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The luminosity curve for Class I and Class II colour normals
THE LUMINOSITY
CURVE FOR CLASS I AND
CLASS 11 COLOUR
NORMALS
STEPHENR. COBB Department of Psychology, University of Glasgow, Scotland (Rrceiced
II .4wpst
1973)
I) The luminosity curves have been measured at eight wavelengths throughout the spectrum for a Z field for both of the Unique Green classes of male. (2) The curve for the Class II normals shows less luminosity response at certain regions of the spectrum especially at wavelengths of 570 nm. (3) A possible explanation of the differences between the luminosity curves for Class I and Class II males is given involving a visual pigment. It is not however, possible with the present evidence to suggest a biochemical structure for such a pigment. .Abstract-(
While investigating the phenomenon of Unique Green and its genetics it was decided to ascertain whether the Class I and Class II normals described by Richards ( 1967) had any appreciable difference in their respective luminosity curves. The probabe mode of inheritance has been shown by Waaler (1967) and confirmed by the present writer (Cobb, 1972 and unpublished observations). The work of Richards shows a differing performance of the two classes with respect to additivita tests on the calorimeter and to chromatic coordinates for white. It was hoped that from earlier and present work it would be possibie to suggest a cause. METHODS S
The observers. who were all males, were selected from those available (technicians. students and academic staff) in the Adam Smith Building during August. 1972. The colour vision of each man was tested under a Macbeth daylight lamp by the Ishihara and Farnsworth 100 hue plates. He was rejected if he had any errors in the Ishihara tests or more than six errors in the Farnsworth tests. or any history of ocular disease inv-olving the retina or the media. These unusually stringent requirements for normality were set in order to eliminate the possibility that any differences between the Class I and Class II normals subsequently found could be ascribed to abnormality of colour vision. Twenty-three persons were tested and of these five were rejected on the basis of the colour vision tests. The acceptable age range was I%-30 yr. Division oj’obserwrs
into Class I and Class II normals
The normal observers were placed in categories Class I or Class II according to the results of the tests using the Nagel II anomaloscope and the serial technique of Waaler (1965) and with the author’s spectrometer and technique. In the writer’s technique the observ-er looks into a spectrometer field in which most of the spectrum is visible. He is then asked to adjust a knob until the position of a pointer in the field is neither at blue-green nor yellow-green but at green.
It is found that with a British population of males. the distribution is bi-modal with a distinct break between the two modes. Under the first of these modes the distribution lies between 495 and 523 nm. These are the Class I individuals and comprise approx 57 per cent of the whole normal male population in Britain. Under the second of these modes the distribution lies between 525 and 540 nm. These are the Class II individuals comprising the remaining 13 per cent. Luminosiry meastvemenE.5
The luminosity measurements were made on the colorimeter after the observer had been dark-adapted for IO min. The W’right colorimzter (1946) was used to produce a square field subtending 1’ at the eye. The observer adjusted the luminance of the top field so that it matched the lower field (which was kept constant) in brightness. Measurements were made at eight different wavelength settings for the particular field at intervals throughout the spectrum. Normally. eight determinations were made at each wavelength and never more than 14 in one session. There was only one session in a day, for considerable concentration on the part of the observer was needed. If a number of observations was increased. the results became inconsistent, and the observer complained of fatigue. In most of the experiments. the lower (comparison) field was red and it was the practice to start making observations with the upper field also at the red end of the spectrum since the matching was not here complicated by hue differences and helped the subject to accustom himself to the idea of matching brightnesses and not colours. Wright (1916) adopted the technique of keeping the test at a fixed energy level in this case 0.13 Ix and plotting the luminosity curve in relation to it. Thus the reciprocal of the energy of the experimental field is the luminosity for that particular point of the spectrum. The energy levels were measured in a similar manner to that described by Wright (1946). information values were checked by means of a-light-dependent resistor R7P71 and R7P5g connected to a Digital Multimeter D.&M? manufactured by Advance. While this method has the advantage of enabling the results to be specified simply, the results can be deceptive. It does lead to a very untidy method in the event of the
SEPHEN
j30
R. Coeu
luminosity contribution of the red end of the curve with a comparison field of 643 nm being markedly different as between one observer and another. Since some observers are weak in the red end of the spectrum it might be better
to use a comparison field in the blue-green area. RESULTS
The results of the experiments are shown in Figs. l3. Figure l(a) shows the luminosity curves for one observer (Class I) and l(b) for one observer (Class II) with a comparison field at both 643 and 508 nm. Althou~ the observation with a blue-green comparison field is considerably more diflIcult than with a red test field, demanding higher degrees of concentration, there is not a substantial difference between the curves apart from the peak position. which seems to be further in the long-wave direction for the comparison field in each case. Wright states that the difhculties in concentration are probably due to the lack of blue response at the fovea tending to pull the eye off fixation. It may also be that the chromatic aberration of the human eye, which leads to the eye being considerably more myopic for wave-lengths normally associated with blue as opposed to yellow and red, contributes to this. This question may depend on a more general ability to concentrate on the part of the subjects.
