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Empirical Evidence for Unique Hues? Empirical evidence for unique hues? Article (Accepted Version) Bosten, J M and Boehm, A E (2014) Empirical evidence for unique hues? Journal of the Optical Society of America A, 31 (4). A385-A393. ISSN 1084-7529 This version is available from Sussex Research Online: http://sro.sussex.ac.uk/id/eprint/52509/ This document is made available in accordance with publisher policies and may differ from the published version or from the version of record. If you wish to cite this item you are advised to consult the publisher’s version. Please see the URL above for details on accessing the published version. Copyright and reuse: Sussex Research Online is a digital repository of the research output of the University. Copyright and all moral rights to the version of the paper presented here belong to the individual author(s) and/or other copyright owners. To the extent reasonable and practicable, the material made available in SRO has been checked for eligibility before being made available. Copies of full text items generally can be reproduced, displayed or performed and given to third parties in any format or medium for personal research or study, educational, or not-for-profit purposes without prior permission or charge, provided that the authors, title and full bibliographic details are credited, a hyperlink and/or URL is given for the original metadata page and the content is not changed in any way. http://sro.sussex.ac.uk Empirical evidence for unique hues? Bosten JM1,2,* snd Boehm AE1,3 1Department of Psychology, UC San Diego, McGill Hall, La Jolla, CA 92093 2Department of Experimental Psychology, University of Cambridge, Downing Street, Cambridge, CB2 3EB, United Kingdom 3Vision Science Group, UC Berkeley, 380 Minor Hall, Berkeley, CA 94720-2020 *Corresponding author: [email protected] Received Month X, XXXX; revised Month X, XXXX; accepted Month X, XXXX; posted Month X, XXXX (Doc. ID XXXXX); published Month X, XXXX 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 find, surprisingly, that results from the two conditions are largely equivalent. In Experiment 2, we investigated the effect of instruction on subjects’ settings of unique hues. We find that altering the color terms given in the instructions to include intermediate hues leads to significant shifts in the hue that subjects identify as unique. The results of both experiments question subjects’ ability to identify certain hues as unique. © 2012 Optical Society of America OCIS codes: (330.1690) Color; (330.1720) Color vision; (330.5510) Psychophysics; (330.5020) Perception psychology http://dx.doi/org/10.1364/AO.99.099999 1. Introduction behavioral evidence to support this view has been hard to come by. One source of evidence that is often cited to justify the special Five hues: red, green, blue, yellow and white, have been given status of unique hues [11-13], is hue scaling. Hue scaling as a special status by color scientists as “unique.” The unique hues are method is based on sensory scaling of multiple stimulus said to be phenomenologically pure, while all other hues are said attributes introduced by Jameson and Hurvich [14]. The subject to be phenomenologically mixed, containing various proportions is presented with colored stimuli and asked to consider how of the five unique hues. much of various given “primary” colors the stimulus contains. Unique hues have a long history in psychology. It was the The subject assigns a numerical rating to each primary that belief that unique hues must have special status in color reflects his judgment of how much of that primary is contained in representation that led Hering [1] to propose red, green, blue and his perception of the stimulus that is presented to him on a yellow as the poles of his opponent mechanisms. Following the particular trial. The idea is that if a hue is unique, it will contain tradition established by Hering, and later by Jameson and one primary, but not any other. Hurvich [2], when color opponent cells were discovered in the Results from hue scaling experiments typically show that primate lateral geniculate nucleus, they were described as being there are four points around the hue circle where subjects report selective for red and green, and blue and yellow [3]. Though it is seeing only one primary. For example, at “unique” green, subjects now clear that the chromatic tuning of color-opponent report seeing green, but not blue, yellow or red. However, since mechanisms revealed psychophysically does not map on to the the primaries to which subjects are asked to assign ratings are Hering primaries [4], and that the color-sensitive cells in the typically themselves the unique hues, it is perhaps unsurprising lateral geniculate nucleus encode intermediate color directions [5- that ratings peak for red, green, blue and yellow in regions where 6], the conviction that unique hues must have prominence in neighboring hues approach the zero level. Red, yellow, green and neural color signals has led to persistence of the idea that color- blue are set apart roughly 90° around the hue circle, so it may be opponent cells encode the Hering primaries, and to a search for simply the perceptual distance between them that results in one higher-order color mechanisms that are selective for red, green, not being seen in another. blue and yellow [7–10]. Sternheim and Boynton [15] did hue scaling for monochromatic lights between 530 and 620 nm. They introduced A. Hue scaling orange as an available primary in some of their conditions, and Despite broad consensus among color scientists that red, concluded that they had evidence to separate yellow, green and green, blue and yellow should be considered unique, solid red, but not orange, as unique. In four conditions subjects were those that have been obtained by many other researchers. The given i) red, green and blue, ii) red, yellow and green, iii) red, function for each primary should peak at the hue angle of its orange and green, and iv) red, orange, yellow and green, to use as associated unique hue, where the functions for the three other primaries. When yellow was unavailable as a primary, there primaries should be at zero. At the positions of intermediate hues were certain wavelengths that Sternheim and Boynton’s subjects two primaries should be reported. The prediction for the were unable to adequately describe by mixtures of red, green and intermediate primaries is very different. Functions for each blue. In contrast, when orange was unavailable, light of intermediate primary should peak at the hue angle of that wavelengths in the “orange” part of the spectrum could be primary. But at the positions of the (subjectively unitary) unique adequately described as mixtures of red and yellow. hues all functions should be at zero. We believe that Sternheim and Boynton’s comparison between conditions was not entirely balanced. In conditions where orange was unavailable as a primary, two closely neighboring hues, red and yellow, were available. But in conditions where yellow was missing, at least one of the two alternative primaries provided was further from yellow on the hue circle than either red or yellow from orange. It could be simply be the spacing of the primaries in color similarity, rather than their uniqueness, that allows adequate description of other hues. Neither Sternheim and Boynton, nor others who have since used hue scaling to investigate color appearance [16-20], have introduced a true control condition. In order to conclude that hue scaling results demonstrate the uniqueness of unique hues, it is necessary to show that the same results cannot be obtained using only non-unique primaries roughly evenly distributed around the hue circle. In Experiment 1, we report the results of hue scaling under two different conditions: a standard “unique” condition, where the four unique hues are given as primaries, and a control Figure 1. Predictions for hue scaling with unique primaries (panel (a)), and “intermediate” condition where four intermediate hues (purple, with intermediate primaries (panel (b)). Along the x-axis we plot hue angle orange, lime, and teal) are given. around the hue circle, and on the y-axis the predicted numerical ratings from 0-9. The dashed vertical lines indicate the unique and binary hue B. Effect of instruction on settings of unique hues primaries, labeled in uppercase: Purple (P), red (R), orange (O), yellow (Y), In experiments where subjects are required to identify or lime (L), green (G), teal (T) and blue (B). Predicted functions of ratings for provide settings of unique hues, other unique hues are invariably each primary as a function of the hue angle of the test stimulus are given in the instructions [21-26]. A subject may be asked to set a indicated by the solid curves, labeled in lowercase. This figure is available yellow that is neither reddish nor greenish [27], or for in color online. measurements of unique green, to decide whether a given color is B. Stimuli too bluish or too yellowish [17]. A representation of a particular trial is given in Figure 2(a).
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