J. Physiol. (1968), 199, pp. 443-456 443With 6 text-figure8Printed in Great Britain

ARE THERE TWO TYPES OF DEUTERANOPES?

BY M. ALPERN, J. MINDEL AND S. TORII*From the University of Michigan, Ann Arbor, Michigan, U.S.A.

(Received 1 July 1968)

SUTMMIARY

1. The colour vision of a Type I deuteranope who fulfils both ofWillmer's criteria (normal foveal luminosity curve, two cone mechanismsin the central fovea revealed by a small 10' test flash) has been studied.

2. Spectral sensitivity curves (at threshold) on bright red or greenbackgrounds are identical in the red-green range.

3. Heterochromatic brightness-match luxminosity curves measured afterbright red or green bleaches are identical in the red-green range.

4. Study of prereceptor light losses show normal colour of the ocularmedia; spectral reflexion coefficient measurements reveal no evidence ofmacular pigment.

5. Luminosity curves measured through a filter which artificially re-places the missing macular pigment is identical to the deuteranopic(Type II) curve. Lack ofmacular pigment explains the 'normal' luminositycurve.

6. Red and violet backgrounds raise the thresholds for 10' red andviolet tests by different amounts because two cone (the red and the blue)mechanisms are concerned.

7. Reducing the size of the test to 4' eliminates the contribution of theblue cone mechanism to threshold. Now only the red mechanism deter-mines the threshold.

8. It is concluded that this subject has only a single red-green conepigment, normal erythrolabe, like other (Type II) deuteranopes.

INTRODUCTION

Deuteranopic dichromatism, a colour vision defect of 1-1 % of the maleand 0x01 % of the female population, is characterized by a spectrumneutral point (matched to Illuminant C) at 498-4 + 5-8 nm (Walls & Heath,1956). It has been suggested by various authors, beginning with Aitken(1873), that deuteranopes have both the normal red and the normal green

* On leave from the Department of Psychology, Tokyo University of Agriculture andTechnology, Tokyo, Japan.

M. ALPERN, J. MINDEL AND S. TORII

pigment and that the nerves from the two kinds of cones are 'fused'.Rushton (1965a) has now shown by retinal densitometry that, on thecontrary, deuteranopes have the normal red-sensitive pigment, erythro-labe, and lack the green-sensitive pigment, chlorolabe. In this, deuteranopesare exactly analogous to protanopes who lack the red-sensitive erythrolabebut have the normal green-sensitive chlorolabe. However, Willmer (1949)proposed that there were two different types of deuteranopes: a 'loss'type (Type II), now clearly documented by the densitometer, and a'fusion' type (Type I). If this were true, it is possible that the latter hasjust never come under densitometer study. Wald (1965), using a variantof Stiles's two-colour threshold experiments, has failed to find any evidencefor more than one red-green cone pigment in thirteen deuteranopicsubjects.To examine the matter in detail, it is necessary to recall that Willmer

(1949) established two criteria for Type I (or fusion) deuteranopes:(a) normal foveal luminosity curve, and (b) equally bright red and violetbleaches raise the foveal thresholds for small (10') red and violet testflashes by different amounts.We have so far examined foveal luminosity of eleven deuteranopes and

fourteen deuteranomalous subjects. Only one of these has a normalluminosity curve (Fig. 1). That subject, J.M., has a neutral point at496-8 nm, well within the range established by Walls & Heath (1956) fora deuteranopic match to Illuminant C. He can match a green light (527 nm)with a mixture of red (650 nm) and a small amount of blue (445 nm). Inthe Nagel anomaloscope, he matches any proportion of R/R +G on thescale (from 0 to 1-0) merely by varying the intensity of the monochromaticyellow, and these intensity settings (even when the mixed field is set at theextreme red or green limit) are never very different. These three indepen-dent criteria all establish that J.M. is a valid deuteranope, although hisluminosity curve is well within normal limits. Subsequent measurements(Fig. 6) show that he also fulfils the second criterion of Wilimer's Type Ideuteranope. In the present paper, we examine the factors which differen-tiate him from other deuteranopes.