90
so t
Fig. Z. Luminosity curve for two Class II observers and the mean luminosity curve for six Class I observers with S.D. bars. Figure 2 shows the mean luminosity curve for six Class I subjects, with S.D. bars, and individual luminosity curves for two of the CIass II subjects who are most distinctive of this class. These curves are piotted in absolute relation to each other and so some order needs to be made of them if there is to be any possibility of postuIating any cause arising from the genetic information now av%iiabIe. One way of comparing the curves is to adjust all the heights of each curve to have max luminosity of 100. This would be helpful if it is assumedthat the differences in luminosity response at the wavelength of the comparison field in this case mostly 643 nm, is different for each of the Unique Green classes. On the other hand, if this assumption is not jus&ed the whole shape of the curve expected for the peak will lack validity. 100
100
!--
b
90 1
WEl0 -
a f g ,{ z ._ 'i
4-
70 so%I40Jo-
2 20$0
I
4ca
Fig. I(a). Luminosity curve of a Class I subject with a bfue and a red test field. (b). Luminosity curve of a Class iI subject with a blue and a red test field.
I
L
4
I xx3
Wavelength,
I
I
1
:,
1
1
1
600 nm
Fig. 3. Mean luminosity for six Class I observers three Class II observers.
and
Luminosity curves for colour normtlls Figure 3 shows the mean of the Class I and the mean of all three Class II subjects plotted separately. It would be possible to show both these curves made up to luminosity of 100 which would mean that it is not here assumed that the response ofeach type is the same in the red at the comparison wavelength used, but that at the peak the curve is not different for each class. In an attempt to see whether this assumption was justified a comparison field in the blue region at 508 nm was run for some of the subjects, both Class I and Class II, and compared with the curve for the same subjects for the red comparison field. As can be seen from Figs. l(a) and (b). there is not a large difference in the position of the peaks between comparison fields of 643 nm but nevertheless with the blue comparison field the peak seems to be moved towards the short wavelength for both Class I and Class II. DISCUSSION
The luminosity curve is substantially the same whether the comparison field has a wavelength of 643 nm in the red, which is easiest for the subject, or of 508 nm in the blue, as far as the greater part of the descending and ascending parts of the curve are concerned, both for Class I and Class II subjects. This wouid indicate that under the fairly bright conditions used (0.3 1Ix) for the comparison field there is little difference in response at the ends of the spectrum between Class I and Class II subjects, The general high levels of S.E.‘s for the experiment shown in Fig. 2 are in line with the N.E.‘s for luminosity curve work. These are stated by Wright to be in the order of + 10 per cent with further errors in the blue region as energy levels have to be interpolated there. The error levels reached here are similar. The peaks of the curves, however, show relatively large differences between the Class I and Class II subjects, especially around the region UC-580 nm where the luminosity response is consider-
100
c
531
ably lower for the Class II subjects than for the Class
I subjects. Indeed for all the Class II subjects the luminosity at 1% nm was actually iower than the measurements at 495 nm whereas this is not the case in any of the Class I subjects. If the Class II curve is subtracted from the Class I curve the curve shown in Fig. 4 is obtained. It is now believed that the Unique Green characteristic is carried on the X-chromosome, that neither the Class I nor the Class II conditions are dominant. and that they breed true (Cobb. 1972; Waaler. 1967). This might be explained by assuming a visual pigment is present in the Class I case and absent in the Class II case and that its absorption characteristics are those of the curve shown in Fig. 4. The hypothesis of an additional visual pigment would explain a number of phenomenon which at present are difficult on the strict Young-Helmholtz Theory of Colour Vision, and these may now be considered brieily. Ecidencefiom
adaptation ejj’kts
