Surface color naming in dichromats

Surface color naming in dichromats

I’ision Pergamon Rc\. Vol. 14. No. CopyrIght c 16. pp. ?I37 19Y4 Prmtcd I” Great Britam 0042-6989(93)E0061-B 2151. 1994 Elsevirr Science L...

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I’ision

Pergamon

Rc\.

Vol.

14. No.

CopyrIght

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16. pp. ?I37 19Y4

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0042-6989(93)E0061-B

2151.

1994

Elsevirr Science Ltd All

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Surface Color Naming in Dichromats* ETHAN Rtwiwd

D. MONTAGt I7 F&uury

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27 August

1993

In previous experiments, Montag and Boynton [(1987) Vision Research, 27, 2153-21621 found that many dichromats can categorize colors using color naming in fair agreement with color-normal subjects. The contribution of rods to color vision was suspected as underlying this ability. Here we follow up on these experiments by having dichromats name colors under various conditions. When the stimuli are limited to a brief presentation time (60 msec) the dichromats’ categorization in the three dimensions of the OSA color space is impaired. Using high light levels so that the rods are saturated does not impair performance. The dichromats named colors during the period of the cone plateau following a rod bleach. Contrary to Montag and Boynton (1987) there was no deficit. These results suggest that an anomalous third cone pigment is responsible for the categorization in three dimensions. It is concluded that the receptors containing the anomalous pigment require greater temporal and spatial summation in order to contribute to the dichromats’ color categorization. Color

perception

Dichromacy

Color naming

Surface colors

INTRODUCTION A true dichromat is an observer who is missing one of the three photopigment types contained in the cone photoreceptors in the retina. Such an individual would only need two suitably chosen primaries to match any colored light. The standard diagnostic tool for determining whether a subject is dichromatic, or more specifically protanopic (missing the long-wavelength sensitive pigment) or deuteranopic (missing the middle-wavelength sensitive pigment), is the anomaloscope. The typical anomaloscope has a small 1-3’ bipartite field. One half consists of a variable intensity yellow light and the other half contains a mixture of a red and a green light, the ratio of which can be varied. A red-green dichromat can adjust the intensity of the yellow to match any ratio of red to green in the other side of the field. Approximately 2% of males are dichromats based on this type of test (Pokorny, Smith, Verriest & Pinckers, 1979). However, psychophysical experiments with dichromats have shown that many are trichromatic (e.g. Smith & Pokorny, 1977; Nagy, 1980; Breton & Cowan, 1981; Frome, Piantanida & Kelly, 1982). At mesopic levels, large-field color matches can be mediated by the rods and the remaining normal pigment types (Smith & Pokorny, 1977). At higher light levels where the rod signals saturate the matches seem to be mediated by a residual anomalous pigment (Nagy, 1980; Breton & Cowan, 1981). Frome, Piantanida and Kelly (1982)

*This paper is adapted from the author’s Ph.D. dissertation (Montag, 1991). A preliminary report based on part of this study was presented at the 1988 annual meeting of the Association in Research in Vision and Ophthalmology. tCenter for Visual Science. University of Rochester. 274 Meliora Hall. Rochester. NY 14627, U.S.A.

Rods

measured the temporal and spectral sensitivity of “forbidden” anomalous pigments in dichromats using flickering lights. Color naming experiments have also shown that with large fields many dichromats are trichromatic. Scheibner and Boynton (1968) demonstrated residual red-green discrimination with 3 deg fields in a task in which dichromats named spectral colors. Nagy and Boynton (1979) had protanopes and deuteranopes name four spectral colors at two different retinal illuminance levels with and without a rod bleach. Their data support the evidence from matching experiments that dichromats exhibit residual color discrimination that depends on a cone type previously thought to be missing. They found that performance was slightly better in the dark than with rods bleached proving that rods also contribute to the ability to discriminate these colors. The protanopes in this experiment were poorer at hue naming than the deuteranopes. Jameson and Hurvich (1978) found the opposite to be true and attributed to its differences in spectral sensitivity. That is, protanopic sensitivity to long wavelengths is much less than the deuteranopic sensitivity. This would provide additional lightness cues for protanopes that deuteranopes cannot use. The subjects in Nagy and Boynton’s experiment, however, could not use this type of information because the stimuli were individually matched for brightness at each of the two levels. Montag and Boynton (1987) studied the ability of dichromats to name a broad array of surface color samples intended to sample the color space of normal subjects uniformly. They found that with the standard large 4 deg field and essentially unlimited viewing time dichromats were able to categorize colors in all three dimensions of color space. Not surprisingly, their

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performance was poorer than that of color-normal controls, but it was much better than might be expected for classical red-green dichromats. The purpose of this paper is to follow up on the experiments of Montag and Boynton (1987) in an effort to determine whether rods, residual anomalous cones or a combination of both allow dichromats to categorize color space in three dimensions, In the earlier work. when the contribution of rods was eliminated by presenting the stimuli in a small I deg field and with a brief exposure (approx. 50 msec), the ability to categorize the colors in the red-green dimension was substantially degraded. One deuteranope and one normal subject were also tested during the cone plateau phase following a Ganzfeld bleach. This also resulted in a loss of categorization in the red---green dimension for the deuteranope. These results suggested that rods were contributing to a third dimension of psychological color space. It was hypothesized that this rod contribution plus the availability of lightness cues allowed the dichromats to categorize the surface colors in good agreement with color-normal subjects. It will be shown below that this hypothesis is wrong. Part of the impetus for Montag and Boynton’s (1987) experiment was to examine the differences in categorical color perception of dichromats as compared to normal trichromats. Following the anthropological studies of Berlin and Kay (1969). there has been increasing evidence supporting the notion that there are I I basic color terms used to categorize psychological color space (see Boynton & Olson, 1987, 1990). The I I basic color terms are black, white, red, blue, yellow, green, purple. pink, orange. brown and gray. Ratliff (1976) argues that the apparent universality of the basic color terms and order of their temporal appearance in the development of any language suggests that there may be a psychophysiological basis that is inherent in genetically determined structures and processes of the visual system. Boynton and Olson (1990) used three measures to confirm the salience of the basic color terms finding support for the notion that the basic color terms refer to fundamental sensations for which there is a specific physiological basis. The three measures were consistency, response time and consensus. Consistency refers to the usage of the same term for a color sample upon second presentation. Subjects were more consistent in their use of every basic color term than any nonbasic term. The same was true of dichromats tested with the same procedure by Montag and Boynton (1987). In fact. dichromats used nonbasic terms very infrequently, suggesting that these nonbasic terms do not have the “stability of reference across occasions of use” that is one of Crawford’s ( 1982) linguistic criteria for determining whether a color term is basic. Like the normal trichromats in Boynton and Olson (1987, 1990), the dichromats’ mean response times were generally faster for naming samples with basic color terms and for naming them consistently. Again this shows that there is the same sort of dichotomy between basic and nonbasic terms for dichromats as for color-normal subjects. Con-

