August 1967

JOURNAL OF THE OPTICAL SOCIETY OF AMERICA VOLUME 57, NUMBER 8 AUGUST 1967

Differences Among Color Normals: Classes I and II*WHITMAN RIcHARDS

Department of Psychology, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139

(Received 3 February 1967)

Color-normals may be divided into two distinct groups by studying (i) the spectral location of uniquegreen, (ii) the additivity of spectral lights, (iii) the rate of recovery of sensitivity, and (iv) the colorimetriccoordinates for "white." The results of any one of these tests may be used to predict an individual's per-formance on the remaining tests. Taken together, the results suggest that, for some individuals, 540-mmcone activity innervates the same channel activated by the 440-nm cones. This desaturation effect of the540-nm cones cannot be entirely neural, however, because of certain differences in color matches.INDEX HEADING: Color vision.

OVER the past one hundred years, the literature oncolor vision has been peppered with reports of

differences which occur among people with normal colorvision.'- 7 Many of these observer differences seem to

* Presented in part at a meeting of the Optical Society ofAmerica in San Francisco, Calif., October 1966 [J. Opt. Soc. Am.56, 1451A (1966)]. This work was supported under grant NSG 496from the National Aeronautics and Space Administration andgrant MH 05673 from the National Institutes of Health, bothawarded to Professor H. L. Teuber, MIT Psychology Depart-ment, Cambridge, Massachusetts. Dr. Deane B. Judd and Dr.S. M. Luria provided helpful criticisms of the text. Dr. Judd'scomments were particularly penetrating and led to a modificationof the original hypothesis.

I H. von Helmholtz, Treatise on Physiological Optics III, J. C.Southall, Ed., (Dover Publications, Inc., New York, 1962), pp.505, 526.

2 W. de W. Abney and W. Watson, Phil. Trans. Roy. Soc.London, Ser. A, 216, 91 (1916).

3 P. W. Cobb, J. Exptl. Psychol. 5, 227 (1922).4J. H. Parsons, An Introduction to the Study of Colour Vision

(Cambridge University Press, Cambridge, England, 1924), p. 55.6 T. M. Forbes, Am. J. Psychol. 40, 1 (1928).6 F. L. Dimmick and M. R. Hubbard, Am. J. Psychol. 52, 242

(1939).7W. D. Wright, Researches on Normal and Defective Colour

Vision (H. Kimpton, London; C. V. Mosby, St. Louis, Mo.,1946), p. 237.

8H. de Vries, Genetica 24, 199 (1948).9 D. B. Judd, J. Res. Natl. Bur. Std. (U.S.) 43, 227 (1949);

J. Opt. Soc. Am. 39, 945 (1949).'5 E. N. Willmer, J. Physiol. (London) 111, 69 (1950).11 M. Tessier and F. Blottiau, Rev. Opt. 30, 309 (1951).

come from bimodal distributions. Bimodality suggeststhat people may differ with respect to some veryspecific inherited traits, rather than differing merely byrandom fluctuations in the inheritance of several traits.For example, observations appear to indicate that thereare some rather clear and dichotomous differencesbetween individuals with respect to (1) binocularsynthesis of yellow,1 ' 4 (2) spectral location of "uniquegreen,"'5 (3) recovery of sensitivity following adapta-tion to intense light,7 and (4) color matches.0 In anattempt to make some sense out of these and otherobserver differences, several experimenters (Abney andWatson, Forbes, Talbot)2 5",4 have proposed that color-normals may be subdivided into only two groups, asopposed to a large number of groups formed by per-muting all types of bimodal differences. To date,however, there has been no major attempt to prove thishypothesis, or to discover the structural differenceswhich may underlie the perceptual differences.

One objective of this study is to show first that

12 R. W. Pickford, Individual Differences in Colour Vision(Routledge & Kegan Paul, London, 1951),

13 H. Juricic, Rev. Opt. 31, 177 (1952).14 S. A. Talbot, Science 115, 220 (1952).11 M. L. Rubin, Am. J. Ophthalmol. 52, 166 (1961).16 W. S. Stiles and G. Wyszecki, J. Opt. Soc. Am. 52, 58 (1962).17 P. W. Trezona, J. Physiol. (London) 150, 67 (1960).

