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Number and variations of the red and green visual pigment genes in Japanese men with normal color vision.
PURPOSE: We analyzed the red/green visual pigment genes in color-normal Japanese men to understand the relationship between color anomalies and genetic defects. METHODS: DNA from 120 color-normal Japanese men was subjected to polymerase chain reaction (PCR)-amplification for exons 2-5 of the red/green visual pigment genes and the PCR products were sequenced. The red:green gene ratios were estimated from the sequencing electropherograms of exon 5 and also from MvaI-restriction fragment analysis of the same exon. The first gene and the downstream genes in the pigment gene array were separately analyzed by PCR, direct sequencing, and/or single-strand conformation polymorphisms. RESULTS: The red:green gene ratios estimated from the ratios of peak heights of nucleotides on the sequencing electropherograms coincided with those estimated from the MvaI-restriction fragment analysis. Among the subjects analyzed, they were 1:1 in 43% (n = 52), 1:2 in 41% (n = 49), 1:3 in 6% (n = 7), and 1:>3 in 9% (n = 11). The first gene in the pigment gene arrays was red in all subjects. Only 1 subject (N22) had a green-red hybrid gene. Exons 2 and 4 had 2 haplotypes each, but exon 3 was highly polymorphic. Exon 5 of the green genes had one polymorphism at codon 283 with a frequency of 32%. CONCLUSIONS: The features of visual pigment genes in color-normal Japanese men were revealed. The data and establishing techniques may be useful for analyzing these genes in color-deficient subjects in the Japanese population.
The selectivity and timing of motion processing in human temporo-parieto-occipital and occipital cortex: a transcranial magnetic stimulation study.
An extrastriate visual area near the human temporo parieto occipital junction (TPO) may selectively mediate motion processing, while contributing little to the perception of color or form. This TPO area may be the human analogue of the monkey middle temporal (MT or V5) and medial superior temporal (MST) extrastriate visual areas. The selectivity of the effect of transcranial magnetic stimulation (TMS) on motion processing was unknown, as was the timecourse of occipital to TPO motion processing. In the first experiment, unilateral TMS was delivered over TPO 50-250 ms after the onset of a random dot motion discrimination display that was presented in the right or left hemifield. TMS reduced the correct discrimination of motion direction only when it was delivered in a discrete time window 100-175 ms following the onset of the display. TMS did not significantly affect hemifield spatial acuity in the same time window. In the second experiment, bilateral TMS delivered over occipital cortex also degraded the discrimination of motion-defined form (MDF) in a discrete time window following the onset of a display presented foveally. Bilateral focal TMS delivered over TPO disrupted the discrimination of MDF in a time window beginning 20-40 ms later than the effect of TMS delivered over occipital cortex. Bilateral focal TMS delivered over TPO also degraded the discrimination of color-defined form, motion direction and color. TMS can trace the timing of visual processing from occipital to extrastriate visual areas.
Tritanopic color matches and the middle- and long-wavelength-sensitive cone spectral sensitivities.
Tritanopic color matches (i.e. matches that depend on the middle- (M) and long- (L), but not short- (S) wavelength-sensitive cones) were made between two half-fields: one illuminated by either a 405 or a 436 nm Hg spectral line; the other by a light of variable wavelength and radiance. Our purpose was to test between rival M- and L-cone spectral sensitivities, which should predict the tritanopic matches. The observers were tritanopes, in whom functioning S-cones are lacking, or normal trichromats, in whom artificial tritanopia was induced by a strong, violet adapting field. The wavelengths found to match the 405 and 436 nm lights agreed poorly with those predicted by the cone spectral sensitivities of Smith and Pokorny (1975) [Vision Research, 15, 161], while the 405 nm matching wavelength agreed poorly with that predicted by Stockman, MacLeod and Johnson (1993) [Journal of the Optical Society of America, A10, 2491]. Both matching wavelengths agreed well, however, with the predictions of the Stockman and Sharpe (2000) [Vision Research] M- and L-cone spectral sensitivities, which lie within the range of measured matches.
The spectral sensitivities of the middle- and long-wavelength-sensitive cones derived from measurements in observers of known genotype.
