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PubMed1996

Gene conversion and natural selection in the evolution of X-linked color vision genes in higher primates.

During higher primate evolution, gene conversion seems to have occurred often between the red and green photo-pigment genes, which are tandemly linked on the X chromosome. To understand this phenomenon better, intron 4 sequences of the red and green pigment genes of a male human (an Asian Indian), a male chimpanzee, and a male baboon were amplified by PCR and sequenced. The data show that the intron 4 sequences between the two genes have been strongly or completely homogenized in the three species studied. Apparently recent gene conversion events have occurred in introns 4 of the red and green pigment genes in humans and chimpanzees. Two or more conversion events may have occurred at different times in introns 4 of the two pigment genes in baboons. The divergence between the two genes is significantly lower in intron 4 than in exons 4 and 5 in each species, contrary to the usual situation that introns evolve faster than exons. It is most likely that strong natural selection for maintaining the distinct functions of exons 4 and 5 of the red and green pigment genes has acted against sequence homogenization of these exons.

Animals↗
PubMed2001

Chromatic contrast thresholds as a prognostic test for visual improvement after macular hole surgery: color vision and macular hole surgery outcome.

PURPOSE: To evaluate the relationship between preoperative chromatic contrast thresholds, postoperative visual acuities (VA), and visual improvement after macular hole surgery. METHODS: A consecutive series of patients with Stage II to IV macular holes was studied before macular hole surgery. Preoperative chromatic contrast thresholds, VA, and reading acuity were analyzed in relation to the postoperative visual function. The chromatic contrast thresholds were measured using a computerized cathode ray tube-based system along red-green and tritan confusion axes. RESULTS: Preoperative and postoperative chromatic contrast thresholds were elevated significantly in affected eyes (P < 0.001). Preoperative VA showed a strong correlation with postoperative VA (r = 0.66, P < 0.001) but a weak correlation with visual improvement (r = -0.33, P = 0.03). Red-green contrast threshold correlated strongly with both the distance visual improvement (r = -0.77, P < 0.001) and reading visual improvement (r = -0.61, P < 0.001). Tritan contrast threshold, however, showed a slightly weaker correlation (distance: r = -0.63, P < 0.001; reading: r = -0.47, P < 0.005). CONCLUSIONS: These results suggest that chromatic contrast thresholds, especially the red-green contrast threshold, represent a better prognostic guide for visual improvement after macular hole surgery than VA measurement.

Aged↗
PubMed1996

Infant color vision: motion nulls for red/green vs luminance-modulated stimuli in infants and adults.

Four-week-olds, 9-week-olds, and adult subjects were tested with low spatial frequency sinusoidal gratings moving at a speed of 25 deg/sec. Luminance-modulated and red/green gratings were presented either separately, or superimposed and moving in opposite directions in a chromatic motion nulling paradigm. An adult observer judged the direction of the slow phase of the subject's eye movements. Luminance-modulated gratings elicited directionally appropriate eye movements in all three age groups, with contrast thresholds decreasing markedly with age. For red/green gratings alone, 4-week-olds responded only marginally, but 9-week-olds and adults produced consistent directionally appropriate eye movements. In the motion nulling condition, 15% contrast luminance-modulated gratings were about equally effective in nulling the motion of the red/green gratings in all three age groups. A formal model of the motion nulling paradigm, separating threshold and equivalent luminance contrast parameters, was developed and applied to the data. Model fits showed that equivalent luminance contrast was constant or nearly constant across age groups. This outcome is consistent with the hypothesis that, with respect to adults, infants show a uniform rather than a differential loss of sensitivity to moving red/green vs luminance-modulated stimuli.

Adult↗
PubMed1997

Infant color vision: moving tritan stimuli do not elicit directionally appropriate eye movements in 2- and 4-month-olds.

The purpose of the present study was to investigate the capacity of infants to code the direction of motion of moving tritan-modulated gratings. Infant and adult subjects were tested with 0.2 c/d sinusoidal gratings moving at a speed of 20 deg/sec. Three conditions were tested: luminance-modulated gratings, tritan-modulated gratings, and luminance- vs tritan-modulated gratings superimposed and moving in opposite directions in a chromatic motion nulling paradigm. Two-month-old infants were tested in all three conditions, while 4-month-olds were tested in only the first two conditions. For infant subjects, an adult observer reported the direction of the slow phase of the infant's eye movements; adult subjects judged the perceived direction of motion of the stimuli. Luminance-modulated gratings produced directionally appropriate eye movements (DEM) in all age groups. Tritan gratings presented alone did not produce DEM in either 2- or 4-month-olds, but did so in adults. Mean equivalent luminance contrasts were near zero in 2-month-olds, and small but reliably above zero in adults. In sum, the present study provides no evidence that infants can code the direction of motion of moving tritan gratings.

Adult↗
PubMed1998

Molecular genetics of spectral tuning in New World monkey color vision.

Although most New World monkeys have only one X-linked photopigment locus, many species have three polymorphic alleles at the locus. The three alleles in the squirrel monkey and capuchin have spectral peaks near 562, 550, and 535 nm, respectively, and the three alleles in the marmoset and tamarin have spectral peaks near 562, 556, and 543 nm, respectively. To determine the amino acids responsible for the spectral sensitivity differences among these pigment variants, we sequenced all exons of the three alleles in each of these four species. From the deduced amino acid sequences and the spectral peak information and from previous studies of the spectral tuning of X-linked pigments in humans and New World monkeys, we estimated that the Ala --> Ser, Ile --> Phe, Gly --> Ser, Phe --> Tyr, and Ala --> Tyr substitutions at residue positions 180, 229, 233, 277, and 285, respectively, cause spectral shifts of about 5, -2, -1, 8, and 15 nm. On the other hand, the substitutions His --> Tyr, Met --> Val or Leu, and Ala --> Tyr at positions 116, 275, and 276, respectively, have no discernible spectral tuning effect, though residues 275 and 276 are inside the transmembrane domains. Many substitutions between Val and Ile or between Val and Ala have occurred in the transmembrane domains among the New World monkey pigment variants but apparently have no effect on spectral tuning. Our study suggests that, in addition to amino acid changes involving a hydroxyl group, large changes in residue size can also cause a spectral shift in a visual pigment.

