The use of photopic saturation in determining the fundamental spectral sensitivity curves

The use of photopic saturation in determining the fundamental spectral sensitivity curves

THE USE OF PHOTOPIC SATURATION IN DETERMINING THE FUNDAMENTAL SPECTRAL SENSITIVITY CURVES P. E. KISG-SMITH and J. R. WEBB Department of Ophthalmic Opt...

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THE USE OF PHOTOPIC SATURATION IN DETERMINING THE FUNDAMENTAL SPECTRAL SENSITIVITY CURVES P. E. KISG-SMITH and J. R. WEBB Department of Ophthalmic Optics, C.M.I.S.T., P.O. Box SY,Sackvills St.. Manchester M60 1QD. En&d

Abstract-4 new method is described for determining the spectral sensitibities of the red. green and blue colour mechanisms. First. evidence is presented that the red. green and blue mechanisms ma! be saturated independently by a flashed background: it follows that. for example. :I bright yellow background may be used to saturate simultaneously the red and green mechamsms. so th~lltthe spectral sensitivity for a superimposed test Bash must correspond to the blue mechanism. SimilarI! the spectral sensiti\iry of the green mechanism may be determined using a Rashed purple background: a rather more compIev procedure was used to isolate the red mechanism. The new method is discussed in relation to preyious psychophysical methods.

The dichromatic vision of certain “colour blind” is now general acceptance of Young’s (1802) trichromatic theory, i.e. that normal colour vision is observers may also be used to determine the fundamentals; the evidence of densitometr! (Rushton. 1963a. dependent on the responses of three “colour 1965,) supports Konis’s hypothesis that the protanope mechanisms”. each most sensitive in a different region has only the normal “green” and “blur” sensitive of the spectrum but each responding to a broad range mechanisms while the deuteranope has only the norof the spectrum on either side of the maximum. The mal “red” and “blue” mechanisms. As the blue results of retinal densitometry (Rushton. 1963a, I965a; mechanism is very insensitive to r,ipid flicker (BrinBaker and Rushton, 1963; Ripps and Weale, 1963) and of microspectrophotometry (Marks, Dobelle and Mac- dley. DLLCroz and Rushton, 1966) the photopic sensiNichol, 1964; Brown and Wald. 1964) confirm the tivity curves of those subjects. determined by flicker idea that the three colour mechanisms correspond to photometry (Pitt. 1935: Sperling, 1961) should correthree visual pigments each present in a different type spond to the spectral sensitivities of the green and red mechanisms respectively. Less direct arguments based of cone. As well as the objective studies mentioned above, a on the colour confusion Iines of dichromats in the chromaticity diagram, lead to similar spectra1 sensitivity large number of psychophysical studies have been devoted to determining the “fundamentals”, i.e. the curves (Pitt, 1951; Vos and Walraven. 1971). There is a moderate degree of agreement between spectral sensitivities of the three colour mechanisms. The more recent studies include determinations of the the spectral sensitivity curves determined by all these different techniques. both objective and subjective. The spectral sensitivities for test targets on bright coloured backgrounds (Stiles, 1953; Wald, 1964); studies of the peak sensitives of the red. green and blue mechanisms occur at about 570. 540 and 4-IOnm respectively. spectral sensitivity for monochromatic background In this paper we describe a new technique ivhich we lights which are just strong enough to reduce a fixed coloured test flash to threshold (de Vries, 1948; Stiles, believe may yield precise measures of the three 1953); studies of the spectral sensitivities for test targets fundamentals. At this point. the reader ma)- reasonabl>after adapting to bright coioured lights (Brindley, 1953; ask what value there is in further estimates of the Walraven, van Hout and Leebeek, 1966); and studies fundamentals when there is already a rough agreement between the results of so many different methods. of inter-ocular colour matching after monocular adaptation to bright coloured lights (Wright, 1934; In reply we wouid make the following points: (I) We believe that a precise determination of the Walters, 1942). The three spectral sensitivity curves fundamentals is not just an academic exercise. but is may be shown to be linear sums of the three colour matching functions (see, for example. Le Grand. 1968); something of general value. Thus. for example. spectral sensitivity curves determined under any photopic conthus colour matching provides valuable information ditions (e.g. photopic lL;minos~tv curves) would be about the fundamentals but does not specify them uneasier to analyse if we had a pre&r knotvledge of the iquely. However, the spectral sensitivity of the “blue” fundamentals. mechanism may be derived from colour matching data if some reasonable assumptions are made (see Hollins (2) There is considerable evidence that normal suband Montabana, 1973). _.jects may differ somewhat in their fundamentals (and There