The work of Wright (1936. 1946) and Brindley (1953) has shown that under adaptations from bright lights a match requires more red to be maintained. This according to Brindle)- is true regardless of the wavelength and the intensity of the light used provided that the intensity is high. Owing to the fact that matches are always upset in the same direction it is necessary on the strict Young-Helmholtz theory to postulate and elaborateaselfscreening hypothesis.Objectivemeasurements of the density of the pigment and differential density of the pigment between the red and green sensitive receptors made by both reilection and transmission in retinal densitometry. provide no evidence for this hypothesis. Neither do these techniques provide any evidence for the photolability necessary for a self screening hypothesis. Although the fact is that these techniques do not at the moment provide evidence of the necessary conditions for this hypothesis to function they do not of course disprove it. It does nevertheless seem better to advance the positiv-e theory whereby this new proposed pigment becomes bleached at an earlier stage than the conventional three cone pigments do and at high intensities ceases to feed a signal into the red channel. Eridence porn the positions of the maximum ~pcctrcrl srnsiticit,v of the cone pigments bJ subjective and objectire techniques and their relation to the Bezold-Brucke efict
d
3ok 20
10t
Wavelength.
nm
Fig. 1. Curve derived from the subtraction of the Class II curve from the Class I curve of Fig. 3.
One of the unsatisfactory features about modern researches into the placing of the point of maximum spectral sensitivity of the pigments in the cones on the Young-Helmhoi~ theory has been the lack of cIose agreement. While they have some features in common and tend to support in general the Young-Helmholtz hypothesis there are serious divergences. The different curves all place the positions of masima1 sensitivity of both the red and green sensitive pig-
ments on the Young-Helmholtz theory on the straight portion of the chromaticity chart near the spectral locus and the blue pigment is placed usually at around +lO nm. In the famous work of Marks, Dobelle and McNichol (1964) where transmission retinal densitometry was undertaken the positions of maximal spectral absorbance were 445,535 and 570 run respectively. The curves derived from subjective mixing data (Wright, 1946; Walters. 1942: Konig. 1903) place the blue fundamental response between 440 and 460 nm and the green at 540 ntn and the red between 530 and 600 nm. As can be seen, the one value that contrasts strongly between the objective and subjective techniques is that for the red. Transmission densitometry probably bleaches entirely any pigment that is highly photolabile, SO that it cannot provide any signal for the digital computer to respond to. Thus we nnght postulate that a photolabile pigment is bleached in transmission densitometry but functions at the intensities of normal calorimetry. If this is so we can say that it must have an absorption maximum well into the red, possibly as far as about 620 nm, and that it feeds into the red channel. It is possible that the 570-nm curve obtained by transmission densitometry arises from a metastable product. A further difficulty with the placing of the red response at 570 run is the explanation of the Stiles-Crawford effect of the second type. The magnitude of this effect varies according to the wavelength used, and for monochromatic lights the subjective hue appears to vary. Since the degree of absorption depends on the density of pigment, and since the absorption shape of the sensitivity curve presumably depends on that of the absorption spectrum of the cone pigments, we can conclude that the effective optical density of the pigment in the cones varies with the direction of incidence. While other explanations are possible. it does seem that some such hypothesis as an extra photolabile pigment with maximal spectral sensitivity at about 620 nm must be considered when there is this serious discrepancy between the placing of the red-sensitive pigment at 570 nm by transmission densitometry. a finding made several times by different workers. and the placing of the red-sensitive pigment nearer to 590 nm by reflection densitometry, and on account of various visual phenomena and calorimetric data. Ecidencefrom colour defects and their relation ro C’niqtte Green Findings
Any viable theory of colour vision must take into account the phenomena of colour vision defect. It is known that where one gene is involved this must specify the protein and so an opsin. It follows from this that where the evidence points to one gene being involved in a certain change of the pigments then only one pigment can be varied. It is extremely difficult to see how any drastic neural change could automatically