1047

WHITMAN RICHARDS

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FIG. 1. Distribution of spectral values for unique green. Foreach graph, the unique-green points on the abscissa are indicatedin terms of wavelengths, and the ordinate shows the number ofindividuals who chose a particular wavelength as their uniquegreen. The upper graph has been taken from Rubin.15 The lowergraph shows current results for a 40 test field of 75 cd/min seenagainst a surround of about 3 cd/in (color temperature 25000 K).The stippled area includes observers who were subsequentlyclassified as belonging to Class II.

observers may, in fact, be consistently classified bytheir observations of several phenomena which havebeen previously reported to give bimodal results. Thevarious phenomena examined, some new and some old,were chosen primarily in the hope of elucidating anystructural differences between the color-vision mecha-nisms of individuals. The result of some preliminarysifting yielded four major methods for classification andpossible correlation: (1) spectral location of uniquegreen,' 5 (2) additivity of spectral lights," (3) rate ofrecovery of sensitivity,7 and (4) colorimetric coordinatesof a "white". 9

Apparatus

All of the following tests were conducted with thesame apparatus, described previously by Wasserman.'8

Briefly, the optical system consisted of a xenon arc,B&L monochromator (half-bandwidth 4.4 nm), a 90adapting field, and accessory choppers and circularneutral density wedges. Provisions were also made tointroduce a second light of high purity into the systemby inserting various Wratten or interference filters,specified below under the appropriate sections. Thesubject had a maxwellian view (2-mm effective pupil)of the test stimuli and of a 90 circular adapting field

18 G. S. Wasserman, J. Opt. Soc. Am. 56, 242 (1966).

having a correlated color temperature of 25000K and aluminance of 3 cd/m2 . This 90 field was on continuously.All viewing was with the right eye, held in alignmentwith the aid of a reduction tube and chin rest.

Subjects

Practically all of the subjects were local universitystudents and included both males and females. Maleswere always tested for normal color vision with Ishiharaplates. When, necessary, hyperopia and myopia werecorrected with accessory lenses, but no subject wasaccepted if the necessary correction exceeded 2 diopters.

THE SPECTRAL LOCATION OF UNIQUE GREEN

The concept of "unique green" has been defined inmany ways. Our definition is similar to that used byRubin,'5 who found that the wavelengths representing"pure green" were bimodally distributed for 278 in-dividuals, as shown in the top part of Fig. 1. In Rubin'sexperiment, the subject adjusted a monochromator tolocate the wavelength which appeared the purest green,i.e., a green which was neither bluish nor yellowish, butone which "embodied only one kind of huedness." Themonochromatic light illuminated one half of a split 6°circular field. The opposite half of the split field con-tained a comparison "white" light (65000K) of about3 cd/m2 . Fixation was maintained between the twohalf-fields while the subject manipulated the luminanceand the wavelength of the test half of the field until asatisfactory "pure green" was found.

Methods

Rather than allowing the subject to be continuouslyexposed to the test light, we presented our wave-lengths for only 2 to 3 sec against an adapting field witha correlated color temperature of 25000 K. The sub-ject's task was to judge whether the wavelengthpresented was an example of a "yellow-green" or of"blue-green." Even though the concept of "uniquegreen" was defined for the subject in a manner analog-ous to that used by Rubin, we did not permit the use ofa "pure-green" category in the forced-choice testprocedure. The wavelengths were presented semi-randomly over a range of 495 nm to 550 nm. The actualspread varied for each individual and for each condition.In general, a running-average estimate of the locationof the pure green wavelengths was made, and onlywavelengths plus or minus about 15 nm about thispoint were used, varied in 2-nm steps. For about 90%of the observers, the end points or the preceding wave-lengths had no effect on the following judgments. Thestandard deviation of the determination of the spectrallocation of the pure green wavelength averaged about3 nm, as compared with a 2-nm standard deviation ofmeasurements found by Rubin. Generally, each ob-server was tested once on each of two separate days.

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DIFFERENCES AMONG COLOR NORMALS

Most of the variability represented a slight change ofcriterion between sessions.

Unique-green determinations were made for circularstimuli of the following diameters: 40, 20, and 10. Thesecolored disks had a luminance of about 75 cd/mi2 , seensuperimposed on the 3 cd/in2 adapting field. Approxi-mately 10 min dark adaptation preceded each session,and familiarization tests were made during this period,with the stimuli presented alone without the adaptingfield.

Results

Essentially identical values for the spectral locationsof unique green were found for the 10, 20, and 40fields.19 The 40 results for 90 observers are shown in thelower portions of Fig. 1. The distribution of observersappears bimodal, with no differences between the sexes,as previously reported by Rubin."5 Because all ob-servers were subsequently reclassified by another,independent test, the stippling has been used to separatethe observers more clearly. Note that the modes andspread of the distribution of Rubin's observations andthe current 40 observations do not agree exactly. Thisdiscrepancy is not due to field size, however, becauseour 20 and 40 distributions were essentially identical.More likely reasons for the differences between Rubin'sand the present observations are (1) differences of thepurities of the stimuli used, (2) differences of the cor-related color temperatures of the adapting fields, and(3) difference in stimulus luminance. In spite of thesedifferences, however, the general similarity of theresults plus the occurrence of a minimum near 520 nmin both portions of Fig. 1 is strong evidence that thetests are approximately equivalent.