The spectral sensitivities of middle- (M-) and long- (L-) wavelength-sensitive cones have been measured in dichromats of known genotype: M-cone sensitivities in nine protanopes, and L-cone sensitivities in 20 deuteranopes. We have used these dichromat cone spectral sensitivities, along with new luminous efficiency determinations, and existing spectral sensitivity and color matching data from normal trichromats, to derive estimates of the human M- and L-cone spectral sensitivities for 2 and 10 degrees dia. central targets, and an estimate of the photopic luminosity function [V(lambda)] for 2 degrees dia. targets, which we refer to as V(2)*(lambda). These new estimates are consistent with dichromatic and trichromatic spectral sensitivities and color matches.
Study of colour discrimination with comb-filtered spectra.
Techniques that involve the use of comb-filtered spectra to study human colour vision have been developed in previous work (Bonnardel, V., Bellemare, H., Mollon, J.D., 1996. Measurements of human sensitivity to comb-filtered spectra, Vision Research 36, 2713-2720; Bonnardel, V., Ruderman D.L., Barlow, H.B., 1997. A fast determination of the Spectral Modulation Sensitivity Function: a comparison between trichromats and deuteranopes. In: C.R. Cavonius (ed.), Color vision deficiencies XIII. Dordrecht: Kluver 415-424). These techniques are applied in the present study to measure colour discrimination among deuteranomalous observers and normal trichromats, with the aim of determining the spectral position of the anomalous cone fundamentals. Results show that comb-filtered spectra are useful in determining the extent to which variability in colour discrimination among anomalous and normal trichromatic colour observers is accounted for by the spectral properties of photoreceptors.
Chromatic and achromatic defects in patients with progressing glaucoma.
To evaluate the pattern of losses associated with glaucomatous injury in patients with progressing glaucoma, functional losses were examined in 14 patients with progressing glaucoma using tests for which detection should be selectively mediated by one of three psychophysical mechanisms. Red-on-white increments, blue-on-white increments and critical flicker frequency were used to isolate the responses of the red-green chromatic mechanism, the blue-on chromatic mechanism, and the high-frequency flicker achromatic mechanism. For our 3.1 degrees circular stimuli, chromatic defects were found in a greater number of the patients with glaucoma than were achromatic defects. We evaluated these defects in terms of two existing hypotheses: preferential loss and reduced redundancy. The greater sensitivity to glaucomatous injury of chromatic tests, compared to achromatic tests, found in this and other studies and the apparent discrepancy between anatomical and psychophysical studies can be parsimoniously explained by differences in cortical summation of ganglion cell responses for the chromatic and achromatic pathways.
Variations in long- and middle-wavelength-sensitive opsin gene loci in crab-eating monkeys.
We analyzed variations in long (L)- and middle (M)-wavelength-sensitive opsin gene loci in crab-eating monkeys. Unlike humans, most monkeys have a single L and a single M gene. Two variant genotypes, one with only one opsin gene (dichromatic) and one with tandemly arrayed multiple genes, were also found in the monkeys. However, the frequency of the former was 0.47%, and that of the latter was 5% in the monkeys, while 2% and 66%, respectively, in Caucasian males. The two variants were found only in Java Island, Indonesia, and South Thailand, respectively. The data suggest that the frequency of each genotype is different among Old World primates.
Colour thresholds in dichromats and normals.
Studies indicate dichromats detect large, long duration spectral increments presented on bright white backgrounds with a blue-yellow colour opponent mechanism. Since opponent processes signal colour, we hypothesized that under these viewing conditions dichromats should perceive spectral increments as coloured at detection threshold. Psychophysical detection and colour discrimination thresholds were determined for normal and dichromatic humans. Test stimuli were 2 degrees, 200 ms increments presented upon a white, 1000 td, spatially coincident background. As expected, normal observers were able to discriminate between white and spectral flashes at intensities near detection threshold intensities. Dichromatic observers required suprathreshold ( approximately 0.30 log units) stimulus intensities to discriminate between the white and spectral flashes. The results do not support our hypothesis and alternative explanations for the elevated colour discrimination thresholds in dichromats are discussed.
Dichromacy characterized by chrominance planes.
