JOURNAL OF THE OPTICAL SOCIETY OF AMERICA

Some Visual Functions of a Unilaterally Color-Blind Person. I. CriticalFusion Frequency in Various Spectral Regions*

EDA BERGER,t C. H. GRAHAM, AND YUN HsiADepartment of Psychology, Columbia University, New York, New York

(Received December 23, 1957)

A new case of unilateral dichromatism is described. She is ayoung woman with normal color vision in one eye and dichromaticvision of a primarily deuteranopic type in the other.

Critical fusion frequency functions for a centrally fixated 28-minfield were determined in ten spectral regions, ranging from onehaving a spectral centroid at 452 m to one with a spectralcentroid at 682 my, on both eyes of this unilaterally dichromaticsubject. Determinations were also made with white light. Measure-ments extended over a range of approximately 3.5 log milli-lamberts. For all colors except red, the curve of critical fusionfrequency vs log luminance for the color-blind eye is displaceddownward on the critical fusion frequency axis with respect tothe curve for the normal eye. For any given spectral region, thedisplacement is approximately constant over the luminance rangetested and the two curves do not reach the same maximumfusion frequency. The magnitude of the shift varies with wave-length. It is greatest in the green, next in the blue-green, yellow-green, blue, and yellow; there is a slight loss in the orange and

no detectable loss in the red. The data for white light data alsoshow reduced critical fusion frequencies for the color-blind eye.

These findings are taken to reflect a reduction in the dichromaticeye of the number of receptors (of a type especially sensitive togreen) available for excitation by the spectral range from about450 mg to 625 mis.

Some measurements of critical fusion frequency with a 1 greenand a 2 white field are also reported. They display the samegeneral trends as the data for small fields, but the extent of thedownward shift of the color-blind function is in each case lessthan that for the corresponding pair of curves with the 28-minarea. The reduction in the amount of downward displacementwith larger test fields comes about through a proportionatelygreater increase in critical fusion frequency with area for thecolor-blind than for the normal eye. The results are formulated interms of a nonuniform distribution of color receptors across thefovea.

INTRODUCTION

THIS paper deals with further investigations ofthe visual functions of a unilaterally color-blind

subject, an individual with normal vision in one eyeand color-blind vision in the other.t§

Several basic visual functions have been determinedfor this subject. Graham and Hsial obtained thresholdluminosity curves for both eyes. The two functions

* This work was supported by a contract between the Office ofNaval Research and Columbia University and by a grant fromthe Higgins Fund of Columbia University.

t Now at the Research Laboratory of Electronics, Massachu-setts Institute of Technology, Cambridge, Massachusetts.

: For a history and analysis of work on the problem see D. B.Judd, J. Research Natl. Bur. Standards 41, 247 (1948). See alsoE. Berger, "Some comments on the visual discriminations ofunilaterally color-blind persons" (in preparation). The latestresearch on unilateral color blindness reported by Judd is that ofLouise L. Sloan and Lorraine Wollach, J. Opt. Soc. Am. 43, 890(1948).

The four following references are the earliest, the second, third,and fourth being concerned with the famous case of HermannGoldenberg, the nature of whose color blindness still remains incontroversy: 0. Becker, Arch. Ophthalmol. Graefe's 25, 205(1879); A. Von Hippel, Arch. Ophthalmol. Graefe's 26,175 (1880);F. Holmgren, Proc. Roy. Soc. (London) 31, 302 (1881); A. VonHippel, Arch. Ophthalmol. Graefe's 27, 47 (1881).

§ We are greatly indebted to our subject's parents for sub-mitting to tests of their color vision at Columbia University.The mother gave normal responses for each eye on the Ishihara,Stilling, and Dvorine tests and gave settings that fell in themiddle of the distribution of anomaloscope scores. The fathergave normal responses for both eyes on the Ishihara, the Stilling,and the Dvorine tests. His performance on the anomaloscope wasnormal for the right eye. However, his settings for the left eyeindicated that he required a somewhat greater than averageamount of red to match a yellow. The settings, nevertheless,probably lie nearer the distribution for normal than for protanom-alous eyes.

' C. H. Graham and Y. Hsia, Proc. Natl, Acad. Sci, V. S. 44,46 (1958).

were found to differ in absolute sensitivity values inthe green and blue regions of the spectrum, the sensi-tivity of the normal eye in these regions being consider-ably greater than the sensitivity of the color-blind eye.At the red end of the spectrum, the sensitivities ofboth eyes are similar. These results are in line withresults Hecht and Hsia2 and Hsia and Graham3 obtainedin comparisons of the luminosity functions of a groupof deuteranopes with those of normal subjects: deu-teranopes can, and usually do, (e.g., in five out of Hsiaand Graham's six cases), show a loss of sensitivity inthe green and blue as compared with normal subjects.Future reports4 on the unilaterally color-blind subjectwill deal with her color naming, binocular color match-ing, hue discrimination, and color mixture. The readeris referred to the paper by Graham and Hsia1 fordetails that are introductory to the present account.

The present paper is concerned with fusion frequencyas a function of luminance in various spectral regionsfor both the dichromatic and normal (trichromatic)eye of our subject.

The experiment is performed to examine the questionof luminosity loss in the blue and green regions of thespectrum for this subject's color-blind eye as comparedwith her normal eye, particularly for luminances con-siderably above threshold such as those required toprovide high rates of critical flicker frequency. It isthought that such data may have a bearing on thenature of the sensitivity loss. Presumably the loss in

2 S. Hecht and Y. IIsia, J. Gen. Physiol. 31, 141 (1947).3 Y. Hsia and C. H. Graham, Proc. Natl. Acad. Sci. U. S. 43,

1011 (1957).4 Graham, Sperling, Hsia, and Coulson (in preparation). See

also Graham and Hsia, Science 127, 675 (1958).