sensus (the between subject agreement In use of color terms) was not specifically examined in the dichromats’ data because the centroids. the central locations of the regions named by a particular color term, were located idiosyncratically for each subject indicating individual differences much greater than for color-normals. Montag and Boynton (1987) argue that the predominance, consistency, and speed of basic color term usage supports the idea of an underlying physiological basis for the basic colors because learning to associate color names to different, arbitrary regions of color space would not explain the increased usage and consistency of the dichromats’ use of basic color terms. Despite an impoverished or weakened sensory system, these dichromats still appear to have the psychophysiological mechanisms that lead them to attempt to categorize color space into the same eleven basic categories as normal trichromats. EXPERIMENT 1: DURATION EXPERIMENT lnntroduction

As mentioned above, Montag and Boynton (1987) used a small stimulus field of brief duration in an attempt to eliminate the contribution from rods to the color naming task. The rationale behind this was that the central rod-free area of the fovea would be fixated on the target and the brief duration would make it impossible for subjects to scan the stimulus and thereby bring rods into play. The duration of the presentation was approx. 50 msec, which is not enough time to permit a saccade (Fuchs. 1976). The stimulus field was I deg. Estimates of the size of the rod free area vary from I to 2deg in diameter (Wyszecki & Stiles, 1982). The dichromats’ ability to categorize in the red-green dimension was degraded, leading to the conclusion that rods are responsible for the ability to categorize in this dimension. Compared to their large field, unlimited time data, there was no such effect for normal subjects. This conclusion was not justified because the small field/short duration of the presentation may have resulted in impaired performance for reasons other than the elimination of rods. For example, Nagy (1989) showed that there may be abnormalities in the temporal properties of color coding in some color deficients which may be related to poor color discrimination. In the experiment to be reported here, the short duration is paired with both large and small field sizes. In this experiment there are three conditions: (I) Prolonged duration, Large field (PL); (2) Brief duration, Large field (BL); (3) Brief duration, Small field (BS). If only condition BS produces a loss in categorization then the short duration itself is not responsible for the effect and the rod hypothesis is supported. If the short duration is responsible for the loss of categorization then conditions BL and BS should yield the same results showing reduced categorization when compared to condition PL. This would not resolve the question of

DICHROMATIC

whether rods or residual anomalous pigments are responsible for the ability to categorize in three dimensions but it would indicate that either mechanism is impaired by brief stimulation. Methods Stimuli. The stimuli selected from the 424 regular two-unit samples of the Uniform Color Scales set, developed by the Optical Society of America (Nickerson, 1981) were also those used in Montag and Boynton (1987). Each surface color sample is intended to be perceptually equidistant from its 12 nearest neighbors (MacAdam, 1974, 1981). These twelve samples are the vertices a cuboctahedral shell around that central sample. Each vertex is the center of another cuboctahedral cluster extending the lattice in the three dimensions (Nickerson, 1981; Billmeyer, 1981). The color space is represented by a coordinate system centered on middle gray with a vertical axis of lightness, denoted L, and two chromatic axes, ,j and g, corresponding roughly to yellow-blue and green-red, respectively. Only the 2 15 samples found at the even lightness levels (L = -6, -4, -2, 0, 2, 4) were used. This so-called “half-set” was used to speed the collection of data. An extensive comparison of results of color naming by color-normal observers showed no systematic differences between the full- and half-set conditions (Olson, 1988) (although a similar control was not carried out on dichromats). Because there are many fewer samples, the distributions of color term usage will not be directly comparable to Montag and Boynton (1987). It also should be noted that a smaller number of judgments will be analyzed for the different measures used. This could have the effect of increasing the variability and decreasing the validity of these data as compared to the data in Montag and Boynton (1987). Appuratus. Subjects were seated facing the back wall of an enclosed booth in front of a slanted table. A single blade shutter, driven by a rotary solenoid was used to expose the color samples through a square opening in the table. This aperture was 3.8 cm square for conditions PL and BL, the large field conditions. For condition BS. the small field condition, the aperture was a circle 0.75 cm diameter. The large field subtended about 4 deg of visual angle and the small field subtended about 1 deg. The shutter was controlled manually by the experimenter. For condition PL, the experimenter flipped a switch to open and then close the shutter. For the brief durations of conditions BL and BS, the experimenter depressed a foot switch that was connected to the shutter via a digital timer that closed the circuit for approx. ‘The designation of the subjects will follow a consistent format throughout this paper. Each subject will be identified by a letter and number combination. The letter indicates the type of color vision (for example. N for normal and D for deuteranope). The numbers were assigned in order as subjects participated in the diKerent experiments. A particular letter and number pair always refers to the same subject in different experiments. The experiments arc not reported in the same order in which they were run so that the \uhjcct’s numbers are not in sequence.

