Clinical Genetics 1915: 7: 214-279
The Unique Green phenomenon and colour vision STEPHEN R. COBB
Department of Psychology, University of Glasgow, Scotland The mode of inheritance of the Unique Green phenomenon is investigated by a new technique. This involves a spectrometer which generates the stimulus in such a manner that the exact position that is neither blue-green nor yellow-green can be compared with neighbouring positions in the spectrum. This technique is used in conjunction with the single field techniques of Waaler to confirm the mode of inheritance he has suggested: that it is an effect carried on the X chromosome. The placing of the Unique Green point in the spectrum is shown to correlate with the mid-matching point in the blue-yellow axis on the Pickford Nicolson Anomaloscope. It is concluded that there may be a highly photolabile visual pigment present, which absorbs maximally at about the yellow region of the spectrum. Received 8 October 1973, revised 30 September, accepted for publication 7 October 1974
The Unique Green phenomenon has been successfully elicited by Ruben (1961), Richards (1967), Cobb (1973), and also by Waaler (1967) with a slightly different technique. In the first of these techniques, the subject is asked to consider a spectrometer generated patch of light of a wavelength between 500 and 540 nm, and t o state whether the colour seen is yellow-green or blue-green on a semi-random, forced choice basis. The Unique Green phenomenon is the occurrence whereby the point in the spectrum that is judged to be pure green, i.e. neither blue-green nor yellow-green, has a distinctly bimodal distribution in a male population. In a female population, however, the distribution is nearly normal. Hurvich et al. (1968), Verriest (1970), Kalmus & Case (1972), and most recently Mezt & Balliet (1973) failed to demonstrate successfully the Unique Green phenomenon. Nevertheless, it is my belief that in all these
cases, factors were present which led to the phenomenon not being apparent. This is discussed at length by Cobb (1973); in brief, all unsuccessful experiments involved a field of 2" or less and there was no comparison field. The successful techniques listed above all involved fields of angular subtense greater than 2", and in some cases of Waaler's (1967) bipartite field technique and the present spectrometer technique, there was a comparison field. Methods and Results
A Hilger and Watts Constant Deviation Spectrometer was set up in such a manner as t o offer the subject a semicircular field of 3" angular subtense presenting a portion of spectrum of approximately 60 nm in width with a pointer in the centre. The instrument was lit from a 40 W tungsten, domestic light bulb. Two techniques have been used to take readings, self-setting and
275
UNIQUE GREEN PHENOMENON AND COLOUR VISION
forced choice. The first of these was used on the pedigrees and was found to give substantially the same results as the forced choice. It was also very considerably less time consuming, taking only about 4 as opposed t o 30 min. The self-setting technique is described here. The room in which the apparatus was placed was only dimly illuminated at a level of not more than 11 lx at the bench. The subject stood in this room for at least 4 min before measurements were commenced, while personal details were recorded. The wavelength drum was set to 540 nm and the subject was instructed to look into the instrument and t o move his head until the pointer was in the centre of the displayed portion of the spectrum. The subject was then asked whether the pointer indicated blue-green or yellow-green. When the subject answered that it was yellow-green, the wavelength was turned t o 500 nm and the subject was asked if he agreed that the colour of light at the pointer was some sort of blue-green. When the subject again answered in the affirmative, he was instructed: "Set the wavelength down so that just where the pointer points it is neither blue-green nor yellow-green but greengreen". Then at least five readings were taken, the experimenter having reset the wavelength down to 500 nm each time. The first reading was ignored; sometimes more than five readings were needed before they became consistent. The reliability of the test for Unique Green point was checked by method of test retest reliability coefficient over 12 subjects, who were retested at intervals between a week and a fortnight. This gave a result of 0.56 which, although it could have been higher, is nevertheless highly significant. Much more importantly, no subject was reclassified on the retest. It was found that children under the age of 10 years were unreliable and therefore were excluded. The
populations tested consisted of 253 males and 187 females, all of whom were undergraduates aged between 18 and 25 years. Most subjects were selected from practical classes by a tutor on the basis that they were inattentive o r had completed their course work. Care was taken that the criteria of selection could not be related in any way to vision.