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VOLUME 48, NUMBER 9 SEPTEMBER, 1958

September1958 VISUAL FUNCTIONS OF UNILATERALLY COLOR-BLIND. I

sensitivity may reflect a loss within a class of colorreceptors, or it may simply involve a question of raisedthresholds within this class. Color receptor lossesmight (in line with the idea that critical flicker is afunction of the number of activated neural elements) beexpected to be reflected in corresponding critical flickerfrequency losses in the deuteranopic eye over theentire luminance range. Elevations in thresholds ofgiven color receptors could, on the other hand, indicateheightened luminance requirements for the color-blindeye without, however, changes in maximum criticalflicker frequency. In any case, it seems desirable toexamine the characteristics of our subject's luminositydiscriminations at levels considerably above threshold.

THE SUBJECT

The subject was a young woman, 25 years of age atthe time testing commenced. Until shortly before shewas brought to our attention, she had been entirelyunaware of any defect in her color vision. An extendedexamination involving the administration of a series oftests designed to detect color deficit was undertaken inorder to establish the type of defect exhibited by thesubject.11

Red-green color blindness in the left eye was indicatedby the subject's performance on the following pseudo-isochromatic tests: Stilling,5 AO,6 Ishihara,7 andDvorine8 ; moreover, the responses on the Ishihara andDvorine plates indicated that she was deuteranopic.These same tests were passed by the subject's righteye without error. The results from the Farnsworth-Munsell 100-Hue Test9 plot as a deuteranopic line forthe left eye provided no time restrictions are imposedon the test. The right eye turned out to be normal onthe test.

The proportions of red and green required in thematch on yellow in the Hecht-Shlaer anomaloscope 10

by the subject's right eye fell at the peak of the distribu-tion in the normal range. The left eye yielded a basicallydeuteranopic response; by suitable variation of theintensity of the yellow light, the entire range of red

11 Dr. Gertrude Rand and Miss Catherine Rittler have alsoexamined our subject. They concluded that the responses of thesubject's left eye were not completely typical of the classical typeof deuteranopia. They found the subject's right eye to be entirelynormal. See reference of footnote 4 for further analysis (based onvisual functions determined by us) to be fully reported in laterpapers.

I J. Stilling, Stilling's pseudo-Isochlromatische Tafeln zur Prlyfungdes Farbensinnes, edited by E. Hertel (Thieme, Leipzig, 1936)19th edition.

6 American Optical Company, A-o Pseudo-isoclzromatic Plates(Beck Engraving, Philadelphia, 1940).

7 S. Ishihara, Tests for Colour-blindness (Kanehara, Tokyo,1932), sixth edition.

8 I. Dvorine, Dvorine Pseudo-isochromatic Plates (WaverlyPress, Baltimore, 1953), second edition.

I D. Farnsworth, Farnswortlz-Munsell 100-Hue Test (MunsellColor Company, Baltimore 1949).

1 M. P. Willis and D. Farnsworth, MRL Report No. 190, U. S.Submarine Base, New London, 11, 1 (1952).

and green mixtures could be matched perfectly toyellow. However, in matching the extreme of the redscale with yellow, the subject complained that theyellow could not be made bright enough to secure asatisfactory match. When the red portion of the fieldwas covered with a Jena glass neutral filter (density1.0), a good match was readily obtained.

Further tests revealed that a mixture of two primaries(460 my and 650 m/u) sufficed in order for the subject tomatch all spectral colors seen by her color-blind eye.¶

The neutral point for the subject's left eye, as deter-mined with a modified Helmholtz color mixer whichprovides a 5200° K white comparison field, was foundto be at 502 m,4.

An ophthalmological examination by Dr. R. L.Pfeiffer of the Eye Institute, College of Physicians andSurgeons, Columbia University, failed to reveal anyevidence of organic eye disease. The fundus of the lefteye was normal. Both eyes were found to be myopic,a -2 diopter lens being required for the right eye anda -4 diopter lens for the left eye to render the subjectartificially emmetropic.

From these various tests it was concluded that thesubject's right eye is normal and that her left eye isclassifiable as deuteranopicl with certain complicationsin the violet part of the spectrum. When, during thecourse of the present account, we speak of our subjectas deuteranopic, our use of the term as implying anapproximate classification should be kept in mind.

APPARATUS

The optical system employed was of the type pro-viding a Maxwellian view. A concentrated-filamentautomobile headlight bulb, monitored at 4 amperesand 6-volts dc, serves as the light source. The lightbulb is housed in an asbestos cylinder containing asmall circular aperture through which the illuminationenters the optical system. Light from the sourcediverges to a lens (L1) and thereafter converges in theplane of a rotating disk shutter which intercepts thebeam at its narrowest cross section. After passingthrough a small opening in a metal stop (S 1), close tothe shutter, the beam is first made parallel by anachromatic lens (L2) placed at focal distance from Si,then brought to a focus at the eye by another achroma-

¶ A later paper will demonstrate that, in her color-blind eye,our subject can match any narrow band of the spectrum with amixture of two primaries, 460 and 650 ma. For this reason, weshall refer to her color blindness as dichromatism. It is probablybasically deuteranopic in nature; certainly she shows a majorloss of luminosity in the green. One method of conducting colormixture experiments on her provide data that seem to be typicalof deuteranopes (cf, F. H. G. Pitt, Trans. Brit. Med. ResearchCouncil, Spec. Rep. Ser., 1935, No. 200) although it is true thatshe shows no small negative dichromatic coefficients as do Pitt'ssubjects. On the other hand, another method of performing colormixture gives data in the blue that are not similar to thoseobtained by Pitt. Her hue discrimination thresholds are lower inthe extreme blue than are those for the usual deuteranopes (seereference for footnote 4).