Dichromacy is described in terms of dichromatic opponent colour spaces. By means of the perceptual criteria 'equally bright', 'neither blue nor yellow' and 'neither red nor green' and embedding in a three-dimensional colour space, it is possible for each type of dichromat to quantify a null-chrominance plane and a null-luminance plane, both of which intersect in the missing colour. These two null planes (or the trace of their intersection with the chromaticity chart) are the chromaticities of the dichromatic opponent primaries. Since a null-luminance plane contains only colour ('chrominance'), it is simply a chrominance plane. Under the assumption that the retinal short-wavelength cones do not contribute to luminance, the chrominance planes of the three types of dichromats intersect in a common straight line, the 'blue' fundamental primary vector. This constellation may serve as a general characterisation of dichromacy.
Temporal analysis of the chromatic flash VEP--separate colour and luminance contrast components.
Temporal analysis of the chromatic flash visual evoked potential (VEP) was studied in human subjects with normal and anomalous colour vision using a deterministic pseudo-random binary stimulus (VERIS). Five experiments were carried out on four normal subjects investigating heterochromatic red-green exchange and single colour/achromatic (either red/grey or green/grey) exchange over a wide range of luminance ratios for the two stimuli, the effects of lowered mean luminance on the chromatic VEP and the effects of colour desaturation at constant mean luminance and constant luminance contrast. Finally, the performance of three dichromats, a protanope and two deuteranopes, on heterochromatic exchange VEP and on colour desaturation were investigated. In contrast to the chromatic electroretinogram, which shows great symmetry with respect to luminance ratio on opposite sides of the isoluminant point, the chromatic VEP demonstrated a distinct asymmetry when the colours exchanged included red. On the red side of isoluminance (red more luminant than green), a wave with longer latency and altered waveform became dominant. The effects of green stimulation were indistinguishable from those of achromatic stimulation at the same luminance contrast over the whole range of chromatic contrast and for all levels of desaturation studied. Desaturation of red with constant luminance contrast (desaturated red/grey stimulation) resulted in a systematic alteration in the evoked waveform. Subtraction of the achromatic first- and second-order responses from responses recorded in the red desaturation series resulted in remarkably uniform waveforms, with peak amplitudes growing linearly with saturation. The absence of interaction between achromatic and coloured components for all (including the most intense colour) stimulus parameters used suggests that the generators of these components are separate. Recordings from the dichromats showed that the contrast response minimum shifted from the point of photopic isoluminance to the point of zero cone contrast (at the silent substitution point) for the remaining cone type. The waveforms recorded with a series of luminance ratios were much simpler than those recorded from trichromats and symmetrical with respect to their isoluminant points. Despite the indication of the presence of L cones of apparently normal spectral sensitivity in the deuteranopes (on the basis of flicker photometry), there was no evidence for a red-sensitive component in the desaturation or heterochromatic stimulation series. The results are discussed in terms of the possibility of separate generation of chromatic and achromatic contributions to the VEP.
A perspective on color vision in platyrrhine monkeys.
Studies carried out over the past two decades show that many platyrrhine (New World) monkeys have polymorphic color vision. This condition results from the sorting of allelic versions of X-chromosome cone opsin genes at a single gene site, yielding a mixture of dichromatic and trichromatic phenotypes in the population. Two genera of platyrrhine monkey are known to deviate significantly from this pattern. Examination of color vision, photopigments, and photopigment genes of all of these monkeys have stimulated a renewed interest in understanding the evolution of primate color vision.
Relating color discrimination to photopigment genes in deutan observers.
Deutan observers are a heterogeneous group, varying nearly continuously from deuteranomalous trichromats with fine chromatic discrimination in the red/green range to deuteranopes who have none. We sought to relate chromatic discriminative ability among deutans measured psychophysically (phenotypes) to observers' separation between long-wave visual pigments inferred from visual pigment genes (genotypes). If middle-wave pigment genes are assumed not to be expressed in these deutan observers there is a clear relation between phenotype and genotype.
Color vision in two observers with highly biased LWS/MWS cone ratios.
Two sisters, heterozygous carriers for congenital X-linked protanopia, were diagnosed as normal trichromats by the Rayleigh match on the anomaloscope. The heterozygous state was established by molecular analysis of their visual pigment genes. The normal color match establishes that the spectral sensitivities of their long-wavelength-sensitive (LWS) and middle-wavelength-sensitive (MWS) cone visual photopigments are within normal variability. Their FM 100-hue test error scores were low, demonstrating superior chromatic discrimination. Heterochromatic flicker photometric (HEP) spectral sensitivities were like those of protanopes. The estimated LWS/MWS cone ratios from the HFP data were 0.09/1 and 0.03/1, compared with ratios in the range of 0.6/1 to 10/1 for typical normal trichromats. Measurements of chromatic grating acuity on chromatically selective backgrounds were performed to study the cone mosaic. The data were consistent with a sparsity of LWS cones. Both protan carriers showed normal spectral sensitivities for all three cone types under cone isolating chromatic adaptation and normal three-peaked curves for increment thresholds on a white pedestal. Hue estimation, run on one carrier was normal. The equilibrium yellow locus was measured in the other carrier and was in the range of normal trichromats. The data indicate that normal color vision can occur even when the LWS/MWS cone ratio is quite abnormal.
