The characteristics of a visual defect associated with abnormal responses to both colour and luminance

THE CHARACTERISTICS OF A VISUAL DEFECT ASSOCIATED WITH ABNORMAL RESPONSES TO BOTH COLOUR AND LUMINANCE B. G. BE?UDER and K. H. RUDDOCK Applied Optics Section. Physics Department, Imperial College, London. SW7 282 (Receiwd

i2 September

1973)

Abstract--This paper deals with the visual response characteristics of a single male subject. .-\lthough members of his maternal family were found to be ted-green defective. the subject’s responses are not consistent with those of the recognized classes ofcolout vision defect. Thus his fovea1 cotour matching is dichromatic: his fovea1 threshold spectral sensitivity is between _7.5 and 5.0 log units less than that of normal observers and his critical fusion frequency saturates at ISlY Hz. Increment threshold measurements reveal the activity of only two spectral mechanisms and for long waveiength stimuli. the increment threshold level falls sharply as the iI~umination level of a superimposed background field is increased beyond a critical level. This last observation correlates with the observer’s subjective report that objects which appear red to normal observers appear to him as “grey” or “black”. with no well-defined form. Experiments with stimuli located up to I I’ off-axis on the temporal retina showed that the subject’s visual responses are essentially invariant with respect to the stimulus location and that characteristic rod responses arcentirely absent. We have proposed ;I functional model for the interpretation the:Verimen-

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In this report we describe some visual response characteristics ofa single male observer, MW, aged 21 yr. His defect first came to our notice during screening tests carried out by an employer, in the course of which he failed to identify correctly any but the first plate of the Ishihara pseudo-isochromatic test for defective colour vision. This first plate. an orange figure 12 on a blue background, is identified by all subjects except those with monochromatic colour vision. However, MW’s performance with the remaining plates was quite inconsistent with that expected from a subject with one of the standard forms of defective colour vision. The results reported here confirm the singular nature of his fovea1 colour vision and show also that his fovea1 increment threshold response characteristics are of a type which. to the best of our knowledge. has not been previously observed. A preliminary report of this work has been published by the authors (Ruddock and Bender. 1972). In the present paper. we demonstrate that these abnormal responses are not restricted to his fovea) vision and that there is no rod contribution to the non-fovea1 responses. We also propose a model for the interpretation of our experimental observations. ESPERISIEWTAL

METHODS

In this study we employed experimental techniques developed by Wright and hts co-workers (Wright. 1916. ch. ‘t-29). but many modifications were required because of the unusual nature of MW’s visual defect. Colour matches were established for a bipartite SO’ x So’ field [Fig. I(a)] using matching stimuli of wavelength 600 and -160nm. These

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Fig. I. Field con~gurations used in the investigation: (a) SO‘ x 80’ bipartite calorimeter field for fovea1colour matching; i. represents the monochromatic test stimulus, R the “red” (600 nm) and B the “blue” (160 run) matching stimulus; (b) lo” field for colour matching; (c) Field configuration for the determin&ion of increment threshold spectral response functions, The circular I ’ test field is superimposed on ;I semi-circular IO background field .tnd a darkened grid marks the location of the test field: td) Field zrrangcmcnt for measuring non-foreal sensitivity: rhe fixation point is adjustable between 0 and I i :twa! from the test tield.

matching stimuli were chosen after considerable preliminar) investigation of his subjective responses to spectral stimuli which showed that he was able to distinguish -‘blues*‘ ifor spectrai stimuli of wavelength i. f 520 nm), “sreens” (for spectral stimuli of wavelength 520nm 5 i 5 6lOnm) and “grey” for 1. r 610nm. The observer adjusted the illumination levels of the two matching stimuli to obtain a match with the test stimulus presented in the louer half-field. Colour matches were also determined for n IO’ bipartite field [Fig. I(b)]. Wavelength discrimination was measured for the visual field configuration of Fig. I(a) with monochromatic stimuii presented in both halves of the field. The observer was instructed to chance the wavelength of the lower half of the field until he couldcjust detect a colour difference between the two halves, which were initially set at the same wavelength. We was also instructed to mamtain equality of apparent brightness beruezn the two half-fields by adjusting the illumination levri of the upper field. so that the waveiength discrimination steps. di. were determined for conditions of constant brightness. For reasons discussed later (see Results section) it was not possible to determine a flicker spectral sensitivit) 1I’,,)curve for subject MW. Threshold response characteristics were determined for MW by applying the methods previously described ina study of unilateral defective colour vision (Bender. Ruddock. de V&s-de 4101and Went. 1972). Spectral sensitivity functions for increment threshold detection were measured in terms of the illumination level of a 10’ dia semi-circular background field required to bring a I’ dia circular test field of fixed wavelength and illumination to threshold [Fig. l(c)]. The illumination level of the test field was maintained at about 04 log units above its threshold level in the absence of ;1 backzround Geld. and the field was presented in a 0.5 set on- O? set off-Aash and recovery cycle to facilitate its detection. The position of the test field within the maintained background field was marked by a darkened grid. and the observer was instructed to adjust the level of the background field until the centrally-fixated test field could no longer be detected. By repeating this measurement for a series of wavelengths of the background field. spectral sensitivity functions were determined. Increment threshold measurements were also performed in which the monochromatic background field stimulus was maintained at a constant illumination level and the increment threshold level of the test field determined for a series ofstimulus wavelengths. i. Both methods of investigating the threshold spectral sensitivity functions of the (n-) mechanisms which subserve human colour vision were developed by Stiles (1939, 1953. 1939).In addition. threshold sensitivity values for zero background field were determined, the observer being instructed to adjust the illumination level of the test field until it just disappeared. In this case the test field was again presented in a 03 set on- 0.5 set o&flash and recovery cycle, but its location was marked by four dim, continuously visible white light fixation points positioned at the corners of the field. For non-fovea1 investigations, a white Iight fixation point was provided [Fig. l(d)]. the position of which could be adjusted to alter the retinal location of the test field.