500
520
510
530
wawlenglh n m
Fig. 1. Distribution of 253 males with respect to the Unique Green point.
I
-l L 520
War.lenOlh
530
n rn
Fig. 2. Distribution of 187 females with respect to the Unique Green point.
276
COBB
It was found that with the male subjects (Fig. l), distribution with respect to the Unique Green point showed a sudden zero frequency in the 520-525 nm cell, whereas the female distribution (Fig. 2) is close to that of a normal curve. The dip for the males is significant, P = 0.01, and remains so with all simple transformations of the data. Waaler (1967) showed with a large number of pedigrees that this is due to a single effect, carried on the X chromosome, which breeds true, neither condition being dominant to the other. Thus in the male, the two homozygous conditions correspond to the two modes, with a distinct gap in the
frequency distribution, whereas in the female, although the two homozygous conditions would manifest distinctly, the heterozygotes are intermediate and fill the gap. I have confirmed the fundamental correctness of Waaler's (1967) hypothesis with 50 pedigrees, which are detailed in Table 1 and summarized in Tables 2 and 3. These pedigrees were families known directly or indirectly to the author and who were willing to cooperate; four families refused. GI and G;1 are used to denote the possible genotypes for the male, according t o which mode Unique Green group he belongs. G11 and G a are the possible genotypes for fe-
Tabie 1 for females for the Class on Richards' (1967) notation self-setting technique with classification into types I and II for males and I, II and 1-11 for females for the Class on Richards (1967) notation Name
Father
Mother Daughter
Whitehead Darle Williams Lehman Strettan Jones Giilet Fry Austin Slade Stretton Shicker Mayfair Laurie Timens Shiels Laidlaw Roberts Rothschild Jordan Jeffry Lee Ferris Brooks Cobb Churchman McConnel
527 II 524 I1 521 I 526 II 528 II Daut. 490 I 520 I 494 512 487 512 487 495 516 528
509 480 515 509 533 540 537 516 512 518 514 516 526 512 508 490 516 508 508 504 5MI 518 488 505 512 528 487
Smith
-
516 490 516 509 498 508 508 498 514 511
I I I I I I I I I I
519 I
I I I I 1-11 II II 1-11 I I I 1-11 1-11 I I I I I I I I I I I I II I
484 I
508 524 523 521 487
Son
I' 1-11 I 1-11 I*
Name
Father
Mother
I
Hyhton Griffiths Curtis Timens Westlake
504 518 510 469 507
Holland Buchanan
519 I 508 I
523 1-11 530 II
Transman
505
546 II
Dickson
518
-
Reichman Reader Parval Philips
522 522 510 503 I
511 506 513 498
Hattle Erskine
524 II 515 I
505 I 512 I
I
I I I I
514 508 513 523 521
I I 1-11 1-11
527 I I 496 I 533 II
Daughter 490 508 489 528 519 514 529 525
I I I 1-11 I I 1-11
528 508 514 508 514 502 496 505 497 530 508 502 512
1-11 I' I I I I I I I 1-11 I I I
1-11
480 508 498 508 508 489 489 518 495 I 498 505 506 490 522 522 509 509 516 488 513 510
I I I 1
I I I
I I' I I
485 520 513 508 (512 516
I I I I I)** I
Erskine Cuthbert Philips Allan
521 I 498 I
I
* These pedigrees suggest that sometimes Class I may be dominant. * * Son of mother only: natural father deceased. Deut. = deuteranomalous.
