Dichoptic difference thresholds for chromatic stimuli

Surround masking reveals binocular adding and differencing channels

Recent studies suggest that binocular adding S+ and differencing S- channels play an important role in binocular vision. To test for such a role in the context of binocular contrast detection and binocular summation, we employed a surround masking paradigm consisting of a central target disk surrounded by a mask annulus. All stimuli were horizontally oriented 0.5c/d sinusoidal gratings. Correlated stimuli were identical in interocular spatial phase while anticorrelated stimuli were opposite in interocular spatial phase. There were four target conditions: monocular left eye, monocular right eye, binocular correlated and binocular anticorrelated, and three surround mask conditions: no surround, binocularly correlated and binocularly anticorrelated. We observed consistent elevation of detection thresholds for monocular and binocular targets across the two binocular surround mask conditions. In addition, we found an interaction between the type of surround and the type of binocular target: both detection and summation were relatively enhanced by surround masks and targets with opposite interocular phase relationships and reduced by surround masks and targets with the same interocular phase relationships. The data were reasonably well accounted for by a model of binocular combination termed MAX (S+S-), in which the decision variable is the probability summation of modeled S+ and S- channel responses, with a free parameter determining the relative gains of the two channels. Our results support the existence of two channels involved in binocular combination, S+ and S-, whose relative gains are adjustable by surround context.

Peripheral vision is mainly for looking rather than seeing

Vision includes looking and seeing. Looking, mainly via gaze shifts, selects a fraction of visual input information for passage through the brain's information bottleneck. The selected input is placed within the attentional spotlight, typically in the central visual field. Seeing decodes, i.e., recognizes and discriminates, the selected inputs. Hence, peripheral vision should be mainly devoted to looking, in particular, deciding where to shift the gaze. Looking is often guided exogenously by a saliency map created by the primary visual cortex (V1), and can be effective with no seeing and limited awareness. In seeing, peripheral vision not only suffers from poor spatial resolution, but is also subject to crowding and is more vulnerable to illusions by misleading, ambiguous, and impoverished visual inputs. Central vision, mainly for seeing, enjoys the top-down feedback that aids seeing in light of the bottleneck which is hypothesized to starts from V1 to higher areas. This feedback queries for additional information from lower visual cortical areas such as V1 for ongoing recognition. Peripheral vision is deficient in this feedback according to the Central-peripheral Dichotomy (CPD) theory. The saccades engendered by peripheral vision allows looking to combine with seeing to give human observers the impression of seeing the whole scene clearly despite inattentional blindness.

Binocular luster elicited by isoluminant chromatic stimuli relies on mechanisms similar to those in the achromatic case

The phenomenon of binocular luster can be evoked by simple dichoptic center-surround stimuli showing a luminance contrast difference between the eyes. Previous findings support the idea that this phenomenon is mediated by a low-level conflict mechanism that integrates the monocular signals from different types of contrast detector cells. Also, isoluminant stimuli with different chromatic contrasts between eyes can trigger sensations of luster. Here, we investigate whether the lustrous impression in such purely chromatic stimuli depends on interocular contrast differences and in particular on interocular contrast polarity pairings in a similar way as in the achromatic case. In our experiments, we measured the magnitude of the lustrous response using a series of isoluminant dichoptic center-ring-surround stimuli with varying ring width whose chromatic properties were varied along the red-green and blue-yellow cardinal directions. The trends in the data were very similar to those of our former study with achromatic stimuli, indicating similar mechanisms in both cases. The empirical luster data could also be predicted fairly well by a chromatic version of our interocular conflict model (with overall R2 values between 0.577 and 0.639), for which two different receptive field models were used, simulating the behavior of color-sensitive double-opponent cells in V1.

Investigating critical brain area for EEG-based binocular color fusion and rivalry with EEGNet

Introduction

Binocular color fusion and rivalry are two specific phenomena in binocular vision, which could be used as experimental tools to study how the brain processes conflicting information. There is a lack of objective evaluation indexes to distinguish the fusion or rivalry for dichoptic color.

Methods

This paper introduced EEGNet to construct an EEG-based model for binocular color fusion and rivalry classification. We developed an EEG dataset from 10 subjects.

Results

By dividing the EEG data from five different brain areas to train the corresponding models, experimental results showed that: (1) the brain area represented by the back area had a large difference on EEG signals, the accuracy of model reached the highest of 81.98%, and more channels decreased the model performance; (2) there was a large effect of inter-subject variability, and the EEG-based recognition is still a very challenge across subjects; and (3) the statistics of EEG data are relatively stationary at different time for the same individual, the EEG-based recognition is highly reproducible for an individual.

