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Psychophysical estimates of optical density in human cones
vlsfon Res. vol. 13, pp. 11994202.
hrgamon
Rsu
1973. Printed in Chat hMa.
LETTER TO THE EDITORS
ESTIMATES OF OPTICAL CONES
DENSITY
IN HUMAN
(Received 31 Octuber 1972)
Srrmnzs of luminosity at bleaching intensities in dichromats have produced conflicting results. MITCHELL and RUSHTON (1971a) reported that at 40,000 td, protanopic luminosities obtained by brightness matching to a 580 nm standard are identical to matches obtained at 200 td. Deuteranopic luminosities obtained by brightness matching to a 580 nm standard at ~,~ td showed a shift of the entire curve to shorter wavelengths. MILLER(1972) found that luminosity measurements made at 250,000 td using heterochromatic tlicker photometry to a 570 nm standard for deuteranopes and a 535 nm standard for protanopes, gave evidence of narrowing of both sensitivity curves in accordance with pigments bleached from an initial concentration of optical density 04-0~5 for protanopes and 05-0*6 for deuteranopes. The discrepancy between the two studies has an important implication for the use of psychophysical data in estimating optical density in human cones. We have compared the two methods directly with three protanopes and four deuteranopes. A two-channel system placed a 2 mm image of a 75 W Xenon source at the plane of the observer’s pupil. Interference filters were used to control the spectral com~sitio~, and fixed metallic neutral density filters were used to control the retinal illuminance. A two-log unit metallic ND wedge interposed in one channel at a point optically conjugate to the artificial pupil was used to determine the match points and match ranges. The interference and neutral density filters were calibrated using a Carey-14 recording densitometer. In addition, the neutral density filters and wedge were checked in place for each of the intetierence filters used in the study. For this task a calibrated photomultiplier-microammeter was placed at the pupil. The linearity of the measuring device was established by using additive combinations of low density filters. For brightness matching we used a 25 deg bipartite field. The technique was similar to that of MrrcruzL and RUSHTON (1971a). Observers made side-by-sidematches between a 580 nm standard and a variable wavelength field. The matches were first made at 40 td. The field was then increased to 40,000 td by removing a nominal 3.0 log neutral density filter at the eyepiece. After a I-min adaptation, the experimenter established a new brightness match and match width. The data were consistent with the MITCHEJL and RUSHTON Q97fa) report. The protanopes showed little change in match values. The largest changes between the 580 nm standard and the variable field were < O-05 log unit and the match widths were about 0.10 log unit. However, there was a tendency, not seen in MITCHELL and RUSHTON’S (1971a) average data, for the average protanopic data to show increased sensitivity at wavelengths below 580 run and decmased sensitivity above 580 nm (changes Q 0.03 log unit). The deuteranopes showed increased sensitivity for wavelengths below 580 run and ’ This researchwas sup~~rtcd by NH ResearchGrant PHS EY 00523. 1199
decreased sensitivity for wavelengths above 580 mn, consonant with a shift of the curve to shorter wavelengths. Matches to wavelengths below 580 nm ,were variable, showing large match widths (> 0.10 log unit) and poor repeatability. For heterochromatic photometry, a beam-splitter replaced the mirror used to produce the bipartite fields. The beam-splitter reduced the maximal retinal illuminance at 654 nm to 29,000 td and thus we obtained only a 65 per cent bleach in the 654 nm HFP comparison using deuteranopes. The method was essentially that of MrLm (1972). The 25 deg field was viewed against an 8” broad-band blue (Wratten 45) background. The experimenter adjusted the wedge to find heterochromatic flicker photometric matches and match widths at a range of luminance levels, starting at 400 td. Observers adapted to the luminance of the flickering field for 1 min before the match was established. The alternation rate was adjusted concurrently to give the minimum rate at which flicker was imperceptible. The alternation rate ranged from 10 cycles at 400 td to 30 set at the highest retinal ill~inan~. The match widths were less than O-04 log unit at all retinal illuminan~s. For deuteranopes, a 654 nm field was alternated with a 580 nm standard, for protanopes a 610 nm field was alternated with a 540 nm standard. A measurement was also made alternating 580 and 504 nm for deuteranopes and 540 and 487 nm for protanopes. The data showed evidence of narrowing in accordance with MILLER’S (1972) data. Figure 1 shows the change in relative luminance of the variable field as the luminance is increased for a 580 and 654 nm field for the deuteranopes and a 540 Protanopes, 540nm
standard 487nm
6lDnm 0.12
f;
-
0.10
-
0~00 0.06
-
0’ B
is ’
fILg?5c
Retinal iil~rni~ance* log td
Deutrranopes, 580nm
standard 504 nm
654 nm 0.