Ocular Effects

As will become clearer later when other bimodallydistributed tests are correlated with the unique-greenmodes, ocular differences, such as ametropia, are notresponsible for the bimodal distribution of unique-greenwavelengths. Approximately 40% of the observersrequired accessory lenses, which were selected to matchtheir eyeglasses. These observers who required cor-rection lenses were distributed equally between the twounique-green distributions.

Variation of the density of the macular pigment alsoseems an unlikely explanation of the bimodal effect. Thebulk of the energy of the adapting field (25000 K) waslocated at 500 nm or longer, and a rather large differenceof macular pigment density would be required toproduce a sizeable color shift of the surround againstwhich the test wavelengths were judged. Subsequenttests presented in later sections will support this con-clusion that the unique-green differences cannot beattributed to macular pigmentation.

19 For test fields smaller than 1', the location of unique greenmay shift toward longer wavelengths for some observers.

ADDITIVITY TESTS

A. Sharpening the Unique-Green Modes

The large spread of wavelengths for the variousobservers who seem to belong to the same class isdisturbing and prevents a precise classification. If theunique-green point represents a balance between the"blue" and "yellow" sensations, then a possible reasonfor the large spread of the distribution could be that the"blue" and "yellow" or "red" color mechanisms areinteracting differently for the various observers. Forexample, a retina having a relatively large populationof 580-nm cones might produce a dominant red me-chanism which would attenuate the activity of the bluesystem. If this is the case, then such nonreciprocalinteractions between independent color mechanismsmight be revealed by considering the brightness effectsof adding small amounts of one wavelength to a largeamount of another. Tessier and Blottiaull have alreadyshown that observers differ considerably with respectto the linearity of these additions.

Method

The apparent brightness of a light composed of about10% of 450 nm plus 90% of 645 nm was comparedsimultaneously with a light of 645 nm. The observersaw two 10 fields separated vertically by 30, both seenagainst the 90 surround (25000 K). For these and thesubsequent additivity tests, the 1° lights were notsuperimposed upon the surround, which was eliminatedin the 1° areas illuminated by the test lights. Thestandard 645-nmlight (Wratten filter no. 92) appearedequivalent in brightness to about 50 cd/n 2 . By manipu-lating a knob, the observer could change the luminanceof the upper field, containing only the 645-nm standardlight, so that the brightnesses of the upper and lowerfields could be equated. First, to establish unit lumi-nances for each light, the observer made heterochro-matic matches between the 450-nm and 645-nm lightswith each light presented simultaneously in either theupper or lower field. It was not necessary that thesematches exactly equate the two lights. After com-pleting the heterochromatic matches, 10% of the unitluminance of the 450-nm light was then presented in thelower 10 field, together with 100% of the unit luminanceof the 645-nm light. The upper 10 field contained onlythe 645-nm light, and the luminance of this light wasthen adjusted to match the combination seen in thelower field. At random intervals, the 10% of 450-nmlight was removed from the lower field, leaving only the645-nm light. These matches between identical 645-nmlights provided the comparison luminance against whichthe 450-nm plus 645-nm combination was compared.

Of interest was whether or not the 10% addition of450 nm increased or decreased the apparent brightnessof the unit luminance of the 645-nm light. At least sixmatches were obtained for each combination in order to

August 1967

WHITMAN RICHARDS

as indicated by the straight lines and by the crosses andcircles the observers may be divided into two separatelgroups. Each group has a comparable range for thered-blue index. However, depending upon the group, adifferent unique-green wavelength is associated with

0 xthe same red-blue index. This suggests that the vari-o \ations of the unique-green wavelengths about the two

modes may be due to independent, individual differ-ences in the relative strengths of the red and blue

500 510 520 530 540 mechanisms. For example, a strongly dominant blueWAVELENGTH,nm mechanism may displace the modal value of unique-

areen teither QA0 nn nr tn R4nf nrn Apnprlina linn-nFIG. 2. Correlation between the "red-blue" additivity index