METHODSLuminoity curve. The luminosity curve is measured by heterochromatic brightness-

matching method by an apparatus and procedure already described in detail (Alpern & Torii,1968b). Briefly, spectral brightness matches were made for 0.50 monochromatic test com-pared with a 10 annular surround foveally fixed. The test wave-length (provided by atungsten lamp illuiminating a Hilger-Watts constant-deviation prism monochromator, withadditional coloured gelatin filters to obviate monochromator stray light) was varied fromthe blue (400 nm) to the red (700 nm) end of the spectrum and back in 10 nm steps. Themonochromator entrance slit width was varied in the experiment so that the test bandwidth

444

ARE THERE TWO TYPES OF DEUTERANOPES? 445

stayed within the limits 2-6-6-5 nm. The wave-length of the surround was changed 6 timesduring the spectrum traverse (A = 402, 430, 470. 520, 590, 650 nm) so that the test neverdiffered by more than 50 nm from the surround. The subject adjusted a calibrated neutralwedge to make the test equal in brightness to the surround, ignoring differences in huewhenever they occurred. A colour-blind subject makes the settings quite easily because thehue differences are never very large for him.

In the usual condition of measurements, the standard was kept at 2-0 td, as seen throughthe rectangular artificial pupil (1 x 3-5 mm). Measurements were also made within 15 secafter intense red and green exposure for 45 sec. The adapting field in such experimentscovered the central 3.20 area and provided a retinal illuminance of 4-2 log1o td. The intensityof the standard (230 td) was considerably higher in such cases, and the limitation of intensityof the tungsten filament then made it impossible to obtain measurements for wave-lengthsless than 500 nm.

Adaptation. The threshold was measured for monochromatic red (675 nm) or violet(445 nm) test flashes seen against monochromatic violet (421 nm) or red (650 nm) back-grounds. The background field wjas 100 in diameter and remained on continuously duringthe testing. The test and background fields were provided by tungsten ribbon filamentsseen in Maxwellian view through a beam splitter. They were rendered monochromatic bynarrow-band interference filters. The test flash was a circular spot 10' or 4' in diameterexposed for 200 msec, once every second. To obviate contamination from rods, the subjectwas first exposed to a very bright (about 6-0 log10 td of tungsten light) background field for45 sec. After about 5 min in the dark, the cones had regained full sensitivity and remainedmore sensitive than the rods for about 10 more minutes. During this time interval, threemeasurements of test flash threshold against no background were obtained. The backgroundwas then turned on, but at its lowest intensity, and three more measurements were made.The background intensity was increased by 0-5 log1o units and three more threshold mea3ure-ments obtained. The process was repeated in this way until the background field reachedits maximum value or the test flash threshold was raised above the limits of our light source.In practice, a given run required only a single full bleach, for by 15 min after the bleach(when, in the dark, rods and cones are about equally sensitive) the background had alreadyreached a sufficiently high intensity that contamination of cone threshold for violet lightby rods was no longer a serious possibility. The experiment was repeated 5 times for eachbackground test combination. Fixation was maintained accurately by two small spots oflight just sufficiently bright to be clearly visible, one about 0.50 above, the other 0-50 below,the test.The equivalence of the red (650 nm) and violet (421 nm) backgrounds in elevating the

thresholds of the different colour mechanisms (Stiles, 1939) was determined by the methoddescribed in detail by Alpern & Rushton (1965). Briefly, foveal thresholds against the twodifferent backgrounds were measured for a 10 monochromatic test, flashed for 200 msec,once a second. To isolate iT5 (i.e. the red mechanism), a red (625 nm) test was used and to

isolate 7T4 (i.e. the green mechanism), a blue-green (476 rm) test was used. The equivalenceof the red and violet backgrounds in elevating these thresholds was measured directly bythe amount of lateral displacement required for coincidence of the respective thresholdversus intensity curves. To isolate 7rT (the blue mechanism), two backgrounds were used inconjunction with a 476 mrn test. The auxiliary background (580 mm) raised the thresholdfor the green mechanism sufficiently high so that the 476 nm test at threshold excited only7rI. The main background, containing either the red or the violet field, was then super-imposed on the auxiliary yellow background and the red and violet background equivalencefor elevating the threshold for nl determined in the usual way.

