns are perceptually ambiguous; they could be either reflectance or illuminance changes. In more complicated patterns, apparent reflectances are less ambiguous, letting us place test and standard patches on surrounds perceived to be different grays. Our subjects matched the apparent amounts of light coming from the patches (brightnesses), their apparent reflectances (lightnesses), or the brightness differences between the patches and their surrounds (brightness contrasts). The three criteria produced quantitatively different results. Brightness contrasts matched when the patch/surround luminance ratio of the test was approximately equal to that of the standard. Lightness matches were illumination invariant but were not exact reflectance matches; the different surrounds of test and standard produced a small illumination-invariant error. This constant error was negligible for increments, but, for decrements, it was approximately 1.5 Munsell value steps. Brightness matches covaried substantially with illuminance.     

Discrimination of structure: II. Feature binding.

In 3 experiments, pigeons acquired a discrimination between patterns comprising the same features. Thus vertical green bars beside horizontal red bars might have signaled food, and horizontal green bars beside vertical red bars might have signaled no food. The solution of this discrimination can be explained by assuming each pattern is represented either by a template or by structural features that are sensitive to combinations of color and line orientation. The 1st explanation predicts subjects should react to a training pattern rotated 90° in the same way as the pattern on which it is based. The 2nd explanation predicts these patterns should be treated as if they signal opposing outcomes. The experiments confirmed the 2nd of these predictions. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Optimal modulation of a Brownian ratchet and enhanced sensitivity to a weak external force

We studied the dynamics of a Brownian particle moving in a spatially anisotropic potential acted on by multiplicative temporal modulations so that V(x,t) = g(t)U(x). Using the concept of the "thermodynamic action," we show that the class of modulation that maximizes the flow is a square-wave in time. We also show that adding a weak, homogenous force F in synergy with the square-wave modulation can cause particles of slightly different size to move in opposite directions. The synergetic change in velocity caused by F can be much greater than the drift velocity that would be caused by F alone.     

Contrast-invariant orientation tuning in cat visual cortex: thalamocortical input tuning and correlation-based intracortical connectivity

The origin of orientation selectivity in visual cortical responses is a central problem for understanding cerebral cortical circuitry. In cats, many experiments suggest that orientation selectivity arises from the arrangement of lateral geniculate nucleus (LGN) afferents to layer 4 simple cells. However, this explanation is not sufficient to account for the contrast invariance of orientation tuning. To understand contrast invariance, we first characterize the input to cat simple cells generated by the oriented arrangement of LGN afferents. We demonstrate that it has two components: a spatial-phase-specific component (i.e., one that depends on receptive field spatial phase), which is tuned for orientation, and a phase-nonspecific component, which is untuned. Both components grow with contrast. Second, we show that a correlation-based intracortical circuit, in which connectivity between cell pairs is determined by the correlation of their LGN inputs, is sufficient to achieve well tuned, contrast-invariant orientation tuning. This circuit generates both spatially opponent, "antiphase" inhibition ("push-pull"), and spatially matched, "same-phase" excitation. The inhibition, if sufficiently strong, suppresses the untuned input component and sharpens responses to the tuned component at all contrasts. The excitation amplifies tuned responses. This circuit agrees with experimental evidence showing spatial opponency between, and similar orientation tuning of, the excitatory and inhibitory inputs received by a simple cell. Orientation tuning is primarily input driven, accounting for the observed invariance of tuning width after removal of intracortical synaptic input, as well as for the dependence of orientation tuning on stimulus spatial frequency. The model differs from previous push-pull models in requiring dominant rather than balanced inhibition and in predicting that a population of layer 4 inhibitory neurons should respond in a contrast-dependent manner to stimuli of all orientations, although their tuning width may be similar to that of excitatory neurons. The model demonstrates that fundamental response properties of cortical layer 4 can be explained by circuitry expected to develop under correlation-based rules of synaptic plasticity, and shows how such circuitry allows the cortex to distinguish stimulus intensity from stimulus form.     

Quantitative analysis of triphasic (H3) horizontal cell-cone connectivity in the cyprinid fish (roach) retina

Although horizontal cells encode chromatic information by means of a variety of spectrally opponent light-evoked response patterns, their synaptic connections with the different spectral classes of cone are not completely understood. In the cyprinid fish retina, where a hierarchical set of interactions between horizontal cells and cone types has been proposed, a particular type of horizontal cell generates light-evoked triphasic (red-hyperpolarizing/green depolarizing/blue-hyperpolarizing) responses. In the present study, we have studied the cone connectivity of these cells by intracellular recording and staining in the roach retina. The horizontal cells were first identified electrophysiologically using spectral stimuli, and then stained intracellularly with horseradish peroxidase. Light microscopy revealed that the cells had consistent H3-like morphologies. At an ultrastructural level, these horizontal cells were deduced to contact selectively blue-sensitive cones. Within the cone pedicles, the majority (approximately 80%) of the contacts were "central" to synaptic ribbons. Some 50% of the "lateral" processes were large and engulfed cone cytoplasm. Spinules were present within the contacted pedicles but not upon the dendrites of the stained horizontal cells, although previous work had suggested that horseradish peroxidase would not interfere with spinule dynamics. The results are discussed in terms of existing modes of horizontal cell-cone connectivity in cyprinid fish retinae.     

