Binocular Color Matching
In the context of this article, “binocular color matching” refers to color vision with two eyes, focusing on the differences between when two eyes are used to view colors (i.e., binocular or dichoptic color) and when only one is used (i.e., monocular color), as well as phenomena (largely) unique to binocular vision such as fusion, rivalry, and stereopsis.
Binocular color fusion: How similar do the chromatic properties of visual signals in each eye have to be in order to be perceived as one?
Binocular color rivalry: How different do the chromatic properties of visual signals in each eye have to be in order to evoke binocular rivalry, which is an alternation in visibility of the visual information going to each eye?
Color and stereopsis: Stereopsis is the perception of depth that is obtained by analyzing the differences between the images going to each eye. To what extent is this perception of depth affected by the chromatic properties of visual information?
Binocular color summation: The processing of visual signals is affected by whether or not a scene is viewed with one or two eyes: to what extent do the chromatic properties of visual signals influence this?
Binocular color appearance: Do colors look different when viewed with one eye or two?
Note that the field of binocular vision and stereopsis has been thoroughly reviewed by Prof. Brian Rogers of the University of Oxford, UK, and Prof. Ian Howard, late of York University, Canada , and for all but the most recent studies, the interested reader should refer to them for further information.
Binocular Color Fusion
A different approach was taken by Malkoc and Kingdom , who, following on from an earlier study by Yoonessi and Kingdom , measured dichoptic color difference thresholds (DCDTs), which are the thresholds for detecting a color difference between the two eyes, coinciding with the detection of a peculiar phenomenon called binocular luster. They found that these thresholds were higher than those for detecting color differences between two stimuli when they were presented side by side, although lower than those required to provoke binocular rivalry (see below). Malkoc and Kingdom  also found that these thresholds were best predicted by the perceived color difference between the two stimuli, rather than any considerations based on cardinal or unique hue mechanisms. Kingdom and Libenson  specifically investigated the processing of interocular differences in saturation (or color contrast). They found that the appearance of the mixture obtained crucially depended on the relative amounts of luminance and chromatic contrast. With purely chromatic differences between the two eyes (i.e., lights with the same hue but different saturations), the more saturated/higher contrast stimulus dominated the percept, but the presence of a luminance pedestal forced the colors to blend and therefore reduced the saturation of the resultant. Kingdom and Libenson  argue from these results that the appearance of a dichoptic color mixture depends on whether or not the brain interprets the information from the two eyes as coming from the same object or not, a phenomenon which they term the “object commonality hypothesis.”
Binocular Color Rivalry
As rivalry is, to a certain extent, the obverse of fusion, most of the issues pertinent to this theme have already been discussed above. It is certainly true that the presentation of saturated red stimuli to one eye and saturated green to the other is often almost paradigmatic in studies where the aim is to evoke binocular rivalry (see, e.g., ; also, again, ). O’Shea and Williams  demonstrated that S-cone-isolating stimuli could induce binocular rivalry, suggesting that rivalry was not solely confined to luminance or red-green chromatic pathways. A detailed study of the wavelength sensitivity of binocular rivalry was performed by Sagawa . Note that rivalry does not occur for briefly presented stimuli. In these situations the dichoptic stimuli tend to superimpose, although are still distinguishable from monocularly superimposed stimuli . There is some evidence that the chromatic system is more affected by binocular rivalry suppression than the achromatic system [11, 24]. Mullen et al.  found that the visibility of chromatic grating stimuli presented to one eye was affected by the presence of luminance stimuli in the other, suggesting that, when stimuli differ between the two eyes, the suppression of one eye by the other is independent of whether the stimulus contains color or luminance contrast (which is not the case for monocular vision, or when the stimulus is the same in both eyes, when the suppression of one stimulus by another, masking, stimulus is more selective).
Color and Stereopsis
There are two issues which have dominated research on color and stereopsis. The first is whether or not there is actually a functioning chromatic stereopsis mechanism. The second is whether or not chromatic information helps to solve the stereo correspondence problem. In both cases the arguments have been similar to those in color motion perception. Historically, a good way of testing whether or not a purely chromatic mechanism exists has been to test performance at isoluminance, when the visual patterns in question are theoretically defined solely by chromatic contrast, without any luminance contrast being present. The earliest study of stereopsis at isoluminance suggested that stereoscopic depth perception was very weak and possibly nonexistent at red-green isoluminance . Subsequent studies provided conflicting results, depending on the precise stimulus characteristics . Around the early 1990s, the extreme views on this issue were represented by two studies: Livingstone and Hubel  and Scharff and Geisler . Livingstone and Hubel  claimed that all previous demonstrations of intact stereoscopic depth perception were artifactual, and down to the technical difficulties associated with removing all luminance information from the stimulus. Their view that stereopsis was essentially “color blind” was consistent with their theories on the parallel processing of visual information by magnocellular- and parvocellular-mediated visual pathways, with stereopsis mediated by the “color-blind” magnocellular stream. Scharff and Geisler , on the other hand, claimed that their careful calibration of the stimulus, accounting for all potential luminance artifacts, not only demonstrated that stereopsis was possible at isoluminance but that it was as good as it could be, given the limits on obtainable color contrast due to the overlap in spectral sensitivities of long- and medium-wavelength-sensitive cones. In a series of studies published between 1994 and 2002, Simmons and Kingdom updated this view of the status of stereopsis at isoluminance to demonstrate that there exists a rudimentary chromatic stereopsis mechanism which is less contrast sensitive and has a more limited disparity range, poorer stereoacuity, and poorer ability to encode a stereoscopically defined shape than its luminance counterpart ([15, 17, 33, 34, 35]; reviewed in ). While this view was challenged in a study by Krauskopf and Forte , who argued, similarly to Livingstone and Hubel , for a complete disappearance of stereopsis at isoluminance, their data could be partially explained by the presence of high spatial frequency luminance artifacts in their stimuli. Having said that, it is very difficult to control for these artifacts, especially given the high contrast sensitivity of luminance-based stereopsis [31, 37]. While most of the above studies were carried out at red-green isoluminance, Grinberg and Williams  demonstrated that stereopsis is also possible at “blue-yellow” isoluminance.
Binocular Color Summation
To what extent does viewing a chromatic stimulus with two eyes affect its visibility or the performance obtained with it? The scientific literature presents a somewhat confusing picture, with Anstis and Rogers  claiming that “two eyes are worse than one,” Tyler and Cavanagh  claiming that “two eyes” are “as sensitive as one,” and Simmons  finding evidence for binocular contrast summation close to “full summation,” when contrast is effectively linearly added between the eyes, and so a binocular stimulus is twice as detectable as when presented to both eyes rather than one (this would be “two eyes are twice as good as one”!). The conditions under which dichoptic color mixing is obtained have already been outlined above. It therefore suffices to say that the extent of binocular summation for a given chromatic stimulus will depend on the task and the precise chromatic and spatiotemporal parameters of the stimulus.
Binocular Color Appearance
Another aspect of binocular color appearance that has recently received attention is how presenting different colors to the two eyes can be an easy way to simulate surface glossiness in stereoscopic visual displays, without the need to create a detailed reflectance model .
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