Blue-sensitive cones in the primate retina: microspectrophotometry of the visual pigment

Uniformity of colour vision in Old World monkeys

It is often assumed that all Old World monkeys share the same trichromatic colour vision, but the evidence in support of this conclusion is sparse as only a small fraction of all Old World monkey species have been tested. To address this issue, spectral sensitivity functions were measured in animals from eight species of Old World monkey (five cercopithecine species and three colobine species) using a non-invasive electrophysiological technique. Each of the 25 animals examined had spectrally well-separated middle- and long-wavelength cone pigments. Cone pigments maximally sensitive to short wavelengths were also detected, implying the presence of trichromatic colour vision. Direct comparisons of the spectral sensitivity functions of Old World monkeys suggest there are no significant variations in the spectral positions of the cone pigments underlying the trichromatic colour vision of Old World monkeys.

Photoreceptor spectral sensitivities: common shape in the long-wavelength region

Previous measurements of mammalian photoreceptor spectral sensitivity have been analysed, with particular attention to the long-wavelength region. The measurements selected for study come from rod and cone systems, and from human, monkey, bovine and squirrel sources. For the spectra from photoreceptor electrophysiology and from psychophysical sensitivity, the frequency scaling applied by Mansfield (1985, The visual system, pp. 89-106. New York: Alan Liss) provides a common shape over a range of at least 7 log10 units of sensitivity, from low frequencies (long wavelengths) to frequencies beyond the peak. The same curve is applicable to the absorbance spectrum of bovine rhodopsin, although the absorbance can only be measured down to about 2 log10 units below the peak. At the longest wavelengths the results exhibit a common limiting slope of 70 loge units (or 30.4 log10 units) per unit of normalized frequency. A simple equation is presented as a generic description for the alpha-band of mammalian photoreceptor spectral sensitivity curves, and it seem likely that the equation may be equally applicable to retinal1-based pigments in other species. Despite the lack of a theoretical basis, the equation has the correct asymptotic behaviour at long wavelengths, and it provides an accurate description of the peak. It also accounts accurately for the experimentally observed "yellowing" of long-wavelength lights that occurs beyond 700 nm.

Interactions of spectral, spatial, and temporal mechanisms in the human pattern visual evoked potential

The human pattern onset-offset visual evoked potential (VEP) was studied with different colours and spatial frequencies presented on a steady homogeneous intensive yellow background. Under this condition a broad, late negative wave (N2) dominated by the blue-sensitive mechanism and a sharper earlier positive wave (P1) dominated by the red-green-sensitive mechanism can be observed. With a 460 nm pattern N2 shows a strong amplitude tuning at low-medium spatial frequencies. With 550 nm an early negative component (N1) is added showing an amplitude tuning at high spatial frequencies. Different spatial resolutions of the two colour mechanisms are thus indicated. With pattern-reversal stimuli the 550 nm stimulus shows a higher temporal resolution than the 460 nm stimulus.

Two cone types of rat retina detected by anti-visual pigment antibodies

The presence of two distinct cone types was demonstrated in the retina of the rat using two cone-specific monoclonal anti-visual pigment antibodies. Cones labelled by antibody COS-1 constituted the large majority (about 93%) of cones, and are most probably responsible for the green photopic sensitivity of the rat. About 7% of the cones were recognized by antibody OS-2, and are thought to be blue-sensitive elements. While OS-2 positive cones were evenly distributed throughout the retina, there were slight differences in the distribution of COS-1 positive cones. The cones made up about 0.85% of all photoreceptor cells. Although the OS-2 positive cones occur in a very low number (0.05% of all photoreceptors) and probably do not appreciably contribute to the photopic system of the rat, their presence in the rat strengthens the presumption that most mammalian species exhibit a dual cone system with a shortwave and a middle-to-longwave sensitivity.