615

BERGER, GRAHAM, AND HSIA

tic lens (L3). A metal stop at S2, between lenses L 2 andL3, and placed at focal distance from L 3, limits the sizeof the stimulus so that it appears as a circular areasubtending a visual angle of 28 min at the observer'seye. Stimulus fields having angular subtenses of 1° and20, respectively, may be obtained by substituting otherappropriate metal stops at S2. Negative lens correctionsfor refractive error are introduced at the eye piece inconjunction with an artificial pupil having a diameterof 2.5 mm. Filters for regulating the spectral composi-tion and the luminance of the stimulus are inserted ina filter box between L 2 and L 3.

The entire apparatus is rigidly mounted and issealed against light beyond S,, the eye piece beinglocated in a completely darkened cubicle in which thesubject sits while making observations.

Intermittent stimulation is achieved by means of therotating shutter which consists of a black aluminumdisk from which two alternate 900 sectors have beenremoved. The axis of rotation is parallel to the stimuluslight path. The disk is mounted on the shaft of a dcmotor (Bodine, type NSH-34) whose axle is in turncoupled by a universal joint to a Weston tachometer.The voltage generated by the tachometer is read froma dc voltmeter. The speed of rotation of the disk iscontrolled by means of a variac connected in serieswith the motor. The relation of the voltage output ofthe tachometer to the rate of interruption of stimulationby the sectored disk, as determined with a strobotac,was found to be F=5.63V, where F refers to thenumber of total cycles per second, (each cycle consistingof a light and dark period), and V is voltage.

In order to specify the spectral composition and therelative visual effectiveness of the test fields used inthe measurements, the following calibrations wereundertaken.

The color temperature of the source was measuredwith a Leeds and Northrup optical pyrometer andfound to be 2700'K. The relative visual effectivenessof this source can be gauged by multiplying the relativespectral energy distribution of a Planckian radiator at2700'K by the relative spectral sensitivity of the eye(i.e., by the luminosity curve). Measurements of thephotopic threshold luminosity curves for both eyes ofthe subject had previously been obtained by Grahamand Hsia'; therefore, the luminosity curve' for hernormal eye has been used in these computations. Asall data to be reported pertain to this one subject,prime interest being centered on comparing the per-formance of her two eyes, the use of her own normaleye as a reference for such comparisons, rather thanthat of the standard observer of the CIE system of

11As the measurements for the subject's luminosity curveextended only to 430 m, values between 430 and 400 mp wereobtained by extrapolating the subject's curve on the basis of therecently published luminosity data of L. C. Thomson and W. D.Wright, J. Opt. Soc. Am. 43, 890 (1953). The two sets of dataagreed closely otherwise.

colorimetry,"2 seems warranted. Actually, the differencesresulting from the use of the subject's normal eyeluminosity curve instead of the standard CIE photopicluminosity function are slight, except in the blue, sothat they would not invalidate comparisons of her datawith other measurements that might have used theCIE curve as a weighting factor for the energy distribu-tion of the source.

The various spectral regions investigated in thisstudy were isolated by means of appropriate colorfilters, Corning glass filters, Wratten gelatin-in-glassfilters, or combinations of these, so chosen that theirspectrol centroids were distributed throughout thespectrum between 450 and 680 msu at approximatelyequal wavelength intervals. The choice of color filtersas a means of producing colored lights, instead of theuse of monochromatic radiation, was dictated by theluminance range to be covered; the luminances availablein the narrow spectral bands isolated by a monochroma-tor are not sufficient to permit exploration at highluminance levels. Since such exploration was one of theprime objectives of this study, it was deemed preferableto use filters having relatively wide spectral bandpasses in order to attain the requisite luminance levels,even though this procedure entailed some sacrifice ofspectral purity. In Table I are listed the individualfilters or the filter combinations by means of which tenregions of the spectrum were isolated. The spectraltransmittances of these filters were measured on aBeckman spectrophotometer. The half-band widths of

TABLE I. Filters for isolating spectral regions (C refers to a Corningglass filter; W, to a Wratten gelatin-in-glass filter).

Half-band width Spectral centroidbColor filters mjA m

C 5113 85 452C 5562+ 50 475W 48C 4784+ 35 500W 75C 3384+ 40 520C 4303+C 5031C 4010 62 539C 3482+ 33 568C 4303W 73 24 591C 2418+ 35 620C 4784C 2408+ 42 640C 4784C 2030 - 682

a That portion of the percent transmittance vs wavelength curve fallingbetween maximum of curve and half that value.

b See text for the description and computations.,Filter C 2030 is a sharp cut-off filter, absorbing all radiant energy

below 640 mu and transmitting at all wavelengths above that value;therefore, its half-band width cannot be determined.

12 D. B. Judd, Handbook of Experimental Psychology, edited byS. S. Stevens (John Wiley and Sons, Inc., New York, 1951), pp.811-867.

616 Vol. 48

September1958 VISUAL FUNCTIONS OF UNILATERALLY COLOR-BLIND. I

spectral transmittance are indicated in the table as wellas the spectral centroids computed by the followingmethods.