M- and L-cones in early infancy: III. Comparison of genotypic and phenotypic markers of color vision in infants and adults.
Genetic analyses were performed on five male children (approximately 3 years), two suspect color-normals and three suspects for congenital color vision deficiencies. These classifications were based on visually-evoked potential (VEP) responses to M- and L-cone-isolating stimuli obtained in a previous study when each subject was either 4- or 8-weeks old. The present analyses were performed in a blind study to characterize the genotypes of these subjects. Four male adults with various color vision phenotypes were also tested as a control. DNA was isolated using a non-invasive technique followed by polymerase chain reaction (PCR) amplification and restriction enzyme analysis to examine the genomic DNA of each subject. The genetic analyses confirmed the VEP identification of two color defective infants, and were consistent with the diagnosis of two other infants as color normal. A third infant was predicted by VEP analysis to have a protan defect, but he did not have a gene array typically found in protan observers.
Development of the spatio-chromatic visual evoked potential (VEP): a longitudinal study.
Most prior visual evoked potential (VEP) research on the development of color vision has employed pattern-reversing stimuli that are not optimal for producing chromatic responses. We measured infant VEPs using low spatial frequency, onset-offset stimuli, modulated along the three axes of a cone-based color space (Derrington et al. [J. Physiol 1984;357, 241-265.]). Three color-normal infants were tested in a longitudinal design over the first postnatal year. One red/green color-deficient infant was also tested at 197 days. We found that VEP responses to S-axis (tritan) stimuli have their initial onset later than responses to red/green (L-M) or achromatic stimuli, and that developmental changes in VEP waveforms are more complex and longer lasting for chromatic than for achromatic stimuli. Possible mechanisms underlying these changes are discussed.
S-cone ERGs elicited by a simple technique in normals and in tritanopes.
PURPOSE: To measure changes in the relative spectral sensitivities of the dark adapted and light adapted ERG and thus to establish the possible contribution of rods to the 'blue cone' ERG elicited by flashes of blue light. BACKGROUND: Short wavelength stimuli in the light-adapted eye evoke small rounded b-waves which have been considered to be S-cone responses. We have recorded such responses from tritanopes, which called the assumptions into question. METHODS: Small ERGs were recorded to blue and green flashes. The stimulus was a Ganzfeld which employed light emitting diodes. ERGs were obtained in both the dark-adapted eye and after light adaptation to intense orange light (peak wavelength 610 nm). The change in sensitivity with light adaptation and the relative spectral sensitivity was determined from the voltage/log light intensity functions, using a 10 microV criterion. RESULTS: (1) peak times and changes in sensitivity did not help distinguish light-adapted rod from possible S-cone responses; (2) analysis of the change in the ratio of blue:green sensitivity from darkness to 4.4 log Td. 610 nm background suggests that in seven normal subjects, 90% or more of the ERG evoked by 440 nm flashes is generated by S-cones; (3) three tritanopes have insignificantly reduced S-cone responses. CONCLUSIONS: (1) clinical techniques used to isolate S-cone ERGs are appropriate; (2) there are at least two types of tritanope and in those we investigated, functional S-cones are probably displaced into the retinal periphery.
Morphology of transient VEPs to luminance and chromatic pattern onset and offset.
Characteristics of the visual evoked response to chromatic and luminance-modulated stimuli reflect the activity of underlying neural mechanisms, although selective neuronal activity depends upon stimulus parameters. In the present study, the behaviour of the transient visual evoked response to low spatial and temporal frequency chromatic stimuli is investigated at a range of colour luminance ratios. Our results show that the response to pattern-offset may be used in addition to the pattern-onset response as part of the signature of the evoked response to luminance-modulated or isoluminant chromatic stimuli.