(ordinate) and the spectral location of unique green (abscissa) fora: 1f ield. The additivity index is a measure of the reciprocity ofthe brightness interactions between 450- and 645-nm lights. TheClass 1 observers (crosses) form one distribution, 'whereas theClass II observers (circles) form a similar distribution, which isdisplaced toward longer wavelengths. These results suggest thatthe variations of the unique-green settings about their respectivemodes are due to individual differences in the interactions betweenthe 440-nm and 580-nm cones.

give an additivity index, which was merely the ratio ofthe luminance of the 645-nm standard required tomatch the 450-nm plus 645-nm combination, dividedby the luminance of the 645-nm standard required tomatch the 645-nm light alone. Tihs, an index of 0.8meant that the addition of 10% of 450-nm to the645-nm light of unit luminance resulted in a light whichcontained more energy but which appeared about 20%less bright than the 645-nm light alone. In addition tomeasuring the effect of small amounts of 450-nm onlarge amounts of 645 nm, the reverse additivity effectswere also determined in a similar manner. In this case,the standard light of 450 nm was obtained with aWratten 47B filter. The actual values measured for 16observers for the two conditions are shown in the graphat the top of Fig. 3. Of particular interest at this point,however, was the nature of the reciprocity in the inter-actions between the blue and red mechanisms. Ameasure of this reciprocity is the ratio between theadditivity indexes for 10% of 450 nm plus 645 nmcompared with the 10% of 645-nm plus 450-nmcombination:

red-blue index

Apparent brightness of 10% 450 nm plus 645 am

Apparent brightness of 10% 645 nm plus 450 nm

If neither the blue nor the red mechanism predominates,then any brightness interactions will be reciprocal andthe red-blue index will be 1.0. On the other hand, if theblue mechanism dominates the red and drives itssensitivity down, then the red-blue index should beless than 1.0.

ResuvIs

Figure 2 shows a plot of the red-blue index vs thesame individual's unique-green wavelength. Clearly,

the observer's group. It is important that thesered-blue interactions appear independent of the basicmechanism which divides the observers into the twogroups.

Typology

The spectral location of unique green, based upon thered-blue index, can now be used as a clear basis fordividing observers into two groups. In order to tietogether in a preliminary way the results of the follow-ing sections, those observers who belong in the short-wave component of the distribution with mode near513 nm will be classified as Class I, whereas thoseindividuals falling in the long-wave portion of thedistribution with mode near 527 nm will be identifiedas Class II.

B. Further Additivity Tests

At the same time that the additivity effects of 450-nmand 645-nm lights were investigated, a 530-nm lightwas also included in the series. These three lights werepaired in the same manner as before, using largeamounts of one with small amounts of the other, givinga total of six possible pairs.

Method

The procedure for each of the six pairs was identicalto that described above for the 450-nm and 645-nmcombinations. As before, the standard lights were645-nm (Wratten 92 filter), 450-nm (Wratten 47Bfilter), plus the additional 530-nm light (B & L inter-ference filter with half-bandwidth 9 nm). A B & Lmonochromator was used to produce the second set ofnarrow-band lights of the same wavelengths. As before,the parameter of interest was an additivity indexindicating whether the small amount of the secondwavelength increased or decreased the brightness of thecombination, as compared with the unit luminance ofthe standard alone. An additivity index of 1.4 meantthat 40% more energy of the standard alone was neededto match the addition of 10% of a second wavelengthto a unit amount of the standard.

Resulls

Figure 3 shows the values for the additivity indexesfor 16 observers for the six possible pairs of lights. Each

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DIFFERENCES AMONG COLOR NORMALS

graph compares the effects of only two lights on eachother. For example, the top graph shows that there isno systematic correlation over individuals between theadditivity effects of small amounts of 450 nm added tolarge amounts of 645 nm, as compared with the inversemixture. On the other hand, we already know that theseadditivity values can be correlated with the unique-green settings, as shown in Fig. 2. The crosses representthe Class I observers (unique green: 513 nm), whereasthe circles indicate the Class II observers (uniquegreen: 527 mn).

The middle graph of Fig. 3 reveals a correlation overindividuals when the additivity effects of 530-nm and645-nm lights are compared. These lights probablystimulate primarily the 540-nm and 580-nm cones andthe corresponding "green" and "red" mechanisms.These mechanisms appear to act reciprocally upon oneanother, as indicated by the linear distribution ofpoints. In the ideal case, i.e., if the red and greenmechanisms were stimulated independently by the twolights, the slope of the line should be one, and should gothrough the point 1.1, 1.1 if the red and green mecha-nisms act in a truly reciprocal manner upon oneanother.