For any given color filter or filter combination, thetransmittance values (Tx) were multiplied at 10-muintervals, by the "white" light curve (EVx), that is,by the product of the energy distribution for the2700° K color temperature of the lamp (Ex) and theluminosity curve for the subject's normal eye (Vx).The resulting product curve (ExVxTx) thus representsthe relative values of luminous flux transmitted by agiven filter.

The absolute luminance of the test field in whitelight was measured by means of a monocular matchingtechnique. A brightness match was first made betweenthe test field and a standard source of the same areawhen both of these were viewed monocularly. Theluminance of the standard (with a known filter in thebeam) was then measured with a Macbeth illuminome-ter, and the maximum available luminance of theincandescent-lamp light was determined to be 729 000millilamberts. The maximum luminance for each of thecolored test lights was then computed from the ratioof the sum of the white product curve (Ex Vx) to thesum of the respective color product curve (ExVxTx).The spectral centroids () represented in Table I werecomputed from the relation

X,= (ZExVxTX)/(2;ExVxTx),

where all the symbols have the previously assignedmeanings.

In the subsequent discussion, whenever reference ismade to a specific wavelength, it is to be understoodthat the number cited refers to the spectral centroid ofthe spectral region isolated by a given color filter ascomputed for our subject's normal eye.

Wratten neutral tint filters were used to vary thebrightness of the test lights. The transmittance of eachof these filters was measured on a Martens polarizationphotometer.

PROCEDURE

The test field was viewed monocularly, an adjustablechin rest being used to aid in locating and stabilizingthe position of the subject's eye with respect to theeyepiece. The eyes were tested on an alternatingschedule in which, at a given luminance, critical fusionfrequencies were determined first for the right (normal)eye, then for the left (color-blind) eye. Any one experi-mental session involved the determination of a criticalflicker fusion function for each eye. Only one spectralregion was explored in any one session, the measure-ments extending over a luminance range of approxi-mately 3.5 log millilamberts.

A ten minute period of dark adaptation precededeach session. The test field was then illuminated at thelowest available level of luminance at which the subject

could both correctly identify its color under intermittentillumination with her normal eye and could also make asetting from flicker to fusion with confidence. Atluminance levels lower than this, the subject experiencedgreat difficulty in making settings, expressed un-certainty as to whether fixation was truly central, andhad but little confidence in the accuracy of her settings.Data collected at such levels showed marked variability.For these reasons, the two criteria listed above wereused to define the lowest level of the luminance rangeat which testing was commenced.

Considerations which served to fix the upper limit ofthe luminance range were primarily based on the inter-ference set up by the flicker in the visual field surround-ing the test patch. When a bright spot in a dark field ismade to flicker, the dark field itself is seen to flickeralso. This field flicker becomes more pronounced withincreasing luminance of the test object, until a point iseventually reached where the flicker rates for the fieldand the test object are equal. The flicker of the visualfield at high test object luminances, which Bartley1 3

attributes to entoptic scatter and dispersion in theocular media, can effectively be ignored by an appro-priately instructed subject, provided the two rates offlicker are sufficiently disparate. When the two ratesare closely similar or equal, the interference set up bythe visual field flicker becomes sufficiently disturbingto render meaningless a judgment of fusion of the testpatch; the subject can no longer discriminate what isbeing fused, the flicker of the test object or that of thevisual field surrounding it.

These factors served to determine the luminancerange over which measurements were made as well asthe instructions given to the subject. The subject wastold to fixate the test patch centrally, so that its imagefell in the central fovea. She was permitted to adaptfor one minute to the prevailing luminance level,following which the experimenter started the sectoreddisk. By means of a suitably located knob connectedwith the variac, the subject then adjusted the frequencyof interruption until the field appeared just fused. Abracketing procedure was used in making the settings,the final adjustment always being made in the samedirection, from flicker to just noticeable fusion. Priorto the next determination, the disk was set at a differentfrequency of interruption. These changes were maderandomly, so that on successive trials the subjectsometimes started her settings from frequencies abovefusion, at other times, from frequencies below fusion.A total of four, occasionally six, judgments was made.The first of these was regarded as a practice settingand was not included in the data on which the averagecritical fusion frequency for that luminance wasdetermined. The luminance levels were presented in theorder of increasing magnitude. At the higher levels,where the subject reported the presence of flicker in

13 S. H. Bartley, J. Exptl. Psychol. 19, 342 (1936).

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BERGER, GRAHAM, AND HSIA

TABLE II. Critical fusion frequency for the right (normal) and left (color-blind) eyes of a unilaterally color blind subject as a functionof log luminance for different wavelengths (central fixation; 28-min test field).