be associated with mutatton in one gcnz causmg a change in the pigment. This is especially so when one considers that the neural network does in fact derive from a different embryological layer from that of the pigments. The Young-Helmholtz theory accounts for the various types of colour defect only moderately well and has difficulty in explaining all the observed differences, especially those with regard to colour anomaly. where the colour vision is still trichromatic as opposed to the extreme defect where the colour vision becomes dichromatic. The possibility of such a visual pigment being present in addition to the three cone visual pigments postulated on the Young-Helmholtz theory of colour vision or in conjunction with one of the pigments postulated on this theory is discussed at length by Cobb (1972). It is suggested there that such a visual pigment would have a maximum absorption at a wavelength of about 570 nm and a very wide band absorption and be highly photolabile. If such a pigment does in fact exist it would have the advantage of explaining rhz experiments of Richards (1967) on additivity. recovery from dark adaptation and colour co-ordinates for white. Richards advances a neurological hypothesis which he admits himself is unsatisfactory. It was hoped that luminosity measurements might. in conjunction with other evidence, make it possible to suggest a biochemical structure. This has not however been possible. ;Icknorvirdgrmrnrs-I am greatly indebted to Professor R. W. Pickford. Department of Psychology. Lmversity of Glasgow. who has unsparingly given encouragement and advice throughout the work. I must also thank all the subjects who showed great interest and persevered while making observations which demanded a considerable degree of concentration.
REFERESCES
Brindlcy G. S. (1953) The effects on colour viston of adaptation to very bright lights. J. Phr:siot.. Land. 122, 332. Cobb S. R. (1972) tinpublished Ph.D. thesis. l..niversity of Glasgow. Konig (1903) Phpsioloyisichon OpriL. Leipzig. M:larks W. B.. Dobelie W. H. and McSichol E. F-. Jr. (1964) Visual pigments of single primate cones. Scznce. :V.E 113. 1lYI. Richards W. J. (1967) Differences among colour normals Class I and II 1. opr. Sot. Am. 37, 1047. Waaler G. H. MM.(1967) The heredity of normst and defective colour vision. .&ch. P. Yorskr Bide&-.-tkad 1, No. 9.
Walters .A. V. (1942) Some experiments on the tribromate theory of vision Proc. R. So;. B 131. 27. Wright W. D. (1936) The breakdown of a colour match with high intensities of adaptation. J. Ph_~~iol..Land. 87, 123. Wright W. D. (1946) Xormal and d&tire cototrr uisim Kempton. London.
Luminosity curves for colour normals R&m&-On mesurr les courbes d‘eflicacite lumineuse pour huit longueurs d’onde r;tparttes dans tout le spectre et un champ de 2’ chez its deuv classes de sujets miles 3 “vert unique”. Pour 1~s SUJtb ~OTITW.IY de la classe II, la riponse de luminosite est basse dans certaines rigions du spectre. specialement ii 5-O nm. I1 est possible d’cxpliquer ;i partir du pigment bisuel Ies d&rences des courbes d‘etlicacits entre les classes 1 et II. mais ii est actuellement prsmarure de suggPrer une structure bioshtmrqupour un tel pigment. Zusammenfawng-( 1) Hellempfindungskutven wurden mit einem 2’ Feld bei Y LVellenliingen fiir beide Klassen von reinem Griin bei miinnlichen Beobachtern gemessen. (2) Die Kurven tGr die Klasse Ii Normalen zeigen in manchen Bereichen des Spektrums. besonders bei ~~lien~~ngen urn 570 nm. eine geringere Heiiigkeitsantwort. (3) Eine miigliche Erkliirung fiir die L’nterschiede zwischen den Heilemfindungskurven mCnnlicher Beobachter der Klassen I und II liegt in cinem Sehstotf. &Iit den gegenwiirtigen Dnten ist es miiglich. eine biochemische Struktut fur einen soichcn Schstoff vorzuschl,agen. PeXOMt?+)
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CURVE FOR CLASS I AND
CLASS 11 COLOUR
NORMALS
STEPHENR. COBB Department of Psychology, University of Glasgow, Scotland (Rrceiced
II .4wpst
1973)
I) The luminosity curves have been measured at eight wavelengths throughout the spectrum for a Z field for both of the Unique Green classes of male. (2) The curve for the Class II normals shows less luminosity response at certain regions of the spectrum especially at wavelengths of 570 nm. (3) A possible explanation of the differences between the luminosity curves for Class I and Class II males is given involving a visual pigment. It is not however, possible with the present evidence to suggest a biochemical structure for such a pigment. .Abstract-(
While investigating the phenomenon of Unique Green and its genetics it was decided to ascertain whether the Class I and Class II normals described by Richards ( 1967) had any appreciable difference in their respective luminosity curves. The probabe mode of inheritance has been shown by Waaler (1967) and confirmed by the present writer (Cobb, 1972 and unpublished observations). The work of Richards shows a differing performance of the two classes with respect to additivita tests on the calorimeter and to chromatic coordinates for white. It was hoped that from earlier and present work it would be possibie to suggest a cause. METHODS S