Computerized simulation of color appearance for dichromats

We propose an algorithm that transforms a digitized color image so as to simulate for normal observers the appearance of the image for people who have dichromatic forms of color blindness. The dichromat's color confusions are deduced from colorimetry, and the residual hues in the transformed image are derived from the reports of unilateral dichromats described in the literature. We represent color stimuli as vectors in a three-dimensional LMS space, and the simulation algorithm is expressed in terms of transformations of this space. The algorithm replaces each stimulus by its projection onto a reduced stimulus surface. This surface is defined by a neutral axis and by the LMS locations of those monochromatic stimuli that are perceived as the same hue by normal trichromats and a given type of dichromat. These monochromatic stimuli were a yellow of 575 nm and a blue of 475 nm for the protan and deutan simulations, and a red of 660 nm and a blue-green of 485 nm for the tritan simulation. The operation of the algorithm is demonstrated with a mosaic of square color patches. A protanope and a deuteranope accepted the match between the original and the appropriate image, confirming that the reduction is colorimetrically accurate. Although we can never be certain of another's sensations, the simulation provides a means of quantifying and illustrating the residual color information available to dichromats in any digitized image.     

On the significance of temporally structured activity in the dorsal lateral geniculate nucleus (LGN)

Higher organisms perceive information about external or internal physical or chemical stimuli with specialized sensors that encode characteristics of that stimulus by a train of action potentials. Usually, the location and modality of the stimulus is represented by the location and specificity of the receptor and the intensity of the stimulus and its temporal modulation is thought to be encoded by the instantaneous firing rate. Recent studies have shown that, primarily in cortical structures, special features of a stimulus also are represented in the temporal pattern of spike activity. Typical attributes of this time structure are oscillatory patterns of activity and synchronous discharges in spatially distributed neurons that respond to inputs evoked by a coherent object. The origin and functional significance of this kind of activity is less clear. Cortical, subcortical and even very peripheral sources seem to be involved. Most of the relevant studies were devoted to the mammalian visual system and cortical findings on temporally structured activity were reviewed recently (Eckhorn, 1994, Progr. Brain Res., Vol. 102, pp. 405-426; Singer and Gray, 1995, Annu. Rev. Neurosci., Vol. 18, pp. 555-586). Therefore, this article is designed to give an overview, especially of those studies concerned with the temporal structure of visual activity in subcortical centers of the primary visual pathway, which are the retina and the dorsal lateral geniculate nucleus (LGN). We discuss the mechanisms that possibly contribute to the generation and modulation of the subcortical activity time structure and we try to relate to each other the subcortical and cortical patterns of sensory activity.     

The selectivity of motion-driven visual attention in infants

The onset of motion was used to study stimulus-driven visual attention in 14-week-olds. The movement of an object did not capture attention reflexively at 14 weeks of age. The attention-getting properties of a moving stimulus depended significantly on its color in combination with the colors of other objects in the visual field. Specifically, detection of a green moving target was masked in the presence of mixed red and green static objects. No such masking was observed when the moving target was red or when the green target moved in a visual field that was populated only with green objects. The same effect was observed to a lesser extent when the green bars were replaced with gray bars. The number of distractors in the visual field exerted an effect on the accuracy of detection only when their appearance in the visual field was coincident with the onset of target motion. Attention to motion at this age is not independent of the structure of the visual field; chromatic preferences play a role in how readily infants attend to a moving object. These effects may be mediated by a difficulty in disengaging attention (from distractors) or in suppressing attention to competing objects once attention is engaged on a target.     

Immediate colour constancy

Colour constancy is traditionally interpreted as the stable appearance of the colour of a surface despite changes in the spectral composition of the illumination. When colour constancy has been assessed quantitatively, however, by observers making matches between surfaces illuminated by different sources, its completeness has been found to be poor. An alternative operational approach to colour constancy may be taken which concentrates instead on detecting the underlying chromatic relationship between the parts of a surface under changes in the illuminant. Experimentally the observer's task was to determine whether a change in the appearance of a surface was due to a change in its reflecting properties or to a change in the incident light. Observers viewed computer simulations of a row of three Mondrian patterns of Munsell chips. The centre pattern was a reference pattern illuminated by a simulated, spatially uniform daylight; one of the outer patterns was identical but illuminated by a different daylight; and the other outer pattern was equivalent but not obtainable from the centre pattern by such a change in illuminant. Different patterns and different shifts in daylight were generated in each experimental trial. The task of the observer was to identify which of the outer patterns was the result of an illuminant change. Observers made reliable discriminations of the patterns with displays of durations from several seconds to less than 200 ms, and, for one observer, with displays of 1 ms. By these measures, human observers appear capable of colour constancy that is extremely rapid, and probably preattentive in origin.