AH experiments were performed with the trichromatic calorimeter designed by Wright (1927-3: 1946, chaps. 1 and 3) incorporating the achromatizing field lens described by Thomson and Wright (1947). For some measurements the normal 80’ x KY calorimeter field was converted to a

10’ circular field bc means of the larg tieid ctttachment designed bq Clarke t 1965). The instrumental sit pupil size is of 1 mm dia for the vtsual smull-field arrangement and @6 mm for the large-tield set-up. The ndnprarion source of the colorimrter gives ;L 7’ diameter visual field. u-hich can be superimposed on the visual field of the colorimctsr via a semi-reflector placed across the exit beam. The size of the adaptation field was controlled by means of a tield stop and the white light stimulus provided by the adaptation source was rendered near monochromatic b? Interference (Balzsr B~O)filtersofappro~imately IOnm half-intensity handaidth. The observer used a dental clamp throughout the ohssrvations to assist in maintaining correct visual alignment. AlI fovea1 observations were preceded h> 5 min darkadaptation and non-fovea1 observations bq 3 i-hr period of dark adaptation.

Spectral energy calibrations were carried out with an EMI photomultiplier, type 91 IA, which was irsesifcalibrated with reference to a Hilger-Schwarz thermopile Wavelength calibrations were performed with a line spectrum. produced by a Phillips mixed gas spectral source. type Hg-Zn-Cd 0.9h 87. Ohsrrtrrr As stated in the Introduction. the subject oithe investingtion is a 21-qr-old male, ,LfW. His fundus and ocular meuia appear normal under ophthalmoscopicexamination and his visual actlit). measured with test charts. is 1.2. He has no medical record of serious disease or injury. nor has he been subjected to treatment with unusual drugs. Preliminary examination showed that he was unable to identify correctly an! but the first of the Ishihara colour vision test plates. Both maternal and paternal families hate a histor! of defective colour vision, his father giving dcn:.in responses to Ishihara plates. whilst male members oi his mother’s family were classed as red-green defective by their Ishihara responses. One sister was classified from Ishihara tests 3s being mildly red-green defective. which is consistent *ith the fact that both sides of the family carry genes for defectilc colour vision. However. the otitstandin~ festure of MiV’s subjective responses is his almost total failure to detect red objects. which appear black or “silvery gre”. with no welldefined form. This subjective response is not reported b) any other member of the farnil?. Results for two other observers, BGB and GJB. both aged Z-! yr and possessing normal trichromatic colour vision. are given for comparison purposes.

RESULTS Unless otherwise stated, all results refer to observer MW. Wavelength discrimination data for the I’ 20’ fovea1 field are given in Fig. 2, the values i- 6i and - di being plotted separately. Atso shown are the corresponding data for normal colour vision (Wright and Pitt, 1934). It is apparent that MW’s wavelength discrimination is grossly disturbed. the discrimination step sizes 6;. being much greater than those of normal colour vision for all values of the comparison wavelength. Further. when 2. is < 500 nm. MW is unable to distinguish betu-een any pair of stimuli. .A single minimum of wavelength discrimination step. Si. occurs for

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Fig. 2. Wavelength discrimination steps, + A i plotted as a function of the comparison wavelength, i. Values + A li are denoted by full circles and - A i. by open circles. The visual field arrangement is shown in the inset. The broken curve gives values for normal colour vision. Observer MW. _I

620 nm which is the wavelength at which deuteranomalous observers give a minimum discrimination step (Nelson, 1938). The normalized fovea1 colour-matching co-efficients (chromaticity co-ordinates) are plotted in Fig. 3, together with a “white” point for a standard source of colour temperature 2854X. The dichromatic nature of the colour matching is consistent with the existence of a single minimum in the wavelength discrimination data; a similar situation exists with the standard forms of dichromatic colour vision (Wright, 1946, chap. 26; 1952). The fovea1 threshold sensitivity function (Fig. 4) is plotted with comparison data for the normal trichromat BGB, and it is apparent that MW’s sensitivity is suppressed throughout the visible spectrum compared a comparison

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Fig. 3. The red (r) and blue (b) matching coeficients for fovea1 small field colour matches plotted as a function of test stimulus wavelength, L. Coefficients are normalized such that r = 6 = O+ for L equal to 550 nm. The wavelength E., denotes the “neutral” wavelength equivalent to white tight of colour temperature 2854X Observer MW.

