Rod influence in dichromatic surface color perception

Vition Rev. Vol. 27, No. 12. pp. 2153-2162.

1987 Copyright 0

Printed in Great Britain. All rights reserved

ROD INFLUENCE IN DICHROMATIC COLOR PERCEPTION

0042-6989/87 $3.00 + 0.00 1987 PergamonJournalsLtd

SURFACE

ETHAN D. MONTAG and ROBERT M. J~IYNTON

Department of Psychology, University of California, San Diego, La Jolla, CA 92093, U.S.A. (Received 19 January 1987; in revisedform 10 June 1987) Abstract-Two protanopes, two deuteranopes, and two normal subjects named 424 OSA Uniform Color Scales samples using single-word color terms of their choice under three different experimental conditions. When viewing a stimulus field subtending about 4deg, the performance of the dichromats revealed a substantial ability to discriminate colors along the red-green axis. When the stimuli were limited to the central fovea, or when rods were excluded with a bleach, dichromats could no longer categorize colors in the red-green dimension. The different conditions did not affect the performance of the normals. The results suggest that rods contribute signals used by dichromats, along with lightness cues, to help discriminate and categorize surface colors. Color perception

Surface color

Dichromat

Categorical perception

INTRODUCI’ION

Color naming paradigms have been used previously to study the ability of dichromats to discriminate colors. Jameson and Hurvich (1978) asked dichromats to name the colored caps from the Farnsworth Dichotomous Test (D-15) and attributed their performance, which was nearly normal, to an ability to discriminate colors using lightness cues. Nagy and Boynton (1979) found that, with large, annular stimuli seen in the aperture mode, the color-naming ability of dichromats tended toward that of normal trichromats when they were asked to name spectral colors matched for luminance, with and without rods bleached. Although the performance of the dichromats was better in the unbleached condition, the bleaching of rods did not abolish red-green discriminations. The authors concluded that the ability of dichromats to discriminate colors using color names could result from a contribution of weak signals from a small number of the peripheral L or M cones previously thought to be missing. Our experiment is designed to study the colornaming ability of dichromats for a broad array of surface colors intended to sample uniformly the subjective color space of normal subjects. Following the anthropological studies of Berlin and Kay (1969) an increasing amount of evidence has been accumulated to support the concept of a set of 11 basic color terms used to

Rods

Color naming

categorize colors in psychological color space. These 11 basic terms are: white, black, red, green, yellow, blue, brown, purple, pink, orange, and gray. It has been argued that each of these terms may relate to sensations determined by a specific physiological substrate (Ratliff, 1976). Recent studies of color naming from this laboratory have supported this concept (Boynton and Olson, 1987) by showing that basic color terms were the only ones used with consensus by all subjects; moreover, for each subject basic terms were used with greater consistency and with shorter mean response times than any nonbasic term. The insistence by dichromats that they enjoy a unique percept of red led Smith and Pokomy (1977) to study large-field color matching in dichromats, who showed evidence of trichromacy. More recent studies have demonstrated the existence of an anomalous type of cone which, in addition to rods, can aid color discrimination and color matching by those classified as dichromatic by conventional tests (Nagy 1980; Breton and Cowan, 1981; Frome et al., 1982). Color naming differs from other psychophysical methods, such as matching and the determination of thresholds, by testing an ability to categorize sensations rather than to discriminate slight differences. This ability to categorize sensations can be revealed by analyzing the patterns of associations between color samples and color names. An important underlying

2153

2i54

ETKSH D. MONTAG and ROBERTM. BOYNTOK

assumption is that the names convey valid information about the sensations designated by them. The first experiment investigated the ability of dichromats to name colors under our standard conditions” We expected that (a) their color naming would be inconsistent with respect to the use of terms that dichromats often misuse in everyday life (such as red, green, pink, brown and gray), (b) they might tend to avoid the use of some of these terms, and (c) their color centroids would align themselves in a plane determined by yellow-blue and lightness axes, with little or no information represented in the red-green dimension. Because these expectations were not confirmed, two additional expe~ments were done, leading to the conciusion that dichromats are able to use input from rods to achieve a degree of disc~mination of color along the red-green continuum.