The fact that the best-fitting line drawn on the graphgoes through a mean point of 0.95, 0.95 and not through1.1, 1.1 deserves closer investigation. First, it suggeststhat for the average observer, the red and greenmechanisms inhibit one another. Secondly, the slope ofthe line suggests that small increases of the activity ofthe green mechanism generally have a greater effectthan comparable increases of the activity of the redmechanism. Finally, both the crosses (Class I observers)and the circles (Class II observers) lie along the sameline, suggesting that these red-green interactions areindependent of the basic Class I-Class II difference.

The graph at the bottom of Fig. 3 has provided amost revealing indication of the basic differencesbetween the Class I and Class II observers. This graphcompares the effects of adding 450-nm and 530-nmlights, which should primarily stimulate the blue andgreen mechanisms. Clearly, for these lights, the Class Iand Class II observers differ, as seen by the circlesgenerally lying above the crosses on this graph. Thewide distribution of the points shows that the smallamounts of 450-nm and 530-nm lights added to eachother may either increase or decrease the effectivebrightness, depending upon the observer. Character-istically, however, the effect of a small amount of530 nm added to 450 nm for the Class II observer is toraise the apparent brightness of the combination abovethe value expected on the basis of the Class I results.The average vertical displacement of the Class IIcircles is more than 8%, comparable to the actualphysical increase of luminance when the 10% of 530-nmlight was added to 450 nm. This could indicate, there-fore, that, for the Class IL observer, the 530-nm additionstimulated both an independent green mechanism (as

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in the Class I case), plus a separate green mechanismwhose activity summated with the blue mechanism.2 0

In other words, for the Class II observer, the smalladdition of 530 nm appears to have a double effect;first, with respect to the customary blue-green inter-actions, and, second, by increasing the activity of theblue mechanism.

20 Rod activity may also feed the blue mechanism. [W. A.Richards and S. M. Luria, Vision Res. 4, 281 (1964).] However, arod contribution to the blue mechanism does not seem to be thesignificant factor causing the class differences reported in thispaper. First, the spectral location of unique green never decreaseswhen small ( A0 ) foveal fields are examined. If the unique-greenpoint changes, it increases toward longer wavelengths, suggestingthe addition of a longer-wave desaturant in the central fovea. Thiswould exclude rods. Second, the modes for the ' ' foveal fieldagree with those for the 40 field, suggesting that the same type ofreceptor is desaturating the blue mechanism in both cases.

August 1967

WHITMAN RICHARDS

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FIG. 4. Curves showing the recovery of sensitivity for Class I(upper set of points) and Class II (lower set of points) observers.The two solid lines describe similar data collected by Wright,7 whodivided observers into two major groups, one having a "direct"recovery curve and the second having a "delayed" recovery curve.(The lower points and curve have been displaced downward by afactor of 2.) In the present tests, the Class I observers always gavea direct curve, whereas the Class II observers gave delayed curves.The recovery curves are for a 10', 50-msec flash of 640-nm light,following adaptation to a light of about 1000 cd/m 2 (color tem-perature 2400'K).

RECOVERY OF SENSITIVITY

Wright 7 some time ago reported considerable in-dividual differences of the rate of recovery of sensitivityfollowing adaptation to very bright lights. The recoverycurves obtained by Wright can be broadly classifiedinto two general types; one shows a recovery processthat sets in as soon as the adapting light is removed(direct recovery), and a second type with the recoverydelayed for a few seconds after extinction of the adapt-ing light (delayed recovery). Wright's results for about90 observers were obtained under the following condi-tions: a 5° adapting light of 6000 td (color temperature=4400 0K) was viewed for about 3 min, and then therecovery of sensitivity for a "red" 20 test patch wastracked by haploscopic matching to a comparison patchof 30 td seen with the other eye. The two types of curvesobtained by Wright using this procedure are shown bythe solid lines in Fig. 4. The delayed curve has beendisplaced downward on the ordinate by a factor of 2.

Method

For convenience, we deviated slightly from Wright'smethod when obtaining recovery curves from the Class

I and Class II observers. First, the adapting light waslimited to 20, and, more importantly, had a colortemperature of about 24000K, with a luminance ofabout 1000 cd/m2 (2-mm effective pupil). Therefore, agreater proportion of the 580-nm and the 540-nmphotopigments were affected by our adapting field, ascompared with Wright's field which stimulated rela-tively more 440-nm photopigment. Secondly, the"red" test field was presented as test flashes of a 640-mmlight in a 10' diameter spot. Each flash lasted 50 msecand occurred 1.5 times a second. These flashes werepresented in the center of a bull's-eye target formed bya narrow 20 annulus and a dark spot in the adaptingfield. When on, the adapting light appeared as a 2°field centered 40 above the bull's-eye where the testflashes appeared.