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60 msec. Measurements made with a photodiode showed that no part of the sample was exposed for more than 100 msec. Each 2 in. square sample was mounted on a 5 in. square of acid-free bristol board. These were inserted one at a time into a slot from outside the booth so that only the sample would appear through the aperture. The back of the booth, the surface of the table and the shutter were painted with a flat gray paint (PPG 4752 “Gray Velvet”) that approximately matches Munsell gray 5 corresponding to a 20% reflectance or approximately L = -2 in the OSA coordinate system. The remaining surfaces of the booth were painted flat white. A single 200-W photoflood lamp. rated at 3200 K, illuminated the booth, providing a luminance of the gray background of approx. 74 cd/m’. The light was mounted on a horizontal platform above the subject’s head and was baffled so that the color sample was indirectly illuminated by reflection from the walls and ceiling of the booth, resulting in a negligible specular reflection from the glossy surfaces of the samples. For the large held conditions, the table was slanted at an angle of 20 deg upward from horizontal. For the small field condition, the table was slanted at an angle of 40 deg in order to decrease the shadow. The viewing distance was approx. 60 cm and was fixed by a nonintrusive, large-size aperture suspended from the horizontal platform above the subject’s head. Subjects viewed the stimuli monocularly through this aperture. The samples had the realistic appearance of surface colors exposed below the opening in the table. Procedure. In all three conditions. and in all the experiments below, samples were presented in a random order until all 215 had been seen. The samples were then presented a second time in the reverse order. Subjects were instructed to respond with any monolexemic (single-word) color name. If subjects responded incorrectly with a compound color term. then the trial was rejected and repeated sometime later in the sequence. For condition PL, subjects were instructed to respond as quickly as possible and had up to 5 set to give a response. After 5 sec. the trial was rejected and repeated sometime later in the sequence. When the subject responded, the experimenter would close the aperture and record the response. Response time was recorded manually by the experimenter. For conditions BL and BS, the samples were presented for the brief duration and subjects had an unlimited amount of time to respond. A microcomputer was used to determine the random order of the samples, to time and record responses and to facilitate data analysis. Subjects were run in a series of 2 hr sessions. Su/~jrcts. Five subjects* took part in this experiment. There were two deuteranopes, a female and a male (D4 and D5, respectively). two male protanopes (Pl and P2), one female tritanope (Tl) and one male normal trichromat (N3). These subjects as well as all the subjects in this paper were paid volunteers solicited by advertisements in the student newspaper and signs hung around campus.

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FIGURE I. Average response times and 95% confidence intervals for the use of basic and nonbasic color names divided into consistent and inconsistent use for each subject. Response times were recorded only in the prolonged duration, large field (PL) condition. For subject PI, there was no consistent use of any nonbasic color term, therefore there is no open square plotted.

All the subjects were naive about the specific purpose of the experiment and were told only that the experimenter was trying to find out how color-blind individuals name colors so well. Subjects were screened and classified according to their performance on the Farnsworth Munsell lOO-Hue test, the American Optical pseudoisochromatic plates and Rayleigh matches on a Schmidt-Haensch anomaloscope. In addition, the tritanope was screened with the Farnsworth F, Tritan Plate* (National Research Council Committee on Vision, 1981) and by demonstrating a tritan error in her matching behavior using the La Jolla Analytic Colorimeter (Boynton & Nagy, 1982). Subsequently, members of Tl’s family were screeened for color and genetic material was analyzed deficiencies showing a dominant single-nucleotide substitution encoding the S-cone pigment associated with the pattern of inheritance of the tritanopia (Weitz, Miyake, Shinzato, Montag, Zrenner, Went & Nathans, 1992). In addition, Bailey and Montag (1992) have recorded Ganzfeld electroretinographic responses to colored flashes for Tl and one other family member with the tritan deficiency. S-cone responses (b-wave) were evident but were reduced compared to color-normal controls. RESULTS Response times. In order to confirm the findings of Montag and Boynton (1987), average response times recorded in condition PL are presented in Fig. 1 with 95% confidence intervals for each category of response. Consistent use occurs when a particular sample is named by the same term on both presentations. For all subjects, *The F, Tritan plate was provided complimentarily by S. M. Luria, Naval Submarine Medical Research Laboratory. Groton. Conn.

basic terms are used more quickly than nonbasic ones. For all but one subject (Tl). samples named consistently are named faster than inconsistently named samples regardless of whether basic or nonbasic. Protanope P2 and deuteranope D4 name the colors just about as quickly as the normal subject does. These results are in agreement with Montag and Boynton ( 1987). suggesting that the use of the half-set stimuli did not alter the pattern of results. Consistency qf‘ color naming. The basic terms were used much more predominantly than nonbasic ones. No subject used basic color terms less than 75% of the time to name the samples. Subject PI used basic terms for more than 90% of his responses. The proportion of basic responses did not change across conditions. The most consistent use of basic color terms (84%) was by normal, N3, in condition PL. The least consistent performance (56%) for the use of basic color terms was by subject D4 in condition BL. The most consistent use of nonbasic terms was by N3 in condition BS (51%). Subject PI did not use any nonbasic term consistently in conditions PL and BS. All subjects were more likely to use basic than nonbasic terms consistently. The dichromats and the trichromats named colors with about the same consistency. There is no evident change in consistency for the different conditions. Therefore, any differences in color-naming performance for the different conditions is not attributable to a failure in the ability to name colors altogether. That is, if the dichromats were no longer able to tell the colors apart and were using a guessing strategy, it would be expected that there would be less consistency. However, despite this level of consistency, dichromats use pairs of terms to name a particular sample that normal trichromats never use together. For example, all the protanopes and deuteranopes used orange and green to name the same sample at least once and the tritanope used pink and yellow to name the same sample. The normal trichromat never used these pairs together to designate a color sample. Centroids. The location of the basic colors in color space can be characterized by the location of centroids for the basic colors in the OSA space for each subject in the three conditions. The centroid for a color is the central tendency of the location of samples named by that color term in the OSA space calculated by averaging the coordinates of all the samples named with that term weighted by whether the sample was named consistently (twice) or inconsistently (once). Figure 2 shows the value on the L coordinate of the centroids for the basic colors for all subjects. Conditions PL, BL and BS are represented by circles, triangles and squares, respectively. For all the subjects, in all three conditions, there is reasonably good agreement in the lightness level of the centroids. These remaining differences are likely attributable to chance variability in performance rather than the effects of experimental manipulations, because the centroids for a particular color term do not show any consistent changes in lightness level between conditions.

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FIGURE 2. Centroid locations on the L dimension of the OSA color space for the I I basic color terms, shown separately for each of the six subjects as indicated at the bottom. All three conditions are shown: condition PL+ircles; condition BL-triangles; and condition BSsquares. The term “black” was only used by subject D4 in all three conditions and by subject Tl in condition BL. Open and solid symbols are used for clarity in separating the colors.