Cycles per second Cycles per second Cycles per secondWavelength Right Left Wavelength Right Left Wavelength Right Left

mg Log mL eye eye M, Log mL eye eye mL Log mL eye eye

452 -1.31 14.1 10.5 539 -1.02 14.8 7.55 620 -1.05 16.6 15.9-0.93 15.6 12.0 -0.45 18.5 9.18 -0.37 18.9 17.6-0.63 20.8 16.6 0.15 21.0 11.7 0.13 21.6 20.4-0.05 23.7 20.1 0.83 26.4 15.8 0.81 24.8 23.8

0.64 28.7 25.1 1.41 30.9 21.0 1.20 28.3 26.51.32 32.6 29.5 2.10 36.3 26.2 1.88 29.9 28.71.71 33.3 30.2 2.78 38.7 29.1 2.27 31.3 30.02.09 35.2 33.3 2.65 30.0 29.3

475 -0.88 16.2 13.0 568 -0.64 19.6 15.1 640 -0.88 17.1 17.0-0.58 18.1 14.1 0.04 22.4 17.8 -0.20 20.2 20.6

0.00 22.6 17.9 0.63 25.8 21.4 0.39 24.1 23.70.69 24.7 20.3 1.22 29.6 24.7 0.98 26.6 26.21.37 31.2 26.2 1.61 32.8 27.2 1.37 27.7 27.21.76 32.2 27.0 1.99 33.7 28.7 1.75 30.4 30.02.14 34.6 30.3 2.29 34.2 29.2 2.05 28.4 28.02.44 34.6 29.9 2.68 31.9 26.6 2.44 29.1 29.02.74 33.3 27.9 2.82 28.0 27.9

500 -0.42 19.5 14.6 591 -1.18 18.7 15.4 682 -1.19 16.8 17.40.08 22.1 16.6 -0.58 22.5 19.5 -0.81 19.7 19.40.76 25.5 20.4 0.10 25.1 22.8 -0.22 23.1 22.61.15 29.1 23.8 0.68 29.8 25.8 0.37 27.2 26.11.83 33.1 27.5 1.37 33.7 30.9 1.14 29.0 29.42.22 35.8 30.1 1.75 36.8 34.1 1.83 31.4 31.52.60 38.1 33.1 2.05 38.2 34.8 2.51 29.9 30.22.90 37.6 32.8 2.44 37.1 35.8

520 -0.36 18.1 13.00.32 22.0 16.20.82 25.0 18.51.50 29.1 22.41.89 32.4 26.42.57 35.8 30.02.96 33.7 27.4

the field outside the test object, she was told to attendonly to the flicker of the central patch and to adjust itto fusion, these instructions being frequently repeatedduring the course of the settings. When the point wasreached where the visual field flicker became so pro-nounced that the subject could no longer ignore it andreported uncertainty as to whether or not the straylight phenomena were influencing her settings, testingwas discontinued.

A minimum of three experimental sessions wasdevoted to obtaining the data with any one color filteror filter combination. From seven to nine luminancelevels were used for each color. All the data for onecolor filter were collected in consecutive sessions beforetesting on the next color was started. No systematicschedule was adhered to, however, with respect to theorder in which filters having different selective spectraltransmissions were tested, so that the various spectralregions explored were represented randomly over theentire course of the experiment.

The data to be reported are based on a total of 30experimental sessions, 3 for each of the 10 selectedspectral regions with the 28-min test field. In addition,measurements were made of the critical flicker fusionfunctions for incandescent-lamp light with test fields of

28 min and 2 in angular subtense, respectively, andalso for green (539 m) light with a 1 field.

RESULTS

The data are summarized in Table II. This tableshows the mean critical fusion frequency values forboth eyes at each of the spectral regions for a centrallyfixated 28-min circular test field. Each entry is theaverage of the measurements made in three separateexperimental sessions and is based on a minimum ofnine settings.

A graphical representation of these data is providedin Fig. 1, where the average critical fusion frequencyvalues for the right (normal) eye are shown by opencircles, those for the left (color-blind) eye by solidcircles. The smoothed curves drawn through thesepoints are empirical. The number in the upper left-hand corner of each plot refers to the spectral centroidof the region isolated by a given color filter or filtercombination.

The shapes of all of these functions are similar; thecurves rise slowly and uniformly until the highestluminances are reached where most of the curves tendto flatten out before declining. This type of relation

618Vol. 48

UNILATERALLY COLOR-BLIND. I

is the one typically found for small centrally fixatedfields falling within the fovea.' 4

For all spectral regions except 640 and 682 mAu, two

curves have been drawn, one for each eye. Inspectionof any given pair of curves reveals that they may betaken as parallel over the luminance range tested, thecurve for the color-blind eye being displaced downwardon the critical fusion frequency axis with respect to thenormal eye curve. The magnitude of this displacementvaries with wavelength. It is maximal in the green at539 m/ where a difference of approximately 9.5 cycles

per second separates the two functions. From the greenonward toward the short wavelength end of thespectrum, the separation between any pair of curvesdecreases, the displacement at 452 mju correspondingto a downward shift of about 3.5 cycles per second.

Similarly, from the green onward toward the long wave-length region, there is a progressive diminution in theextent of the displacement of the color-blind curve

4 452mj / 475m30.

20 X

I0

Z 40o 50O mp 5 2Omju

w 30 ,

30 2w0.(.0

-J40> 5 39mju 568 mp

30

Z 0

0lowLL 4

Z 5 91rrp 620 mj

0 3 o

20

I-

O 0640mp 682 m

30

20

1O

IC.? 0.0 1,0 2.0 3.0 -1.0 0.0. 1.0 2.0 3.0

LOG LUMINANCE (MILLILAMBERTS)

FIG. 1. Critical fusion frequency functions for the normal (opencircles) and color-blind (solid circles) eyes of a unilaterallydichromatic subject in various spectral regions. Central fixation;28-min test field.

14 R. Granit and P. Harper, Am. J. Physiol. 95, 211 (1930);S. Hecht and E. L. Smith, J. Gen. Physiol. 19, 979 (1936); R. T.Ross, J. Gen. Psychol. 18, 111 (1938); V. V. Lloyd, Am. J.Psychol. 65, 346 (1952).

