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Spectral sensitivity and visual pigment absorbance
Vision Rrr. Vol. 13 .pp. 1797-1798.
Pzrgamon
F’ICSS 1973. Printed
in Great
Brirain.
LETTER TO THE EDITORS
SPECTRAL SENSITIVITY
AND VISUAL PIGMENT
ABSORBANCE
(Received 26 March 1973)
BOWMAKER (1973) discusses the agreement between reliable spectral absorbance curves and physiological action spectra of receptor systems triggered by light absorption in corresponding pigments. He refers to the apparent lack of agreement between the scotopic sensitivity curve for man (CRAWFORD,1949) and the absorbance data due to WALD and BROLVN (1958), and calls the latter an enigma. A study of the original papers and of more recent work may offer a solution. Wald and Brown write: “We have recently obtained a number of human eyes, within several hours of enucleation. No special precautions were taken to adapt them to the dark”. The phrase “within several hours” lends itself to a number of quantitative interpretations, and the fact that “the retinas were removed under dim red light and stored in the dark at - 10°C until used” is unlikely to dispel one’s suspicion that Wald and Brown’s data may be distorted by the presence of long-lived and possibly photosensitive products of photolysis (WEALE, 1957, 1962 ; MATTHEWS,HUBB~UU),BROW and WALD, 1963 ; RIPPS and WEALE, 1969; ALPERN, 1971). In view of the pH of their solution (6.5), metarhodopsin I may also have been present. Precisely because Crawford’s data are “the best-documented” ones, to use Bowmaker’s justified phrase, they deserve re-examination. The measurements were done with a circular 20” bipartite field, the observers being instructed to hxate one end of the dividing diameter. It seems to me that this must have involved fovea1 and other cone activity, contributing to the overall visual performance. That this cannot have been insignificant is proved by ancillary measurements given by Crawford in the same paper. Data were obtained at a lower luminance level than the standard ones, and reveal the onset of a Purkynt! shift. It is likely that there is a finite luminance range within which large test-fields fail to reveal an appreciable contribution due to cones (AGUILARand STILES,1954). It would seem, though, that Crawford worked near its upper limit (10m3 td). Furthermore, part of the retinal area he studied is likely to have been covered by macular pigment which absorbs maximally at h m 460 nm (RUDDOCK,1963; BONEand SPARROCK, 1971). The effect of this filter probably balances the over-correction (WEALE,1961) due to a possible misapplication of the data of LUDVIGHand MACCARTHY(1938). In fine, it is likely that the fully dark-adapted rod sensitivity of man peaks at a wavelength shorter than the 497 nm quoted by Bowmaker: an acceptable comparison with pigment absorbance data is yet to be produced. R. A. WEALE Department of Visual Science, Institute of Ophthalmology, Judd Street, London W. C. 1 1797
1798
LETTERTO THE EDITORS
REFERENCES ACUILAR, M. and STLLES,W. S. (1954). Saturation of the rod mechanism of the retina at high levels of stimulation. Opticaacra 1,59-65. ALPERN, M. (1971) Rbodopsin kinetics in the human eye. J. Physiol., Land. 217, 47-471. BONE, R. A. and SPZJU(OCK,J. M. B. (1971). Comparison of macular pigment densities in human eyes. Vision Res. 11,1057-1064.
J. (1973). Spectral sensitivity and visual pigment absorbance. Vision Res. 13, 783-792. CRAWFORD,B. H. (1949). The scotopic visibility function. Proc. phys. Sot. B62, 321-334. LUDVIGH, E. and MACCARTHY, E. F. (1938). Absorption of visible light by the refractive media of the human eye. Arch. ophthal. 20,37-51. MAITHE~S, R. G., HUBBARD,R., BROWN, P. K. and Wm, G. (1963). Tautomeric forms of metarhodopsin. ~WMAKER,
J. gen. Physiol. 47,215~MI. RIPPS, H. and WEALE, R. A. (1969). Rhodopsin regeneration in man. Nature, Lond. 222, 775-777. RUDDOCK, K. H. (1963). Evidence for macular pigmentation from coiour matching data. Vision Res. 11, 41749. . WALD, G. and BRONX, P. K. (1958). Human rhodopsin. Science, N. Y. 127,2X-226. WEALE, R. A. (1957). Observations on photochemical reactions in living eyes. Brit. J. Ophthal. 41,461-474. WEALE, R. A. (1961). Notes on the photometric significance of the human crystalline lens. Vision Res. 1,
183-191. WEALE,R. A. (1962). Photochemical
changes in the dark-adapting
human retina. Vision Res. 2,25-33.