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Fig. 4. Logarithm of relative fovea1 threshold sensitivity to a 1’ circular test field, log .S, plotted as a function of the stimulus wavelength, {. Closed circles give data points for observer MW and open circles those for observer BGB. Error bars show the totaf spread of three indjvidual determinations. The broken Iiw represents the data for observer MW, displaced to give the same peak sensitivity as for observer BGB. The location of the test field, which was presented in a I/2 set on., i/2 set off Rash and recovery cycle, was marked by four continuously visible fixation points (see inset for field arrangement).

with that of the normal observer. The most notable feature of these results is the rapid fall in sensitivity between 610 nm and 6.50 nm, there being a reduction of some 25 log units in sensitivity over this 40 nm waveband. Such a change in spectral sensitivity is completely uncharacteristic of the absorption spectra of visual pigments (Dartnail, 1952, 1957) and suggests that some factor other than photopigment abnormality contributes to MW’s response characteristics. Fovea1 increment threshold values for a test stimulus of wavelength i superimposed on a constant highluminance conditioning field of wavelength p are shown in Fig. 5 and again comparison data for observer BGB are also given. The results for MW show evidence of only two sensitivity peaks, approximately at wavelengths 440 and 560 nm, whereas for the normal observer BGB, three peak sensitivities are indicated, approximately at wavelengths 440, 540 and 570 MI. Spectral response functions of the increment threshold (x-f m~~nisrn measured in terms of the luminance of the background field (rq~ method) are shown in Fig. 6.

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Tnoci‘~tssesoirrsponse:L\rrS found. one: [Fig. 6(a)] being ClOsei>similar to the z5 re5ponse ofnormal colour vision first described by Stiles { 1939. I%?) m-d tabnlatxi bq Wyszecki and Stllcs (19671. The second [Fig. 6(b)] is a bro.td b,m_i response function. quit;: ditI&nt in nature ikam the zt-mechanisms i;,und in normal coiour vision (Stiles. 1939. 1953. 19591. As the lon,a wavelength section of this function is similur to zd [Fig. 6(b)]. we attempt& to iji’l;lte ;t short \\:t\c‘length mschznism. equivalent to thr norm:\1 z: function b\ =mploqing the method developed b> Stiles; for the &aration of nil and xr (Stiles. 1953). The spectral response function obtained in this way shows ritasonablt _ = oood agreement Lvith ths normal ;I~ mechanism IF@. 7). and it seems most probabls that the broad-band function shown in Fig. G(b) is a combination of the x1 and nz; mechanisms shown in Figs. 6(a) and 7 xspcctively. Increment threshold values. I,. for a 1’ dia test stimulus superimposed on a background iield of illsmination level I, arc plotted in Fig. 5. The stimulus wavelength was the samt: for both test and background fields and data are given for a number of wavelengths. The most significant feature of these data is the sudden fall in the thrsshold illumination level of the test stimulus, I,, which occurs as the illumination 1~~4, I,, of the background field is incrsased bs>ond a critic31 value. This effect. which is most marked for long waveieneth stimuli. does not appear to have been rsportzd previottslv. The critical value of bxkground illumination letel at i\ hlch the fall in I, occurs coincides approximateli \vith that at which the backpround &Id becomes visible to ths obssrvsr MW’. The data for Lvavelsngth 650 nm has been pIottzd szparat&>- in Fig. 9. together with comparison data for the normal trichromats BGB and GJB. and this plot serbes to ?mphasize the abnormal responses to long uave!ensth stimuli uhich ars of !+LfW’svisual characterthe most surprisin, 0 irature . istics. In addition to the extraordinarily low threshold sensitivity (some 5 log units lowsr than that of the norITId observers). the variation of I, for ML%-is restricted to a small range of ~~lucs at the upper part of the reSpOIW? range for normal vision. Critical fusion frequency M’as determined as a function of illumination level for an SO’ x -U fovsal field Lvhich was IGO per cent squarsnave modulated. The relationship bets-ecn CFF and stimulus illumination level is shown in Fig. 10 for test ~~avelsngth 590 nm, together with comparison valuss for the normal trichromnt BGB. The CFF values for obser\sr MW never rise above a saturation ievrl of I_?- IS Hz. tvhersas over a comparable range of supra-threshold illumination levels. CFF values for observsr BGB increase continuousl> to more than 40 Hz. A similar result was obtained with stimulus wavelengths -MO and 600 nm. The saturation of flicker rzsponsc at 15-18 Hz is similar to that found in normal obsstl-ers for the rod (Hrcht and Shlaer. 1936) and ;1) cone mechanisms (Brindlry, du Croz and Rushton. 19661. Abnormal responses for flicker wsrc not unespsctsd. since it is pr~+x\htc that the threshold mrchani5m \\ hich stts the

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Fig. 5. Relative increment threshold illumination level, I,, of a I’ dia foveally fixated test field of wavelength i., supetimposed on 7’ circular background field of wavelength /I and illu~~~tion level f,u. Data for diRerent back~ound field wavelengths. JC have been displaced vertically for clarity of presentation and values of/l (in nm) and 1,~(in log td) are given with each set of data points. Fig. 5(a) refers to observer MW and 5(b) to observer BGB. Data points are the mean of three or four individual rrtadings, total spread of values being between *@OS and k@? log units in each

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(b) (4 Fig. 6. Spectral sensitivity functions determined by measuring the illumination level of a 10” semicircular background field required to bring a 1’ test field to threshold [Fig. I(c) and inset]. The sensitivity. S. expressed as the inverse of the required illumination level, is piotted as a function of the wavelength of the background field. p. The test stimulus was set at about 0.8 log units above the threshold illumination levei for zero background iilumination level. Each point represents the mean of three readings. Fig. 6(a) gives results for test wavelengths 590 nm (open circles). 551 nm (closed circles) and 495 nm (crosses) and the continuous line represents the normal zs function. Fig. 6ib) gives results for test wavelengths 650 nm (open circles) and 438 nm (closed circles) and the broken curve corresponds to the normal z5 function. The spread of individual results was between kO.05 and i:@15 log units. around the mean values.

flicker fusion limit is the same as. or closely related to.