Methods The methods were identical to those used by Boynton and Olson (1987). Stimuli. We used the 424 reguiar two-unit sampies of the Unifo~ Color Scales set, deveioped by the Optical Society of America to sample three-dimensional subjective color space uniformly (Nickerson, 1981). Each color sample is intended to be perceptually equidistant from its f2 nearest neighbors (MacAdam, 1974, Biiimeyer, 1981). The OSA color space is represented within a coordinate system centered upon middle gray with a vertical axis of iightness, denoted by L, and two chromatic axes, j and g, corresponding roughly to yellow-blue and green-red, respectively. Apparatus. Subjects were seated facing the back wall of an enclosed booth in front of a slanting table. A single-bladed shutter driven by a rotary solenoid was used to expose the color samples. Each 2-in square sampk was mounted on a 5-m square of acid-free bristol board; these were inserted one at a time in a slot from the outside of the booth. The inside back of the booth, the surface of the table, and the shutter were painted flat gray (PPG 4752 “Gray Veivet” ) which approximately matches Munsell gray 5 ~rresponding to a 20% reflectance or approximately L = -2 in the OSA coordinate system. The remaining walls and ceiiing were painted flat white.

A single 200-W photoflood lamp. rated at 3200K, illuminated the booth, providing a luminance of the gray background of 40 cd/m’. The light was mounted on a horizontal platform above the subject’s head and was baffled so that the color sample and its surroundings were indirectly illuminated by reflection from the walls and ceiling of the booth, resulting in a negligible component of specular reflection from the sample. The samples appeared as surface colors exposed just below a 3.8-cm square opening in a table that was slanted at an angle of 20deg upward from horizontai. Subjects viewed the stimuli binocularly at an angle of about 30deg from normal. At a viewing distance of approximately 60 cm, the exposed portion of the color sample subtended about 4 deg of visual angle. Procedure. Samples wore presented in a random order until ail 424 had been seen; the samples were then seen a second time in the reverse of that order. Following each exposure, subjects were instructed to respond with any monoiexemic (single-word) color name within 5 set of the onset of stimulus presentation. Noncompliance was signaled by sounding a bell to indicate that the trial had been rejected (it would be repeated sometime later in the sequence of trials). A m~cr~omputer was programmed to determine the random order of the samples to be presented, to time and record responses, and to facilitate data analysis. Subjects were run in a series of 2 hr sessions. Subjects. The subjects were students between the ages of 18 and 25. Six subjects were used: 2 female normals (N I and N2), 2 male protanopes (PI and P2), and 2 male deuteranopes (Dl and D2). Five were naive as to the purpose of the experiment; one of the normal control subjects (N2) was generally knowledgeable about this program of research. Subjects were screened and classified according to their performance on the American Optical pseudoisochromatic plates and Rayleigh matches on a SchmidtHaensch anomaioscope. Resdts

Consistency of color naming. Consistent coiornaming is said to occur when a subject uses the same term to name a sample on both presentations. All basic color names were used consistently at least twice by each subject, with the exception of orange for one subject, gray for two, and black for three. The rightmost column of Table 1 show that ail subjects were more

2155

Dichromatic surface color perception Table 1. Consistency of color naming within basic and nonbasic color-naming categories, also showing the distribution of responses between these categories for Experiment 1. The total of basic plus nonbasic responses equals 848 (424 samples named twice each) for each subject B Consistent nonbasic Total nonbasic

A Consistent Basic Total basic

Subiect Nl N2 Dl D2 PI P2

(2741435) = (392/535) = (562/847) = (618/843) = (498/781) = (3841678) =

(196/413) = (168/313) = (O/l) = (O/S) = (18/67) = (781170) =

0.630 0.733 0.664 0.733 0.638 0.566

likely to use basic than nonbasic terms consistently, and that the dichromats, who used basic color terms much more often than normals, did so with equal consistency. Response times. Table 2 shows the mean response times and their standard deviations for basic and nonbasic use of color names divided according to consistent and inconsistent use for each subject. In agreement with Boynton and Olson (1987) there is for all subjects a speed advantage for naming samples with basic color terms, as well as for naming them consistently. There were no significant differences in the speed of response between normals and dichromats. Centroih. The centroid location of a color in the OSA space occurs at a position that minimizes the weighted average distance of all samples named with a particular color term. A color centroid is computed by averaging the L, j, and g values for all the samples named with a particular term, weighted according to whether the term was used inconsistently (once) or consistently (twice). The open circles in Fig. 1 show the L coordinate of centroids for each of the basic colors for each subject. The squares and triangles refer to experiments to be described subsequently. For all three experiments, there is good agreement