497 508 525 509
I I
I I
I I 1-11 I
Son
489 508 518 528 512
I I I II I
511 I 510 I 514 I 508 I
515 I 512 I 503 I 506 I 503 I 498 I
UNIQUE GREEN PHENOMENON AND COLOUR VISION
Table 2
Genotypes of mothers and their sons: all sons have genotypes which can arise from the X chromosome of their respective mothers. (From the pedigrees in Table 1) Sons
Mothers GI 28 6
Gii
28
Gn
3
3
G22
G2
4 3
Table 3
Genotypes of fathers and mothers producing daughters of particular genotypes: the daughters have genotypes which can arise from the X chromosome Parents
18
2 6 5 1 0
Gi X Gii Gi x Gi2 Gi x G n
Gz x Gii G2 x Giz G2 x G n
Daughters
24
2 2' 1' 1'
3 4 4 1
' These pedigrees suggest that sometimes Class I may be dominant.
males, which correspond to the male G1 and G2, respectively, and G12 forms an intermediate group with a small overlap (see Waaler 1967, Cobb 1973). The pedigrees were tested by an apparatus slightly different from that described above, since the laboratory spectrometer, which had been developed to give as clear-cut a result as possible, could not be transported to the locations of the families concerned without risk of damage and disturbance to the calibration. The apparatus used for the pedigrees was a Nagel 11 anomaloscope similar to that used by Waaler (1967), and his single field technique was repeated. This involved setting the luminance and mixture screws at zero and asking the subject to set
277
the upper field, by means of the main screw, until the colour seen was neither blue-green nor yellow-green but green-green. The first 10 families in Table 1 had their Unique Green points measured using both apparatuses and in each case the classification was the same, although in the case of the Nagel Model anomaloscope, the spread of readings for each subject was greater. Thus, data shown in Figs. 1 and 2 and Table 4 were collected by the laboratory spectrometer technique, whereas those in Tables 1, 2 and 3 were collected by Waaler's (1967) single field technique with the Nagel Model 11 anomaloscope. The method of classifying a male subject as Class I or Class I1 presented certain problems within the pedigrees, since the Nagel Model 11 used in the field did not give such clear-cut results as the spectrometer apparatus and there was n o clear zero frequency such as shown in Fig. 1. The most satisfactory method was found t o be to take readings below 522 nm as belonging t o Class I, and those above 523 nm t o Class 11. I n the pedigree Williams, this system produced an anomaly: the father at 521 nm and the daughter at 523 nm have both been placed in Class I because the mother was a clear Class I at 515 nm. It is not known whether this is a pedigree which does not fit, or a bad experimental plot; in any case, it does not fall very far outside the Class I criterion. It is assumed for the purposes of Tables 2 and 3 that Class I arises from G1 in the male and GI1 in the female, and that Class 11 arises from G2 in the male and GZ2in the female. In some cases, parents were not available for testing and are absent from Table 1. In the pedigree Brooks, the father of the son in the pedigree was dead and the mother had remarried. Thus in the pedigree, the father was that of the daughter not of the son, the son and daughter being half brother and sister.
COBB
278
Table 4 Correlation between the midmatching point for the Rayleigh equation for that particular axis on the Pickford-Nicolson anomaloscope and the Unique Green point for 91 female art students: significant at the P = 0.01 level Green r = 0.057 Green r = 0.111 Yellow r = 0.264
Red Blue Blue
Since this characteristic breeds true and appears to be sex-linked, it strongly suggests that a visual pigment is being directly affected from a gene on the X chromosome. Further study was undertaken in an attempt t o elucidate what visual pigment might be responsible. A Pickford-Nicolson (Pickford & Lodowski 1960) filter anomaloscope was set up and a population of 94 women, none of whom had more than four errors or hesitations on the Ishihara test for colour blindness, were tested on three axes: Red Green matching yellow; Blue Green matching blue green; BIue Yellow matching white. Data were collected with regard to both the mid-matching point and matching range, and these were correlated with findings on the test for Unique Green (Table 4). The correlation for the blue-yellow is significant, P 0.01. This is particularly important since, for the anomaloscope used, the retest reIiability coefficient is only 0.611 (n = 32) for this axis (Pickford 1951). Furthermore, in colorimetry in general, it is for this axis that the lowest reliability is found on the chromaticity chart, probably owing to the individual variations in macula pigmentation and cone characteristics at the fovea.