The locations of the centroids in the j and g axes of the OSA space are shown in Fig. 3(a-f). Each panel shows the results for one subject in all three conditions. Circles, triangles and squares again represent conditions PL, BL, and BS, respectively. As a guide for the eye, lines are drawn connecting the centroids for the colors that typically fall on the perimeter of the hue circle in the order blue, green, yellow, orange, red, pink, purple and back to blue. Since these figures plot only in the j and g dimensions, the distances between centroids in the figures do not show the actual distances of centroids in all three dimensions of the OSA space. By inspection, the locations of the centroids for the color-normal subject, N3, change little from one condition to the next. For most of the other subjects there is considerable change in centroid location for different conditions. For all the dichromats the centroids for condition PL are more dispersed in the chromatic dimensions. More specifically, for the protanopes and the deuteranope the centroids in the brief duration conditions seem to migrate toward the line connecting the centroids for yellow and blue. For the tritanope, the brief duration conditions resulted in some movement of the centroids toward the line that would connect green and red. For all dichromats, both brief duration conditions appear to have the same effect on the centroid locations despite the difference in field size. Also, the average of the distances between all pairs of the centroids of the colors on the periphery of the hue circle for condition PL is greater than those of conditions BL and BS, indicating a greater spread of location of the centroids with the prolonged stimulus duration. Condition PL (the “standard condition” in these experiments) can be considered the condition eliciting optimal performance for this experiment because the color space is divided into categorical regions whose centers are furthest apart in the color space implying greatest distinctness and separateness, most like that of color-normal subjects. Despite the shift in the locations of the centroids, the dichromats are still able to categorize colors in the OSA color space in a systematic manner in conditions BL and BS, rather than losing the ability to categorize altogether. The

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consistency of response discussed above is also evidence of this. Movement of centroids. The position of a centroid for a color term changes from one condition to another so that the distance between these centroid locations may be calculated. For each of the basic colors for all six subjects, three distances were calculated, one for each possible pair of experimental conditions: the distance between conditions PL and BL, BL and BS and PL and BS. Figure 4 shows the average of these distances for the basic color terms used by each subject in all three conditions. All distances were taken as positive. If the centroids in one condition were in exactly the same place as the centroids in a second condition, the mean distance between centroids would be represented by a bar with zero height. Normal N3 shows the least average change in all three comparisons (shortest bars). For all six subjects, the mean distance between centroids for conditions BL and BS is smaller than either of the other pairs of conditions. This means that the brief duration conditions are more similar than the other pairs of conditions in location of centroids. Best fitting plane. The change in the locations of the dichromats’ centroids between the prolonged and brief duration conditions indicates that a brief stimulus presentation results in a degradation of the ability to categorize colors in all three dimensions of color space. It appears that these subjects retain the ability to categorize in the lightness dimension and one chromatic dimension but lose the ability to categorize in a second chromatic dimension. The directions in which the centroids migrate are consistent with the opponent-color models of color vision (Boynton, 1979). The loss of the M-cone (deuteranopia) or the L-cone (protanopia) input leads to the loss of a red&green opponent system but leaves an achromatic luminance channel and chromatic yellowblue channel providing for a two-dimensional color space. Similarly, lack of an S-cone (tritanopia) leads to a two-dimensional color space fed by a red-green opponent channel and a luminance channel. Inspection of Fig. 3 suggests that for those dichromats affected, the loss is roughly in the dimension predicted by this model. Even though deuteranope D5 shows this pattern of change in centroid location to a much smaller extent [Fig. 3(e)], his results are still consistent with the model because even under the optimal condition his centroids fell close to a plane in the OSA space. D5 performs most like a true dichromat. Whatever the mechanism is that allows the other dichromats to categorize in all three dimensions of color space is either absent or weak in D5. This mechanism is ineffective in the other dichromats when the stimulus is limited to a 60 msec exposure. The following method was used to help characterize the location of centroids in the OSA space based on the above model that centroids migrate toward a plane consisting of the L dimension and a single chromatic dimension in the short duration conditions. The best fitting plane to the centroids for a subject in a given condition and a measure of the goodness of fit of this plane to the centroids was calculated. Since the L. j

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FIGURE 3. Locations of the centroids in the j and g dimensions of the OSA color space. Lines connect the ceutroids in the order blue, green, yellow, orange, red, pink, purpk and back to blue as a guide to the eye for inspecting the graphs. (a)-(f) Normal N3, protanopes. PI and P2. deuteranopes, D4 and DS. and tritanope TI. respectively. Not all the sub+ts used all I1 basic color terms in all three conditions.

DICHROMATIC

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Stimulus

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SUBJECT FIGURE 4. The position of a centroid for a color term changes from one condition to another and the distance between the two positions is calculated. The average of these distances for all the basic color terms used by a subject in a pair of conditions is represented by the height of the bar. The striped bar represents the mean distances between conditions PL and BL. The open bar represents the mean distances between conditions BL and BS. The shaded bar is for conditions PL and BS.

and g coordinates of the centroids are observed values that are subject to error, all three variables are treated as “dependent” variables for fitting a plane. Pearson (1901) presents such a method for finding a plane of closest fit to a system of points in space and determining the mean square residual which is an index of goodness of fit. The smaller the mean square residual, the better the points fit a plane. Since optimal performance is associated with a dispersion of centroids in all three dimensions of color space, performance is inversely related to the goodness of fit and therefore positively 250

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related to the mean square residual. By looking at the change in the mean square residual, we can characterize the degree of collapse of the centroids in the short duration conditions. The mean square residuals of the fits are plotted in Fig. 5. For all the dichromats, a plane fits the centroids better in the brief duration conditions than in the prolonged duration condition in agreement with the model. Discussion As in Montag and Boynton (1987) the results of this experiment can be taken as support of the notion of the special nature of the 11 basic colors. The dichromats use the basic color terms predominantly, consistently and more quickly to categorize colors in color space. The predominance and consistency of basic term use helps support the validity of the analyses made on the color naming data. Though there is considerable individual variability (e.g. deuteranope D5 exhibits poor categorization in all three conditions), many dichromats (including the tritanope presented here) demonstrate the ability to categorize the OSA surface colors in three dimensions of

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FIGURE 5. Mean square residuals of the best fitting planes to the centroids of the basic colors for each subject in each condition. PL is the prolonged duration, large field condition. BL is the brief duration, large field condition. BS is the brief duration. small field condition.