4”__

P. E. KIUG-SMITH and J.R. WEBB

thus in their colour matching. Wright. 1946). The studies of Verriest (1971) on the spectral sensitivities of protanopes and deuteranopes indicate that these variations can be quite considerable for the red and green mechanisms. Thus. if precise knowledge of a subject’s fundamentals is required, one cannot rely on published data, but a direct hetermination should be made for that subject. (3) The theoretical justification for some of the methods listed above is often uncertain. For example, it is not clear why the bright blue background in Waid’s (1964) studies should totaly abolish the response of the green mechanism to the test flash thus leaving only the red mechanism responsive, The spectral “field” sensitivity method depends on the assumption that the threshotd of, say, a red test spot depends only on the stimuiation of the red (ns) mechanism by the background; the underlying assumption of the independence of the mechanisms is a useful approximation but is unlikely to be strictly true (Stiles, 1953, 1959; Boynton, Ikeda and Stiles, 1964). Indeed, the spectral field sensitivity of the red mechanism (Stiles, 1953) does not correspond to, say, the photopic luminosity curve of the deuteranope (Pitt, 1935; Sperling, 1961). Criticism of the binocular matching method of Wright (1934) and Walters (1942) has been considered by Brindley (1970). Perhaps the direct methods which are easiest to justify theoretically are the colour matching method (Hollins and Montabana. 1973) and Brindlev’s (19.53)“artificial monochromacy” method. unfortunately, the colour matching method can only be applied to the blue mechanism: Brindley‘s method may be used for the red and green mechanisms but requires repeated adaptation to bright violet lights which can cause long-lasting after images or even permanent damage (Harwerth and Sperling, 1971).

The present method makes use of the phenomenon of photopic “saturation” (Alpern, Rushton and Torii. 1970); they showed that if a fovea1 incremental threshold curve was derived with a flashed rather than a steady background, the threshold intensity increased very rapidly as the background intensity was increased above a certain level. PresumabIy, the visual system saturates or overloads at these high intensities and so becomes very insensitive to the test flash. Our method depends on the assertion that saturation occurs independently in the red, green and blue retinal mechanisms. Evidence for this assertion is provided in Part I of this paper, but, a priori. it seems probable for the following two reasons: (1) Alpern and Rushton (1965) showed that. in the “contrast-flash” effect, the threshold for a test flash exciting one colour mechanism depends on the response of only that colour mechanism to the surrounding contrast flash. There thus seem to be independent pathways from the surround for the three colour mechanisms. Alpern, Rushton and Torii (1970) showed that saturation occurs in the surround response; it

seems likely therefore that saturation must take place independently in the separate pathways from the surround. (2) EIectrical recordings from receptors show that little increase in hyperpoiar~ation is caused by brief light stimuli above a certain intensity (e.g. Baylor and Fuortes, 1970; Boynton and Whitten. 1970; Werblin, 1971; Grabowski, Pinto and Pak, 1972: Kleinschmidt, 1973). In this situation, the response to a superimposed test flash must be very weak; if this is the eiectrophysiological basis of photopic saturation then saturation must occur independently in the red. green and blue mechanisms. The results ofPart I provide further evidence for the theory that saturation occurs independently in the three mechanisms. If this is so, it should be possible to “isolate” one coIour mechanism by using a test flash which causes complete saturation in the other two mechanisms; for example, a bright yellow background flash could perhaps saturate both the red and green retinal mechanisms, so that only the blue mechanism could still respond to the test flash. In that case, the spectral sensitivity determined from the threshold intensities for the test gash should correspond to the spectral sensitivity of the blue mechanism. In Part 2 of this paper we provide evidence that the blue and green spectral sensitivity curves can be determined in this manner. By a modification of this technique (Part 3). the spectra! sensitivity of the red mechanism may also be derived. METHODS

,A two-channel Maxwellian view system was used and is represented in Fig. l(a). Both beams were derived from B. a 50 W quartz iodine bulb driven by a stabilized power supp1y. Both the test beam (denoted by subscript t) and the background beam (subscript b) pass successively through lenses, L, neutral wedges, W. shutters, Sk colour filters F (if necessary) and field stops, S. The mirrors. M. were frontsurfaced. The two lenses L, and L, had focal length 55 cm and u’ere placed at a distance of about 3 cm from B: they thus formed enlarged virtual images of the filament and correspondingly increased the total light flux through the stops. The two beams were combined by the beam combining cube, bc., and images of the filament. from both the test and background beam were formed by the achromatic doublet lens L, at the centre of the subject’s pupil (at E) which had been dilated with cyclopentalate. These filament images measured approximately 4 by 2 mm. The maximum retinal illumination for a white background beam was measured by the method of Rushton (1965b) and found to be in the range 741-7.51 log td on different occasions. The stops S, and St, were positioned in the focal plane of L, and so appeared to be at infinity.