Cones in the retina of the Mongolian gerbil, Meriones unguiculatus: an immunocytochemical and electrophysiological study

Immunocytochemistry revealed in the retina of the Mongolian gerbil three immunologically distinct photoreceptor cell types. Rods comprising about 87% of the total receptor population were selectively recognized by an antirhodopsin serum (AO). The most abundant cone type (11-13% of photoreceptors) was labeled by the monoclonal antibody COS-1, specific in mammals to the middle-to-long-wave sensitive cone visual pigments. A minor cone population (2.5-5% of the cones) reacted with the monoclonal antibody OS-2, shown earlier to bind to the blue cones in mammalian species. Color substitution experiments revealed on the ERG level a color discrimination capability which must be attributed to the cooperative activity of green-sensitive (COS-1 positive) and blue-sensitive (OS-2 positive) cones. We conclude that the Mongolian gerbil has a well developed cone system, and that it may possess dichromatic green-blue color vision.

Polymorphism in normal human color vision and its mechanism

Earlier we reported that Rayleigh matches made by males with normal color vision fall into distinct groups, and proposed that this behavior reflects an X-chromosome linked polymorphism in the spectral positioning of cone pigments (Neitz & Jacobs, 1986). In the present experiments two different color matches were obtained from each of 60 color normal males. Comparisons of the data from these two matches allowed variations in color matching that are produced by individual differences in the spectral positioning of middle and long wavelength cone pigments to be distinguished from color vision variations caused by other factors. Considered together with findings from molecular biology and spectral measurements of individual cone pigments, these results suggest that among color normal humans: (1) there are discrete variations in both middle and long wavelength cone pigments, and (2) most individuals have more than three different cone pigment types.

The effects of reverse monocular deprivation in monkeys. I. Psychophysical experiments

Monkeys had one eye closed at about 30 days of age for 14, 30, 60, or 90 days, then opened, and the fellow eye closed for another 120 days. The animals then had at least 10 months of binocular visual experience before behavioral training and testing were begun. All subjects were used in a series of psychophysical investigations during the next two years. The results of the behavioral studies indicated that the initially deprived eyes (IDE) of the two monkeys that were subjected to initial deprivation periods of 14 or 30 days recovered normal or near-normal spatial contrast sensitivity. In contrast, the two animals which underwent longer periods of initial deprivation showed incomplete recovery, especially for high spatial frequency stimuli. All of the monkeys exhibited a reduction in spatial contrast sensitivity for their reverse deprived eyes (RDE); the earlier the onset of the reverse-deprivation procedures (i.e., the shorter the initial period of deprivation), the greater the deficit in the RDE's spatial contrast sensitivity. Measurements of temporal contrast sensitivity showed that all of the subjects' IDEs had normal or near-normal sensitivity levels. However, the reverse-deprivation procedures initiated at 90 days of age or earlier produced a frequency-dependent reduction in the RDE's temporal modulation sensitivity. The measures of increment-threshold spectral sensitivity revealed that only the RDE of the monkey that had the shortest initial deprivation period had an abnormal spectral sensitivity function. The results demonstrate that many of the severe behavioral deficits produced by early monocular form deprivation can be recovered via reverse-deprivation procedures. However, depending upon the length of the initial deprivation period and the age at which the reversal procedure is initiated, the second deprivation period can also adversely affect the functional capacity of the RDE.

Identification of the blue-sensitive cones in the mammalian retina by anti-visual pigment antibody

Monoclonal antibodies to visual pigments produced in our laboratory were applied to analyze the distribution of color-specific photoreceptor cells in the retina (photoreceptor mosaic). We demonstrated in two ways that the monoclonal antibody OS-2 specifically recognized the blue-sensitive cone cells in the mammalian retina. First, rabbit photoreceptors damaged selectively by intense blue light were recognized by OS-2 antibody. Second, OS-2-positive cones in the ground squirrel were those with thick inner segments, which is known to be characteristic of the blue-sensitive cones. In addition, the OS-2-positive cones in monkeys have a distribution and pattern characteristic of blue-sensitive cones in that species. In several other species (human, rabbit, cow, and pig), the OS-2-positive cones represent an appropriate minority of the population of photoreceptor cells. The visual pigment recognized by the OS-2 antibody had a relative molecular weight of 36,000, as shown by immunoblotting of 3 mammalian species. All other cones were recognized by another monoclonal antibody, COS-1, which is regarded as specific to middle-to-long-wavelength-sensitive photoreceptors.