FIGURE centroids

6. Mean square residuals of the best fitting planes to the of the basic colors for each subject in each condition in Expt 2.

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color space when given an essentially unlimited viewing time. By limiting the exposure of the stimulus to 60 msec the performance deteriorates so that categorization is degraded along the chromatic axis which is presumed to be missing according to the opponent model of color vision. The mechanism. either rods or a residual third cone mechanism (or both), responsible for the third dimension of categorization is unable to function efficiently given a brief stimulus presentation. Why might this be? Rod responses are known to be more sluggish than those of cones. Schnapf and Baylor (1987) showed that the peak response of a cone occurs about 150 msec before that of a rod with dim flashes. Although this difference decreases as flash intensity increases. there is still a considerable latency between peak response. Sharpe, Stockman and MacLeod (1989) showed that phase lags between rods and cones following flickering lights corresponded to delays of over 60 msec. Rod--cone interactions were studied by Gouras and Link (1966) by recording ganglion cell responses to brief flashes of light in macaque monkeys. They found that only the cone response was elicited when a scotopic and photopic test flash were presented simultaneously because the initial fast cone response blocked the slower rod response. Makous and Boothe (1974) showed psychophysically that the cones can interfere with detection of a test flash by rods. Therefore, it is plausible that any contribution rods may make to color perception in dichromats is lost with brief duration stimuli because the signals from the rods and the cones are unable to combine in the early stages of processing where presumably comparisons of the receptor outputs are made. There is no reason to suspect that residual anomalous cones would be slower than that of any of the other cones. However, short duration stimuli may interfere because of abnormalities in post-receptoral processes (see Nagy, 1989). Longer stimulus durations may be needed in order to integrate the signals from these residual cones as is the case for larger stimulus fields in the space domain. An analogy can be made with the short wavelength sensitive cone mechanism in normal trichromats. It has been shown (Weitzman & Kinney, 1967; Kaiser, 1968) that color naming becomes increasingly tritanopic as the stimulus duration (and field size) decreases. The S-cone contribution is diminished when spatial and temporal integration is prohibited. Like the S-cones (Williams, Collier & Thompson, 1983; McCrane, de Monasterio, Schein & Caruso, 1983), the putative residual cone mechanism may be sparsely distributed in the retina1 mosaic. [See Mollon, Astell and Cavonius (I 992) for a discussion of “tachistoscopic tritanopia.“] EXPERIMENT

2: LIGHT LEVEL EXPERIMENT

Introduction As light level increases the rod system saturates so that it is no longer able to respond to an incremental stimulus (Aguilar & Stiles, 1954). At these high levels of adap-

tation, the rods are believed to no longer contribute to vision. In the present experiment, three different levels of ambient illumination will be used to set whether rods contribute to color naming in dichromats. At the lowest experimental levels (the Low light level condition) the ratio of rod to cone activity is highest so that if rods do contribute. best performance ought to be seen here. At the highest level (the High light level condition), saturation will prevent rod contribution. The intermediate light level (the Medium light level condition) is the standard illumination used in the previous color naming experiments on dichromats. Meth0d.y The methods were identical to those above except for the following changes. Subjects. Seven subjects participated in this experiment. There were two color-normal subjects, a male and a female (Nl and N2. respectively), three deuteranopes (Dl, D2 and D3), one protanope (PI) and one tritanope (Tl). Subjects N2, PI and TI dropped out of the study before participating in the Low light level condition. The data for the Medium light level for subjects PI and Tl who participated in the duration experiment in the previous experiment are the same data as the prolonged duration, large field condition (PL), presented in the previous experiment. Appurutus. The Medium light level condition is the standard presentation method used previously. An aluminum baffle was placed over the single bulb to reduce the li’ght level for the Low light level condition. The average luminance of the gray background was approx. 25 cd/m’. Five overhead photoflood lamps were used to produce the High light level of approx. 360 cd/m’ on the gray background. For subjects PI and Tl, six overhead lamps produced an adapting luminance of approx. 410 cd/m’. Procedure. In each condition, samples were presented following the procedure described in condition 1 (PL) of the previous experiment. Subjects PI and TI viewed the stimuli monocularly at a distance of approx. 60cm through a nonintrusive, large-size aperture suspended from the horizontal platform above the subject’s head. All other subjects viewed the stimuli binocularly and were instructed to keep their heads at the correct distance. The sequence of the light-level conditions was: Medium, High, Low. Results Anulysis of’ light levels. Aguilar and Stiles (1954) used an incremental threshold technique to measure the intensity of light needed to saturate the rod mechanism. They found that at a field intensity of about 100 scat td the sensitivity of the rods begins to fall off rapidly until they saturate at about 2000-5000 scat td (corresponding to 3.3-3.7 log Scot td). This saturation level corresponds to a luminance of white light (daylight) of about 12&3OOcd/m*. Measurements from single rods from monkeys show that their response saturates at corresponding levels (Baylor. Nunn & Schnapf, 1984)

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indicating that rod saturation is due to response compression at the level of rods themselves. Retinal illuminance was calculated by converting luminance measured in cd/m’ to scat td using the analysis of pupil size data collected by deGroot and Gebhard (1952) [which does not correct for the StilessCrawford effect of the first kind (Stiles & Crawford, 1933)], and using the black body radiators of different temperatures provided by Wyszecki and Stiles (1982). Table I shows the retinal illuminance for the background in scat td for the different conditions in the experiment. Also tabulated are the retinal illuminances for the darkest achromatic OSA sample with coordinates (-6,O.O) and the lightest sample with coordinates (4,0,0). In order to calculate these values, it is assumed that the spectral reflectances of the background gray and the achromatic color samples are relatively flat so that the spectral distribution of the light reflected from these surfaces is approximately the same as the distribution from the source. To estimate the effect of light level on the rod system, the effectiveness of the darkest and lightest samples as rod stimuli were calculated. Using the data from Aguilar and Stiles (1954) and Wyszecki and Stiles (1982) the ratio of increment threshold to field radiance (the Fechner fraction) for the background in Low light level condition is 0.54. This means that samples must have a luminance of at least 2.78 log scat td for rods to detect them against the background. In the decremental direction, the darkest sample’s illuminance, taken as the adaptation level, corresponds to a increment Fechner fraction of about 0.24. This means that the background must be greater than 2.3 log scat td in order to be above the increment threshold. It is reasonable to assume that at the Low light level, there is enough variation in the rod signal to influence color vision since rods can signal the differences between both the darkest and lightest samples and the background. For the Medium light level the Fechner fraction is 2.2 indicating that a sample need be greater than 3.5 log scat