12
c
/
Retinal illuminance, log td &tbg of comparison fiald as a function of retinal ~~i~~. Positivevahresnesd rcktiwly greater intwity in the uxwarkm fkfd at the match. SymboIs for protmapes: 0, AD; 0, KS; A, DP; 1), avera& value from side-by-sidematching procedure. @mbols for dcutcranopes: 0, KG; 0, MB; A, i-d% 0, ES; 0, a-w vabx
~$0. I. change in VJ&~
from side-by-side matching procedure.
LmTERTomEELmoRs
1201
and 610 nm field for the protanopes. The dashed line is the predicted change in wedge setting for a pigment bleaching from an initial concentration of optical density O-4for deuteranopes and 0-3 for protanopes. The predictions were obtained using the VOSand WALRAVEN (1971) primaries corrected to represent absorption spectra (WYSZECKIand STILES,1967) and the equations developed by BIUNDLEY(1955) and RUSHTON(1972) and MILLER (1972) with 25,700 td as the 50 per cent bleach intensity. The closed symbols are the average values obtained from the brightness matching experiment (taking total difference between 540 and 610 run settings for protanopes). Also shown are the values of the 487 run comparison for protanopes and the 504 MI comparison for deuteranopes. Side-by-side heterochromatic matching errors was well as differences in methodology account for the discrepancies between the MITCHELLand RUSHTON(197la) and MILLER(1972) data. In MILLER’s (1972) experiment, a total density change after a 90 per cent bleach of 0.19 was found for one deuteranope for 570 and 650 nm and O-l6 fdr one protanope for 535 and 625 nm. The corresponding optical densities are.0.54.6 for the deuteranope and O+O-5 for the protanope. In our measurements, the average density change was O-09for four deuteranopes (65 per cent bleach) and O-07 for three protanopes (75 per cent bleach), corresponding to an optical density of O-4 for deuteranopes and 0.3 for protanopes. At similar bleaching levels the difference between the two laboratories is less than 0.06 log unit. Microspectrophotometric data give O-3 as a probable lower limit of optical density for primate cones (DOBEJLE,MARKSand MACNICHOL,1959). Retinal densitometry has given estimates of O-35 for protanopes (RUSHTON,1963) and O-4 for deuteranopes (KINGSMITH, 1971). Color matching data (BRINDLEY,1955; TERSTIEGE,1967) and theoretical analyses of Stiles-Crawford data (WALRAVEN,1962; EN~CH and S~LZ~, 1961) have given estimates between O-5 and l-0. In two of the psychophysical studies (BRINDLEY,1955; ENCCH and STILES,1961), there is evidence that the density estimates are highly sensitive to individual observer difference. TERSTIEGE(1967) showed a range of acceptable density estimates of 0-5-l *Ofor his own eye. We have calculated that the intra-observer variability of normal Rayleigh equation midpoints (WILLIS and FARNSWORTH,1952; SCHMIDT, 1955; MITCHELL and RUSHTON, 197lb) is too small to allow for more than a O-2 range of density in either pigment, unless the concentrations of the middle- and long-wavelength sensitive pigments varied reciprocally about some average value. Further, normal and dichromatic pigments must be within a similar O-2 range, or primaries based on dichromatic copunctal points would not predict normal color matching. Since MORELAND(1972) has shown that normal variation in inert ocular pigments can account for about 90 per cent of variability of Rayleigh matches, the role of concentration differences is probably much smaller than the possible range cited above. Thus the O-3-1*0 range of densities, estimated from the various psychophysical studies, is unlikely to represent either a range of initial concentrations in human cone pigments or different concentrations in dichromats from normals. We would like to emphasize that estimates of optical density based on psychophysical data are highly sensitive to experimental and observer error, and to choice of primary shape. It would appear the psychophysical attempts to estimate optical densities must be viewed with some liberality. They are indeed only estimates. VIVUNNEC. SMITH Eye Research Laboratories, JOELPOKORNY 950 East 59th St., Chicago, III. 60637
1202
Lm~~r~olli~E~rro~s
REFERENCES B-m, G. S. (1955). A photochemical reaction in the human retina. Proc. phys. Sot. Load. 698,862-870. DoBELU. W. H., MARKS,W. B. and MACNICHOL,E. F. (1969). Visual pigment density in shtgle primate foveal cones. Science, N. Y. 166,1508-1510. ENOCH,J. M. and Srnes, W. S. (1961). The colour change of monochromatic light with retinal angle of inc&nce. Optica Actu 8,329-358. JCruo-SMITH,P. E. (1971). The optical density of erythrolabe determined by retinal densitometry. J. Physiof., Lond. 218, loo-101p. MI= S. S. (1972). Psychophysical estimates of visual pigment densities in red-green diihromats. J. Phydol., Lotid. 222, w-107. MITCHJXLL, D. E. and Rt~strro~, W. A. H. (1971a). Visual pigments in dichromats.Vision Res. l&1033-1043. MITCHELL, D. E. and RUSHMN,W. A. H. (1971b). The red/green pigments of normal vision. Vision Res. 11, 1045-1056.