Spectral sensitivity at threshold. The spectral sensitivity curves were also obtained bymeasuring threshold for monochromatic tests against bright red or green coloured back-grounds. The same monochromatic field used in brightnes3s-matching was used for these

446 M. ALPERN, J. MINDEL AND S. TORIIexperiments. It was flashed (for 200 msec, once a second) in the centre of a 3.20 circularbackground field which was 3-6 log10 td either red (650 nm) or green (500 nm). Subjectsadjusted the neutral wedge over the test field for threshold. A single spectrum traversecould be completed in about 45 min.The testing conditions for both the luminosity and threshold measurements differ in

detail from those used by Willmer (1949). However, the fact that the results agree withthose obtained by him on Type I deuteranopes suffices to justify the comparison.

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100

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400 450 500 550 600 650 700Wave-length (nm)

Fig. 1. Heterochromatic brightness-match luminosity curves for normal (M.A.,open circles), for Type I deuteranope (J.M., squares), and for seven Type IIdeuteranopes (filled circles connected by dotted line). The smooth curve is theCornmission Intemationale de l'Eclairage (C.I.E.) photopic luminosity curve. Theresults are shifted vertically for agreement at the red end of the spectrum. Theintensity of the standard was 2-0 td.

RESULTS

The luminosity curve obtained on this deuteranope (J. M.) is illustratedin Fig. 1 by the squares. For comparison, the results of the same experi-ment on one of us (M.A.) (a colour normal) is shown by the open circlesin the same figure. These measurements compare within the limits foundby Wilimer (1949) (for normal subjects and Type I deuteranopes) andwithin the limits of experimental error to the Commision Internationalede l'Eclairage (C.I.E.) photopic curve (continuous line). All three curves

ARE THERE TWO TYPES OF DEUTERANOPES?

are considerably higher in the blue-green and blue parts of the spectrumthan the measurements of deuteranomalous and deuteranopic luminosity.This is shown by the filled circles which are the geometric means of themeasurements on seven deuteranopes.

Two-colour thresholds. If the Type I deuteranope's luminosity curve isnormal because he has both red and green visual pigments and if his colour

400 450 500 550 600 650 700Wave-length (nrm)

Fig. 2. Spectral sensitivity curves at threshold for a 0.50 foveally fixed target on a3.2° background field (3-6 log10 td) either 650 nm (open circles) or 500 nm (filledcircles) for normal observer M. A. (upper pair of curves) and for Type I deuteranopeJ. MI. (lower pair of curves).

blindness results because the nerves from his red and green cone mechan-isms are 'fused', then the presence of the red and green pigments perhapswould be revealed by two colour thresholds, as indeed Willmer (1949)suggested from his results with only two test wave-lengths and two back-ground fields. Wald (1965) examined thirteen deuteranopes with completespectral traverses on a variety of different backgrounds which should have

447

M. ALPERN, J. MINDEL AND S. TORIIrevealed the spectra of the two red-green pigments if any were to be found.All of his observers revealed only a single spectrum in the red-green range.However, it is not clear that any of these observers fulfilled Willmer'scriteria for Type I deuteranope. We have therefore repeated Wald'sexperiment on J. M. By this criterion, J. M. also reveals no evidence formore than one red-green pigment. Figure 2 shows the results from typical

20 r

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400 450 500 550 600 650 700Wave-length (nm)

Fig. 3. Heterochromatic brightness-match luminosity curves matched to a standardof 230 td within 15 sec immediately following 45 sec exposure of background fieldof 4-2 log10 td either 605 nm (open circles) or 527 nm (filled circles) for normalobserver M.A. (upper pair of curves) or Type I deuteranope J.M. (lower pair ofcurves).

experiments of many of a similar kind which we have done in order totease out action spectra of more than one kind of red-green cone in hisretina. It is evident that though different colour backgrounds uncoversignificantly different relations between the blue cone mechanism and thered-green cone mechanism, all backgrounds produce essentially the samered-green spectral threshold curve. On the normal subject (M.A.), on the

448

ARE THERE TWO TYPES OF DEUTERANOPES? 449

contrary, the spectral threshold curve in the red-green range on a redbackground is clearly different from that measured on a green background.Hence, by this test, J.M. behaves, like a deuteranope Type II, as thoughhe had only a single red-green visual pigment.