The procedure was as follows: The observer staredat the adapting light for approximately 3 min. Upon asignal, he then looked down into the center of thebull's-eye, maintaining fixation by keeping his after-image centered within the black annulus. At the sametime, he was required to attend to the brightness of theflashing light. Initially, only 3 to 5 flashes were pre-sented, in order that the luminance of the test flashcould be set near threshold at the onset of recovery(i.e., when the adapting light had been extinguished).When the test flash was just above threshold, it usuallywas reported as "pink," not red. Once the necessaryluminance of the test flash had been determined, theobserver was instructed to note its brightness immedi-ately after the adapting light had been extinguished andafter his fixation had shifted to the center of the bulls-eye. The observer's task now was to maintain thebrightness of the test flash at its initial level by mani-pulating a knob which controlled a circular neutraldensity wedge inserted in the appropriate optical path.As sensitivity recovered, the increased brightness of thetest flash was offset by turning the wedge. These wedgesettings were recorded as a function of time, and pro-vided the estimate of the rate of recovery of sensitivityfor the 640-nm test flash. After several practice runs,four records of the wedge settings were superimposedand averaged to yield a rate-of-recovery curve.

Results

As shown by the upper curve in Fig. 4, all elevenClass I observers gave direct recovery curves, withtheir mean result in close agreement with the recoveryrates obtained by Wright (solid line). On the otherhand, all six Class II observers studied gave delayedrecovery curves (X2 = 13, p<0.001). (The lower pointsand Wright's delayed curve have been displaced down-ward by a factor of 2.) Even though the Class LI ob-servers seemed to recover slightly more rapidly thanWright's delayed observers, there was still no overlapbetween the Class I (direct) and Class II (delayed)curves during the initial stages of recovery.

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DIFFERENCES AMONG COLOR NORMALS

COLORIMETRIC COORDINATESFOR A "WHITE"

In order to demonstrate conclusively that macular-pigmentation differences were not responsible for theClass I and Class II differences, the chromaticitycoordinates of the surround were measured for a Vfield. These measurements led to some rather surprisingresults.

Method

The "white point" corresponding to the surround(correlated color temperature 25000 K) was measuredfor a 10 field following a method used by Sinden.?1 Thismethod involves finding the wavelength which com-plements certain standard wavelengths. The chroma-ticity coordinates of the observer's "white point" canthen be found by the intersection of the lines joining thecomplementary wavelengths necessary to give a matchto the "white" surround. Four standard wavelengthswere used: 450 nm (Wratten 47B), 470 nm (Wratten48), 605 nm (Wratten 72B), and 645 nm (Wratten 92).Additional neutral density filters were also used inconjunction with these color filters, and the finalestimates of the dominant wavelengths of the lightswere made by a visual color match against mono-chromatic wavelengths produced by the B & L mono-chromator. The observer saw a to field composed of twosuperimposed "monochromatic" lights (one producedby the Wratten filters, the other obtained with themonochromator) of variable luminance, and this 10field appeared in an opaque portion of the 90 surroundwhich had a luminance of 30 cd/m2 . For each of the four"standard" wavelengths (450, 470, 605, and 645 nrm)various settings of the monochromator were examinedas possible candidates for the complementary colorwhich, together with the standard, yielded a "white"which matched the surround in both brightness andchromaticness. The necessary luminance adjustmentsof the two monochromatic lights were made with twocircular neutral density wedges, one manipulated by theobserver and the second by the experimenter.

Results

The white points for nine Class I observers (crosses)and five Class II observers (circles) are shown in Fig. 5,which represents a portion of the standard CIE chroma-ticity diagram. For the particular "white" used, theClass II observers lie significantly above the Class Iobservers (x2= 10, p<0.005). An insert in Fig. 5 showsthe expected directions of displacement for macular-pigment differences (xanthophyll) and for differencesdue to other common ocular factors (cornea, lens,humors).Y The Class I and Class II differences are

21 R. H. Sinden, J. Opt. Soc. Am. 35, 737 (1945).2a D. B. Judd, in Handbook of Experimental Psychology, S. S.

Stevens, Ed. (John Wiley & Sons, Inc., New York, 1951), p. 813.

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clearly not in either of these directions. Also of interestis the effect of field size, which is shown for one observer(WR) by the positions of the 4°, 1°, and j0 symbols.Note that the effect of field size is in the general direc-tion of the spread of the Class I (or the Class II)observers.