+ +
+
The Unique Green phenomenon and colour vision STEPHEN R. COBB
Department of Psychology, University of Glasgow, Scotland The mode of inheritance of the Unique Green phenomenon is investigated by a new technique. This involves a spectrometer which generates the stimulus in such a manner that the exact position that is neither blue-green nor yellow-green can be compared with neighbouring positions in the spectrum. This technique is used in conjunction with the single field techniques of Waaler to confirm the mode of inheritance he has suggested: that it is an effect carried on the X chromosome. The placing of the Unique Green point in the spectrum is shown to correlate with the mid-matching point in the blue-yellow axis on the Pickford Nicolson Anomaloscope. It is concluded that there may be a highly photolabile visual pigment present, which absorbs maximally at about the yellow region of the spectrum. Received 8 October 1973, revised 30 September, accepted for publication 7 October 1974
The Unique Green phenomenon has been successfully elicited by Ruben (1961), Richards (1967), Cobb (1973), and also by Waaler (1967) with a slightly different technique. In the first of these techniques, the subject is asked to consider a spectrometer generated patch of light of a wavelength between 500 and 540 nm, and t o state whether the colour seen is yellow-green or blue-green on a semi-random, forced choice basis. The Unique Green phenomenon is the occurrence whereby the point in the spectrum that is judged to be pure green, i.e. neither blue-green nor yellow-green, has a distinctly bimodal distribution in a male population. In a female population, however, the distribution is nearly normal. Hurvich et al. (1968), Verriest (1970), Kalmus & Case (1972), and most recently Mezt & Balliet (1973) failed to demonstrate successfully the Unique Green phenomenon. Nevertheless, it is my belief that in all these
cases, factors were present which led to the phenomenon not being apparent. This is discussed at length by Cobb (1973); in brief, all unsuccessful experiments involved a field of 2" or less and there was no comparison field. The successful techniques listed above all involved fields of angular subtense greater than 2", and in some cases of Waaler's (1967) bipartite field technique and the present spectrometer technique, there was a comparison field. Methods and Results
A Hilger and Watts Constant Deviation Spectrometer was set up in such a manner as t o offer the subject a semicircular field of 3" angular subtense presenting a portion of spectrum of approximately 60 nm in width with a pointer in the centre. The instrument was lit from a 40 W tungsten, domestic light bulb. Two techniques have been used to take readings, self-setting and
275
UNIQUE GREEN PHENOMENON AND COLOUR VISION
forced choice. The first of these was used on the pedigrees and was found to give substantially the same results as the forced choice. It was also very considerably less time consuming, taking only about 4 as opposed t o 30 min. The self-setting technique is described here. The room in which the apparatus was placed was only dimly illuminated at a level of not more than 11 lx at the bench. The subject stood in this room for at least 4 min before measurements were commenced, while personal details were recorded. The wavelength drum was set to 540 nm and the subject was instructed to look into the instrument and t o move his head until the pointer was in the centre of the displayed portion of the spectrum. The subject was then asked whether the pointer indicated blue-green or yellow-green. When the subject answered that it was yellow-green, the wavelength was turned t o 500 nm and the subject was asked if he agreed that the colour of light at the pointer was some sort of blue-green. When the subject again answered in the affirmative, he was instructed: "Set the wavelength down so that just where the pointer points it is neither blue-green nor yellow-green but greengreen". Then at least five readings were taken, the experimenter having reset the wavelength down to 500 nm each time. The first reading was ignored; sometimes more than five readings were needed before they became consistent. The reliability of the test for Unique Green point was checked by method of test retest reliability coefficient over 12 subjects, who were retested at intervals between a week and a fortnight. This gave a result of 0.56 which, although it could have been higher, is nevertheless highly significant. Much more importantly, no subject was reclassified on the retest. It was found that children under the age of 10 years were unreliable and therefore were excluded. The
populations tested consisted of 253 males and 187 females, all of whom were undergraduates aged between 18 and 25 years. Most subjects were selected from practical classes by a tutor on the basis that they were inattentive o r had completed their course work. Care was taken that the criteria of selection could not be related in any way to vision.