This result, however, does not exclude the possibility that J. M. mayhave two red-green pigments, one ofwhich cannot be revealed by thresholdagainst red or green coloured backgrounds. Deuteranomalous trichromats,by this test, also reveal only one red-green pigment (Rushton, 1965 b;Wald, 1966; Alpern & Torii, 1968c); yet they clearly have two. Alpern &Torii (1968c) found that they could reveal the presence of two red-greenpigments in deuteranomalous trichromats by measuring heterochromaticbrightness-match luminosity curves within 15 sec after very bright red orgreen bleaches. We have therefore repeated those experiments on J.M.

Effect of chromatic adaptation on brightness-match luminosity curves. Thebrightness-match luminosity curves of J.M. after red and green bleachesare shown in Fig. 3. For comparison, the results on M.A. show how theluminosity curve of the normal eye is altered by the same procedures.While the heterochromatic brightness-match luminosity curve of M.A. isvery similar to, if not exactly the same as, that of J.M., under standardtesting procedures, the two observers behave quite differently when themeasurements are made after very bright red and green bleaches. M.A.'scurve after a red bleach is much less sensitive in the red than that obtainedafter a green bleach. This is because this luminosity curve is synthesized bytwo cone mechanisms in the red-green range. Deuteranomalous show asomewhat similar result and for the same reason, although the differencesare very much smaller than those found for the normal subject (becausethe Amax of the red and green pigments are closer together than is the casefor the normal). On the other hand, J.M., though his luminosity curve isessentially normal, behaves quite differently. His luminosity curves afterequally bright red and green bleaches are identical in the red-green range.In this respect, he differs not at all from all other deuteranopes we havestudied. According to this test, like those already described, J.M. behavesas though he has a single cone visual pigment in the red-green range.

Prereceptor luminosity curve distortions. The results of the experimentsdescribed contain what appears to be a contradiction: the luminositycurve of J. M., as usually measured, is essentially normal, suggesting thecontribution of two red-green photolabile cone pigments; but after redor green bleaches, his luminosity curves are identical in the red-greenrange as though he, like all other deuteranopes that we have so far testedin this way, had only one red-green cone pigment. This contradictionmight be resolved, however, if J.M.'s luminosity curve, like that of theother deuteranopes, were determined by only a single red-green cone

29 Physiol. I99

M. ALPERN, J. MINDEL AND S. TORII

pigment, i.e. normal erythrolabe, provided that the prereceptor light lossesin his eye differ enough from those in other deuteranopes' eyes.

The prereceptor light losses of relevance must vary in wave-length, ofcourse, or they could not account for the results. There are two principalsources of prereceptor colour absorption in the eye, namely in the lens and

30

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450 500 550Wave-length (nm)

600 650 700

Fig. 4. A. Spectral reflexion coefficient of the 100 temporal and macular fundusas measured by the method of Brindley & Willmer (1952). The topmost three curves

are for the macula, the lowermost three are for the peripheral fundus. The resultsare the means for four normals (open circles), three Type II deuteranopes (dashedline), and Type I deuteranope J.M. (filled circles). B. The absorption spectra ofthe macular pigment of the normals, Types I and II deuteranopes calculated fromthe results illustrated in 4A. The ordinate is the log10 of the reflexion coefficient ofthe macula subtracted from the log10 of the reflexion coefficient of the peripheralfundus (corrected for double passage by dividing by two).

in the macular pigment, both having a similar, though by no means

identical, yellow colour.The possibility that J. M. lacks a normal amount of a yellow lens pigment

(or, in fact, that his ocular media are abnormally coloured in any way) has

450

ARE THERE TWO TYPES OF DEUTERANOPES?