The observers. who were all males, were selected from those available (technicians. students and academic staff) in the Adam Smith Building during August. 1972. The colour vision of each man was tested under a Macbeth daylight lamp by the Ishihara and Farnsworth 100 hue plates. He was rejected if he had any errors in the Ishihara tests or more than six errors in the Farnsworth tests. or any history of ocular disease inv-olving the retina or the media. These unusually stringent requirements for normality were set in order to eliminate the possibility that any differences between the Class I and Class II normals subsequently found could be ascribed to abnormality of colour vision. Twenty-three persons were tested and of these five were rejected on the basis of the colour vision tests. The acceptable age range was I%-30 yr. Division oj’obserwrs
into Class I and Class II normals
The normal observers were placed in categories Class I or Class II according to the results of the tests using the Nagel II anomaloscope and the serial technique of Waaler (1965) and with the author’s spectrometer and technique. In the writer’s technique the observ-er looks into a spectrometer field in which most of the spectrum is visible. He is then asked to adjust a knob until the position of a pointer in the field is neither at blue-green nor yellow-green but at green.
It is found that with a British population of males. the distribution is bi-modal with a distinct break between the two modes. Under the first of these modes the distribution lies between 495 and 523 nm. These are the Class I individuals and comprise approx 57 per cent of the whole normal male population in Britain. Under the second of these modes the distribution lies between 525 and 540 nm. These are the Class II individuals comprising the remaining 13 per cent. Luminosiry meastvemenE.5
The luminosity measurements were made on the colorimeter after the observer had been dark-adapted for IO min. The W’right colorimzter (1946) was used to produce a square field subtending 1’ at the eye. The observer adjusted the luminance of the top field so that it matched the lower field (which was kept constant) in brightness. Measurements were made at eight different wavelength settings for the particular field at intervals throughout the spectrum. Normally. eight determinations were made at each wavelength and never more than 14 in one session. There was only one session in a day, for considerable concentration on the part of the observer was needed. If a number of observations was increased. the results became inconsistent, and the observer complained of fatigue. In most of the experiments. the lower (comparison) field was red and it was the practice to start making observations with the upper field also at the red end of the spectrum since the matching was not here complicated by hue differences and helped the subject to accustom himself to the idea of matching brightnesses and not colours. Wright (1916) adopted the technique of keeping the test at a fixed energy level in this case 0.13 Ix and plotting the luminosity curve in relation to it. Thus the reciprocal of the energy of the experimental field is the luminosity for that particular point of the spectrum. The energy levels were measured in a similar manner to that described by Wright (1946). information values were checked by means of a-light-dependent resistor R7P71 and R7P5g connected to a Digital Multimeter D.&M? manufactured by Advance. While this method has the advantage of enabling the results to be specified simply, the results can be deceptive. It does lead to a very untidy method in the event of the
SEPHEN
j30
R. Coeu
luminosity contribution of the red end of the curve with a comparison field of 643 nm being markedly different as between one observer and another. Since some observers are weak in the red end of the spectrum it might be better
to use a comparison field in the blue-green area. RESULTS
The results of the experiments are shown in Figs. l3. Figure l(a) shows the luminosity curves for one observer (Class I) and l(b) for one observer (Class II) with a comparison field at both 643 and 508 nm. Althou~ the observation with a blue-green comparison field is considerably more diflIcult than with a red test field, demanding higher degrees of concentration, there is not a substantial difference between the curves apart from the peak position. which seems to be further in the long-wave direction for the comparison field in each case. Wright states that the difhculties in concentration are probably due to the lack of blue response at the fovea tending to pull the eye off fixation. It may also be that the chromatic aberration of the human eye, which leads to the eye being considerably more myopic for wave-lengths normally associated with blue as opposed to yellow and red, contributes to this. This question may depend on a more general ability to concentrate on the part of the subjects.