Fig. 7. As for Fig. 6, but for test wavelength 438 nm and with an auxiliary background field of wavelength 590 nm and fixed illumination level 3.0 log td. Points are the means ofthree readings. and error bars give the total spread of the results. The full curve corresponds to the normal z I function. Observer MW.

that which determines increment threshold levels I, (de Lange. 1945). The poor Nicker response associated with MW’s vision and the physical discomfort which he experienced during CFF measurements prevented our measuring flicker Vi responses. The recovery of threshold sensitivity following adaptation to white light (Fig. 11) was normal, which indicates that the bleaching properties of the cone photopigments are not abnormal. although we were unable to determine this function for long wavelength (650 nm) light. because of the technical difficulty of providing sufficient light for the test stimulus. The results given so far refer to IMW’Sfovea1 visual responses, but we also carried out measurements with non-fovea1 fields, both to check rod function and to ensure that the fovea1 results were not the resuit of a locaiized scotoma. We note that MW found no difficulty in fixating the visual fields for fovea1 measurement; thus, n priori. it seems unlikely that a central, local scotoma is the cause of the responses described

E. G. BESDER and K. f-f. RCDCXXX

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Fig. LO.Critical fusion frequency values for a 100 per cent square-wave modulated -IO’x 80’ test field of wavelength 590 nm. Full circles give data for the normal trichromat BGB and open circles those for observer MW. The relative illumination level. log I. has been adjusted so that fovea1 threshold for the test field coincides for the two observers at the origin. Thus log f measures the suprathreshold illumination level of the Hicker stimulus. Each point is the mean of three readings. and individual results were spread over 0

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Fi_s. 8. Log increment threshold illumination level of a I’ circular zest stimulus plotted as a funcrion of rhr troiand illumination level, f.u. of a 7’ circular background field (see inset). The wavelen_eth of the test and background fields was the same for each set of data and the value of this wavelength in nm is given with each set. Each point represents the mean of three readings, and error bars represent the total spread of individual results. Threshold levers for zero background field (log f = - x) are given by full circles, other values by open circles. Data for different wavelengths have been displaced arbitrarily in a vertical direction for clarity. Observer

retina. The results (Fig. 11)show that recovery is complete within S-S min, which is characteristic of cone adaptation and closely similar to the corresponding fovea1 response (also shown in Fig. I I). There is no sign of the long-term recovery of threshold sensitivity associated with rod responses (Kohlrausck 1922, 1931). 7

previously. The recovery of threshold sensitivity foilowing adaptation to white light was measured for an 80’ x 40’ test field located 1I” off-axis on the temporal

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Fig. 9. As Fig. 8, for test and background stimuli of wavelength 650 nm. Operr circles; observer MW; full circle, observer BGB and crosses, observer GJB. The threshold illumination level for zero background field level is marked T for each set of data.

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Fig. II. The time course of log increment threshold illumination, I. following 30 set adaptation to a white light source of colour temperature approximately 2700°K and illumination level 5.6 log td. The 40’ x 80’ test iieid was iixated fobeally for points denoted x (test wavelength 440 nm), o (test wavelength 500 nm) and l (test wavelength 550 nm). Points denoted 0 refer to a test wavelength of 5 10 nm, with the test field located 11’ off-axis on the temporal retina. The test field was presented in a 10 set on, 1!2 xc off flash and recovery cycle for all measurements. Errors were of the order of +i31 log units for each point. Observer MW.

The characteristics of a visual defect The threshold spectral sensitivity function was measured for a 40 x SO’ field located 11’ off-axis and although the sensitivity function (Fig. 12) differs from that for fovea1 threshold measurements. it is not a typical rod function. When the stimulus is moved off-axis there is little change in sensitivity except at 11’ off-axis. where there is a reduction for all wavelengths investigated {Fig. 13). The expected increase in threshold sensitivity for blue stimuli presented off-axis (Stiles, 1949; Kishto, 1970) which results from rod activity is not observed for the subject MW. Thus, the off-axial spectral sensitivity function (Fig. 12), the time course of dark adaptation for off-axis stimuli (Fig. 11) and the variation of threshold sensitivity with retinal Iocation (Fig. 13)all fail to show the contributions of a rod-type mechanism. We conclude that for observer MW the rods are either non-functional or their responses are suppressed. The relatively small change in threshold sensitivity across the fovea1 and parafoveal retina, which was observed for all stimulus wavelengths, shows that MW’s visual responses are fairly uniform for retinal locations within I I’ of the fovea centralis. This is confirmed by colour matches carried out with a 10” matching field, the results of which (Fig. 14) are very similar to those for the 120 fovea1 field (Fig. 3). In the course of these measurements- MW reported that the large field colour matches were uniformly satisfactory over the 10” field. which is consistent with the conclusion that his abnormal cone responses are not confined to the fovea1 region. DISCUSSION