been excluded simply by measuring his scotopic (rod) spectral sensitivitycurve. If eye media colour were responsible for the 'distortion' of J.M.'sluminosity curve, his rod curve would show an increased sensitivity in theblue (450 nm) of the order of 0 5 log10 units over that of normals. His rodspectral sensitivity curve is, on the contrary, perfectly normal.To investigate the macular pigment, we measured the spectral reflexion

coefficient of the macular and peripheral (100 temporal) fundus using themethod described by Brindley & Willmer (1952). The results are shown inFig. 4 as the filled circles for J. M., as the dashed line for three typicalType II deuteranopes, and as the open circles for four normal eyes. InFig. 4A the three topmost curves show the macular measurements, thelower three the peripheral results. Figure 4B shows the macular pigmenta-tion (half the difference between the log10 of the macular and peripheralreflexion coefficients) in these three groups. The three deuteranopes showslightly more macular pigment than our normals, but these small differencesare probably not significant. On the other hand, the results for J. M. areclearly different from both! There is little evidence in his eye of anymacular pigment. The small increased absorbance of this macular area at450 nm, compared with the peripheral retina, is essentially the same asthat found at 650 nm.To demonstrate that this lack of macular pigment can account for

J.M.'s normal luminosity curve in the presence of only a single red-greencone pigment, erythrolabe, one can measure his luminosity curve whilelooking through a filter which has precisely the colour characteristics ofthe missing macular pigment.The skin of fresh carrots, ground in the presence of chloroform and then filtered, providesa solution which has the appropriate absorption spectrum (Alpern & Torii, 1968a). If theamount of chloroform is adjusted so that the absorbance at 455 nm is about 0 4, the solutionhas precisely the absorption spectrum needed to make up for the differences in macularpigment of J.M. compared with the three Type II deuteranopes illustrated in Fig. 4. Acuvette of such a solution was mounted immediately behind the artificial pupil of theluminosity meter, and J.M. made brightness-match luminosity measurements while lookingthrough this solution.

The mean results of three such experiments are shown in Fig. 5 in whichJ. M.'s results, obtained in this way (continuous line), are compared withthe usual measurements (without cuvette of added 'macular pigment')made on the three Type II deuteranopes whose macular pigment resultswere compared to J.M.'s in Fig. 4 (open circles). The dashed line showsJ.M.'s usual curve from Fig. 1. When the macular pigment which Fig. 4shows he lacks is replaced, J.M. has a Type II deuteranopic luminositycurve like all other deuteranopes so far examined.

T.v.i. curves on red and violet backgrounds. All of the above results areaccounted for if J. M. has only a single red-green pigment, erythrolabe,

29-2

451

M. ALPERN, J. MINDEL AND S. TORII

but lacks his normal share of macular pigment. But one difficulty remains.Willmer (1949) measured foveal thresholds for 10' red and violet testtargets after red and violet adaptation. For Type II deuteranopes, equallybright red and violet bleaches raised the test threshold by equal amountsas though only a single pigment with an absorption spectrum nearlyidentical to the luminosity curve influenced the threshold for these smalltargets. For normals (and for Type I deuteranopes) this was not the case;for them, at least two different cone visual pigments were required toexplain foveal thresholds.

20

400 450 500 550 600 650 700Wave-length (nm)

Fig. 5. Heterochromatic brightness-match luminosity curve of J. M. measuredwhile viewing through a solution of artificial macular pigment, the absorbance ofwhich (0 4 at 455 nm) is precisely that required to make up for the differencesbetween J.M. and the three Type II deuteranopes illustrated in Fig. 4B (solidline connecting filled circles). The dashed line shows the curve obtained for J. M.under usual viewing method (squares in Fig. 1). Open circles are the mean log10luminosity (by usual viewing method) for the three Type II deuteranopes whosemean macular pigment curve is shown by the dotted line in Fig. 4B.

Our results in a similar series of excperiments are shown in Fig. 6.Following Wrillmer (1949), we have arbitrarily equated the thresholds inthe dark for the red and violet test flashes. The results for the 10 min testtarget for both M.A. and J.M. cannot be described by single smoothcurves, suggesting in each case that the action spectrum of more than onecone mechanism determines threshold.