Amino Acid Substitution↗
PubMedDate not supplied

How long range is contour integration in human color vision?

We quantified and compared the effect of element spacing on contour integration between the achromatic (Ach), red-green (RG), and blue-yellow (BY) mechanisms. The task requires the linking of orientation across space to detect a contour in a stimulus composed of randomly oriented Gabor elements (1.5 cpd, sigma = 0.17 deg), measured using a temporal 2AFC method. A contour of ten elements was pasted into a 10 x 10 cells array, a ndbackground elements were randomly positioned within the available cells. The effect of element spacing was investigated by varying the mean interelement distance between two and six times the period of the Gabor elements (lambda = 0.66 deg) while the total number of elements was fixed. Contour detection was measured as a function of its curvature for jagged contours and for closed contours. At all curvatures, we found that performance for chromatic mechanisms declines more steeply with the increase in element separation than does performance for the achromatic mechanism. Averaged critical element separations were 4.6 +/- 0.7, 3.6 +/- 0.4, and 2.9 +/- 0.2 deg for Ach, BY, and RG mechanisms, respectively. These results suggest that contour integration by the chromatic mechanisms relies more on short-range interactions in comparison to the achromatic mechanism. In a further experiment, we looked at the combined effect of element size and element separation in contour integration for the Ach mechanism. We found that the critical separation decreases linearly with the spatial frequency, from about 5 deg at low spatial frequency (larger elements) to about 1 deg at high spatial frequency (smaller elements) suggesting a scale invariance in contour integration. In both experiments we also found no differences between closed and open jagged contours detection in terms of element separation. The neuroanatomical implications of these findings relatively to area V1 are discussed.

Color Perception↗
PubMed1999

The evolution of trichromatic color vision by opsin gene duplication in New World and Old World primates.

Trichromacy in all Old World primates is dependent on separate X-linked MW and LW opsin genes that are organized into a head-to-tail tandem array flanked on the upstream side by a locus control region (LCR). The 5' regions of these two genes show homology for only the first 236 bp, although within this region, the differences are conserved in humans, chimpanzees, and two species of cercopithecoid monkeys. In contrast, most New World primates have only a single polymorphic X-linked opsin gene; all males are dichromats and trichromacy is achieved only in those females that possess a different form of this gene on each X chromosome. By sequencing the upstream region of this gene in a New World monkey, the marmoset, we have been able to demonstrate the presence of an LCR in an equivalent position to that in Old World primates. Moreover, the marmoset sequence shows extensive homology from the coding region to the LCR with the upstream sequence of the human LW gene, a distance of >3 kb, whereas homology with the human MW gene is again limited to the first 236 bp, indicating that the divergent MW sequence identifies the site of insertion of the duplicated gene. This is further supported by the presence of an incomplete Alu element on the upstream side of this insertion point in the MW gene of both humans and a cercopithecoid monkey, with additional Alu elements present further upstream. Therefore, these Alu elements may have been involved in the initial gene duplication and may also be responsible for the high frequency of gene loss and gene duplication within the opsin gene array. Full trichromacy is present in one species of New World monkey, the howler monkey, in which separate MW and LW genes are again present. In contrast to the separate genes in humans, however, the upstream sequences of the two howler genes show homology with the marmoset for at least 600 bp, which is well beyond the point of divergence of the human MW and LW genes, and each sequence is associated with a different LCR, indicating that the duplication in the howler monkey involved the entire upstream region. [The sequence data described in this paper have been submitted to GenBank under accession nos. AF155218, AF156715, and AF156716.]

Animals↗
PubMed2001

Asynchronous processing in vision: color leads motion.

It has been demonstrated that subjects do not report changes in color and direction of motion as being co-incidental when they occur synchronously. Instead, for the changes to be reported as being synchronous, changes in direction of motion must precede changes in color. To explain this observation, some researchers have suggested that the neural processing of color and motion is asynchronous. This interpretation has been criticized on the basis that processing time may not correlate directly and invariantly with perceived time of occurrence. Here we examine this possibility by making use of the color-contingent motion aftereffect. By correlating color states disproportionately with two directions of motion, we produced and measured color-contingent motion aftereffects as a function of the range of physical correlations. The aftereffects observed are consistent with the perceptual correlation between color and motion being different from the physical correlation. These findings demonstrate asynchronous processing for different stimulus attributes, with color being processed more quickly than motion. This suggests that the time course of perceptual experience correlates directly with that of neural activity.

Color Perception↗
PubMed1987

Comparison of color sensation in dichoptic and in normal vision.

Color vision in humans is independent over a wide range of the spectral composition of the illuminating light (Young 1807; Hering 1879). The retinex theory accounts for this color constancy by assuming that for each of the three waveband channels determined by the retinal cones a global lightness record of the scene is first computed by the visual system. The three records then serve to generate color at every point (Land 1983). Where do these computations take place? In this report a scene consisting of fourteen colored fields was viewed while one band of wavelengths enters one eye and a different band enters the other (dichoptic case) or while both bands enter both eyes (normal case) under otherwise identical conditions. The perceived color of every field is very similar in both cases although the physical stimulation of the eyes differs. It is also found that color constancy is maintained under dichoptic conditions. The results show that the cortex is crucial for the computation of color.

Adult↗