90
so t
Fig. Z. Luminosity curve for two Class II observers and the mean luminosity curve for six Class I observers with S.D. bars. Figure 2 shows the mean luminosity curve for six Class I subjects, with S.D. bars, and individual luminosity curves for two of the CIass II subjects who are most distinctive of this class. These curves are piotted in absolute relation to each other and so some order needs to be made of them if there is to be any possibility of postuIating any cause arising from the genetic information now av%iiabIe. One way of comparing the curves is to adjust all the heights of each curve to have max luminosity of 100. This would be helpful if it is assumedthat the differences in luminosity response at the wavelength of the comparison field in this case mostly 643 nm, is different for each of the Unique Green classes. On the other hand, if this assumption is not jus&ed the whole shape of the curve expected for the peak will lack validity. 100
100
!--
b
90 1
WEl0 -
a f g ,{ z ._ 'i
4-
70 so%I40Jo-
2 20$0
I
4ca
Fig. I(a). Luminosity curve of a Class I subject with a bfue and a red test field. (b). Luminosity curve of a Class iI subject with a blue and a red test field.
I
L
4
I xx3
Wavelength,
I
I
1
:,
1
1
1
600 nm
Fig. 3. Mean luminosity for six Class I observers three Class II observers.
and
Luminosity curves for colour normtlls Figure 3 shows the mean of the Class I and the mean of all three Class II subjects plotted separately. It would be possible to show both these curves made up to luminosity of 100 which would mean that it is not here assumed that the response ofeach type is the same in the red at the comparison wavelength used, but that at the peak the curve is not different for each class. In an attempt to see whether this assumption was justified a comparison field in the blue region at 508 nm was run for some of the subjects, both Class I and Class II, and compared with the curve for the same subjects for the red comparison field. As can be seen from Figs. l(a) and (b). there is not a large difference in the position of the peaks between comparison fields of 643 nm but nevertheless with the blue comparison field the peak seems to be moved towards the short wavelength for both Class I and Class II. DISCUSSION
The luminosity curve is substantially the same whether the comparison field has a wavelength of 643 nm in the red, which is easiest for the subject, or of 508 nm in the blue, as far as the greater part of the descending and ascending parts of the curve are concerned, both for Class I and Class II subjects. This wouid indicate that under the fairly bright conditions used (0.3 1Ix) for the comparison field there is little difference in response at the ends of the spectrum between Class I and Class II subjects, The general high levels of S.E.‘s for the experiment shown in Fig. 2 are in line with the N.E.‘s for luminosity curve work. These are stated by Wright to be in the order of + 10 per cent with further errors in the blue region as energy levels have to be interpolated there. The error levels reached here are similar. The peaks of the curves, however, show relatively large differences between the Class I and Class II subjects, especially around the region UC-580 nm where the luminosity response is consider-
100
c
531
ably lower for the Class II subjects than for the Class
I subjects. Indeed for all the Class II subjects the luminosity at 1% nm was actually iower than the measurements at 495 nm whereas this is not the case in any of the Class I subjects. If the Class II curve is subtracted from the Class I curve the curve shown in Fig. 4 is obtained. It is now believed that the Unique Green characteristic is carried on the X-chromosome, that neither the Class I nor the Class II conditions are dominant. and that they breed true (Cobb. 1972; Waaler. 1967). This might be explained by assuming a visual pigment is present in the Class I case and absent in the Class II case and that its absorption characteristics are those of the curve shown in Fig. 4. The hypothesis of an additional visual pigment would explain a number of phenomenon which at present are difficult on the strict Young-Helmholtz Theory of Colour Vision, and these may now be considered brieily. Ecidencefiom
adaptation ejj’kts