452

ARE THERE TWO TYPES OF DEUTERANOPES? 453

t~~~~~~~~~~~+ <0~~~~~~~~~~~~~~~~~~~~~~~~~~~1

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Log10 Retinal illuminance background (u) td

Fig. 6. Threshold-intensity curves for violet (445 nm) test on 650 nm (open circles)or 421 nm (plus signs) background and for red (675 nm) test on 650 nm (filled circles)and 421 nm ( x signs) backgrounds. The abscissa is the background retinal illumin-ance in (photopic) td. The top two families of curves are for the normal, the bottomtwo for Type I deuteranope J. M. The bottom family of curves for each observer isfor the 10' test, the top for the 4'. For the normal observer the curve drawn throughthe results for the red test on a violet background is that obtained for the red teston the red background but shifted the amount (0-35 logl0 unit) required to give thetwo backgrounds equal effectivity in exciting 7Ti for this observer. For the violettest the curves are shifted the amount needed to give the two backgrounds equaleffectivity in exciting T4. For J. M. the curve through the 10' violet test on violetbackground at low intensities is that obtained for the 10' violet test on the redbackground but shifted the amount required to give the two backgrounds equaleffectivity in exciting it1 for this observer. Only a single curve is drawn through theresults for a red test for the 10' target and for all the results for a 4' target, becausethe effectivity of the two backgrounds in exciting 7T5 for J. M. is, within the pre-cision of our estimates (±0-1 log10 unit), identical to their photopic luminosityequivalence.

M. ALPERN, J. MINDEL AND S. TORIIFor the normal (M. A.), it is clear that these are the red and green cones,

respectively, as Willmer supposed. With the red test, the smooth curvedrawn through the x s is the same curve as that drawn through the dotsbut displaced to the right about 0-35 log1o units. This is the amount neededto give the two backgrounds equivalence for exciting his red mechanism.With the violet test, the curve drawn through the results for a violetbackground is the same curve as that drawn through the results with ared background, but shifted along the abscissa by the amount required togive the two backgrounds equivalence for exciting his X4 (green) cones.Apparently, the 10' test flash is sufficiently small to eliminate thepossibility that the violet test excites the nr, (the blue) mechanism on thisobserver. For equivalence of ir1 for him, the open circles should fallapproximately 0 5 log1o units to the right of their positions in Fig. 6.For J.M. these relations are a bit different. With the red test, the red

and violet backgrounds once more have the action spectrum of his i5 (red)mechanism. The equivalence of these backgrounds for 75is approximatelythe same as their photopic luminosity equivalence, which determines theactual positions of the experimental points along the abscissa scale inFig. 6. But the action spectrum for raising the threshold of the violet testis that of J.M.'s 7r1 (i.e. the blue, not the green) mechanism. The smoothcurve drawn through the plus signs at low intensities is the same curvedrawn through the open circles but shifted to the left the amount neededto give equivalence of the red and violet backgrounds for elevating thethreshold for ir1 on J.M. For violet tests on violet backgrounds, thethreshold for x71 soon becomes greater than that of v5, but on red back-grounds the n, mechanism determines violet test threshold for as high aswe can measure it.

Evidently, Willmer's (1949) assumption that the 10' test is sufficientlysmall to eliminate contributions of J.M.'s blue cones to threshold isunwarranted. If, however, the size of the test target is reduced to 4', thenthe contributions of J.M.'s blue cones are eliminated. Now the actionspectrum of red and violet backgrounds required to raise red and violettests by a fixed amount can be described by the absorption spectrum of asingle visual pigment. The single mechanism of concern is his red mechan-ism (which largely determines his photopic luminosity curve) since allfour curves for the 4' test run together when the backgrounds are equatedfor equal 7T5 effectivity (or, what is here the same, for equal photopicluninosity). On the other hand, for the normal reducing the size of the testflash has no such effect. The action spectrum of the violet and red back-grounds for elevating the 4' violet test is, like the results with the 10'violet test, that of his green mechanism.

Thus, these results, like all the others in this paper, suggest that J. M.

454

ARE THERE TWO TYPES OF DEUTERANOPES?

has only a single cone mechanism in the red-green range. This mechanism,like that of all Type II deuteranopes, contains the normal red pigment,erythrolabe, though its action spectrum as measured on J.M.'s eye issomewhat different since he lacks macular pigment.