FIG. 2. A com-parison of the criti-cal fusion frequencyfunctions for the nor-mal (open circles)and color-blind (solidcircles) eyes of a uni-laterally dichromaticsubject for "white"(incandescent - lamplight) and green testfields of various sizes.Top: "white" light,28-min field; center:"white" light, 20field; bottom: green(539 m) light, 10field. Central fixa-tion.

50

7

zaA

0

U)-

4M

C., 10

50

40

30

20

10

0-2.0 -1.0 00 10 2.0 30

LOG LUMINANCE (mL)

relative to the normal until, when the extreme red isreached, one function adequately describes both sets ofdata.**

Measurements with a 28-min white field are reportedin Table III and shown in Fig. 2. They manifest thesame trend seen in the data for colored light: thefunctions for the two eyes are essentially parallel onthe critical frequency axis, the curve for the color-blindeye being shifted downward by a factor of approxi-mately 8.0 cycles per second.

Some determinations with larger test objects; a 10green (539 miu) and a 20 "white" field produced byunfiltered incandescent-lamp light are also listed inTable III and plotted in Fig. 2. They again show theparallel downward displacement of the color-blindfunction. The magnitude of the shift is in each caseless than that displayed by the corresponding pair ofcurves with the smaller (28-min) test area. The displace-ment for the 10 green field is on the order of 6.5 cyclesper second; for the 2° white field it is around 5 cycles.With the latter field, the identity of the two functionsat the lowest luminance level is probably a reflection

** The slopes of the critical fusion frequency functions are seento vary with wavelength. Similar findings have been reported inother investigations of flicker fusion thresholds for colored lights[cf, H. E. Ives, Phil. Mag. 24, 352 (1912); F. Allen, Phil. Mag.38, 81 (1919); C. Landis, Physiol. Revs. 34, 259 (1954)].

WHTE - 28'

, . I I I

I I I I I

619September1958 VISUAL FUNCTIONS OF

40

30

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BERGER, GRAHAM, AND HSIA

TABLE III. Critical fusion frequency for the right (normal) and left (color-blind) eyes of a unilaterally color-blind subject as a functionof log luminance for different test areas of white and green (539 my) lights (central fixation).

White White 539 mA28-min test field 2° test field 1° test field

Cycles per second Cycles per second Cycles per secondRight Left Right Left Right Left

Log mL eye eye Log mL eye eye Log mL eye eye

-1.17 15.3 9.24 -2.24 15.6 15.9 -1.02 21.0 15.5-0.87 17.6 9.35 -1.26 17.1 15.0 -0.15 24.3 18.1-0.49 20.0 12.6 -0.19 24.6 19.8 0.83 30.4 21.1-0.19 19.2 12.6 0.88 33.4 28.3 1.71 39.0 32.7

0.01 22.5 12.9 1.86 41.9 37.0 2.78 42.9 35.80.20 22.2 14.0 2.74 45.5 40.70.58 24.4 17.4 3.13 41.8 36.00.88 25.9 17.11.18 29.8 21.11.56 31.9 23.11.86 34.2 24.22.15 36.0 27.82.74 37.4 28.8

of rod activity, for rods are known to be present withina centrally fixated 20 area."

A comparison of the 2 white curve for the normaleye with those obtained for a similar test area by Hechtand Smith" and by Lloyd4 shows that, in the presentinstance, the S shape is more drawn out, but the slopeof the linear segment is about the same as that foundin the curves reported by these other investigators.

DISCUSSION

The proven ability of the eye to distinguish samplesof radiant energy by color implies the existence ofprocesses at the receptor level that are capable of re-acting differentially according to the wavelength of theincident light. A minimum of three such color selectiveprocesses is considered necessary by all color theories,but whereas this number is also regarded as sufficientin some systems (e.g., Young-Helmholtz), 6 it is not inothers (e..g., Hering, Hartridge).17

An inspection of the pair of curves drawn throughmeasurements obtained at corresponding spectralregions for each eye of our unilaterally color-blindsubject (Fig. 1) reveals the similarity between thesedata and those typically reported in comparisons ofcritical fusion functions for foveally fixated test objectsof various sizes. Several studies have shown that criti-cal fusion frequency diminishes with a decrease in thearea of foveal stimulation for any given luminance.The curve for the smaller area is shifted downward onthe ordinate, relative to that for the larger area, thedisplacement being maintained over the entire lumi-nance range so that the two functions are roughly

1' S. Polyak, Documenta Opthalmol. 3, 24 (1949).16 H. Helmholtz, Treatise on physiological optics, English

translation by J. P. C. Southall, Handbuch der pysiologischenOptik (Optical Society of America, 1924-5, 1909-1 1), third edition,Vols. 1, 2, and 3.

17 E. Hering, Grundzlge der Lehre vorn Lichtsinn (Julius Springer,Berlin, 1920). H. Hartridge, Recent advances in the physiology ofvision (Churchill, London, 1950).

parallel. Thus, the color-blind eye of our subject seemsto behave as if it were being stimulated by an effectivelysmaller stimulus area than the normal eye.

Physically, of course, the size of the retinal areastimulated was identical for both eyes. However, ifwithin that area there were fewer functional receptorsactivated in one eye than in the other, then it might beexpected that the results for the color-blind eye wouldsimulate those obtainable with a reduced stimulus areain the normal eye. I

The idea that dichromatism represents a loss orinactivation of part of the receptor mechanism whichnormally mediates color vision could provide anadequate explanatory framework within which the find-ings from the present study can be encompassed.