The work of Wright (1936. 1946) and Brindley (1953) has shown that under adaptations from bright lights a match requires more red to be maintained. This according to Brindle)- is true regardless of the wavelength and the intensity of the light used provided that the intensity is high. Owing to the fact that matches are always upset in the same direction it is necessary on the strict Young-Helmholtz theory to postulate and elaborateaselfscreening hypothesis.Objectivemeasurements of the density of the pigment and differential density of the pigment between the red and green sensitive receptors made by both reilection and transmission in retinal densitometry. provide no evidence for this hypothesis. Neither do these techniques provide any evidence for the photolability necessary for a self screening hypothesis. Although the fact is that these techniques do not at the moment provide evidence of the necessary conditions for this hypothesis to function they do not of course disprove it. It does nevertheless seem better to advance the positiv-e theory whereby this new proposed pigment becomes bleached at an earlier stage than the conventional three cone pigments do and at high intensities ceases to feed a signal into the red channel. Eridence porn the positions of the maximum ~pcctrcrl srnsiticit,v of the cone pigments bJ subjective and objectire techniques and their relation to the Bezold-Brucke efict
d
3ok 20
10t
Wavelength.
nm
Fig. 1. Curve derived from the subtraction of the Class II curve from the Class I curve of Fig. 3.
One of the unsatisfactory features about modern researches into the placing of the point of maximum spectral sensitivity of the pigments in the cones on the Young-Helmhoi~ theory has been the lack of cIose agreement. While they have some features in common and tend to support in general the Young-Helmholtz hypothesis there are serious divergences. The different curves all place the positions of masima1 sensitivity of both the red and green sensitive pig-
ments on the Young-Helmholtz theory on the straight portion of the chromaticity chart near the spectral locus and the blue pigment is placed usually at around +lO nm. In the famous work of Marks, Dobelle and McNichol (1964) where transmission retinal densitometry was undertaken the positions of maximal spectral absorbance were 445,535 and 570 run respectively. The curves derived from subjective mixing data (Wright, 1946; Walters. 1942: Konig. 1903) place the blue fundamental response between 440 and 460 nm and the green at 540 ntn and the red between 530 and 600 nm. As can be seen, the one value that contrasts strongly between the objective and subjective techniques is that for the red. Transmission densitometry probably bleaches entirely any pigment that is highly photolabile, SO that it cannot provide any signal for the digital computer to respond to. Thus we nnght postulate that a photolabile pigment is bleached in transmission densitometry but functions at the intensities of normal calorimetry. If this is so we can say that it must have an absorption maximum well into the red, possibly as far as about 620 nm, and that it feeds into the red channel. It is possible that the 570-nm curve obtained by transmission densitometry arises from a metastable product. A further difficulty with the placing of the red response at 570 run is the explanation of the Stiles-Crawford effect of the second type. The magnitude of this effect varies according to the wavelength used, and for monochromatic lights the subjective hue appears to vary. Since the degree of absorption depends on the density of pigment, and since the absorption shape of the sensitivity curve presumably depends on that of the absorption spectrum of the cone pigments, we can conclude that the effective optical density of the pigment in the cones varies with the direction of incidence. While other explanations are possible. it does seem that some such hypothesis as an extra photolabile pigment with maximal spectral sensitivity at about 620 nm must be considered when there is this serious discrepancy between the placing of the red-sensitive pigment at 570 nm by transmission densitometry. a finding made several times by different workers. and the placing of the red-sensitive pigment nearer to 590 nm by reflection densitometry, and on account of various visual phenomena and calorimetric data. Ecidencefrom colour defects and their relation ro C’niqtte Green Findings