Pzrgamon
F’ICSS 1973. Printed
in Great
Brirain.
LETTER TO THE EDITORS
SPECTRAL SENSITIVITY
AND VISUAL PIGMENT
ABSORBANCE
(Received 26 March 1973)
BOWMAKER (1973) discusses the agreement between reliable spectral absorbance curves and physiological action spectra of receptor systems triggered by light absorption in corresponding pigments. He refers to the apparent lack of agreement between the scotopic sensitivity curve for man (CRAWFORD,1949) and the absorbance data due to WALD and BROLVN (1958), and calls the latter an enigma. A study of the original papers and of more recent work may offer a solution. Wald and Brown write: “We have recently obtained a number of human eyes, within several hours of enucleation. No special precautions were taken to adapt them to the dark”. The phrase “within several hours” lends itself to a number of quantitative interpretations, and the fact that “the retinas were removed under dim red light and stored in the dark at - 10°C until used” is unlikely to dispel one’s suspicion that Wald and Brown’s data may be distorted by the presence of long-lived and possibly photosensitive products of photolysis (WEALE, 1957, 1962 ; MATTHEWS,HUBB~UU),BROW and WALD, 1963 ; RIPPS and WEALE, 1969; ALPERN, 1971). In view of the pH of their solution (6.5), metarhodopsin I may also have been present. Precisely because Crawford’s data are “the best-documented” ones, to use Bowmaker’s justified phrase, they deserve re-examination. The measurements were done with a circular 20” bipartite field, the observers being instructed to hxate one end of the dividing diameter. It seems to me that this must have involved fovea1 and other cone activity, contributing to the overall visual performance. That this cannot have been insignificant is proved by ancillary measurements given by Crawford in the same paper. Data were obtained at a lower luminance level than the standard ones, and reveal the onset of a Purkynt! shift. It is likely that there is a finite luminance range within which large test-fields fail to reveal an appreciable contribution due to cones (AGUILARand STILES,1954). It would seem, though, that Crawford worked near its upper limit (10m3 td). Furthermore, part of the retinal area he studied is likely to have been covered by macular pigment which absorbs maximally at h m 460 nm (RUDDOCK,1963; BONEand SPARROCK, 1971). The effect of this filter probably balances the over-correction (WEALE,1961) due to a possible misapplication of the data of LUDVIGHand MACCARTHY(1938). In fine, it is likely that the fully dark-adapted rod sensitivity of man peaks at a wavelength shorter than the 497 nm quoted by Bowmaker: an acceptable comparison with pigment absorbance data is yet to be produced. R. A. WEALE Department of Visual Science, Institute of Ophthalmology, Judd Street, London W. C. 1 1797
1798
LETTERTO THE EDITORS
REFERENCES ACUILAR, M. and STLLES,W. S. (1954). Saturation of the rod mechanism of the retina at high levels of stimulation. Opticaacra 1,59-65. ALPERN, M. (1971) Rbodopsin kinetics in the human eye. J. Physiol., Land. 217, 47-471. BONE, R. A. and SPZJU(OCK,J. M. B. (1971). Comparison of macular pigment densities in human eyes. Vision Res. 11,1057-1064.
J. (1973). Spectral sensitivity and visual pigment absorbance. Vision Res. 13, 783-792. CRAWFORD,B. H. (1949). The scotopic visibility function. Proc. phys. Sot. B62, 321-334. LUDVIGH, E. and MACCARTHY, E. F. (1938). Absorption of visible light by the refractive media of the human eye. Arch. ophthal. 20,37-51. MAITHE~S, R. G., HUBBARD,R., BROWN, P. K. and Wm, G. (1963). Tautomeric forms of metarhodopsin. ~WMAKER,
J. gen. Physiol. 47,215~MI. RIPPS, H. and WEALE, R. A. (1969). Rhodopsin regeneration in man. Nature, Lond. 222, 775-777. RUDDOCK, K. H. (1963). Evidence for macular pigmentation from coiour matching data. Vision Res. 11, 41749. . WALD, G. and BRONX, P. K. (1958). Human rhodopsin. Science, N. Y. 127,2X-226. WEALE, R. A. (1957). Observations on photochemical reactions in living eyes. Brit. J. Ophthal. 41,461-474. WEALE, R. A. (1961). Notes on the photometric significance of the human crystalline lens. Vision Res. 1,
183-191. WEALE,R. A. (1962). Photochemical
changes in the dark-adapting
human retina. Vision Res. 2,25-33.