MORELAND, J. D. (1972). The effect of inert ocular pigments on anomaloscope matches and its reduction. Mod. Probl. Ophthd. 11,12-l&
RUSHTON, W. A. H. (1963). The density of chlorolabe in the fovea1 cones of the protanope. J. Physiol., Land. 16%,360-373.
RUSK~~N,W. A. H. (1972). Visual pigments in man. In Hun&ook of Sensory Physiology VIZ/l Photochemistry of Vision (cd&cd by H. J. A. DARTNAJ.L). pp. 364-394. Springer-Verlag, New York. ScmmDT, I. (1955). Some problems related to testing color vision with the Nape1 anomaloscope. J. opt. Sac. Am. #,514-522.
TIIRSIIMIE, H. (1967). Untersuch~ zum Persistenz- und Koetkicntensatz. Die Farbe 16, l-120. Vos, J. J. and WALIU~EN,P. L. (1971). On the derivation of the foveal receptor primaries. Vision Res. 11, 799-818. WALU~BN, P. L. (1962). On the mechaniims of colour vision. Institute for &rception RVO-TNO. WILLIS,M. and FARNSW~R~,D. (1952). Comparative evaluation of anomabscopes. Medical Resewch Laboratory Report No. 190, Bureau of Medicine and Surgery, Navy Department. WYSZECKI, G. and Srrras, W. S. (1967). Color Science, Wiley, New York.
hrgamon
Rsu
1973. Printed in Chat hMa.
LETTER TO THE EDITORS
ESTIMATES OF OPTICAL CONES
DENSITY
IN HUMAN
(Received 31 Octuber 1972)
Srrmnzs of luminosity at bleaching intensities in dichromats have produced conflicting results. MITCHELL and RUSHTON (1971a) reported that at 40,000 td, protanopic luminosities obtained by brightness matching to a 580 nm standard are identical to matches obtained at 200 td. Deuteranopic luminosities obtained by brightness matching to a 580 nm standard at ~,~ td showed a shift of the entire curve to shorter wavelengths. MILLER(1972) found that luminosity measurements made at 250,000 td using heterochromatic tlicker photometry to a 570 nm standard for deuteranopes and a 535 nm standard for protanopes, gave evidence of narrowing of both sensitivity curves in accordance with pigments bleached from an initial concentration of optical density 04-0~5 for protanopes and 05-0*6 for deuteranopes. The discrepancy between the two studies has an important implication for the use of psychophysical data in estimating optical density in human cones. We have compared the two methods directly with three protanopes and four deuteranopes. A two-channel system placed a 2 mm image of a 75 W Xenon source at the plane of the observer’s pupil. Interference filters were used to control the spectral com~sitio~, and fixed metallic neutral density filters were used to control the retinal illuminance. A two-log unit metallic ND wedge interposed in one channel at a point optically conjugate to the artificial pupil was used to determine the match points and match ranges. The interference and neutral density filters were calibrated using a Carey-14 recording densitometer. In addition, the neutral density filters and wedge were checked in place for each of the intetierence filters used in the study. For this task a calibrated photomultiplier-microammeter was placed at the pupil. The linearity of the measuring device was established by using additive combinations of low density filters. For brightness matching we used a 25 deg bipartite field. The technique was similar to that of MrrcruzL and RUSHTON (1971a). Observers made side-by-sidematches between a 580 nm standard and a variable wavelength field. The matches were first made at 40 td. The field was then increased to 40,000 td by removing a nominal 3.0 log neutral density filter at the eyepiece. After a I-min adaptation, the experimenter established a new brightness match and match width. The data were consistent with the MITCHEJL and RUSHTON Q97fa) report. The protanopes showed little change in match values. The largest changes between the 580 nm standard and the variable field were < O-05 log unit and the match widths were about 0.10 log unit. However, there was a tendency, not seen in MITCHELL and RUSHTON’S (1971a) average data, for the average protanopic data to show increased sensitivity at wavelengths below 580 run and decmased sensitivity above 580 nm (changes Q 0.03 log unit). The deuteranopes showed increased sensitivity for wavelengths below 580 run and ’ This researchwas sup~~rtcd by NH ResearchGrant PHS EY 00523. 1199