A/B 1.33 1.37 inf inf 2.37 1.23

0.475 0.537 0 0 0.269 0.459

among all six subjects concerning the lightness level of the centroids, which makes it profitable to concentrate upon centroid locations in the two chromatic dimensions, i and g, projected onto a horizontal plane. Figure 2 shows the centroid locations of the basic color names plotted in this manner for the two normal subjects. The centroids for the normals agree well with each other, and with those published by Boynton and Olson (1987). Lines have been drawn to connect centroids in the order blue, green, yellow, orange, red, pink, purple, blue. These are colors that typically lie on the perimeter of a hue circle. Adjacent pairs were found by Boynton and Olson to be linked in the sense discussed in the next section. With the exception that red is linked to purple as well as to pink, linkage does not occur for color pairs having one or more intermediate colors between them. The centroid locations for the dichromats (Fig. 3) are less dispersed than those of the normal subjects. However, the general arrangement of colors is similar to that of the normal subjects, and it is clear that the centroid positions have by no means collapsed to form a line. Compared to the close agreement between the two normals, the dichromats’ performance is idiosyncratic. Because of this, there are samples that are labeled with one color name by one

Table 2. Mean response times, and their standard deviations, for samples named twice with basic or nonbasic color terms, divided according to consistent vs inconsistent use of basic and nonbasic terms within each category. Response times were recorded only in Experiment 1 Response times Subject NI N2 Dl D2 PI P2

Use of basic term(s) Consistent Inconsistent Mean SD Mean SD 1.91 1.93 2.25 1.11 1.77 1.96

0.48 0.81 I .03 0.33 0.45 0.84

2.20 2.33 2.82 1.39 2.12 2.52

0.57 0.90 0.93 1.48 0.46 1.42

Use of nonbasic term(s) Consistent Inconsistent Mean SD Mean SD 2.29 2.17

0.58 0.71

2.43 2.34

1.01 0.68

2.49 2.55 4.55 2.04 2.35 2.94

0.60 0.84 0.22 0.48 0.94

ETHAN D. MONTAG and ROBERTM. BOYNTON

2156

WHITE 0 YELLOW

40

o ,“,o

0

ORB&GE

BLUE

L

0

*r

l

BGREEN

* 010

A

. .

RED b

-4.0

.

.

-

-6.0.

I N2 NI

0.2 DI

P2 PI

N,N2

D,02

p, P2

N2 NI

D2pl P2 DI

N2 NI

MP(P2 cl1

NI %

D2 PI 4

SUBJECT

Fig. I. Location of centroids for the eleven basic color terms on the lightness dimension L in the OSA color space, shown separately for each of the six subjects as indicated at the bottom (normals Nl and N2, deuteranopes Dl and D2, protanopes PI and P2). Data for three experiments are shown: circles-Experiment I (the standard condition); triangles-Experiment 2 (small stimuli viewed briefly and monocularly); squares-Experiment 3 (rods selectively bleached).

dichromat that another dichromat (or a normal) might label differently. Linkage analysis. Boynton and Olson (1987) have used the term “linked” to describe colors whose names are jointly used to designate at least one sample inconsistently. Using the average data of seven normal subjects, Boynton and Olson found that colors whose centroids were less than 7.5 OSA units apart were almost always linked, whereas those separated by more than this amount almost never were. Figure 4 shows a linkage analysis done individually for the six subjects of this experiment. The 7.5unit rule holds about 85% of the time. as depicted by the ratio of unshaded to shaded bar area on the EXP

EXP

I

TOP BOTTOM

-

PI

-

P2

-

01

-

D2

I

-NI -Nz

graph, and there is little difference between normals and dichromats in this respect. However, this analysis does not reveal that, whereas no linkages occur for normals that do not also

c

BLUE

GREEN -.---__

YELLOW

Fig. 2. Location of centroids in the j,g dimensions of the OSA color space for normal subjects N 1 and N2 in Experiment I. Centroids for white, gray, and black (which overlap extensively) are all containd within the shaded area around the 0,O coordinate position,

Fig. 3. Centroid locations for color terms used consistently at least once, ploted in the j, g dimensions of the OSA color space for the dichromatic subjects in Experiment I. Top: protanopes PI and P2. Bottom: deuteranopes Dl and D2. in each case, white, gray, and black (when used) plot within an area around the 0,O coordinate about twice as large as that shown for normal subjects in Fig. 2.