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td in order to be detected by the rods. This is greater than the illuminance of the lightest sample. For the darkest sample, an increment must be greater than 2.79 log scat td in order for the rods to distinguish it (a Fechner fraction of 0.54). This is less than the background level. The Medium light level is a high mesopic level. If rods are contributing, the signal is very small. The adaptation level of the High light level condition is within the region in which rod contribution disappears entirely. In this region the Fechner fraction is increasing with a slope approaching infinity (Wyszecki & Stiles. 1982). The rods are unable to distinguish between the background and the stimuli. Centroids. As seen in the previous experiment, the change in centroid location is the most salient feature of the effects of the experimental manipulations. For this reason, analysis will focus on centroids in this and the subsequent experiment. The location on the L axis of the centroids remained fairly stable between conditions and there was reasonably good agreement across subjects for each color term. The locations of the centroids on the chromatic axes of the OSA space, j and g (not shown), do not demonstrate the collapse of the centroids toward a plane seen in the previous experiments (Montag & Boynton, 1987; and the previous experiment) when a short duration stimulus is used. The locations of the centroids remain relatively stable. There seems to be no change elicited by the experimental manipulations. A collapse of centroids to a plane is demonstrated by a decrease in the mean square residual of the best-fitting plane. Figure 6 shows that there is no trend in the data supporting a collapse of the centroids. The change in the mean square residuals are likely due to noise and not an effect of the experimental manipulations. Discussion Neither decreasing the light level in order to increase rod participation nor raising the light level to saturate the rods had a measurable effect on color naming. This contradicts the conclusions of Montag and Boynton (1987) who hypothesized that “the rods contribute a new sensation via the red-green channel which allows discrimination in an otherwise missing channel”. It was shown in the previous experiment that the loss of categorization seen in Montag and Boynton (1987) could have been due to the brief stimulus duration. It is possible that a brief stimulus duration may be less effective for a weak cone mechanism as well as for rods. The question remains why the dichromat tested on the cone plateau after a bleaching stimulus in Montag and Boynton (1987) demonstrated the loss of categorization characterized by the movement of the centroids to a plane in the OSA color space. It seems unlikely, based on the experimental evidence, that rods alone are mediating the ability of dichromats to categorize colors in three dimensions. It is possible that they may play a role at lower light levels, however. another mechanism must contribute when the rods saturate. The negative results in this experiment support

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the notion of a third anomalous cone pigment in many small-field dichromats.

EXPERIMENT

3: DURATION, HIGH LIGHT LEVEL AND BLEACH

Introduction The purpose of the present experiment is to try to replicate and extend the previous findings on the same group of subjects in order to help confirm that a residual anomalous cone type is contributing to produce the good color naming performance seen under the standard conditions. In all conditions the stimulus is large. At the standard light level and at a rod-saturating background level, the stimuli are presented with short and long durations. Perhaps by pairing an intense light level with a short presentation, the signal from the third cone type might be strong enough to contribute so that the performance in this condition will be better than with a short stimulus presentation at the standard light level. In addition, all the subjects were tested at the standard light level during the period of the cone-plateau following a bleaching stimulus in order to eliminate rods and to see if the effect found on the single subject in Montag and Boynton (1987) would replicate. If the dichromats retain their ability to categorize after a rod bleach, then we can support the conclusion from the previous experiment that rods do not contribute to this ability. Metho& The methods were identical to those above except for the following changes. Subjecrs. Seven subjects participated in this experiment. There were two color-normal subjects (N4 and NS), two deuteranopes (D7 and D8) and three protanopes (P4, PS and P6). Apparatus. Eight overhead photoflood lamps were used to produce the high light level of approx. 524 cd/m2 on the gray background. Procedure. All subjects viewed the stimuli monocularly at a distance of approx. 60 cm through a nonintrusive, large-size aperture suspended from the horizontal platform above the subject’s head. The bleach was accomplished using the same procedure used in Montag and Boynton (1987). Before entering the booth, the subject was exposed monoculariy to a bright bleaching light that filled the entire visual field for one minute. The bleach was produced by staring into half a white ping-pong ball mounted on the bottom of a Styrofoam coffee cup fitted over the lens of a Kodak 750-W Carousel slide projector. After bleaching, the subject waited 3 min for the cones to recover. The subject was subsequently tested for 5 min using the standard conditions. In order to minimize the effects of successive bleaching, the subject waited 2.5 min before repeating the procedure. The bleaching condition required two sessions of two hours each. The sequence of the conditions was in this order: Standard, Short, High/Short, High/Long, Bleach.

MONTAG

Results The five conditions of this experiment are:

(1) standard light level, long duration (Standard); (2) standard light level, short duration (Short); (3) high light level, short duration (High/Short); (4) high light level, long duration (High/Long); (5) cone plateau following a bleach (Bleach). Light level analysis. deGroot and Gebhard’s (1952) pupil size analysis was used to convert the luminance measured in the booth to retinal illuminance. The light level of the gray background at the high light level corresponds to a retinal illuminance of 3.65 log scat td. This is well into the region of saturation as determined by Aguilar and Stiles (1954). The rod system is incapable of detecting an incremental stimulus at this level. Bleaching procedure. The dark adaptation curve was measured for the experimenter and one deuteranope with a Maxwellian-view apparatus. The subjects tracked the threshold for a 546nm light presented without a background. The cone plateau extended to about 12 min where rods become more sensitive. The subjects in this experiment named colors between the third and eighth minutes following the bleach. The rods are likely to be unresponsive during the testing interval used in the color naming experiment as determined by the location and steepness of the rod branch at the rod-cone break in the dark adaptation curve. Centroids. The locations of the centroids for the basic colors on the L-axis of the OSA color space are in good agreement between subjects across conditions. To characterize the change in locations of the centroids in the different conditions, the method of fitting a plane to the basic color centroids was used. The mean square residuals of the best-fitting planes are presented in Fig. 7. The mean square residuals for both normal trichromats and deuteranope D7 are high in all conditions showing that none of the conditions, relative to the standard one, caused a loss in the ability to categorize in all three dimensions. (Subject D7 participated in Rayleigh match experiments as well as color naming. Despite his ability to categorize in all three dimensions even in the short duration conditions, the matching data confirm the anomaloscope diagnostic that he is dichromatic with small fields.) Protanopes P5 and P6 had low mean square residuals in all five conditions meaning that their centroids fell close to a plane, demonstrating a poor ability to categorize the color space in all three dimensions in all the experimental conditions. Subjects D8 and P4 show patterns of results similar to one another, but different from the other dichromats. For both subjects, the short duration conditions, whether high or standard intensity, led to a decrement in the spread of centroids in color space. The Bleach condition did not elicit the same collapse of the centroids although for D8 the mean square residual is lower for the bleach than for the Standard or High/Long conditions.

DICHROMATIC

Discussiorl

Summary

of the results:

( I ) The experimental manipulations did not cause any substantial change in the categorization of colors for the normal subjects. (2) Deuteranope D7 showed good categorization in all five conditions. There is a slight collapse of centroids in the short duration experiments. However, this change is small compared to the effects usually seen in dichromats. (3) Protanopes PS and P6 were not able to categorize colors in three dimensions as well as the other dichromats in this study and in the previous experiments. (4) Deuteranope D8 and protanope P4 categorized colors well in the Standard, High/Long and Bleach conditions. Removing rod contribution by testing during the cone plateau did not impare these dichromats’ ability to categorize colors in three dimensions. Poor categorization and the collapse of the centroids to a plane occurred only in the two short duration conditions. (5) Increasing the light level did not improve performance for the dichromats when paired with a short duration presentation. These results support the hypothesis that residual anomalous cones provide small-field dichromats the ability to categorize colors in three dimensions using color terms. Although rods contribute at low light levels providing a basis for discrimination in color matching experiments, these experiments show that rods do not contribute to the ability to categorize. Nagy and Boynton (1979) point out a distinction between ordinary red-green dichromats, who show residual trichromacy and subject KE of their study and subject TJ in the study of Scheibner and Boynton (1968) who may be true protanopes (even with large fields) in the classical sense. In Nagy’s (1980) large-field color matching experiments, there is only one subject, a protanope (subject GR), who is a large field dichromat when rods are eliminated from participation in the matching. Jaeger and Krastel (1987) also found one subject. a protanope, who was dichromatic with fields up to 3 I deg in diameter. In these matching experiments, out of the 30 dichroma ts studied ( 14 protanopes and 16 deuteranopes) only four were dichromats in the classical sense and they were all protanopic. It is possible that the two protanopes who failed to show the ability to categorize in three dimensions even under the Standard condition are likewise true dichromats even with large fields. If this is true, it argues against rods as the mechanism that provides the ability to categorize colors in three dimensions since these subjects presumably have normal rod vision, but were unable to categorize well. Unless these subjects have a post receptoral difference that precludes rod contribution, they ought to do as well as other protanopes. However. the true protanope (that is, protanopic even with large-field stimuli) GR in Nagy (1980, 1982) was able to reject matches when rods were able to participate.

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The parsimonious explanation is that these two protanopes are missing the anomalous pigment found in ordinary dichromats and that rods are not responsible for the good color categorization found in ordinary dichromats. Consistent with this explanation are the results from deuteranope D8 and P4. Both of these subjects showed the characteristic collapse of centroids toward a plane defined by the blue-yellow axis and the lightness dimension. The rods were eliminated from the High/Short and High/Long and Bleach conditions. Since performance was good in the Bleach and High/Long conditions, the rods role in categorization is likely to be small. The deficit in the two short duration conditions most likely is a result of interference in the ability of the residual anomalous cone mechanism to integrate over time. The increase in the light level did not produce the desired effect of boosting this third cone type’s signal. Why did the deuteranope in Montag and Boynton (1987) and perhaps subject D8 show an impairment in categorization in the bleaching condition? The only way that the bleaching and the high light level conditions can cause differential behavior is through differences in the responses from cones. One possibility is that while the rods are recovering from the bleach during the testing interval, they are reducing the sensitivity of cones. It has been shown in psychophysical flicker detection (Goldberg, Frumkes & Nygaard, 1983) flicker ERGS (Arden & Frumkes, 1986) and in threshold color detection (Sugita & Tasaki. 1988) that as rods dark adapt, thresholds for cones increase. These experiments indicate that these effects occur mostly after the rod--cone break in the dark adaptation curve. However, Alexander and Fishman (1984) show substantial decrease in flicker sensitivity during the period of the cone plateau. In order for this to cause a decrement in performance in color naming. the high light level must have been much more efficient than the bleaching condition in eliminating rod contribution. This is unlikely. It is possible that recovery from the bleach in the anomalous pigment is slower than in the other two pigments. Norren and Padmos (1974) showed that the short-wavelength sensitive cones in normal trichromats have a time constant of recovery from a bleach of 2.3 min which is about 1.6 times longer than that of the other two cone types. If this difference is related to the sparsity of S-cones and its greater temporal and spatial requirements, then a residual cone pigment would also take longer to recover from a bleach. If the recovery time extends much past the third minute following the bleach when the subjects are tested then a deficit in color categorization may be seen. Individual differences in recovery rate (Norren & Padmos, 1974) may also explain why some subjects may show a deficit.