Discussion

The critical channels for EEG-based binocular color fusion and rivalry could be meaningful for developing the brain computer interfaces (BCIs) based on color-related visual evoked potential (CVEP).

Detection of vertical interocular phase disparities using luster as cue

Interocular disparities in contrast generate an impression of binocular luster, providing a cue for their detection. Disparities in the carrier spatial phase of horizontally oriented Gabor patches also generate an impression of luster, so the question arises as to whether it is the disparities in local contrast that accompany the phase disparities that give rise to the luster. We examined this idea by comparing the detection of interocular spatial phase disparities with that of interocular contrast disparities in Gabor patches, in the latter case that differed in overall contrast rather than phase between the eyes. When bandwidth was held constant and Gabor spatial frequency was varied, the detection of phase and contrast disparities followed a similar pattern. However, when spatial frequency was fixed and Gabor envelope standard deviation (and hence number of modulation cycles) was varied, thresholds for detecting phase disparities followed a U-shaped function of Gabor standard deviation, whereas thresholds for contrast disparities, following an initial decline, were more-or-less constant as a function of Gabor standard deviation. After reviewing a number of possible explanations for the U-shape found with phase disparities, we suggest that the likely cause is binocular sensory fusion, the strength of which increases with the number of modulation cycles. Binocular sensory fusion would operate to reduce phase but not contrast disparities, thus selectively elevating phase disparity thresholds.

A simple model of binocular luster

The dichoptic combination of simple center–surround stimuli showing a contrast difference between eyes can trigger a lustrous impression in the fused percept, particularly when the contrast polarities in the two input images are of opposite sign. Recent developments suggest that the phenomenon of binocular luster results from a neural conflict between ON and OFF visual pathways at an early binocular level. Support for this idea was found in a previous study in which the empirical luster judgments strongly correlated with the predictions of an interocular conflict model which was based on such ON–OFF pairings. However, our original model could not account for the fact that weaker lustrous sensations can also be evoked by stimuli showing contrast polarities of same sign between eyes. In the present study we present an improved model that also includes ON–ON and OFF–OFF pairings. The predictive power of this model was tested in a series of four experiments, using a total of about 500 different center–ring–surround configurations as test stimuli. We found that, overall, our modified version accounts for more than 80% of the variance in the empirical luster judgments and that the former problems could be largely resolved. Our results further suggest a nonlinear transducer function for the binocular conflict signals.

Binocular luster - A review

Binocular luster is a visual phenomenon that can be elicited by dichoptic stimuli showing an interocular difference in color or luminance contrast. For instance, when the two eyes are presented with simple center-surround stimuli in which the center patch in one eye is brighter and in the other eye darker than the common surround, the center patch in the fused percept assumes a lustrous appearance reminiscent of metal or graphite. Soon after the discovery of this phenomenon in the mid-19th century, it was intensively studied and several explanations were proposed. After this initial phase, however, research interest waned significantly. Stimulated by new insights into related phenomena and the underlying physiological mechanisms, the last 20 years have seen an increase in research activity in this field, which has considerably expanded our understanding of binocular luster. In this paper, we provide a detailed review of research on binocular luster over the past 170 years. We present and discuss the existing findings in a number of separate sections, dealing with 1) the phenomenology of binocular luster, 2) different theories that have been proposed, 3) several factors influencing the lustrous impression, 4) the relationship between binocular luster and binocular rivalry, 5) the current understanding of its neural basis, and 6) potential applications based on binocular luster.

Detection of binocular chromatic fusion limit for opposite colors

When the input colors of the left and right eyes are different from one another, binocular rivalry may occur. According to Hering theory, opponent colors would have the most significant tendency for rivalry. However, binocular color fusion still occurs under the condition that each eye's opponent chromatic responses do not exceed a specific chromatic fusion limit (CFL). This paper detects the binocular chromatic fusion limit for opposite colors within a conventional 3D display color gamut. We conducted a psychophysical experiment to quantitatively measure the binocular chromatic fusion limit on four opposite color directions in the CIELAB color space. Due to color inconsistency between eyes may affect the binocular color fusion, the experiment was divided into two sessions by swapping stimulation colors of left and right eyes. There were 5 subjects and they each experienced 320 trials. By analyzing the results, we used ellipses to quantify the chromatic fusion limits for opposing colors. The average semi-major axis of the ellipses is 27.55 Δ E a b∗, and the average semi-minor axis is 16.98 Δ E a b∗. We observed that the chromatic fusion limit varies with the opposite color direction: the CFL on RedBlue-GreenYellow direction is greater than that on Red-Green direction, the latter being greater than that on Yellow-Blue direction and the CFL on RedYellow-GreenBlue direction is smallest. Furthermore, we suggested that the chromatic fusion limit is independent of the distribution of cells, and there is no significant change in the fusion ellipse boundaries after swapping left and right eye colors.