E.~prrimenral

condirions

The spatial and temporal arrangement of the test and background stimuli are represented m Fig. 1(b1.The test and background spots were centred in the lens L, and the subject fixated the centre of the test spot. Ledex rotary

Phoropic

jatut3tion in determining specrral

Fb

S&b wb Lb

Fig. I. Experimental conditions: (a) Maxwellian-view systern used in the experiments, see text for description; (@spatial and temporal arrangement of the test and background flashes.

solenoids were used for the shutters and their timing was controlled by a digital pulse generator. The inrerval between presentations was adjusted to keep the level of pigment bleaching at a low value-less than 10 per cent for most background intensities and less than 20 per cent For the highest background intensities. This intervaf was caicuiated from the pigment kinetics described by Rushton (1963b, 1965b) and at high intensities, could be as great as 50 sec. The standard error of the threshold determinations was normally about 0.06 log units. For the experiments reported here. the subject was JRW but the main results were conFrrmed on PER-S. Spectral sensitivity curves were derived using a series of 15 Balaer’s interference filters (half bandwidth about 10 nm) placed in the test beam. The special method used for deter: mining the spectra1 sensitivity of the red mechanism will be described in that section (Part 3).

The relative quantum intensity of the test beam for each interference filter was measured at the eye position by means of two photocells; a seienium phorovoltaic cell (previously calibrated by the National Physic& Laboratory) and a Hilger-Sehwarz thermopile. In the latter calibration, the response to i.r. radiation was determined by placing a Schott RG695 filter in front of the photocell; assuming that this filter has a 92 per cent transmission for infra-red, but no transmission for visible light, a correction for infraredcould then be determined for each interference filter. The relative quantum intensities derived From the two photocells agreed within kO.04 log units except for the two extreme wavelengths (.U?l and 672 nm). The mean of the two calibrations was used in calculating spectral sensitivities.

sensitivity

CWY~S

423

The hnsaritv of the neutral wedge in the test beam was checked by a &uaf method using bipartite matching. The optical density of a particular part of the wedge fcorresponding to a density of about 1) was measured for each of the 15 test wavelengths used in an Uvispek H700 spectrophotometer: correction factors were derived from these readings toconvert the nominal wedge readings into the true optical density at each wavelength. P;\RT

1. 1NDEPENDE;LT THREE

COLOCR

SATLRATiON OF THE M&CHAZIS%

By studying incremental threshold curves for spectral test Rashes of various wavelengths on spectral background fields (also of various wavelengths) Stiles (1949, 1953, 1959) has deduced that light adaptation occurs (nearly) ~ndependentIy in each cotour mechanism. With a suitable choice of test and background colaurs. the incremental threshold curve was found to have two or more branches. By studying the displacement of these branches when test or background colours were altered, he could demonstrate that each branch corresponded to a particular mechanism and that adaptation was occurring indepen. dentiy in each mechanism. IIere, we apply a similar analysis to photopic ~turat~on.

Results Red ~~st~~s~~tin orange ~~ckg~~~f~~~~s~. The eiangles in Fig. 2 correspond to an incremental threshold curve determined for a red test flash on a Bashed orange back~ound. The ~n~emen[ai rhreshoid curve shows two branches A and B which saturate at dil%rent intensities as indicated by the dashed lines. Foilowins the anafysis of Stiles. we would expect that the red mechanism should respond at threshold in the low intensity range. Thus branch A may correspond to the red mechanism while branch B might correspond to the green mechanism (which could reasonabIy be expected to have a higher threshold at low intensities hut saturate at a higher intensity of the orange baekground light). The foiiowing observations provided further evidence for this interpretation and for independent saturation in the red and green mechanisms. (I) Curve C in Fig. 2 corresponds to an incremental threshold curve obtained in conditions where only the red mechanism should respond to the test gash (red test iIash on white background). If the lower branch (A) of the basic (red on orange) results also correspahds to the red mechanisms as predicted. then branch A should have exactly the same shape as the curve C, but displaced sideways because it was determined with a different back~ound colour but the same test colour, (This prediction corresponds to one of the displacement rules of Stiles--see Marriott_ 1962.) This prediction is seen to be fulfifted; branch X is, in fact, curve C displaced sideways. (2) Curve D in Fig, 2 corresponds to an incremental. threshold curve obtained in conditions in which the threshold shouid be determined by the green

0

Log

tlWW%oid intensity -1

-2

Red

on white 5

,’ -3

-4

d J

L -5

-4

-3

Log baCkground

prsdictsd.

it should

-3 Log background

intensity

Fig. 2. Incrcmenwl threshold curves derived using Hashed backgrounds. Tk triangles correspond to a red test Hash on an orange background. the circle to the same red test flash on a white background :tnd the squar