Any viable theory of colour vision must take into account the phenomena of colour vision defect. It is known that where one gene is involved this must specify the protein and so an opsin. It follows from this that where the evidence points to one gene being involved in a certain change of the pigments then only one pigment can be varied. It is extremely difficult to see how any drastic neural change could automatically
be associated with mutatton in one gcnz causmg a change in the pigment. This is especially so when one considers that the neural network does in fact derive from a different embryological layer from that of the pigments. The Young-Helmholtz theory accounts for the various types of colour defect only moderately well and has difficulty in explaining all the observed differences, especially those with regard to colour anomaly. where the colour vision is still trichromatic as opposed to the extreme defect where the colour vision becomes dichromatic. The possibility of such a visual pigment being present in addition to the three cone visual pigments postulated on the Young-Helmholtz theory of colour vision or in conjunction with one of the pigments postulated on this theory is discussed at length by Cobb (1972). It is suggested there that such a visual pigment would have a maximum absorption at a wavelength of about 570 nm and a very wide band absorption and be highly photolabile. If such a pigment does in fact exist it would have the advantage of explaining rhz experiments of Richards (1967) on additivity. recovery from dark adaptation and colour co-ordinates for white. Richards advances a neurological hypothesis which he admits himself is unsatisfactory. It was hoped that luminosity measurements might. in conjunction with other evidence, make it possible to suggest a biochemical structure. This has not however been possible. ;Icknorvirdgrmrnrs-I am greatly indebted to Professor R. W. Pickford. Department of Psychology. Lmversity of Glasgow. who has unsparingly given encouragement and advice throughout the work. I must also thank all the subjects who showed great interest and persevered while making observations which demanded a considerable degree of concentration.
REFERESCES
Brindlcy G. S. (1953) The effects on colour viston of adaptation to very bright lights. J. Phr:siot.. Land. 122, 332. Cobb S. R. (1972) tinpublished Ph.D. thesis. l..niversity of Glasgow. Konig (1903) Phpsioloyisichon OpriL. Leipzig. M:larks W. B.. Dobelie W. H. and McSichol E. F-. Jr. (1964) Visual pigments of single primate cones. Scznce. :V.E 113. 1lYI. Richards W. J. (1967) Differences among colour normals Class I and II 1. opr. Sot. Am. 37, 1047. Waaler G. H. MM.(1967) The heredity of normst and defective colour vision. .&ch. P. Yorskr Bide&-.-tkad 1, No. 9.
Walters .A. V. (1942) Some experiments on the tribromate theory of vision Proc. R. So;. B 131. 27. Wright W. D. (1936) The breakdown of a colour match with high intensities of adaptation. J. Ph_~~iol..Land. 87, 123. Wright W. D. (1946) Xormal and d&tire cototrr uisim Kempton. London.
Luminosity curves for colour normals R&m&-On mesurr les courbes d‘eflicacite lumineuse pour huit longueurs d’onde r;tparttes dans tout le spectre et un champ de 2’ chez its deuv classes de sujets miles 3 “vert unique”. Pour 1~s SUJtb ~OTITW.IY de la classe II, la riponse de luminosite est basse dans certaines rigions du spectre. specialement ii 5-O nm. I1 est possible d’cxpliquer ;i partir du pigment bisuel Ies d&rences des courbes d‘etlicacits entre les classes 1 et II. mais ii est actuellement prsmarure de suggPrer une structure bioshtmrqupour un tel pigment. Zusammenfawng-( 1) Hellempfindungskutven wurden mit einem 2’ Feld bei Y LVellenliingen fiir beide Klassen von reinem Griin bei miinnlichen Beobachtern gemessen. (2) Die Kurven tGr die Klasse Ii Normalen zeigen in manchen Bereichen des Spektrums. besonders bei ~~lien~~ngen urn 570 nm. eine geringere Heiiigkeitsantwort. (3) Eine miigliche Erkliirung fiir die L’nterschiede zwischen den Heilemfindungskurven mCnnlicher Beobachter der Klassen I und II liegt in cinem Sehstotf. &Iit den gegenwiirtigen Dnten ist es miiglich. eine biochemische Struktut fur einen soichcn Schstoff vorzuschl,agen. PeXOMt?+)
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