The visual response characteristics of our subject MW do not correspond to those of the known classes I

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ofdefective colour vision, and in order to interpret our experimental data. we make use of the model shown in Fig. 15. In constructing this model, we have attempted to minimize the number of ad hoc assumptions required to explain the experimental results. The principal features of the model are as follows:

Fig. 14. Colour matching coeffcients for a lo” bipartite matching field. The values of the red (r) and biue (b) matchFig. I?. Log relative threshold sensitivity to a 4tY x 80’ test field. located I I off-axis on the temporal retina, plotted as a function of the wavelength, i., of the test field. Points denote the mean of three readings and error bars show the spread of the indi idual results. Observer MW.

ing coefficients are given as a function of the test stimulus wavelength, L, the coefficients being normalized such that r = g = 0.5 for I equal to 550 nm. The matching stimuli were of wavelength 600 nm (R) and 460 nm (G). Observer MW.

8. G. BEMER and K. H. RI_OLWX The experimental data on Mu”s visual responses are related to the function of this model in the following way: (a) Studies on the red- and green-sensitive mechanisms of deuteranomaly show that their relative spectral sensitivity characteristics as measured by increment threshold techniques (Watkins. 1969) and by colour matching methods (Rushton. Powell and Fig. 15. The functional model used to interpret !&w’s visual White. 1973). an similar. Thus the difference signal response characteristics, which is described fully in the test. (Ri - Gf) in the C, channel is small for all wavePoint A represents the functional block in the Lchannel. lengths. In the absence of effective luminance signals. and it is further postulated that the - Gj component of the the output of the system is therefore determined by the (5; - Gj 1channrl disappears. B; signal of the C,-channel except ar longer wavelengths, for which 5;. isaiso very small [xr e.g. values of rrcigiven by Wyszccki and Stiles ( 196_!]. (bj The rod signals are suppressed. since they pass {if There are three spectral classes of cone receptors and also rod receptors present in MW’s retina. The solely along the blocked Lchannel. (c) The threshold spectral sensitivity for zero backcone types correspond to the red (R,) and blue (Bi) mechanisms of normai colour vision and the anomaground field (Fig. 4) is determined by the joint ourput of the C, and CL channels. The absence of L-channel ious green (Gl) mechanism of deuteranomaiy. Our screening tests showed that 51w’s mother must be a output is consistent with the overall IOU threshold sensitivity values for MW. Further. since ths value of (B;) carrier for a red-green colour vision defect. since both as measured by xj (Wyszecki and Stiles. 19673 falls his sister and some materna1 uncles were red-green rapidly for wavelength i. > 520 nm the long wavelength colour defective. The postulate of deuteranomalous threshold values are probablv determined bv- the colour vision is thus supported by genetic evidence. Mitchell and Rushton (1971) demonstrated that the function@, - G”).The large f&in threshold sen&isity spectral response of the green-sensitive mechanism of between 610 and 640 nm is therefore interpreted as a property of this difference function deuteranomaly (Gf) differs from that of normal colour (dj The blue (n,) response mechanism constitutes vision and Rushton. Powell and White (1973) have the only effective excitatory mechanism for much of determined the spectral characteristics of this mechanism. As isdiscussed in Section(b) below, it is not the spectral range. Thus. the observed saturation of necessary to omit rod receptors, despite the fact that CFF at IS to IS Hz (Fig. 101is consistent with thz fact there is no rod contribution to the non-fovea1 visual rethat the xi nicker response saturates 3t about 1s Hz sponses of our subject (Figs. 11, I Z and 13). (Brindiey. du Croz and Rushton. 19661. (ii) Three classes of neural channel receive the recep(ej The fovea1 colour vision associated with the tor signals. two C-type and one L-type. A similar model model is mediated by the cone mechanisms of deuterwas proposed to interpret the non-fovea1 colour vision anomaly. Increment threshold measurements on deuresponses of dichromats (Ruddock, 1971). and the teranomalous subjects show that rr, mechanisms of extensive physiological evidence for such organization normal colour vision cannot be isolated. but in addiwas reviewed in that publication. As was proposed for tion to the normal rri and x5 me~hanjsms a third mechanism with spectral characteristics closely similar the earlier model, the rods are connected only to the to those of the normal rcI:is observed (IVatkins. 1969). L-channel, with an inhibitory connection from the blue-sensitive cone mechanism. It should be noted that Our increment threshold data I’M IIW rev4 tu’o indesignating the C-channels (R; - G>,jand (B, - G’,). spectral mechanisms. corresponding to the rtI and Zj vve imply only antagonistic combinations of signals functions of normal colour v-ision. Thus. our resulti are from the red and green and blue and green receptor not inconsistent with Watkins data for deuteranomamechanisms respectively and not that the receptor sig- lous observers. as it is improb~lble rhar results for a nals are combined linearly. single observer could resolve the differences benvcen (iii) We postulate two special features of MW’s the normal TINand the anomalous third mechanism of t-isuaf system. to which we attribute the unusual deute~anomaly. It is significant that despite the grossly nature of his response characteristics. The first is a abnormal threshold spectral function observed with functional block in the L-unit. designated ,A in Fig. 15. zero background field. the increment threshold spectral mechanisms can still be isolated. This suggests that and we discuss below. in Section (hj, the anatomical these mechanisms are entirely determined by visual relocation of this block point. The second feature is that the “blue-green” CZ colour channel propagates a blue sponses arising prior to the point(s) at u hich malfunction occurs in MW’s visual system. (8;) signal, but no opponent signal, i.e. the C, com(fj The model possesses a divarianr output. given by ponent signal designated -(G’) in Fig. 14 is absent. This is examined further in Section (i) below, but it the signals from the C, and C, channels. >LIW”sdichromatic colour matching (Fig. 2) is consistent with a must be admitted that it is a relatively arbitrary propodi\:lriant s\jtem and his ~~a~elsngth discrimination sition.