-4

-2

hare ssactly

the

same shape as curvt D but displaced vertically because a different test colour but the same background colour were used for the two sets of measurement. This prediction is also fulfilled; branch B is curve D displaced upwards. (3) For high intensity orange backgrounds. the subject reported that the red test Rash appeared greenish: this obsersation is. of course. consistent with the theory that branch B corresponds to the green mechanism. and so only this mechanism can respond to the red test Aash under these conditions, All these observations support the theory of independent saturation of the red and green mechanisms. GRWZirsrfirrsir OIZrrllorv hackgrormtf. The triangles in Fig. 3 correspond to incremental threshold measurements using a green test flash on a yellow flashed background. As in the case of a red test flash on an orange background. the curve has two branches, A and B, which are interpreted as corresponding to the green and blue mechanisms respectively for the following reasons,

~-

12

~7

-f

intensity

Fig. 3. Incrrmrntal threshold curves for Hashed backgounds. The triangles correspond to a green test flash on a yellow background. the circles to the same green test flash on a white background and the squares to a blue test Hash on the same yellow background as for the triangles. The green filter used was a 516 nm interference tilter, the yellow filter was a Schott OG 515 (dominant wavelenpth 583 nm), and the blue filter was a glass filter of unknown origin but dominant wavelength 162 nm. Branch A corresponds to CLKVS C displaced sideways while branch B corresponds to curve D displaced upwards (see text for discussion). (1) Curve C corresponds to conditions in which the threshold should be determined by the green mechanism (the same green test flash but on a white background). This provides confirmation that branch A corresponds to the green mechanism, as branch A is curve C displaced sideways as predicted by the displacement rules of Stiies. (2) Curve D corresponds to conditions in which the threshold should be determined by the blue mechanism (blue test flash on the same yellow background as was used for the basic curve). If branch B corresponds to the blue mechanism as predicted, then it should have exactly the same shape as curve D but should be displaced vertically (corresponding to the change in test colour); this is indeed found to be the case. As in the case, discussed earlier, of a red test flash on an orange background, a change in the appearance of the green test flash occurred as the intensity of the yellow background was increased above the ‘*break” in Fig. 3 (i.e. the intersection of branches A and B). At low background intensities the test flash appeared greenish, but for high intensity backgrounds the test flash appeared white or pale blue, In conciusion. the observations of Figs. 1 and 3 support the theory that saturation occurs indtpendentty in the red. green and blue mechanisms.

PART ?. SPECTR.iL BLUE AND

SESSlTI)‘ITIES

GREEX

OF THE

MECHASISMS

If photopic saturation occurs independently in the ‘-red”. “green” and “blue” mechanisms. then it should be possible to saturate the “red” and “green” mechanisms without saturating the “blue” mechanism by means of a bright yellow background gash. A test of this prediction is represented in Fig. 1. In Fig. +a) an incremental threshold curve has been derived for a green test flash (516 nm) on a yellow background (Schott OG 515. dominant waveiength. 583 nm). The Iogarithm of the Weber fraction (d-l/i) has been plotted as this clearly demonstrates the twobranched nature of the incremental threshold curve; we interpret the Low-intensity branch (log I less than -3) as corresponding to the green mechanism while

Fig. 4. isolation of the blue mechanism on a flashed yellow background: (a) incremental thresholds for a green (516 nm) test Hash on a flashed yellow (Schott OG 515) background. The curve through the lower left hand points is thought to be associated with the seen mechanism which saturates at about log I = -3, for high intensity backgrounds the threshold is therefore determined bl; the blue mechanism: (b) spectral sensitivity curves for a test flash superimposed on the same flashed yellow background as in Fig. 4(a). The cirties correspond to a IO%background intensity of log f = 3.6 [indicated by the circle and arrow in Fig. J(a)] at long wavelen,gths, the observed sensitivity is much greater than the senslttvity of the blue mechanism (continuous line corresponding to the field sensitivity of Stiles x3 mechanism) indicating that the green (and probably the red) mechanism also responds to the test flash. The triangles correspond to a higher backg.round intensity of log I = - 2.3 [corresponding to the triangle in Fig. 4(a)]: the spectral sensitivity is seen to be in good aereement with Stiles rr, results (continuous curve) indication that the green and red mechanisms no longer respond to the test Rash.

the other branch should correspond to the blue mechanism (cf. Fig. 3). If this interpretation is correct. both the blue and the green mechanisms (and probably the red mechanism) should be unsaturated for the weaker backgrounds used. This is confirmed in Fig. -l(b) \\ here the circles correspond to the spectral test sensicivit) derived for yellow background flashes of intensity log I = -3.6 [corresponding to the circle and arrow in Fig. 4atJ. The curve corresponds to the spectral iield sensitivity of the blue mechanism (the rc> mechanism of Stiles. 1953): it can readilv be seen that at least two and probably three mecha&ms respond to ths test Rash. Ata higher intensity [log I.= - 2.Z corresponding to the triangle and arrow in Fig. -L(a)] !\< would predict that the green mechanism (and probably the red mechanism) should be saturated by the background flash and so only the blue mechanism should respond to the test flash. The spectral sensitivity for the test flash should then correspond to the spectral sensitivity of the blue mechanism. The measured spectral sensitivity curve for this high intensity background is represented by the triangles in Fig. 4(b); as predicted, these measurements are in good agreement with the continuous curve which corresponds to the spectral field sensitivity of the blue (n,) mechanism. There is some deviation of our measurements for the two shortest wavelengths; this may be related to differences in the lens transmission of the subjects involved which ma> be quite large in this region of the spectrum (Said and Weale. 1959: Cooper and Robson, 1969).