The lower critical fusion frequencies obtained in thesubject's color-blind eye may be likened to the lowerfrequency found when the total number of elementsstimulated is reduced through the use of progressivelysmaller foveal test areas. The loss of critical frequencyas measured by the extent of the separation betweenany pair of corresponding curves for the normal ascontrasted with the dichromatic eye is selective withwavelength. Its magnitude is greatest in the green,next in the blue-green, yellow-green, blue, and yellow;there is some slight loss in the orange and no detectableloss in the red. This result is taken to reflect a cor-responding selective reduction in the number of effectivereceptors with wavelength.

The elements which are postulated to be missing ornonfunctional in our subject's eye are members of theclass of color receptors having a maximum of spectralsensitivity in the green portion of the spectrum, near540 m. How such an hypothesized loss occurs is amatter of speculation. It could come about in a numberof ways. For example, it could conceivably result froma failure of differentiation at the embryological level.Alternatively, it could be that an appropriate photo-chemical substance may either have failed to develop,

620 Vol. 48

September1958 VISUAL FUNCTIONS OF UNILATERALLY COLOR-BLIND. I

may have suffered a reduction in concentration, or mayhave become modified and undergone a change to anonphotosensitive form. In any event, the end resultis an inactivation of receptors that normally functionin color vision.

Light from the incandescent lamp stimulates allreceptors. In the color-blind eye one would predict adrop in the critical fusion frequency function forincandescent-lamp light but of lesser magnitude thanthat for green. This expectation turns out to be thecase. The downward displacement of the color-blindcurve for white light, though marked (about 8.0 cyclesper second), is not so great as that found at 539 myt(i.e., 9.5 cycles).

The data for white and red light constitute evidenceagsinst our acceptance of an unmodified "collapse" or"transformation" system as the basis of deuteranopia.This notion, championed by Walls and Mathews,'8

Pitt, 9 Wright,20 and Judd2 among others, asserts thatin deuteranopia the sensitivity of green receptors hasbecome identical with or similar to that of the redreceptors. Whereas our data for green and blue lightcould be accounted for by such a formulation (the shiftin sensitivity of the green receptors would result in adip in the over-all luminosity curves), the data forwhite and red light are not in accord with a simpletransformation hypothesis. This type of theory wouldpredict that no discrepancy should be manifestedbetween the curves for the two eyes with white light.As regards red light, the curve for the color defectiveeye should be elevated above that for the normal eye,since a greater number of elements responsive to redstimulation exists in the former. Neither expectationis borne out by our data.

Measurements of the fusion frequencies in each eyewith a 10 green (539 mju) and a 20 white field also yieldcurves which display the downward shift of the color-blind function. In each case, the increased fusionfrequencies expected with the larger areas are mani-fested by both eyes. However, the extent of the separa-tion between each pair of curves for these larger fieldsis less than that found with the smaller 28-min field forthese respective lights. The reduction in the separationbetween curves when larger test fields are employedcomes about through a proportionately greater increasein critical fusion frequency with area for the color-blindthan for the normal eye.

An explanation of this phenomenon is to be found inthe differential pattern of receptor type distributionacross the fovea. K6nig2 2 first called attention to the

18 G. L. Walls and R. W. Mathews, "New means of studyingcolor blindness and normal foveal color vision," University ofCalifornia Publications, Psychology 7, No. 1, (1952).

19 F. H. G. Pitt, Proc. Roy. Soc. (London) 132, 101 (1944).20W. D. Wright, Researches on Normal and Defective Colour

Vision (Mosby, St. Louis, 1947).21 D. B. Judd, J. Opt. Soc. Am. 33, 294 (1943).22 A. Konig, Gesammelte Abihandlungen zur physiologischen

Optik (Barth, Leipzig, 1903).

fact that the center of the fovea is relatively lesssensitive to blue radiation than is the fovea as a whole.The foveal center is usually defined as the small areaof 20 or 30 minutes of angular diameter centered aroundthe point of fixation, while the whole fovea is generallythought to encompass a central 2 area.20 K6nig'soriginal finding, ignored for some 50 years, has latelybeen subjected to quantitative testing" that has clearlydemonstrated the reduced sensitivity of the centralfovea to blue stimulation. Moreover, recent work byStiles24 points to a nonuniform distribution of redreceptors as well with maximum sensitivity at thecenter of the fovea. These variations in the relativespectral sensitivities of different color receptors acrossthe fovea are taken to reflect corresponding variationsin their relative numbers, and it is this nonuniformdistribution of receptor types that provides a clue tothe understanding of our data obtained with the largertest areas.

Our small test patch (28 min) fell just within theconfines of the central fovea. Within this area, therelative proportion of green to blue receptors is largerthan it is in a 10 area. Therefore, for the color-blindeye, a greater proportion of functional receptors wasavailable within the 1 green area than was the casefor the smaller field. This condition would then result inthe disproportionate increase in fusion frequency witharea for the color-blind eye as compared to the normal.A similar interpretation applies to the comparativedata for the 28-min and 20 white fields.

Our results seem to indicate that the selectivespectal sensitivity losses found at threshold' for oursubject's dichromatic eye probably reflect correspondingreceptor losses rather than increased thresholds forgiven receptors. The latter alternative would requirethat the critical flicker function for the color-blind eyebe displaced on the luminance axis, relative to the normalfunction, in the direction of higher luminances. Thepair of curves for any given wavelength should havethe same asymptote, both eyes attaining the samemaximum fusion frequency. This, as we have seen, isnot the case. Rather, the reduced sensitivity of thedeuteranopic eye at threshold is interpreted as resultingfrom a loss of color receptors which makes itself feltnot only at threshold but, judged by the data onflicker, at high photopic luminances as well.