cur\e with its single mimimum step (Fig. 2) is 31~0 typical ofdichromatic colour vision systems (JVright. 1946. chap. 26). Deuteranomalous observers also posssss a minimum discrimination step at about 620 nm (N&on. 1938). and thus it is probable that M’IL”swavelength discrimination the (R; - Gf) channel. More detailed analysis of the colour matching and wavelength discrimjn3tion properties of the model is not possible without further assumptions about its functional organization. (g) The remarkable increment threshold characteristics of our subject (Figs. 8 and 9) are not incorporated in the model. since they probably reflect complex properties of the C, and C1 channels. The unusual feature of these data. namely the sudden fall in the threshold level I, of the test field as the level f, of the background field increases in valus. is particularly marked for long wavelength stimuli. This suggests that it is a property of the C, channel and may be associated with the antagonistic centre-surround organization of receptive fields in the (R, - G,) channels of vertebrate colour vision. Such organization is observed electrophysiologically in responses of ganglion cells (Wagnsr. MacXichol and Wolbarsht. 1963: Daw. 1968). lateral genicular units (Wiesel and Hub&. 1966) and cortical units (Dow and Gouras, 1973). Yoon (1970) has reported ganglion cell responses in the cat for which threshold lzvei was lowered by the presence of 3 background field. although not with the “step-function” observed for MW. (h) The separation of the vertebrate visual pathways into L-type and C-type neural channels is observed at retinal level in the electrophysiological responses of the ganglion cells (as referenced above) and OF the horizontal cells (Svartichin, 1956; ~~a~Nicho1 and Svaetichin, 1958). Thus. the block point A couid occur at almost any level of MW’s visual system. However, we found that we could elicit a pupil response with both long and short-wavelength stimuli which were perceptably subliminal for the subject MW. Since the afferent nerves of the pupil control loop are optic nerve fibres that lead to the pretectal region of the brain. where they synapse with motor nerves, this observation suggests that the signals leaving the retina are normal and that the block point A is located centrally in Mw’s visual system. investigation of the spectral characteristics of the subject’s pupil responses is ptanned. in order to obtain further evidence regarding his visual defect. Qualitative descriptions of the visual responses associated with disorders of the visual cortex (Duke-Elder, 1919, chap. 4) are consistent with MW’s subjective responses to sisual stimuli. This lends additional support to the suggestion that his responses are, at least in part, the result of a malflmction at some central point in the visual system. (i) The blue-sensitive cone mechanism appears to fuhction for observer MW. because we can isolate thz x,-spectral mechanism (Fig. 7). However. we found no blue-green wavelength discrimination. hence the postulate that the C:- (B,_- Gj) discrimination channel

carries only ths (B;,) signals. This is a somewhat arbitrary proposal. since as w2 discussed in the preceding Section(h), division into L-and C-t>pe neural channels arises early in the visual pathways. It is therefore not obvious how a selective block of one spectral component of the C1 channel could be associated with a central block. as we propose for the L-channel. In the absence of more detailed physiologica evidence regarding the organization of the visual pathwavs. it does not seem fruitful to discuss altsrnative functional models. The speculative model discussed above does not constitute a unique representation of MW‘s vision and it could be argued that quantitative analysis of the model should be undertaken. We have given consideration to this point. but decided that no useful purpose would be served because of unresolved questions regarding the fundamental nature of deuteranomsly. ft is found experimsntally that both colour-matching and wavelength discrimination characteristics vary significantly from one deuteranomalous subject to another. This suggests that the anomalous mechanism of deuteranomaly does not possess unique spsctral response characteristics, such 3s those given by Rushton, Powell and White (1973). and a hypothesis regarding the variable nature of anomalous trichromacv has been analysed by one of the authors (Ruddock.in preparation). Accordingto this hypothesis. the spectral responseofthe anomalous mechanism (Gj of the model) can take any value between the normal green and normal red spectral response. Thus. the signals (R, - Gl) in th2 CI channel of the model could correspond to any one of a large number of possible functions. Cntil this point is resolved. no useful conclusions can be obtained by quantitative analysis. The value of the model (Fig. 15), which is essentially the same as that used to interpret a quite independent set of experimental data (Ruddock, 1971), is that it gives a basis for inter-relating the many unusual features of MW’s visual responses. Further, by postulating two special features [see (iii) above]. it yields a consistent qualitative description of these responses. The experimental data obtained in this investigation provide additional evidence in support of the hypothesis that post-receptor31 organization of human vision involves L- and C-type channels. One major point which remains unresolved is the cause of the functional block in the L-units and the abnormal function of the C2 discrimination channel. There is no evidence that it has a genetic origin, yet there is no known medical history to which it can be attributed. This point will probably not be clarified unless other observers with similar visual response characteristics are found. We note that during a recent study of colour vision amongst Kurds in Iran. Lightman, Carr-Locke and Pickles (1970) found a group of observers whose performance with the Ishahara screening plates was similar to that of our subject. Ackno,vledgemrrrt~-We are particularI!: indebted to our subject, IMW.for his ready and able co-operation during the