A similar analysis has been performed for the green mechanism using a purple (Kodak Wratten No. 32) filter in the background beam. For a relati\el> weak background flash (log I = -2.75) the spectral sensitivity is represented by the squares in Fig. 5. The corresponding curve represents the spectral sensitivity of the green mechanism (in fact the spectral sensitivity of the protanope determined by flicker photometry-Pitt. 1935. 1944). The squares deviate systematically from the curve indicating that the test flash stimulates not only the _yeen mechanism but also the red mechanism (and possibly the blue mechanism). The triangles in Fig. 5 correspond to the spectral test sensitivity derived with a much brighter purple background (log I = - 1.62).It can be seen that in this case there is good agreement with the curve representing the protanope spectral sensitivity (except at the extreme violet end of the spectrum). It seems that only the green mechanism can respond to rhe test flash. In summary, the results of Part 2 are in good agreement with the prediction that the blue mechanism may be isolated by saturating the red and green mechanisms by a flashed yellow background, and that the green mechanism may be isolated by saturating the red and blue mechanisms with a flashed purple background. We have not, however, been successful in isolating the red mechanism by using. say, a blue back-

respond to the test stimulus. The spectral test sensitivity should then correspond to the red mechanism, Figure 6 illustrates the stimulus presentation. A red background 1IIford 605) was presented first. but this was suddenly replaced after 1 set bv a blue-green background @ford 623) which was e&mated to have approximately the same effect on the red mechanism, but a much stronger effect on the green and blue mechanisms. .A O-05xc test flash was applied 0.05 set after this transition.

2.4

22

29

1.8

16

I.4

wave number rlsi’

Fig. 5 Isolation of the green mechanism on flashed purple backgrounds. The squares correspond to the spectral test sensitivity measured on a relatively weak purple (Kodak Wratten No. 32) background. The triangles correspond to a high intensity background; they are seen to be a much better fit to the continuous curve which corresponds to the’ green mechanism (the spectral sensitivity of the protanope determined by tlicker photom~tr~-Pitt. 1935. 1944).Thus. for this high intensity purple background it is probable that onl? the preen mechanism responds to the test Hash.

Spectral test sensitivity determinations using the above stimulus presentation (Fig. 6) are represented. for a range of background intensities. in Fig. 7. The curves correspond to the spectral sensitivity of the red mechanism-in this case the photopic sensitivity of deuteranopes derived by Bicker photometry by Pitt ( 1935.19-l-i).As we had hoped, there is good agreement between the measured points and the curves for the higher intensity backgrounds(log I = - 1.5. - 2 and 2.5): the agreement is less good for the lower intensity backgrounds (particular log I = -4) and so the green mechanism may also be active at these lower intensities, as espected.

DISCCSSIOZ

Bash to saturate the blue and green mechanisms; we think that the reason for this lack of success is that there is no region of the spectrum where the green mechanism is much more sensitive than the red mechanism so it is not possible to saturate the green mechanism reliably without also saturaKing the red mechanism. ground

PART 3.

ISOLATION

OF THE

RED hlECH.4NISXI

Crawford (1937) has shown that an incremental threshold for a test Bash measured at the instant that a background light is switched on, is considerably greater than the threshold intensity after the eye has adapted to the same background for, say, 0.5 sec. It is probable that switching on the background light causes a large transient response in the retina which in turn causes a transient reduction in sensitivity to a superimposed test Hash. We have tried to make use of this transient effect in an attempt to isolate the red mechanism by saturating the green and blue mechanisms. The principle of the method is to apply the test flash at a moment when there is a sudden increase in the background stimulus to the green and blue mechanisms. but not corresponding increase for the red mechanism. We hoped thdt the transient response in the green and blue mechanisms would be sufficient to cause saturation of those mechanisms so that only the red mechanism could still

The spectral sensitivity curves of Figs. 4. 5 and 7 provide further evidence for the theor? that photopic saturation occurs independently in the red. green and blue coiour mechanisms. .As an example, in Fig. 4(b) the spectral sensitivity for a test spot on a bright yellow background is seen to correspond to the spectral field sensitiv?ity for the blue mechanism (x3); presumably only the blue mechanism can respond to the test flash. confirming the prediction that the red and green

Redme&an&q-““?

lS8C

O.SseC

for isolating the red Fig. 6. Stimuius presentation mechanism. A red @ford 603) background is first exposed for I set and is then replaced by green (Ilford 623) for 02 sec. A 0.05 set test flash is presented @OjSK after this transition from red to green. The lower part of the diagram illustrates that the red and green backgrounds have the same effect on the red mechanism: however, the green mechanism responds much more strongly to the green background than to the red so there is a large transient input to the green mechanism as the background switches from red to green.