decreased sensitivity for wavelengths above 580 mn, consonant with a shift of the curve to shorter wavelengths. Matches to wavelengths below 580 nm ,were variable, showing large match widths (> 0.10 log unit) and poor repeatability. For heterochromatic photometry, a beam-splitter replaced the mirror used to produce the bipartite fields. The beam-splitter reduced the maximal retinal illuminance at 654 nm to 29,000 td and thus we obtained only a 65 per cent bleach in the 654 nm HFP comparison using deuteranopes. The method was essentially that of MrLm (1972). The 25 deg field was viewed against an 8” broad-band blue (Wratten 45) background. The experimenter adjusted the wedge to find heterochromatic flicker photometric matches and match widths at a range of luminance levels, starting at 400 td. Observers adapted to the luminance of the flickering field for 1 min before the match was established. The alternation rate was adjusted concurrently to give the minimum rate at which flicker was imperceptible. The alternation rate ranged from 10 cycles at 400 td to 30 set at the highest retinal ill~inan~. The match widths were less than O-04 log unit at all retinal illuminan~s. For deuteranopes, a 654 nm field was alternated with a 580 nm standard, for protanopes a 610 nm field was alternated with a 540 nm standard. A measurement was also made alternating 580 and 504 nm for deuteranopes and 540 and 487 nm for protanopes. The data showed evidence of narrowing in accordance with MILLER’S (1972) data. Figure 1 shows the change in relative luminance of the variable field as the luminance is increased for a 580 and 654 nm field for the deuteranopes and a 540 Protanopes, 540nm
standard 487nm
6lDnm 0.12
f;
-
0.10
-
0~00 0.06
-
0’ B
is ’
fILg?5c
Retinal iil~rni~ance* log td
Deutrranopes, 580nm
standard 504 nm
654 nm 0.
12
c
/
Retinal illuminance, log td &tbg of comparison fiald as a function of retinal ~~i~~. Positivevahresnesd rcktiwly greater intwity in the uxwarkm fkfd at the match. SymboIs for protmapes: 0, AD; 0, KS; A, DP; 1), avera& value from side-by-sidematching procedure. @mbols for dcutcranopes: 0, KG; 0, MB; A, i-d% 0, ES; 0, a-w vabx
~$0. I. change in VJ&~
from side-by-side matching procedure.
LmTERTomEELmoRs
1201
and 610 nm field for the protanopes. The dashed line is the predicted change in wedge setting for a pigment bleaching from an initial concentration of optical density O-4for deuteranopes and 0-3 for protanopes. The predictions were obtained using the VOSand WALRAVEN (1971) primaries corrected to represent absorption spectra (WYSZECKIand STILES,1967) and the equations developed by BIUNDLEY(1955) and RUSHTON(1972) and MILLER (1972) with 25,700 td as the 50 per cent bleach intensity. The closed symbols are the average values obtained from the brightness matching experiment (taking total difference between 540 and 610 run settings for protanopes). Also shown are the values of the 487 run comparison for protanopes and the 504 MI comparison for deuteranopes. Side-by-side heterochromatic matching errors was well as differences in methodology account for the discrepancies between the MITCHELLand RUSHTON(197la) and MILLER(1972) data. In MILLER’s (1972) experiment, a total density change after a 90 per cent bleach of 0.19 was found for one deuteranope for 570 and 650 nm and O-l6 fdr one protanope for 535 and 625 nm. The corresponding optical densities are.0.54.6 for the deuteranope and O+O-5 for the protanope. In our measurements, the average density change was O-09for four deuteranopes (65 per cent bleach) and O-07 for three protanopes (75 per cent bleach), corresponding to an optical density of O-4 for deuteranopes and 0.3 for protanopes. At similar bleaching levels the difference between the two laboratories is less than 0.06 log unit. Microspectrophotometric data give O-3 as a probable lower limit of optical