Dichromatic surface color perception 60

EXPERIMENT I t > 7.5 LINKED

c40

>7.5 UNLINKED

Y 2 z k

2157

cones containing an anomalous version of photopigment lacking in central vision. Experiment 2 was designed to determine whether one or both of these two peripheral mechanisms could account for the residual redgreen discrimination shown by the dichromats in the first experiment. Apparatus and procedure

57.5 UNLINKED

20

z !i z

0

NI N2

DI 02

PI P2

SUBJECT

Fig. 4. Results of inconsistent color naming in Experiment 1 relative to the 7.5~unit rule, which asserts that basic color pairs whose centroids are less than 7.5 units apart are linked (at least one color sample exists that is called by both names on the two replications) whereas those color pairs whose centroids are more than 7.5 units apart are unlinked. When all 11 basic color terms are used, which was the case for 4 of the 6 cases depicted, there are 55 possible inconsistent response pairs, which determines the overall height of the bar. Unshaded areas represent the fraction of cases consistent with the 7.5 rule; the shaded areas represent the two classes of exceptions to it. Shorter bars result when one of the basic color terms is not used.

occur for dichromats, some linkages not found for normal subjects occur for dichromats. Three of these occurred for all four dichromats; Green-orange, green-brown, and blue-pink. The mean inter-centroid distances between these colors for the protanopes are 6.0, 5.2, and 4.5 respectively; for the deuteranopes, 6.7, 5.4, and 5.7. For normal subjects the corresponding values are 8.9, 8.0, and 8.5. Thus the 7.5-unit rule is confirmed in all cases for these color pairs.

The apparatus was modified so that only the subject’s central fovea would view the stimulus for a duration too brief to permit scanning eye movements. The aperture beneath which the color samples were seen was reduced to a circle 1 deg in diameter and the slant of the table was increased to 40 deg. The viewing distance was approximately 60 cm at an angle of 15 deg to for which conditions specular normal, reflectance from the color sample remained negligible. The viewing distance 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. After a signal from the experimenter, the subject fixated where the stimulus would appear and pressed a button that caused the shutter to open for approximately 50 msec. Subjects responded as in the first experiment, using a monolexemic color term. There was no time limit for response, and response times were not recorded. The procedure was otherwise the same as for the first experiment, and the same subjects participated. Results

Use of basic and nonbasic color terms. Table 3 shows the distribution of basic and nonbasic color terms for Experiment 2, divided according to whether or not they were used consistently. A comparison with Table 1 reveals that limiting EXPERIMENT 2 the field size and the exposure duration did not affect the subjects’ ability to respond consisThe results of the first experiment show that tently. Again the rightmost column shows that dichromats can distinctly categorize regions of subjects were more likely to use basic than color space in all three dimensions using the 11 nonbasic terms consistently. As in the first basic color names, and that they do so in a experiment, dichromats used primarily basic manner that tends to resemble that used by color terms to name samples; however, the normal subjects. distribution of color name usage is different. In Color discrimination, based on matching exgeneral, their use of the terms pink and orange periments, has been shown by Pokomy and decreased and their use of blue and white inSmith (1977) and by Nagy (1980) to be improved with large fields because of a con- creased. However, the changes in the distributribution from rods. Even with rods bleached, tions were different for the individual dichromost dichromats exhibit some residual red- mats and no other general tendancies were green discrimination attributable to peripheral evident, whereas some contradictory ones were.

ETHAN

2158

B~YNTOTS

D. MONTAGand ROBERTM.

Table 3. Consistency of color naming within basic and nonbasic colornaming categories, also showing the distribution of responses between these categories for Experiment 2. The total of basic plus nonbasic responses equals 848 (424 samples named twice each) for each subject

Subject

A Consistent basic Total basic

Nl N2 DI D2 PI P2

(2601422) = 0.6 16 (4181594) = 0.704 (568/841) = 0.675 (584/838) = 0.697 (5361781) = 0.686 (3701681) = 0.543