Photopx s.ltururion in determining

2.4

2.2

1.6

20

1.6

1.4

Wave number pi’

Fig. 7. Isolation of the red mechanism using the stimulus arrangement of Fig. 6. The intensity of the red-green background is indicated beside each set of points. The curves correspond to the spectral sensitivity of the red mechanism (photopic sensitivity of the deuteranope determined by flicker photometry-Pitt. 1935). For the higher intensity msasurements (lower curves) there is good agreement between the points and the curves indicating that the red mechanism has been isolated and that the green and blue mechanisms have been saturated by the background sti-

mulus arrangement mechanisms may be saturated independently from the blue mechanism. In comparison complete isolation of the blue mechanism does not seem possible with steady yellow backgrounds (see Stiles, 1953. Fig. 13). 7%~sire of transient

generation

The results of Fig. 7 indicate that a transient (step type) stimulus may cause photopic saturation (in this case of the green and blue mechanisms) while the steady stimulus to the red mechanism does not cause saturation, even though it has nearly the same strength (in terms of quantum catch. Fig. 6). This suggests that at least some of the transient response oithe visual system is generated at an earlier stage in the visual system than the saturation effect; these transients must therefore be occurring within the red. zreen and blue colour mechanisms. This is consistent with the observation of transient responses in photoreceptors (Toyoda. Nosaki and Tomita 1969, Werblin and Dowling. 1969, Kieinschmidt. 1973). 7% spectral sensiticities blue rnechnnisr77s

ofrhe

red. green and

If one accepts that photopic saturation occurs independently in the red, green and blue mechanisms, then the spectral sensitivities for high intensity back-

sp~cfr~~l

43

sensitivitk cur\es

grounds in Figs. 4. 5 and 7 should correspond to the sensitivities of the three colour mechanisms. It is seen from Fig. qb) that the spectral sensitivity of the blue mechanism determined by this method is in good agreement with Stiles’ (1953) determination of the spectral field sensitivity of the blue (z,) mechanism; similarly our spectral sensitivity determinations for the green (Fig. 5) and red (Fig. 7) mechanisms are in good agreement with spectral sensitivities iflicker photometry) of protanopes and deuteranopes respectively (Pitt, 1935, 1944). For the blue and green mechanisms there seems to be reasonable (but not perfect) agreement between the results of a number of techniques: spectral test sensitivity (Wald. 19641 spectral field sensitivity (Stiles, 1953), the spectral sensitivity of protanopes (Pitt. 1935: Sperling. 1961). the colour matching method (Hollins and Montabana, 1973) and the chromatic adaptation methods of Brindley (1953) and Walraven er (11.(1966). It is probable that all these rest&s are approximately consistent Lvith the colour matching functions (Stiles. 1953). The greatest inconsistency between different methods probably occurs for the red mechanism. The present results are consistent with the spectral sensitivity of the deuteranope (Fig. 5) and also with the “artificial monochromacy” results of Brindley (1953). However. there seem to be discrepancies between these determinations and the spectral field sensitivity results of Stiles (1953), the spectral test sensitivity results of Wald (196-Q and the results of Walrasen et uf. (1966) who used chromatic adaptation. Stiles (1953) has pointed out that the spectral field sensitivity of the red mechanism (x5) does not seem to be a linear sum of the colour matching functions and this is evidence that the technique does not yield a precise ms;xure of the red cone sensitivitv. We believe it is important to know \vhich methods may be used to determine the fundamentals reliably. A precise knowledge of the fundamentals should be of particular importance in the interpretation of spectral sensitivity~urvesdetermined under any p hotopic conditions (e.g. by flicker photometry. or from absolute or incremental thresholds). However. an analysis of all the techniques discussed above is hampered by the fact that it is not possible to assume that the fundamentals are the same in different subjects (e.g. Verriest. 1971). Apparent differences between two techniques could be due to differences between the subjects tested. For this reason. one of us (Webb) is undertaking a systematic study of a number of these methods (including photopic saturation) all in the same subject. It is also intended to use the subject’s colour matching functions as a test of the validity of the different techniques. intensity

levels wed

in d7@rent

methods

Following Harwerth and Speriing’s i1971) demonstration of prolonged colour blindness in monkeys after viewing intense spectral lights. the experi-