The differences of the locations of the "white points"for the two classes of observers shown in Fig. 5 arereminiscent of differences examined by Juddg In orderto account for individual differences for color matchesof two supposed metamers, made from rutile andanatase titanium pigments, Judd was forced to recom-mend a modification of the standard luminous-efficiencycurve. Judd's modifications essentially change theweighting given to the short wavelengths. Whenapplied to the data given in Fig. 5, the effect of thismodification lies roughly in the direction of the arrowlabelled xanthophyll. Therefore, Judd's correction isalso not in the direction of the Class I-Class IIdifferences.

The differences between the white points of theaverage Class I and Class II observers do not seemexplicable in terms of the customary corrections formacular or lens pigmentation. Instead, the differencesshown in Fig. 5 are more in accord with increasedsensitivity for the Class II observer in the 500-540 nnregion of the spectrum. If a screening pigment presentin the Class II observer is responsible for the differencesof the white points, then this pigment must absorbprimarily at the short- and long-wave portions of thespectrum. Alternately the Class I observer couldpossess an additional screening pigment absorbingmaximally near 520 nm. To my knowledge, unless oneof the photopigments or its derivative is involved, nosuch stabile pigments are known to be present in thehuman retina. Therefore, the most obvious alternate

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WHITMAN RICHARDS

explanation for the white-point differences is to assumeeither that the receptor responses of the Class I andClass II observers are different, or that for at least oneof the classes of observers, there are serious departuresfrom additivity of color-mixture equations.

DISCUSSION

The fact that the four indices of the Class I andClass II difference are related appears puzzling. Clearly,the differences cannot be due to macular pigmentation,because such a screening pigment could not cause theadditivity differences shown in Fig. 3, nor couldmacular-pigment differences be responsible for thedirect and delayed recovery rates shown in Fig. 4.

If pigmentation differences are excluded, then perhapsthe next most obvious hypothesis is that the receptorsensitivities (i.e., mixed or different photopigments) ofthe Class I and Class II observers are different. Such ahypothesis must also be excluded, however, because anybimodally distributed differences of photopigmentsbetween observers would also cause the color-mixturefunctions to be bimodally distributed. To date, how-ever, there has been no evidence of bimodality of color-mixture functions which have been determined for areasonable sample under identical conditions.23 -25

In order to account for the results of all four tests, Iwould like to propose that, unlike the Class I observer,the Class II color-normal has 540-nm cones whichcontribute additional activity to the blue mechanism.2 0

This could be accomplished, for example, by packingextra 540-nm cones around the 440-nm receptors so thatthese extra 540-nm cones also innervate the bluemechanism. Such a desaturating effect would be con-sistent with the paler colors reported by the Class IIobserver,26 and could provide a simple explanation ofthe unique-green, additivity, and recovery-of-sensitivitydifferences. As will be shown later, the color-matchingresults may also be interpreted with one additionalassumption.

If there is neural desaturation of the blue mechanismby 540-nm cone activity, then the spectral sensitivityof the blue mechanism for the Class II observer wouldbe displaced toward longer wavelengths, and wouldproduce a corresponding shift in the spectral location ofunique-green (assuming that unique-green represents abalance between the blue and red or yellow mechanism).Similarly, an additional contribution of the 540-nm

23 W. D. Wright, Trans. Opt. Soc. 30, 141 (1928-1929).24 1. G. H. Ishak, J. Opt. Soc. Am. 42, 844 (1952).25 W. S. Stiles and J. M. Burch, Opt. Acta 6, 1 (1959).26 Of the 100 subjects examined to date, one observer has been

found whose left eye is Class I and whose right eye is Class II. Thisdifference is retinal, because both the left and right hemi-fields ofthe left eye are Class I, whereas both the left and right hemi-fieldsof the right eye are Class IT. The subject reports that with his lefteye colors appear more saturated than the same colors seen withthe right eye, and he prefers to use this eye for monocular ob-servations with microscopes. Both eyes are slightly astigmatic; inaddition, the left eye (Class I) requires a -2 diopter correction,whereas the right eye requires only a -1 diopter correction.

cones to the blue mechanism would also explain thedifferences between the Class I and Class II observerswhen the additivity effects of 530-nm and 450-nm lightsare compared. For the Class II observer, the 530-nmlight would affect both the "green" and "blue" me-chanisms, whereas for the average Class I observer, onlythe "green" mechanism would be activated. Therefore,a small amount of 530-nm light added to 450 nm shouldalways appear brighter to the Class II observer than tothe comparable Class I observer, as suggested by thelower graph in Fig. 3.