Spectral sensitivity of primate photoreceptors

The spectral sensitivities of rods and cones in macaque and human retinas were determined by recording the membrane current from single outer segments. In the macaque retina, the wavelengths of maximum sensitivity were at about 430, 530, and 561 nm for the blue, green, and red cones, respectively, and at 491 nm for the rods. The shapes of the spectra of the three cones were similar when plotted on a log wavenumber scale; the rod spectrum was slightly broader. Spectral sensitivities of the red and green cones from a human retina were virtually identical to those of macaque cones. For comparison with human psychophysical measurements, the rod and cone spectra were adjusted to give the sensitivities expected for light incident on the cornea of the human eye. These functions satisfactorily predicted the scotopic and photopic luminosity functions as well as results from human color-matching experiments. The adjusted spectra of the red and green cones also agreed well with the pi-mechanism of Stiles (1953, 1959).

Polymorphism of visual pigments in a callitrichid monkey

Microspectrophotometric measurements of visual pigments have been obtained for a large sample of New World monkeys of the species Callithrix jacchus jacchus. These animals exhibit a polymorphism of visual pigments. The rods (Lmax 499 nm) and the short-wave receptors (Lmax 423 nm) appear to be common to all animals but individuals differ in the number and spectral position of pigments in the green-yellow spectral region. The latter pigments cluster near 545, 559 and 567 nm. Male monkeys draw one pigment from this set and female monkeys may draw one or two. The results are generally consistent with a genetic theory that postulates in Callithrix three possible alleles for a single locus in the X-chromosome. It appears that polymorphisms of cone pigments may be widespread among neotropical primates.

Spectral sensitivity of cones of the monkey Macaca fascicularis

1. Spectral sensitivities of cones in the retina of cynomolgus monkeys were determined by recording photocurrents from single outer segments with a suction electrode. 2. The amplitude and shape of the response to a flash depended upon the number of photons absorbed but not the wave-length, so that the 'Principle of Univariance' was obeyed. 3. Spectra were obtained from five 'blue', twenty 'green', and sixteen 'red' cones. The wave-lengths of maximum sensitivity were approximately 430, 531 and 561 nm, respectively. 4. The spectra of the three types of cones had similar shapes when plotted on a log wave number scale, and were fitted by an empirical expression. 5. There was no evidence for the existence of subclasses of cones with different spectral sensitivities. Within a class, the positions of the individual spectra on the wave-length axis showed a standard deviation of less than 1.5 nm. 6. Psychophysical results on human colour matching (Stiles & Burch, 1955; Stiles & Burch, 1959) were well predicted from the spectral sensitivities of the monkey cones. After correction for pre-retinal absorption and pigment self-screening, the spectra of the red and green cones matched the respective pi 5 and pi 4 mechanisms of Stiles (1953, 1959).

Cynomolgus and rhesus monkey visual pigments. Application of Fourier transform smoothing and statistical techniques to the determination of spectral parameters