500
520
510
530
wawlenglh n m
Fig. 1. Distribution of 253 males with respect to the Unique Green point.
I
-l L 520
War.lenOlh
530
n rn
Fig. 2. Distribution of 187 females with respect to the Unique Green point.
276
COBB
It was found that with the male subjects (Fig. l), distribution with respect to the Unique Green point showed a sudden zero frequency in the 520-525 nm cell, whereas the female distribution (Fig. 2) is close to that of a normal curve. The dip for the males is significant, P = 0.01, and remains so with all simple transformations of the data. Waaler (1967) showed with a large number of pedigrees that this is due to a single effect, carried on the X chromosome, which breeds true, neither condition being dominant to the other. Thus in the male, the two homozygous conditions correspond to the two modes, with a distinct gap in the
frequency distribution, whereas in the female, although the two homozygous conditions would manifest distinctly, the heterozygotes are intermediate and fill the gap. I have confirmed the fundamental correctness of Waaler's (1967) hypothesis with 50 pedigrees, which are detailed in Table 1 and summarized in Tables 2 and 3. These pedigrees were families known directly or indirectly to the author and who were willing to cooperate; four families refused. GI and G;1 are used to denote the possible genotypes for the male, according t o which mode Unique Green group he belongs. G11 and G a are the possible genotypes for fe-
Tabie 1 for females for the Class on Richards' (1967) notation self-setting technique with classification into types I and II for males and I, II and 1-11 for females for the Class on Richards (1967) notation Name
Father
Mother Daughter
Whitehead Darle Williams Lehman Strettan Jones Giilet Fry Austin Slade Stretton Shicker Mayfair Laurie Timens Shiels Laidlaw Roberts Rothschild Jordan Jeffry Lee Ferris Brooks Cobb Churchman McConnel
527 II 524 I1 521 I 526 II 528 II Daut. 490 I 520 I 494 512 487 512 487 495 516 528
509 480 515 509 533 540 537 516 512 518 514 516 526 512 508 490 516 508 508 504 5MI 518 488 505 512 528 487
Smith
-
516 490 516 509 498 508 508 498 514 511
I I I I I I I I I I
519 I
I I I I 1-11 II II 1-11 I I I 1-11 1-11 I I I I I I I I I I I I II I
484 I
508 524 523 521 487
Son
I' 1-11 I 1-11 I*
Name
Father
Mother
I
Hyhton Griffiths Curtis Timens Westlake
504 518 510 469 507
Holland Buchanan
519 I 508 I
523 1-11 530 II
Transman
505
546 II
Dickson
518
-
Reichman Reader Parval Philips
522 522 510 503 I
511 506 513 498
Hattle Erskine
524 II 515 I
505 I 512 I
I
I I I I
514 508 513 523 521
I I 1-11 1-11
527 I I 496 I 533 II
Daughter 490 508 489 528 519 514 529 525
I I I 1-11 I I 1-11
528 508 514 508 514 502 496 505 497 530 508 502 512
1-11 I' I I I I I I I 1-11 I I I
1-11
480 508 498 508 508 489 489 518 495 I 498 505 506 490 522 522 509 509 516 488 513 510
I I I 1
I I I
I I' I I
485 520 513 508 (512 516
I I I I I)** I
Erskine Cuthbert Philips Allan
521 I 498 I
I
* These pedigrees suggest that sometimes Class I may be dominant. * * Son of mother only: natural father deceased. Deut. = deuteranomalous.