B. G. BESDERand K. H. RCDDOCK

.?-I1

long series of arduous observations u hich form the basis of this report. We wish to thank Dr W. S. Stiles. F.R.S.. for v&able discussions. .&it Commodore T. J. G. Price. F.R.C.S.. D.O.?&S.. R.A.F.. who referred ;41W to us for investigation and who carried out the ophthalmo~opic and acuity tests reported here and Dr. G. J. Burton, of this department, for making a number of observations. One of us. BGB. also wishes to thank the Science Research Council ior the award of a research studentship. REFERENCES

Bender B. G.. Ruddock I(. H.. de Vries-de XIol E. C. and Went L. N. (1971) The colour vision characteristics of an observer with unilateral defective colour vision: results and analysis. Vision Res.12,203j_ZO57. Brindley G. S.. du Croz J. J. and Rushton W. A. H. (1966) The flicker fusion frequency of the blue-sensitive mechanism of colour vision. J. Physiol.. Land. 183, 497%&. Clarke F. J. J. (1963) Further studies of extra-fovea1 colour metrics. Oprica Acrn 10, 257-274. Dartnall H. J. A. (1952) Visual pigment 167, a photosensitive pigment present in tenth retinae. J. Phpsiol.. Land. 116, 1%289. Dartnall H. J. .A.(1937) The Visual Pigments. Chap. 1. Methuen. London. Daw N. W. (19683Color-coded ganglion cells in the goldfish retina: extension of their receptive tieids by means of new stimuli. J. Physid.. Land. 197, 567-392. de t’alois R. L.. Abramov L and Jacobs G. H. (1966) Analysis of response patterns of LGN cells. J. opt. Sot. Am. 56,966-977.

Dow 5. M. and Gouras P. (1973) Color and spatial specificity of single units in rhesus monkey fovea1 striate cortex. J. ‘~~~~r~p~~si~~. 36, 7Y-100. Duke-Elder W. S. (1949) Text-Book of Ophthulmology. Vol. IV. Tire neuroiog~ ofrision. Motor and optical anomalies. Chap. 43. Henry Kimpton, London. Hecht S. and Shlaer S. (1936) The relation between intensity and critical frequency for different parts of the spectrum. i. @en.Phrsioi. 19,96>977. Kishio B. N. f 1970) Variation of the visual threshold with retinal location--I: the central 20’ of visual field. vision Rrs. 10, 735-761.

Kohlrausch A. (1922) L’ntersuchungen mit farbigen SchwelIenpciiflichtern iiber den Dunkeladaptationsverlauf des normalen Auges. PJYiigers Arch. ger. Physiol. 196, 113117. Dimmersehen. Kohlrausch A. (1931) Tagessehen. Adaptation. pp. 149~lj94o~~u~r~buch der ffor~~e~ und ~(~r~~o~ogisc~I~f1 Ph@ioijie. Bd. 11 2. Springer. Berlin. De Lange H. Dzn. (1958) Research into the dynamic nature of the human fovea-cortex systems with intermittent and modulated light--I: attenuation characteristics with white and colored light. J. opt. Sot. rim. 48, 777-784. Liahtman S. L.. Carr-Locke D. L. and Pickles H. G. fl970) The frequencv of PTC tasters and males defective in colour vision-in a Kurdish ~pulation in Iran. Human Bid. 42, 66%669.