CONCI,USIONS The fact that many dichromats can name colors with fairly good agreement with color-normal observers does not necessarily mean that the subjective appearance of

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the colors correspond to the sensations enjoyed by normal trichromats. At a first glance, the color naming behavior of the dichromats might argue against the possibility that the chromatic mechanism subserved by the residual anomalous cones is a reduced version of the normal mechanism. Naming the sample green one time and red the next is contrary to Hering-type opponency demonstrated in normal color vision. Do these dichromats mean the same thing as normal trichromats do when they call something green or red? Psychophysical evidence supports the notion that the residual cone mechanism in these dichromats contains an anomalous cone pigment (e.g. Nagy, 1980; Breton & Cowan, 198 I; Frome et al., 1982). Nagy and Boynton (1979) showed that the color naming of large-field spectra1 lights by dichromats, even with rods bleached, is similar to normal observers. These examples support the idea that the weak chromatic mechanism in these small-field dichromats is a reduced version of the normal mechanism. Pokorny and Smith (1977) showed that the variability in hue judgments in anomalous trichromats can be explained by a weakened red-green opponent channel. They were able to get hue estimation functions in color-normals that were similar to the anomalous trichromats’ by having normal trichromats judge spectral hues over a restricted range that was calculated to mimic the reduced input to the red-green chromatic channels in the anomalous observers. They concluded that the actual neural processing in normal and anomalous trichromats does not differ but that the input to the red-green mechanism is reduced for the color deficient observers. By extension, it is possible that small-field dichromats have an even weaker input to the chromatic mechanism. When color-normal subjects judge spectral hues (as in Sternheim & Boynton, 1966), the values of the red and green response functions should be very close to zero near neutral yellow in the spectrum. However in Pokorny and Smith (1977), when color-normals judged colors in a reduced region of the spectrum (or for the judgments of anomalous trichromats), the red and green response functions cross at values indicating that a particular stimulus can be 25% red and 25% green. Clearly, Hering type opponency rules out the existence of such a color, but when subjects are forced to make judgments about weakly chromatic stimuli, seemingly contradictory responses can occur due to among other things small changes in chromatic adaptation and judgment criteria. A similar case may be made for the dichromats in the present study. Because of their reduced chromatic capabilities, judgments of certain colors will produce responses that are apparently incongruous with red-green opponency. It is possible that they possess a similar internal notion of red and green as normal trichromats but because of the weak chromatic input in their visual system, they can never visually realize them. Instead they have learned to categorize colors based on very small perceptual differences.

In studies of the molecular genetics of inherited red--green color vision defects (Nathans, Piantanida, Eddy, Shows & Hogness, 1986; Deeb, Linscy, Hibiya. Sanocki, Winderickx, Teller & Motulsky, 1992) the genotypes of many of those subjects classified as dichromats by screening with small-field anomaloscopes show more than one pigment gene on the X chromosome. Many of these genes are fusion genes composed of fragments of both long-wavelength and mediumwavelength sensitive pigment genes. It is possible that these additional pigments may underlie the trichromacy demonstrated in many small-field dichromats when tested with large field stimuli and the ability of dichromats to categorize colors in three dimensions of color space in the present study. Those subjects with only one pigment gene on the X chromosome may be true dichromats regardless of field size. This true dichromacy may underiy the performance of those subjects who demonstrated poor color naming performance in ail color naming conditions: D5, P5 and P6. The contribution from the residual anomalous cones is weak enough that it has not been conspicuous in the measurements of spectral sensitivity used to derive the action spectra (e.g. Smith & Pokorny, 1972). The post receptoral chromatic mechanisms must be extremely sensitive to the small signals from the anomalous cones. In the past. dichromats have been used to study the “front end” of the visual system because of the presumed loss of a photopigment. Now, the study of the postrecep toral properties of small-field dichromats can lead to insights on how color vision is processed in normal trichromats. REEERENCES Aguilar, M. & Stiles. W. S. (1954). Saturation of the rod mechanism of the retina at high levels of stimulation. Oprico Acre, I, 59-65. Alexander, K. R. & Fishman, G. A. (1984). Rod-cone interaction in flkker perimetry. Brirr31 Journal of Op~fha~~ogy, 68. 303-309. Arden, G. B. & Frumkes, T. E. (1986). Stimulation of rods can increase cone flicker ERGS in man. Vision Research, 26, 71 I .72 1. Bailey, J. E. & Montag, E. (1992). Short wavelength sensitive cone system function in hereditary tritanopia. Investigative Ophlhalmoiogy and Visual Science (Suppl.). 33, 701.

Baylor, D. A., Nunn, B. J. & Schnapf, J. L. (1984). The photocurrent, noise and spectral sensitivity of the rods of the monkey Macaca facicular&. Journal of Physiology, London, IN, 612428. Berlin, B. & Kay, P. (1969). Basic color terms: Their uniwrsiry and a;olution. Berkeley, Calif.: University of California Press. Bilhneyer, F. W. Jr (1981). The geometry of the OSA Uniform Color Scales Committee space. Color Research and Application, 6, 34-37. Boynton, R. M. (1979). Human color vision. San Francisco, Calif.: Holt (Rinehart & Winston). Boynton, R. M. & Nagy, A. L. (1982). La Jolla analytic coiorimeter. Journol of the Optical Society of America, 72 666-667.

Boynton, R. M. & Olson, C. OSA space. Color Research Boynton, R. M. & Olson, C. color terms confirmed by

X. (1987). Locating basic colors in the and Application, 12, 94-105. X. (1990). Salience of chromatic basic three measures. V&ton Research, 30.

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Breton, M. E. & Cowan, W. B. (1981). Deuteranomalous color matching in the deuteranopic eye. Journal of the Optical Sociefy of America, 71, 1220- 1223.

Crawford, T. D. (1982). Defining “basic color terms”. Anthropological tinguiFtics, 2d. 338-343.

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Deeb, S. S., Lindsey, D. T., Hibiya, Y., Sanocki, E., Winderickx, J., Teller, D. Y. & Motulsky, A. G. (1992). Genotype-phenotype relationships in human red/green color vision defects: Molecular and psychophysical studies. American Journal of Human Generics, 51, 687-700.

deGroot, S. G. & Gebhard, J. W. (1952). Pupil size as determined adapting luminance. Journal crf the Optical Society of America,

by 42,

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Acknowledgements-The

author was supported in ganEnge1 Fellow by the Kollmorgan Foundation. Dr Robert M. Boynton for his support and advice of this research and for his helpful comments on

part as a KollmorThe author thanks during the course the manuscript.