mcntr‘r >houiJ Ix iuncerncd aho~~t the possibilit! of retin:tl Jarnags when using wmc of the techniques discussed above. Blue adapting lights seem to be the most hazardous (Brindle!. 195;: Harwerth and Sperling, 19711. so it is interesting to comparz the exposure to blur: lights required bq different methods in making a

spectral sensitivity determination at one test wavelength for the red mechanism. Three techniques require relati\clk large exposures: (I I The present method. The background intensities used in Fig. 7 (three bottom curves) range from 3.6 to 4.6 log td of the blue green light. For a typical determination. the subject viewed up to 20 background exposures of I :2 set: total exposure was therefore 4.6 to 5.6 log td set for the three highest intensities. (2) Brindlcy’s artificial monochromacy. The subject tirst \icwed a drcp violet light of 4.4 log td for 20 set and then a blue green light of 4.9 log td for IO sec. Total exposure was thus about 6.1 log td sec. (3) Wald’s (1964) method (spectral test sensitivity on intense blue backgrounds). Two background levels were used of 5.35 and 6.15 log td respectively; assuming that the subject made a threshold setting in 20 set, these correspond to curves of 6.65 and 7.45 log td sec. In comparison. Harwerth and Sperling (1971) found significant (but not complete) damage to the blue mechanism in monkeys after an exposure of 4.0 log td of blue light for 560 min (about 8.6 log td set). Although all the above figures are not strictly comparable (as different blue lights were used) it would seem that the present method is unlikely to cause detectable damage but that the other two methods may bs rather more doubtful in this respect. .4ck~io~~I~t/yc~~nurrrs--Wr are particularly grateful to Professor iV. .A. H. Rushton and Dr. G. B. Arden for their detailed comments on the manuscript. We \vould like to thank Mr. D. Cards” for expert technical assistance and Professor J. R. Crnnlk-Dillon for the provision of research facilities. This work was supported by a Grant-in-Aid from the Royal Society.

REFERESCES Alpern 41. and Rushton W. A. H. (1964) The specificity of the cone interaction in the after-flash effect. J. Ph.~iol.. Lorrll. 176, 462-371. Alpsrn 51.. Rushton W. A. H. and Torii S. (1970) Signals from cones. J. Ph.~xio/.. Lontl. 207, 463475. Baker H. D. and Rushton W. A. H. (1965) The red-sensitive pigment in normal cones. J. Ph.vsiol.. Land. 176, 56-72. Baylor D. A. and Fuortes M. G. F. (1970) Electrical responses of single cones in the retina of the turtle. J. Physiol.. L.ond. 207, 77-92. BoLnton R. ht.. lkrda M. and Stiles W. S. (1964) Interactions among chromatic mechanisms as inferred from positive and negative increment thresholds. C’ision Rrs. 4, si-I 17. Bobnton R. M. and Whitten D. N. (1970) Visual adaptation in monker cones: recordings of late receptor potentials. S<~i~~r~(~~~. <. 1: 170. l-123-~ 1426.

Brindle! G. S. (1953) The effects

on colour vision of adaptation to very bright lights. J. Phxsiol, Luntl. 122. 3321350. Brindle! G. S. (1970) Ph!sioloy,v vJ_ the Rrrinu and C’isrrc~i P~rhtvn~. pp. Z-L>247. Arnold. London.

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W. A. H. (1966) The 5ickei fusion frequency of the blue-sensitive mechanism of colour vision. J. Ph!sio/.. Dmd. 183, 497-500.

Brown P. K.andWaldG.(l964)Visualpigmentsinsingle rods and cones of the human retina. Science. .V.Y. 144,45-j?. Cooper G. F. and Robson J. G. (1969) The jeilow colour of ths lens of man and other primates. J. Phxsiol.. Land. 203, 411-417. Craaford B. H. (1947) Visual adaptation in relation to brief conditioning stimuli. Proc. R. Sot. 134B. 25.;302. de yries H. L. (1948) The fundamental response curves of normal and dichromatic vision. Physicu 11. 367-350. Grabowski S. R.. Pinto L. H. and Pak W. L. (1972) ;\daptation in retinal rods of Auolotl. Science, N.Y. 176, l?-Ia- 1233. Hsracrth R. S. and Sperling H. G. (1971) Prolonged colour blindness induced by intense specctral lights in rhesus monkeys. Science, N.Y. 174, 5X-523. Hollins and Montabana D. J. (L973) Spectral sensitivity of the fovea1 blue sensitix mechanism dctcrmined b! CO~OLII mixture. 1ision Rex 13, 1391-1393. Kleinschmidt J. (1973) Adaptation properties of intracellularly recorded gekko photoreceptor. In Biochemisrr.v and Phrsiobg!: of Visual Pigments (edited by Langer H.). pp. 219 ‘24. Springer-Verlag. Berlin. Lc Grand Y. 11968) Liyhr. Colour un~l C’lsion. Chapman & Hall. London. Xlarks W. B.. Dobelle W. H. and MacNichol E. F. (1964) Visual pigments of single primate cones. Science. N.Y 143. 1151-1153. blarriott F. H. C. (1962) Colour vision: the two colour technique of Stiles. In Thr E.w (edited bb Davson H). Vol. 2. pp. 25 l-171. Academic Press. New York. PII! F. H. G. (1935) Characteristics of dichromatic vision. \lsd. Res. Count. Soec. Reo. Ser. No. 200.