The role of contrast polarities in binocular luster: Low-level and high-level processes

The binocular fusion of two center-surround configurations, where one center is brighter, the other darker than the common surround, leads to a strong impression of luster in the central patch. Without reversed contrast polarities of the center patches, this impression is much weaker or even absent. However, we observed that in the latter case the perceived luster can be considerably enhanced by enclosing both centers with a thin ring of fixed luminance. Compared to the standard stimulus, this center-ring-surround configuration shows much less binocular rivalry and the luster has also a different, more glass-like material quality. In a psychophysical experiment, we examined how the magnitude of the lustrous response depends on the width of the ring, both in stimuli with reversed and consistent contrast polarities. It has been proposed that binocular luster results from a neuronal conflict between ON and OFF visual pathways. To test this hypothesis with respect to our data, we developed a simple model to estimate the amount of interocular conflict resulting from a given binocular stimulus pair and applied it to all stimuli used in the experiment. We found strong correlations between the interocular conflict measure and the strength of luster observed in the experiment, suggesting that a common low-level mechanism determines the magnitude of the lustrous response. Regarding the differences in the perceived material quality of the lustrous impressions, we discuss evidence indicating that high-level processes are involved that promote the visual system's interpretation of the ring-stimuli as a certain depth-segmented 3D scene.

Interocular difference thresholds are mediated by binocular differencing, not summing, channels

Patterns in the two eyes' views that are not identical in hue or contrast often elicit an impression of luster, providing a cue for discriminating them from perfectly matched patterns. Here we attempt to determine the mechanisms for detecting interocular differences in luminance contrast, in particular in relation to the possible contributions of binocular differencing and binocular summing channels. Test patterns were horizontally oriented multi-spatial-frequency luminance-grating patterns subject to variable amounts of interocular difference in grating phase, resulting in varying degrees of local interocular contrast difference. Two types of experiment were conducted. In the first, subjects discriminated between a pedestal with an interocular difference that ranged upward from zero (i.e., binocularly correlated) and a test pattern that contained a bigger interocular difference. In the second type of experiment, subjects discriminated between a pedestal with an interocular difference that ranged downward from a maximum (i.e., binocularly anticorrelated) and a test pattern that contained smaller interocular difference. The two types of task could be mediated by a binocular differencing and a binocular summing channel, respectively. However, we found that the results from both experiments were well described by a simpler model in which a single, linear binocular differencing channel is followed by a standard nonlinear transducer that is expansive for small signals but strongly compressive for large ones. Possible reasons for the lack of involvement of a binocular summing channel are discussed in the context of a model that incorporates the responses of both monocular and binocular channels.

At least two distinct mechanisms control binocular luster, rivalry, and perceived rotation with contrast and average luminance disparities

When one views a square-wave grating and dichoptically changes the average luminance or contrast of the monocular images, at least three perceptual phenomena might occur. These are the Venetian blind effect, or a perceived rotation of the bars around individual vertical axes; binocular luster, or a perceived shimmering; and binocular rivalry, or an alternating perception between the views of the two eyes. Perception of luster and rivalry occur when the "light bars" in the grating dichoptically straddle the background luminance (one eye's image has a higher luminance than the background and the other eye's image has a lower luminance than the background), with little impact from the "dark bars." Perception of rotation, on the other hand, is related to average luminance or contrast disparity, independent of whether or not the "light bars" straddle the background luminance. The patterns for perceived rotation versus binocular luster and binocular rivalry suggest at least two separate mechanisms in the visual system for processing luminance and contrast information over and above their differing physiological states suggested by their different appearances. While luster and rivalry depend directly on the relation between stimuli and the background, perceived rotation depends on the magnitude of the luminance or contrast disparity, as described by the generalized difference model.