Other data on the critical fusion functions of color-blind individuals are meager. Several investigatorshave measured this function in monochromats,2 " butlittle has been reported on the flicker functions ofdichromats. Allen's work26 is an exception. He foundthat different types of color defectives exhibited losses

23 E. N. Willmer and W. D. Wright, Nature 156, 119 (1945);L. C. Thomson and W. D. Wright, J. Physiol. 105, 316 (1947).

24 W. S. Stiles, Documenta Ophthalmol. 3, 138 (1949).25 A. Ajo and H. Terdskeli, Acta Ophthalmol. 15, 374 (1937);

Hecht, Shlaer, Smith, and Peskin, J. Gen. Physiol. 31, 459 (1948);E. Dodt and L. Wadensten, Acta Ophthalmol. 32, 165 (1954).

26 F. Allen, Phys. Rev. 15, 193 (1902).

-621

BERGER, GRAHAM, AND HSIA

in fusion frequency, as compared to normals, in char-acteristic spectral regions. Similar findings had pre-viously been published by Ferry.27 Deuteranopesshowed decreased fusion frequencies in the green, blue,and yellow regions of the spectrum. These results are inaccord with those of the present study.

27 E. S. Ferry, Am. J. Sci. 44, 192 (1892).

ACKNOWLEDGMENTS

We wish to express our appreciation to ProfessorC. G. Mueller, Professor Carney Landis, and Mr.Aaron Hyman for valuable critical suggestions. Specialthanks are due our subject for the many hours thatshe spent in observation during the course of theseexperiments.

JOURNAL OF THE OPTICAL SOCIETY OF AMERICA VOLUME 48, NUMBER 9 SEPTEMBER, 1958

Some Visual Functions of a Unilaterally Color-Blind Person. II. BinocularBrightness Matches in Various Spectral Regions*

EDA BERGERJ C. H. GRAHAM, AND YUN HsIADepartment of Psychology, Columbia University, New York, New York

(Received December 23, 1957)

Binocular brightness matches in eight spectral regions, ranging from one having a spectral centroid at452 m to one with a spectral centroid at 681 mu, were determined for a unilaterally dichromaticsubject, a young woman with normal color vision in one eye and basically deuteranopic vision in the other.The measurements were made at photopic luminance levels by means of a polarization photometer in whichthe field of view of each eye subtended 1.8°.

For a report of apparent equality of brightness of the two test fields, the luminance requirements for thefield seen by the subject's color-blind eye exceed those for the field viewed by her normal eye in all but thered spectral regions. The luminance loss varies with wavelength; it is greatest in the green, less in the blue,and even less in the yellow. These selective spectral luminosity losses are maintained over a luminance rangeof approximately 2.5 log millilamberts. These data confirm earlier findings on selective luminosity losses atthreshold for this subject's deuteranopic eye.

INTRODUCTION

IN discussions of the characteristics of color-blindvision, it has generally been asserted that the two

major forms of dichromatism may be distinguished bythe fact that protanopes show decreased sensitivity toradiations from the long wave end of the spectrum,whereas deuteranopes do not exhibit spectral sensitivitylosses. Such statements are based primarily on measure-ments of the luminosity functions of these two types ofcolor defective observers. Compared to the normalluminosity curve, the protanopic curve is greatlyreduced in the red end of the spectrum. The luminositycurve of the deuteranope, on the other hand, has usuallybeen described as similar in all essential respects tothat of the normal trichromat.'

In 1947 Hecht and Hsia2 reported that deuteranopesas well as protanopes suffer substantial reductions intheir absolute luminosity functions. More extensive

* This work was supported by a contract between the Office ofNaval Research and Columbia University, and by a grant fromthe Higgins Fund of Columbia University. Reproduction inwhole or in part is permitted for any purpose of the United Statesgovernment.

t Present address: Research Laboratory of Electronics, Massa-chusetts Institute of Technology, Cambridge, Massachusetts.

1 F. H. G. Pitt, G. Brit. Med. Research Council, Spec. Rept.Ser. No. 200 (1935).

2 S. Hecht and Y. Hsia, J. Gen. Physiol. 31, 141 (1947).

work by Hsia and Graham3 in the recent past hasconfirmed this finding. Hsia and Graham investigatedthe luminosity functions of normals and of dichromats.Their data for five protanopes show the expectedreduction in luminosity at the long wavelengths. Inaddition, however, the luminosity curves of sixdeuteranopes exhibit decreased sensitivities in thegreen and blue regions of the spectrum.

Graham and Hsia4 also measured the thresholdluminosity curves for both eyes of a unilaterally color-blind subject, a young woman with primarily deuter-anopic discriminations in her left eye and normal colorvision in her right eye. The two functions were foundto differ in absolute sensitivity values in the greenand blue regions of the spectrum; the sensitivity ofthe normal eye in these regions is considerably greaterthan the sensitivity of the deuteranopic eye. In the redend of the spectrum, the sensitivities of both eyes aresimilar. The data on this subject are thus in line withthe results Hsia and Graham obtained from theircomparison of the luminosity functions of a group ofdeuteranopes with those of a group of normals: deuter-anopes show a loss in sensitivity in the green and blueas compared with normal subjects.

3 Y. Hsia and C. H. Graham, Proc. Natl. Acad. Sci. 43, 1011(1957).

4 C. H. Graham and Y. Hsia, Proc. Natl. Acad. Sci. 44, 46(1958).

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