The third result (Fig. 4), involving the differences inthe rates of recovery of sensitivity to a "red" test flash,may also be plausibly explained, in terms of neuraldesaturation by 540-nm cones, providing that a flashseen near threshold is always detected against thebackground activity of the blue, green, and red mecha-nisms, which act somewhat independently. 2 7-2 9 Aconditioning field of 24000 K normally does not affectthe 440-nm cones to any great extent. Hence, if theactivity of the blue mechanism is also not affected bythe 540-nm cone activity, then the Class I observer canestimate the recovery of sensitivity of the red mecha-nism by comparison with a lower level of steady activityin the blue channel. On the other hand, if the 540-nmcones contribute activity to the blue channel, then theactivity of the blue mechanism of the Class II observerwould be higher, and will change as the 540-nm photo-pigment regenerates. For the Class II observer, extinc-tion of the adapting light would then be followed byrecovery of sensitivity of all three channels. Therefore,the Class II observer would not have the advantage ofan immediate differential change of activity of the redmechanism relative to the blue channel, which isavailable to the Class I observer.

The fourth result, that there are Class I-Class IIdifferences of the calorimetric coordinates for "white,"is the most difficult to interpret. These coordinates maybe changed only by altering the effective sensitivities ofthe photopigments. If the neural-desaturation hy-pothesis is to be preserved, therefore, we must reach theunlikely conclusion that similar genetic factors arebeing manifested in two quite different ways; forexample, by altering neural linkages and by introducinga screening pigment. From a theoretical point of view,such an explanation is quite unsatisfactory; a singlehypothesis which explained the results of all four testswould be much preferred over a multi-factor inter-pretation.

However, there is still at least one other way ofaccounting for the colorimetric differences between theClass I and Class II observers shown in Fig. 5, without

27 W. S. Stiles, Coloquio Sobre Problenmas Opticas de la Vision(Union Internationale de Physique Pure et Appliqu6e, Madrid,1953), pp. 65-103.

28 W. S. Stiles, Proc. Natl. Acad. Sci., U. S., XVash. 45, 100(1959).

29 Mv. Alpern and XW. A. H. Rushton, J. Physiol. (London) 176,473 (1964).

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proposing direct differences confined within the re-ceptors themselves. For example, failures of additivityof the color-mixture equations could occur for one classof observer. Such failures would not require differencesof the color-mixture functions obtained with a maxim-ally saturated field, but there should be serious depar-tures from the additivity principle when these functionsare compared with similar functions obtained by makingmatches to "white." Crawford has already reportedsuch differences.3 0 The nature of the mechanism whichleads to these failures of strict additivity is unknown atpresent. However, Crawford does present evidencewhich suggests that at least part of the differencesbetween the white and maximum saturation fieldconditions cannot be due to the presence of rods.

If the presence of a fourth photopigment is excluded,then the failures of additivity must represent some kindof coupling between the receptors which affects photo-chemical excitability. Such a coupling need not be adirect electrical one, but could also arise from selectivediffusion of substances involved in photolysis.Y1 In orderto illustrate the effect of such a coupling upon colormatches, consider the possibility that when the 540-nmphotopigment is stimulated alone, then the sensitivity

30 B. H. Crawford, Vision Res. 5, 71 (1965).31 G. S. Brindley, J. Physiol. (London) 164, 168 (1962).

of the adjacent 440-nm receptor is reduced by eitherelectrical or photolytic action, and that the converseinteraction may also occur when the 440-nm receptoralone is activated by light. In order that calorimetricadditivity be preserved, we now require that thereceptors still interact in the same way even when theyare both simultaneously stimulated by light. Thiswould not be possible with either an electrical or diffu-sion model for the coupling because the opposingenergy gradients between the receptors would cancel.

Only a very weak kind of coupling between thereceptors would be necessary to destroy the additivityof color matches made under steady-state viewingconditions. If the 540-nm and 440-nm receptors aretightly packed together, then such indirect photo-chemical coupling is possible, in addition to neuralconvergence. The hypothesis that the photoexcitabilityof one receptor may be affected by its neighbor is notnew; it has already been proposed by Brindley1 in orderto account for some unusual color effects which developat the boundaries of after-images. If we accept Brind-ley's suggestion, then all of the observed differencesbetween the Class I and Class II color-normals could bethe consequences of different packing and assemblyarrangements for the various types of receptors,particularly the 440-nm and 540-nm cones.

Dr. George A. Morton, director of the Conversion DevicesLaboratory, RCA Laboratories, second David RichardsonMedalist, at the Meeting in Columbus, April 1967.

August 1967