density for primate cones (DOBEJLE,MARKSand MACNICHOL,1959). Retinal densitometry has given estimates of O-35 for protanopes (RUSHTON,1963) and O-4 for deuteranopes (KINGSMITH, 1971). Color matching data (BRINDLEY,1955; TERSTIEGE,1967) and theoretical analyses of Stiles-Crawford data (WALRAVEN,1962; EN~CH and S~LZ~, 1961) have given estimates between O-5 and l-0. In two of the psychophysical studies (BRINDLEY,1955; ENCCH and STILES,1961), there is evidence that the density estimates are highly sensitive to individual observer difference. TERSTIEGE(1967) showed a range of acceptable density estimates of 0-5-l *Ofor his own eye. We have calculated that the intra-observer variability of normal Rayleigh equation midpoints (WILLIS and FARNSWORTH,1952; SCHMIDT, 1955; MITCHELL and RUSHTON, 197lb) is too small to allow for more than a O-2 range of density in either pigment, unless the concentrations of the middle- and long-wavelength sensitive pigments varied reciprocally about some average value. Further, normal and dichromatic pigments must be within a similar O-2 range, or primaries based on dichromatic copunctal points would not predict normal color matching. Since MORELAND(1972) has shown that normal variation in inert ocular pigments can account for about 90 per cent of variability of Rayleigh matches, the role of concentration differences is probably much smaller than the possible range cited above. Thus the O-3-1*0 range of densities, estimated from the various psychophysical studies, is unlikely to represent either a range of initial concentrations in human cone pigments or different concentrations in dichromats from normals. We would like to emphasize that estimates of optical density based on psychophysical data are highly sensitive to experimental and observer error, and to choice of primary shape. It would appear the psychophysical attempts to estimate optical densities must be viewed with some liberality. They are indeed only estimates. VIVUNNEC. SMITH Eye Research Laboratories, JOELPOKORNY 950 East 59th St., Chicago, III. 60637
1202
Lm~~r~olli~E~rro~s
REFERENCES B-m, G. S. (1955). A photochemical reaction in the human retina. Proc. phys. Sot. Load. 698,862-870. DoBELU. W. H., MARKS,W. B. and MACNICHOL,E. F. (1969). Visual pigment density in shtgle primate foveal cones. Science, N. Y. 166,1508-1510. ENOCH,J. M. and Srnes, W. S. (1961). The colour change of monochromatic light with retinal angle of inc&nce. Optica Actu 8,329-358. JCruo-SMITH,P. E. (1971). The optical density of erythrolabe determined by retinal densitometry. J. Physiof., Lond. 218, loo-101p. MI= S. S. (1972). Psychophysical estimates of visual pigment densities in red-green diihromats. J. Phydol., Lotid. 222, w-107. MITCHJXLL, D. E. and Rt~strro~, W. A. H. (1971a). Visual pigments in dichromats.Vision Res. l&1033-1043. MITCHELL, D. E. and RUSHMN,W. A. H. (1971b). The red/green pigments of normal vision. Vision Res. 11, 1045-1056.
MORELAND, J. D. (1972). The effect of inert ocular pigments on anomaloscope matches and its reduction. Mod. Probl. Ophthd. 11,12-l&
RUSHTON, W. A. H. (1963). The density of chlorolabe in the fovea1 cones of the protanope. J. Physiol., Land. 16%,360-373.
RUSK~~N,W. A. H. (1972). Visual pigments in man. In Hun&ook of Sensory Physiology VIZ/l Photochemistry of Vision (cd&cd by H. J. A. DARTNAJ.L). pp. 364-394. Springer-Verlag, New York. ScmmDT, I. (1955). Some problems related to testing color vision with the Nape1 anomaloscope. J. opt. Sac. Am. #,514-522.
TIIRSIIMIE, H. (1967). Untersuch~ zum Persistenz- und Koetkicntensatz. Die Farbe 16, l-120. Vos, J. J. and WALIU~EN,P. L. (1971). On the derivation of the foveal receptor primaries. Vision Res. 11, 799-818. WALU~BN, P. L. (1962). On the mechaniims of colour vision. Institute for &rception RVO-TNO. WILLIS,M. and FARNSW~R~,D. (1952). Comparative evaluation of anomabscopes. Medical Resewch Laboratory Report No. 190, Bureau of Medicine and Surgery, Navy Department. WYSZECKI, G. and Srrras, W. S. (1967). Color Science, Wiley, New York.