B Consistent nonbasic

For example, the use of the term red increased for Dl, but red is avoided entirely by B2 in Experiment 2. With only a few such exceptions, all 11 basic color terms were used by the dichromats, although some were employed very sparingly and inconsistently. Returning to Fig. 1, note again that the change of conditions from Experiment 1 to Experiment 2 had no significant effect on the lightness values of the centroids for any of the subjects. Cenrroids. The positions of the centroids are shown in Fig. 5 for the normal subjects. The good agreement between the distributions and centroid locations for each of the two normals across the two experimental conditions (compare Figs 2 and 5) indicates that, despite the relative impoverishment of stimulus conditions in Experiment 2, color constancy was well maintained. This was not a necessary result, because the stimuli appeared as surface colors beneath a hole in the inclined table which created small shadows that were more noticeable than in the first experiment; also, the stimuli were viewed monocularly and only briefly. Therefore EXP

2

A/B

Total nonbasic (180/426) = (108/254) = (2/7) = (O/IO) = (10167) = (481167) =

0.423 0.425 0.286 0 0.448 0.287

I .46 I .66 2.36 inf I .53 I.89

changes in the dichromats’ performance in the second experiment cannot be attributed to a general failure of color constancy of a sort that would affect normal subjects as well. Looking at the centroid locations for the dichromats in the second experiment, shown in Fig. 6, there are major shifts in the locations of some of them which result from the use of the small, brief stimulus (compare with Fig. 3). In general, the centroids for blue and yellow remain near their previous locations, but those for other colors tend to migrate toward a -

EXP. 2

PI

TOP

o----o P2 -

D’ D2

BOTTOM

BLUE

PURPLE 9’

,A . \

.

‘9 PINK

.!%__

\

-

-,vELLcnv /

/

I

W

PINK

GREEN

b’‘-.\

\ lo--

BRiWN

Fig. 5. Location of centroids in the j,g dimensions of the OSA color space for normal subjects NI and N2 in Experiment 2, where small, brief, monocular stimuli were viewed.

- -1 GREEN

OYELLOW

Fig. 6. Centroid locations for color terms used consistently at least once, plotted in the j, g dimensions of the OSA color space for the dichromatic subjects in Experiment 2. Top: protanopes PI and P2. Bottom: deuteranopes Dl and D2. White, gray, and black (when used) plot within an area around the 0,O coordinate about twice as large as that shown for normal subjects in Fig. 2.

2159

Dichromatic surface color perception EXPERIMENT

2

> 7.5 LINKED

> 7.5 UNLINKED

(7

5 UNLINKED

(75

II N’Z

DI

02

PI

LINKED

PZ

SUBJECT

position falling on a line connecting the centroids for blue and yellow. Even though the centroids tend to line up between blue and yellow, the color space remains categorically divided among the different color names. Their locations can be described as falling approximately where a perpendicular dropped from a centroid location derived from the first experiment would intersect a line connecting blue and yellow. Although this reduces some of the inter-centroid distances, the 7.5~unit rule still holds equally as well as for Experiment 1 (see Fig. 7). The migration of centroids for P2 was not as complete as for the other dichromats. Subject P2 may have adopted a strategy whereby he

-NORMAL

3

EXPERIMENT

3

Procedure

Fig. 7. Results of inconsistent color naming in Experiment 2 relative to the 7.5~unit rule (see caption to Fig. 4).

EXP

fixated noncentrally on the stimuli allowing either peripheral cones or rods to contribute. It is also possible that his central fovea is populated by a larger number of rods than those of other subjects. Experiment 3 was designed to distinguish between contributions from rods and peripheral cones by selectively eliminating rod function by testing subjects “on the cone plateau”, during the limited period of time where the cones have recovered from a bleach but the rods are still desensitized.

c--rDEUTERANOPE I

The apparatus was configured as in the first experiment. Before entering the booth, a subject was exposed monocularly to a bright bleaching light that filled the entire visual field for one minute. The bleach was produced by staring into half a table tennis ball mounted on the bottom of a Styrofoam coffee cup fitted over the lens of a Kodak 750-W Carousel slide projector. The bleaching procedure and timing sequence were those used by Nagy (1980). After bleaching, the subjects waited for 3 min for the cones to recover. The subjects were subsequently tested for five minutes. Subjects waited another 2.5 min before repeating the procedure in order to minimize the effects of successive bleaching. Subjects were presented with the 424 samples, one at a time, in a random order and were instructed to respond with a monolexemic color term. No response times were recorded. Because of its unpleasant and time-consuming nature, the experiment was otherwise shortened by presenting each sample only once; therefore consistency could not be tested. Only two subjects, N2 and Dl, were used. Results

Fig. 8. Centroid locations in the j, g dimensions of the OSA color space for the two subjects tested in Experiment 3 (one normal and one deuteranope), where rods were selectively bleached. White, gray, and black (when used) again scatter around the 0.0 position.