Differences in Stereoscopic Luster Evoked by Static and Dynamic Stimuli

We compared the classic static stereoscopic luster phenomenon with a recently described dynamic variant ("counter modulation") to investigate whether they are related to the same or different processes. In the experiments, we presented pairs of center-surround stimuli haploscopically and measured the effect of the contrast between center colors on perceived luster. The center colors were either static or temporally modulated. In addition, we examined five color conditions (one achromatic, two equiluminant, and two mixed conditions) and three background conditions that influence the channel-wise polarities of the contrast of the two centers to the common surround. The results for static and dynamic stimuli differed in several ways, suggesting that they depend on different mechanisms: Compared with the static version, in dynamic stimuli, luster was perceived at markedly lower contrasts, did not depend on the sign of the contrast polarities, and appeared more steady. However, both phenomena seem also similar in some respects: In both cases, equiluminant stimuli led to lustrous impressions that were considerably less strong than those evoked by stimuli containing luminance variation, and the strength of the perceived luster was generally boosted with reversed contrast polarities.

Adaptation to interocular difference

Patterns in the two eyes' views that are not identical in hue or contrast often elicit an impression of luster, providing a cue for discriminating them from perfectly matched patterns. Here we ask whether the mechanism for detecting interocular differences (IDs) is adaptable. Our stimuli were horizontally oriented multispatial-frequency grating patterns that could be subject to varying degrees of ID through the introduction of interocular phase differences in the grating components. Subjects adapted to patterns that were either correlated, uncorrelated, monocular (one eye only), or anticorrelated. Following adaptation, thresholds for detecting IDs were measured using a staircase procedure. It was found that ID thresholds were elevated following adaptation to uncorrelated, monocular, and anticorrelated but not correlated patterns. Threshold elevation was found to be maximal when the orientations of the adaptor and test gratings were the same, and when their spatial frequencies were similar. The results support the existence of a specialized mechanism for detecting IDs, the most likely candidate being the binocular differencing channel proposed in previous studies.

Interocular correlation sensitivity and its relationship with stereopsis

Stereoscopic vision uses the disparity between the images received by the two eyes to derive three-dimensional estimates. Here, we were interested in providing a measure of the strength of binocular vision alternate to disparity processing. In particular, we wanted to assess the spatial dependence of sensitivity to detect interocular correlation (IOC). Thus we designed dichoptic stimuli composed of bandpass textures whose IOC is sinusoidally modulated at different correlation frequencies and compared sensitivity to these stimuli to that of analogous stimuli modulated in disparity. We observed that the IOC sensitivity is low pass/band pass and increases with stimulus duration and contrast in a similar way to that of disparity sensitivity. IOC sensitivity is only weakly, though significantly, correlated with disparity sensitivity in the population. It could provide an alternate measure of binocular sensitivity.

Perceptual resolution of color for multiple chromatically ambiguous objects

In a classic study, Kovács 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 Kovács 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 Kovács 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.

Ocularity Feature Contrast Attracts Attention Exogenously

An eye-of-origin singleton, e.g., a bar shown to the left eye among many other bars shown to the right eye, can capture attention and gaze exogenously or reflexively, even when it appears identical to other visual input items in the scene and when the eye-of-origin feature is irrelevant to the observer's task. Defining saliency as the strength of exogenous attraction to attention, we say that this eye-of-origin singleton, or its visual location, is salient. Defining the ocularity of a visual input item as the relative difference between its left-eye input and its right-eye input, this paper shows the general case that an ocularity singleton is also salient. For example, a binocular input item among monocular input items is salient, so is a left-eye-dominant input item (e.g., a bar with a higher input contrast to the left eye than to the right eye) among right-eye-dominant items. Saliency by unique input ocularity is analogous to saliency by unique input colour (e.g., a red item among green ones), as colour is determined by the relative difference(s) between visual inputs to different photoreceptor cones. Just as a smaller colour difference between a colour singleton and background items makes this singleton less salient, so does a smaller ocularity difference between an ocularity singleton and background items. While a salient colour difference is highly visible, a salient ocularity difference is often perceptually invisible in some cases and discouraging gaze shifts towards it in other cases, making its behavioural manifestation not as apparent. Saliency by ocularity contrast provides another support to the idea that the primary visual cortex creates a bottom-up saliency map to guide attention exogenously.