497 508 525 509
I I
I I
I I 1-11 I
Son
489 508 518 528 512
I I I II I
511 I 510 I 514 I 508 I
515 I 512 I 503 I 506 I 503 I 498 I
UNIQUE GREEN PHENOMENON AND COLOUR VISION
Table 2
Genotypes of mothers and their sons: all sons have genotypes which can arise from the X chromosome of their respective mothers. (From the pedigrees in Table 1) Sons
Mothers GI 28 6
Gii
28
Gn
3
3
G22
G2
4 3
Table 3
Genotypes of fathers and mothers producing daughters of particular genotypes: the daughters have genotypes which can arise from the X chromosome Parents
18
2 6 5 1 0
Gi X Gii Gi x Gi2 Gi x G n
Gz x Gii G2 x Giz G2 x G n
Daughters
24
2 2' 1' 1'
3 4 4 1
' These pedigrees suggest that sometimes Class I may be dominant.
males, which correspond to the male G1 and G2, respectively, and G12 forms an intermediate group with a small overlap (see Waaler 1967, Cobb 1973). The pedigrees were tested by an apparatus slightly different from that described above, since the laboratory spectrometer, which had been developed to give as clear-cut a result as possible, could not be transported to the locations of the families concerned without risk of damage and disturbance to the calibration. The apparatus used for the pedigrees was a Nagel 11 anomaloscope similar to that used by Waaler (1967), and his single field technique was repeated. This involved setting the luminance and mixture screws at zero and asking the subject to set
277
the upper field, by means of the main screw, until the colour seen was neither blue-green nor yellow-green but green-green. The first 10 families in Table 1 had their Unique Green points measured using both apparatuses and in each case the classification was the same, although in the case of the Nagel Model anomaloscope, the spread of readings for each subject was greater. Thus, data shown in Figs. 1 and 2 and Table 4 were collected by the laboratory spectrometer technique, whereas those in Tables 1, 2 and 3 were collected by Waaler's (1967) single field technique with the Nagel Model 11 anomaloscope. The method of classifying a male subject as Class I or Class I1 presented certain problems within the pedigrees, since the Nagel Model 11 used in the field did not give such clear-cut results as the spectrometer apparatus and there was n o clear zero frequency such as shown in Fig. 1. The most satisfactory method was found t o be to take readings below 522 nm as belonging t o Class I, and those above 523 nm t o Class 11. I n the pedigree Williams, this system produced an anomaly: the father at 521 nm and the daughter at 523 nm have both been placed in Class I because the mother was a clear Class I at 515 nm. It is not known whether this is a pedigree which does not fit, or a bad experimental plot; in any case, it does not fall very far outside the Class I criterion. It is assumed for the purposes of Tables 2 and 3 that Class I arises from G1 in the male and GI1 in the female, and that Class 11 arises from G2 in the male and GZ2in the female. In some cases, parents were not available for testing and are absent from Table 1. In the pedigree Brooks, the father of the son in the pedigree was dead and the mother had remarried. Thus in the pedigree, the father was that of the daughter not of the son, the son and daughter being half brother and sister.
COBB
278
Table 4 Correlation between the midmatching point for the Rayleigh equation for that particular axis on the Pickford-Nicolson anomaloscope and the Unique Green point for 91 female art students: significant at the P = 0.01 level Green r = 0.057 Green r = 0.111 Yellow r = 0.264
Red Blue Blue
Since this characteristic breeds true and appears to be sex-linked, it strongly suggests that a visual pigment is being directly affected from a gene on the X chromosome. Further study was undertaken in an attempt t o elucidate what visual pigment might be responsible. A Pickford-Nicolson (Pickford & Lodowski 1960) filter anomaloscope was set up and a population of 94 women, none of whom had more than four errors or hesitations on the Ishihara test for colour blindness, were tested on three axes: Red Green matching yellow; Blue Green matching blue green; BIue Yellow matching white. Data were collected with regard to both the mid-matching point and matching range, and these were correlated with findings on the test for Unique Green (Table 4). The correlation for the blue-yellow is significant, P 0.01. This is particularly important since, for the anomaloscope used, the retest reIiability coefficient is only 0.611 (n = 32) for this axis (Pickford 1951). Furthermore, in colorimetry in general, it is for this axis that the lowest reliability is found on the chromaticity chart, probably owing to the individual variations in macula pigmentation and cone characteristics at the fovea.
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+