Table 4 shows the distribution of basic colorterm usage for the two subjects of this experiment. These results are roughly equivalent to those of Experiments 1 and 2 for both subjects. The values for the first and second experiment are halved to allow the comparison to be based on the same number of trials for all experiments. By again examining Fig. 1, we see that for both subjects the centroids did not substantially change location on the lightness dimension. Figure 8 shows that the centroids did not shift location for N2 (compare with Fig. 5), and that

ETHAN D. MONTAG

2160

and ROBERTM. BOYNTON

Table 4. Distribution of US of basic COICX terms for normal N2. and deuteranope Dt, for Experiment 3, in which subjects were presented with each sample once, and for experiments f and 2. where samples were presented twice. Values for Experiments 1 and 2 have been divided by twa to facilitate comparison of data with Experiment 1

Nomsl N2 Color name Green Blue Purple Pink Orange Yellow Brown Red Gray white %Iack Totals

Deuteranope Dl .--~.

Exp. 1

Exp. 2

Exp. 3

Exp. 1

Exp. 2

Exp. 3

77.5 37.5 20 30 39.5 23 10 IS

66 40.5

82 44 I2 35 48 23 6 17 15 0 0 282

178.5 58.5 27.5 66 346.5

i 58.5 f&S 33 43.5 4:

23 12.5 T A 0.5 423.5

3 35 0 28 0 420.5

182 64 28 78 a 45 4 5 0 15 2 423

12.5 2.3 0 261.5

:: 58.5 20.5 :: 20.5 I 0 291

the results for B1 show centroid positions that tend to fall on a fine between blue and yellow, in a fashion similar to Experiment 2. DISCUSSION

The principal restrlts of the three experiments may be summarized as follows:

(1) Both normals and dichromats used the eleven basic color terms names to divide subjective color space categorically. (2) The dichromats used nonbasic color terms very infrequently. (3) For the standard viewing condition (4 deg field and essentially unlimited viewing time) the performance of dichromats was surprisingly good in comparison witb color-normal subjectsshowing d~~erent~atjo~ of colors in ail three dimensions. (4) When small, brief stimuli were used, dichromats continued to employ basic color names to divide color space, but they did so more idiosyncratically with some names very seldom used or omitted. Centroids gravitated toward a piane of two dimensions containing variations in lightness and yellow-blue. (5) Bleaching of rods yielded results for a deuteranope similar to those of the limited exposure condition. (6) The ~rfo~a~~ of normal subjects was basically unchanged across all three experiments. Rod contribution In an opponent~olor model of human color vision, two opponent chromatic channels and

one luminance channel are used to encode color (Boynton, 1979). frotanopes and deuteranopes, whose central vision is believed to lack the photopigments normally found in the long and middle wavelength absorbing cone-types respectively in the fovea, have only two dimensions for the encoding of color, mediated by the blue-yellow and luminance channels. This model would predict an ability to discriminate colors only in these two dimensions. However, the results from the first experiment, using a 4 deg stimulus field, show that dichromats can distinctly categorize digerent regions of color space in ali three d~rne~s~ons using the 1t basic color terms, doing so in a manner resembling that of normal subjects. In the limited-exposure and bleaching experiments, dichromats lost the ability to categorize color in ther red-green dimension, although they were still able to categorize colors along the lightness and blueyellow continuua. Two possible explanations for the dichromats’ red-green discrimination in the first experiment involve an anomalous cone pigment. and rods. Experiments 2 and 3 were designed to determine which of these was contributing information in the third dimension. Experiment 2 eliminated the use of rods and peripheral cones and removed most of their ability to discriminate on the red-green axis. The third experiment sekztively spared co&e function, but the performance of deuteranope D1 was no better than it had been in the second experiment. This evidence points to rods as the receptors that initiate the messages that provide the third dimension of dichromatic color experience. The trichromacy of large-field color matching