Efficient Coding Theory Predicts a Tilt Aftereffect from Viewing Untilted Patterns

The brain is bombarded with a continuous stream of sensory information, but biological limitations on the data-transmission rate require this information to be encoded very efficiently [1]. Li and Atick [2] proposed that the two eyes' signals are coded efficiently in the brain using mutually decorrelated binocular summation and differencing channels; when a channel is strongly stimulated by the visual input, such that sensory noise is negligible, the channel should undergo temporary desensitization (known as adaptation). To date, the evidence for this theory has been limited [3, 4], and the binocular differencing channel is missing from many models of binocular integration [5-10]. Li and Atick's theory makes the remarkable prediction that perceived direction of tilt (clockwise or counterclockwise) of a test pattern can be controlled by pre-exposing observers to visual adaptation patterns that are untilted or even have no orientation signal. Here, we confirm this prediction. Each test pattern consisted of different images presented to the two eyes such that the binocular summation and difference signals were tilted in opposite directions, to give ambiguous information about tilt; by selectively desensitizing one or other of the binocular channels using untilted or non-oriented binocular adaptation patterns, we controlled the perceived tilt of the test pattern. Our results provide compelling evidence that the brain contains binocular summation and differencing channels that adapt to the prevailing binocular statistics.

Stereo chromatic contrast sensitivity model to blue-yellow gratings

As a fundamental metric of human visual system (HVS), contrast sensitivity function (CSF) is typically measured by sinusoidal gratings at the detection of thresholds for psychophysically defined cardinal channels: luminance, red-green, and blue-yellow. Chromatic CSF, which is a quick and valid index to measure human visual performance and various retinal diseases in two-dimensional (2D) space, can not be directly applied into the measurement of human stereo visual performance. And no existing perception model considers the influence of chromatic CSF of inclined planes on depth perception in three-dimensional (3D) space. The main aim of this research is to extend traditional chromatic contrast sensitivity characteristics to 3D space and build a model applicable in 3D space, for example, strengthening stereo quality of 3D images. This research also attempts to build a vision model or method to check human visual characteristics of stereo blindness. In this paper, CRT screen was clockwise and anti-clockwise rotated respectively to form the inclined planes. Four inclined planes were selected to investigate human chromatic vision in 3D space and contrast threshold of each inclined plane was measured with 18 observers. Stimuli were isoluminant blue-yellow sinusoidal gratings. Horizontal spatial frequencies ranged from 0.05 to 5 c/d. Contrast sensitivity was calculated as the inverse function of the pooled cone contrast threshold. According to the relationship between spatial frequency of inclined plane and horizontal spatial frequency, the chromatic contrast sensitivity characteristics in 3D space have been modeled based on the experimental data. The results show that the proposed model can well predicted human chromatic contrast sensitivity characteristics in 3D space.

Empirical evidence for unique hues?

Red, green, blue, yellow, and white have been distinguished from other hues as unique. We present results from two experiments that undermine existing behavioral evidence to separate the unique hues from other colors. In Experiment 1 we used hue scaling, which has often been used to support the existence of unique hues, but has never been attempted with a set of non-unique primaries. Subjects were assigned to one of two experimental conditions. In the "unique" condition, they rated the proportions of red, yellow, blue, and green that they perceived in each of a series of test stimuli. In the "intermediate" condition, they rated the proportions of teal, purple, orange, and lime. We found, surprisingly, that results from the two conditions were largely equivalent. In Experiment 2, we investigated the effect of instruction on subjects' settings of unique hues. We found that altering the color terms given in the instructions to include intermediate hues led to significant shifts in the hue that subjects identified as unique. The results of both experiments question subjects' abilities to identify certain hues as unique.

The categorisation of non-categorical colours: a novel paradigm in colour perception

In this paper, we investigate a new paradigm for studying the development of the colour ‘signal’ by having observers discriminate and categorize the same set of controlled and calibrated cardinal coloured stimuli. Notably, in both tasks, each observer was free to decide whether two pairs of colors were the same or belonged to the same category. The use of the same stimulus set for both tasks provides, we argue, an incremental behavioural measure of colour processing from detection through discrimination to categorisation. The measured data spaces are different for the two tasks, and furthermore the categorisation data is unique to each observer. In addition, we develop a model which assumes that the principal difference between the tasks is the degree of similarity between the stimuli which has different constraints for the categorisation task compared to the discrimination task. This approach not only makes sense of the current (and associated) data but links the processes of discrimination and categorisation in a novel way and, by implication, expands upon the previous research linking categorisation to other tasks not limited to colour perception.