Microspectrophotometric measurements were performed on 217 photoreceptors from cynomolgus, Macaca fascicularis, and rhesus, M. mulatta, monkeys. The distributions of cell types, for rods and blue, green, and red cones were: 52, 12, 47, and 44, respectively, for the cynomolgus, and 22, 4, 13, and 13 for the rhesus. Visual cells were obtained fresh (unfixed), mounted in various media (some containing 11-cis-retinal), and then located visually under dim red (650 nm) illumination. Absolute absorbance (A), linear dichroism (LD), and bleaching difference (BD) absorbance spectra were recorded through the sides of outer segments. The spectra were subjected to rigorous selection criteria, followed by digital averaging and Fourier transform filtering. Statistical methods were also applied to the accepted samples in the estimation of population means and variances. The wavelength of mean peak absorbance (lambda max) and the standard error at 95% certainty of the rod and blue, green, and red cone pigments in cynomolgus were 499.7 +/- 2.5, 431.4 +/- 4.2, 533.9 +/- 2.4, and 565.9 +/- 2.8 nm, respectively. The rhesus pigments were statistically indistinguishable from the cynomolgus, having lambda max of approximately 500, 431, 534, and 566 nm. Statistical tests did not reveal the presence of a lambda max subpopulation (i.e., anomalous pigments). The bandwidth of each alpha-band was determined in two segments, giving rise to the longwave half-density (LWHDBW), shortwave half-density (SWHDBW), and total half-density (THDBW) bandwidths. The LWHDBW was found to have the smallest variance. Both the LWHDBW and the THDBW showed linear dependence on the peak wavenumber (lambda max)-1 for the four macaque pigments.

A comparison of the spectral sensitivities of monkeys with anisometropic and stimulus deprivation amblyopia

Psychophysical procedures were used to determine increment-threshold spectral sensitivity functions for monkeys with either stimulus deprivation amblyopia produced by monocular lid suture or anisometropic amblyopia produced by optical defocus. The spectral sensitivity functions obtained from all of the non-treated control eyes had 3 sensitivity maxima at approximately 445, 520 and 610 nm, and were typical of functions for humans and monkeys with normal trichromatic vision. Relative to their paired control eyes, the amblyopic eyes of all of the lid-sutured monkeys demonstrated reductions in absolute sensitivity. In addition, the amblyopic eyes of the monkeys form-deprived at 1 month of age exhibited alterations in the shapes of their spectral sensitivity functions. The pattern of spectral sensitivity deficits observed in the monkeys form-deprived at 1 month of age varied with the duration of deprivation and indicated that the spectral characteristics of the neural mechanisms mediating stimulus detection in these lid-sutured monkeys' amblyopic and control eyes were substantially different. In contrast, none of the anisometropic amblyopes demonstrated any alterations in the shapes of their spectral sensitivity functions and only one anisometropic subject exhibited a reduction in sensitivity. A comparison of the spectral sensitivity deficits observed in our lid-sutured and anisometropic monkeys reveals that when lid suture and optical defocus are initiated early in life for equivalent periods of time, lid suture produces abnormalities which are both qualitatively and quantitatively different from the abnormalities produced by optical defocus. Therefore, in addition to factors such as the age of onset and the duration of deprivation, it is concluded that the degree of image degradation is important in determining which visual functions are affected and to what extent they are affected.

Acquired dyschromatopsia in glaucoma

Recent studies have suggested that the recognition of blue-yellow color vision deficits may have some predictive value in determining which ocular hypertensives are at risk of developing glaucoma and in monitoring the progress of the disease in glaucoma patients. This article reviews current theories of normal color vision and the differences that may occur in glaucoma, outlining methods of color vision testing and interpretation, and summarizing the results of recent studies.

Two types of trichromatic squirrel monkey share a pigment in the red-green spectral region

Microspectrophotometric measurements have been obtained for individual photoreceptors from four female squirrel monkeys (Saimiri sciureus) that had been shown behaviourally to be trichromatic. Relative to a normal human observer, two of the monkeys required more red light for a Rayleigh match; the other two required more green light than a normal human observer. In the red-green spectral region, the first type of monkey was found to have two cone pigments with peak sensitivities at approximately 536 and 549 nm, whereas the second type was found to have pigments with peak sensitivities at approximately 549 and 564 nm. By maximum likelihood estimation it was shown that the microspectrophotometric data could be described by a model that assumed only three underlying distributions, two of which were present in each type of monkey. The fit of this model was as good as one in which a "double normal" distribution was fitted individually to the data for each animal. This result is consistent with a genetic theory that postulates in Saimiri three possible alleles for a single locus on the X-chromosome; the heterozygous female enjoys trichromacy because Lyonisation ensures that only one photopigment is manufactured in any given cone.