tvlacNichol E. F. and Svaettchin G. ( 1958) Electric responses from the isolated rctmas of tishes. .4nt. J. Ophrhd. 16. so. a pt. 2. X-40. Sfitchell D. E. and Rushton W’..A. H. (1971)The red-green pigments of normal vision. Vision RLIS. Il. ioJ%--10%. Nelson J. H. (1938) .Anomalous trichromatism and its relation to normal trichromatism. Proc. Phys. Sot. 50, 661697. Ruddock K. H. ( 1971) Parafoveal colour vision responses of four dichromats. J’ision Rrr. 11. t-I:+156 Ruddock K. H. i 197 ) The effects on normrtl coiour vision characteristics of mixing the red and green phoropigmen&: analysis of a possible mechanism of anomalous trichromacy. In preparation. Ruddock K. H. and Bender B. G. (19721 On an observer vvirh abnormal colour vision. .\locl. &oh/. Oplnha[. II, 199-204. (hcquired Colour vision deficiencies. Int. Symp.. Ghent 1971.) Rushton W. A. H.. Powell D. S. and W’hite K. D. 11973) Anomalous pigments in the eyes of the red-green colour blind. .Vcnrrr. Lo&. 213, I67-L68. Stiles W. S. (1939) The directional sensitivity of the retina and the spectral sensitivities of the rods and cones. Proc. R. SOC. 127B. 65-105. Stiles W. S. (1949) increment thresholds and the mechanisms of coiour vision. Docttnrrrtm Op~~r~~~~l. 3, 13%163. Stiles W. S. { !953l Further studies of visual mechanisms by the two-colour threshold technique. pp. 6ClO3 of Co/oqnio Sohre Problemas Opricos de la I’isi&f. Union internationale de physique pure et apliquee, XIadrid. Stiles W. S. ( 19591Colour vision: the approach through mcmment threshold sensitivity. Proc. nrrr. ic& Szi. 49. i@%i i-t. Svaetichin G. (IQ%) Spectral response curves from single cones. .&fir pirysioi. Scnnci. 39, suppl. I ?A I T-46. Thomson L. C. and Wright W’. D. (1947) The colour sensitivity of the retina within the central fovea of man. J. P/I!siol.. bnd. 105. 316-331. Wagner H. G.. MacNichoi E. F. and Woibarsht .Lf. L. 11963) Functional basis for “on”-center and “off’-crnter receptive fields in the retina. J. opt. Sot. .&r. 53, 66--n. Watkins R. D. (1969) Fovea1 increment thresholds in normal and deutan observers. b’isiott Rrs. 9. 1iYj_1196. Wiesel T. N. and Hubel D. H. (1966) Spatial and chromatic interactions in the lateral geniculate hod! of the rhesus monkey. .I. 4rnrophrsiol. 29, 1I 1C I 156. Wright W. D. (1927-1928) A trichromatic coiorimeter with spectra1 primaries. Trans. opt. Sot.. Land. 29. 22j_?-tl. Wright W. D. (1946) Researches on .‘v’ortwl and Drf;c~i~ Colour liision. Henry; Kimpton. London. Wright W. D. (1952)The characteristics of triranopia. J. upt. Sot. rim. 42. 509-521. Wright W. D. and Pitt F. H. G. (I93.l) Hut: discrimination in normal colour vision. Proc. Phn. Sot. 46, -23%368. Wyszecki G. and Stiles u’. S. (1967) C&r Science. Chap. 7. John W&y. New York. k’oon >f. 1.1970)Reversal of Weber’s Law for an extraordinary unit in the cat’s retina. E’isionRrs. 10. 769-771.

The characteristics of a visual defect R&urn&On

393

de la reponse visuelle chez un sujet masculin qui, quoique des du type rouge-vert. repond dune facon qui echappe aux classes connues d’anomalies colorees. Ainsi ses equations foveales de couleurs sent dichromatiques. Son seuil fovea1 de sensibihtt spectrale est entre 2,s et 5.0 unites log superieur aux observateurs normaux et sa frequence critique de fusion se sature entre 15 et 18 Hz. Les mesures de seuil differentiel r&lent l’activite de deux mecanismes spectraux settlement et, aux grandes longueurs d’onde, la sensibilite differentielle’ tombre brusquement quand on augmente I’eclairement dun fond superpose au-deli dun niveau critique. Cette dernitre observation s’accorde avec l’impression subjective que des objets vus rouges par des observateurs normaux paraissent au sujet “gris” ou “noirs” sans forme bien definie. Des experiences avec des membres

stimulus

etudie Ies caracteristiques

de sa famille

situ&s a II’

essentiellement

maternelle

de l’axe

invariantes

soit deficients

sur

la ritine

relativement

temporale

montrent

h l’emplacement

des batonnets sont entierement absentes. Nous proposons donnies

experimentales

qui combine

de certains mecanismes

visuels

une vision

colorte

que

du stimulus un modele

deuteranomale

les

reponses

visuelles

du

sujet

sont

et que les reponses caracteristiques fonctionnel

d’interprttation

de ces

un mauvais fonctionnement

avec

centraux.

Zusamtnenfassung-Diese Arbeit handelt von der visuellen Antwort-Charakteristik eines mannlichen Beobachters. Obwohl Mitglieder seiner Familie miitterlicherseits rot-griin-farbfehlsichtig waren. stimmen die Antworten nicht mit denen der bekannten Arten .von Farbfehlsichtigkeiten iiberein. So ist z.B. sein foveaier Farbabgleich nicht dichromatisch und seine foveale Farbschwellempfindlichkeit ist urn 2.5 bis 5.0 Log-Einheiten niedriger als die eines normalen Beobachter. Ausserdem liegt seine kritische Flimmerfrequ& bei 15-18 Hz. Messungen der Unterschiedsschwelle zeigen dass nur zwei spektrale Mechanismen aktiv sind. Ausserdemzeiet sich. dass fur lannwelliee Reize die Unterschiedsschwelle pliitziich abf4lt wenn das Releuchtungsniveau ;ines iiberlag&tenHint~rgrundes iiber einen kritischen Wert hinaus anwichst. Diese letztere Beobachtung stimmt mit der subjektiven Aussage der Versuchsperson iiberein, dass Objekte. die normalen Beobachtern rot erscheinen, fi.ir ihn ‘@au” oder “schwarz” aussehen und keine definierte Form aufweisen. Versuche mit Reizen. die I I’ ausseraxial auf der temporalen Seite der Retina dargeboten wurden, zeigten. dass die visuelle Antwort im wesentlichen unabhangig vom Reizorr ist und dass die charakteristische Stabchenantwort vijllig fehlt. Wir haben deshalb ein Funktionsmodell fiir die Interpretation der experimentellen Daten aufgestellt. in welchem deuteranomales Farbensehen mit der Fehlfunktion einiger zentraler visueller Mechanismen kombiniert wird.

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