Pit; F. H. G. (1944) Tie natuie of normal trichromatic and dichromatic vision. Proc. R. Sot. 132B. 101-I 17. Ripps H, and Weale R. A. (1963) Cone pigments mal human fovea. c’ision Res. 3, 53 I-543. Rushton W. A. H. (1963a) A cone pigment in the 1. Physiol.. Land. 168, 34>359.‘ Rushton W. A. H. (1963b) Cone oiement kinetics tanope. J. Ph.&l.. L.&d. 168; 314388. Rushton W. A. H. (19658) A foceal pigment in anope. J. Ph!..siol.. Land. 176. 24-37. Rushton W. A. H. (1965b) Cone pigment kinetics teranopc. J. Phgsiol., Land. 176, 3%-G.

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Said F. S. and Wsale R. A. (1959) The variation with age of rhe spectral transmissivity of the living human crystalline lens. Grwnroiogia 3, 2 (3-3 I. Sperling H. G. (1961) Prediction of relative luminous eficiency from fundamental sensation curves. Vision Rrs. I, -12-6 1. Sri& W. S. (1949) Increment thresholds and the mechanisms of colour vision. Documrnrn Ophthal. 3, 13% 165. Stiles W. S. (1953) Further studies of visual mechanisms by the two-colour threshold technique. In Colloquia sohre Problemas Opricos de la Vision. Vol. I. pp. 65-103. Union lnternationale dc Physique pure et appliquL;. Madrid.

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senziti\it! c‘ur~c\

1'9

Fundamental response curves of a normal and a deuteranomalous observer derived from chromatic adaptation data. J. opr. Sec. A~rr. 56. 1’_%117. Wcrblin F. S. (1971) .AJaptation in a kertsbrate retina: intracellular recording in .L’ucrurus.J. Smrophysiol. 34, 2% 2-11. Q’crblin F. S. and Dowling J. E. I19691 Orpanization of the retina of the mud puppy. Srctnr~r.s ~m~ulo.w.s-II: intracellular recording. J. .Vewoph!siol. 31. 339-355. u’right W’. D. (193-1) The measurement and analysis of colour adaptation phenomena. Proc. R. SILL..1l5B. -IV-87. LVright W. D. ( 1916) Rrs~wx /IL<\OH .Vorrrd uml &j&tire Colour C’ision.Kimpton. London. \r’oung T. (IS03 On the theory of light and colour. Phil rmns. 92. 12-45.

K&sum&-On dicrit une nou\clle mAho& pour ditcrminer les srnstbilitts spcc~ales dcs mscanismes color& rouge. vert et bleu. On prtsente d’abord des arguments selon lesquels ces mkcanismes peuvent Ptre indipendamment saturis par un fond lumineux; on en dCduit par exemple qu’on peut employer un fond brillant jaune peut servir g saturer a la fois les micanismes rouge et vert. si bien que la sensibilite d’un eclair test superpoe doit correspondre au mecanisme bleu. De m&me la sensibilite spectrale du mecanisme vert peut se dkterminer en utilisant un fond brillant pourpre; on utilise une mCthode plus complexe pour isoler le mecanisme rouge. On discute cette methode nouvelle en la comparant aux m&hodes psychophysiques antlrieures.

Zusammenfassung-Einc neue Methode, mit der man die spektrale Empfindlichkeit des rotcn. priinen und blauen Farbempfindungsmechanismus bestimmen kann. wird beschrieben. Zungchst wird nachgewiexn. dass die Rot-. Griin- und Blaumechanismen unabhsngig von einem mit Blitzlicht aufgehelltsn Hintrrgrund gesgttigt werden; danach folgt. dass beispielsweiss mit einem hellen. gelben Hintergrund die Rotund Grtinmechanismen gleichzeitig geslttigt werden. so dass die spektrale Empfindlichkeit auf einen z4tzlichen Reitzblitz durch den Blaumechanismus bestimmt wird. ihnlich wurde die spektralc Empfindlichkeit des Griinmechanismus durch einen Purpurblitz bestimmt: urn den Rotmechanismus zu isolieren. wird eine weit kompliziertere Methode angewendet. Die neue Methode wird im Zusammenhang mit friiheren psychophysischen Methoden diskutiert.

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