DISCUSSION

In this paper we have studied the ways in which a single deuteranopicobserver who meets all the requirements of a Type I deuteranope differsfrom all the other deuteranopes we have ever studied who fulfil the re-quirements of Type II deuteranopes. It was found that his normalluminosity curve could be explained by the absence of a normal amountof macular pigment in conjunction with only a single red-green pigment(erythrolabe), and that probably because of the absence of macularpigment his foveal blue-blind area was smaller than the 10' area testedby Willmer (1949).

Thus, at least for J.M., there are no grounds for proposing that he hasnormal amounts of chlorolabe as well as erythrolabe, but that the nervesfrom his red and green cone mechanisms are 'fused'. The results of themeasurements, after strong chromatic adaptation, argue strongly againstthis view. J.M. is best explained as having only normal cyanolabe- anderythrolabe-filled cones and lacking completely chlorolabe.Of course, these experiments cannot exclude the possibility that other

dichromats exist who have all three cone visual pigments in normalamounts but in whom the red and green cone mechanisms somehowconverge on to a single neural pathway (i.e. are fused). This paper does,however, outline the sort of experimental test which will be required todocument such a state of affairs. Until we know more about the colourvision of such hypothesized dichromats, it seems quite unsound to referto them as deuteranopes. Until evidence to the contrary is forthcoming,it seems wisest to regard such dichromats with a 'fusion' mechanism asdiffering in colour vision characteristics as remarkably from deuteranopesas we now regard them as differing from protanopes. If they exist at all,they are best regarded as a fourth and completely distinct dichromaticgroup.

We are indebted to Gilbert B. Lee for assistance in several of these experiments. The workwas supported in part by Grant NB 01578-10 from the National Institute of NeurologicalDiseases and Blindness, and by a grant from Research to Prevent Blindness, Inc., New York.

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456 M. ALPERN, J. MINDEL AND S. TORII

REFERENCES

AiTTEN, J. (1873). On colour and colour sensation. Tranm. R. Scott. Soc. Arts 8, 375-418.ALPERN, M. & RUSHTON. W. A. H. (1965). The specificity of the cone interaction in the

after-flash effect. J. Physiol. 176, 473-482.ALPERN, M. & TORTI, S. (1968a). Prereceptor colour vision distortions in protanomalous

trichromacy. J. Physiol. 198, 542-560.ALPERN, Al. & ToRII, S. (1968 b). The luminosity curve of the protanomalous fovea. J. gen.

Physiol. 52. (In the Press.)ALPERN, M. & TORII, S. (1968 c). The luminosity curve of the deuteranomalous fovea. J. gen.

Physiol. 52. (In the Press.)BRINDLEY, G. S. & WILLMER, E. N. (1952). The reflexion of light from the macular and

peripheral fundus oculi in man. J. Physiol. 116, 350-356.RUSHTON, W. A. H. (1965a). A foveal pigment in the deuteranope J. Physiol. 176, 24-37.RuSHTON, W. A. H. (1965b). Discussion on human colour vision. In Ciba FoundationSymposium Colour Vision: Physiology and Experimental Psychology, ed. DE REUCK,A. V. S. & KNIGHT, J., pp. 246-247. Boston: Little Brown.

STILES, W. S. (1939). The directional sensitivity of the retina and the spectral sensitivitiesof the rods and cones. Proc. R. Soc. B 127, 64-105.

WALD. G. (1965). Gibt es zwei verschiedene Typen von Deuteranopie? Ber. Ver8amm.ophthal. Ges. 67, 272-273.

WALD. G. (1966). Defective color vision and its inheritance. Proc. natn. Acad. Sci. U.S.A,55, 1347-1363.

WALLS, G. L. & HEATH, G. G. (1956). Neutral points in 138 protanopes and deuteranopes.J. opt. Soc. Am. 46, 640-649.

WrLLMER, E. N. (1949). Further observations on the properties of the central fovea incolour-blind and normal subjects. J. Physiol. 110, 422-446.