Dichromatic surface color perception

shows that rods do not add new sensations for normal trichromats. Therefore rods must contribute their influence for dichromats through the existing chromatic or luminance channel (Boynton, 1984). Across experimental conditions, centroids did not significantly change their positions in the lightness dimension. The pattern of migration of centroids toward the blue-yellow line in the second and third experiments indicates that discrimination of the blueyellow dimension is not affected by rod contribution. Therefore, it is hypothesized that the rods contribute a new sensation via the redgreen channel which allows discrimination in an otherwise missing dimension. Color naming and categorization. The OSA color space was designed so that, for normal trichromats, approximately equal steps of perceptual color change are achieved as one moves from a sample to one of its nearest neighbors. Although color space is characterized by uniform perceptual changes, observers categorize distinct regions using color names. The results from our study support this notion of categorization and the idea that there are eleven basic and categorically distinct basic color sensations, each of which is given a unique name. Nl used 34 nonbasic color terms and N2 used only 15 of them, yet the centroids for the basic terms agreed very well between them, and across conditions. Dichromats used only 4 nonbasic color names altogether (tan, lavender, cream, and beige), and these very seldom. The OSA color space is based on discrimination data from normal trichromats. In our experiments, the results for dichromats were mapped onto trichromatic coordinates. It has been shown that the 7.5~unit rule, which describes a distance in the OSA space within which colors tend to be linked and beyond which they tend not to be, holds as well for the dichromats as for the normal subjects. Presumably it is because dichromacy is a reduced form of normal color vision that such a representation is useful and meaningful. One way to visualize the relation between subjective color space and color sensation is provided within the framework of fuzzy-set concepts (Zadeh, 1965). Samples called brown, for example, are contained in a region that could be fitted with an ellipsoid with the centroid at its center. For normal subjects, a sample located at the centroid would almost always be called brown, with that probability diminishing toward the outer limits of the

2161

ellipsoid. Basic colors are linked if represented by ellipsoids that intersect; for example, if this were the case for ellipsoids representing brown and orange, then either term might be used to name colors that plot in the intersecting region. When compound terms and modifiers are not permitted, basic color terms do not characterize such intermediate colors well. Therefore subjects may use a nonbasic color term instead, for example sienna to stand for a compound of orange and brown. But the evidence of our experiments shows that such terms simply do not measure up when compared to basic color terms: they are used less often, with less consensus, with less consistency, and with longer response times than are the basic terms. These facts place nonbasic terms, and the sensations they represent, in a category subordinate to that of basic colors. Dichromats exhibit a strong and essentially normal ability to categorize colors on the lightness and blue-yellow dimensions. Allowing rods to contribute permits a limited degree of categorization of colors on the otherwise missing red-green dimension. The dichromats’ performance is idiosyncratic compared to the regularity seen in the results of the normal subjects, and centroids are less widely separated, indicating that the information that dichromats have available to discriminate and categorize colors in the red-green dimension is substantially impoverished compared to that of normals. Whereas normals can use a bivalent continuum of red to green from the opponent channel, dichromats probably receive only a univariant signal from rods which signals either red or green. The perceptual claims of dichromats indicate that this message means red (Boynton and Scheibner, 1967; Smith and Pokomy, 1977). However, other evidence indicates that this additional sensation is green (McCann and Benton, 1969; McKee et al., 1977). Because dichromats use the 11 basic color names predominantly and consistently to categorize color space despite the limited information available to them, their performance can be viewed as evidence to support the special nature of these terms and the idea of an underlying physiological basis for their use. Learning to divide color space arbitrarily to match normal behavior would be unlikely to lead to as much consistency of color naming as these subjects demonstrate. It also would not explain why these terms are so often chosen, even under very

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ETHAN D. MONTAGand ROBERTM. E~YNTON

ambiguous circumstances, in preference to nonbasic terms that are also widely used by those with normal color vision. More research on the interaction of rods and cones, the differences in Color encoding among normals, and the different classes of color deficients would be useful. For example, if the color-naming ability of dichromats is enhanced by rod participation, then color naming performance by dichromats should be further improved in our experimental situation by increasing field size and reducing illumination to mesopic levels. These predictions remain to be tested. research has been supported by Grant EY-10451 from the National Eye Institute. The authors thank Geoffrey M. Boynton for his assistance in stimulus preparation and computer programming.

Acknowledgemenu-This

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