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PubMed1993

The spectral properties of the two rod pathways.

Psychophysical and electroretinographic observations in normal and achromat observers suggest that rod flicker signals have access to at least two retinal pathways: one (pi 0), slow and sensitive, predominating at scotopic luminance levels; the other (pi'0), fast and insensitive, predominating at mesopic ones. We have measured steady-state flicker detection sensitivities on background fields ranging from 430 to 640 nm in normal observers. Our results suggest that cone signals can reduce the sensitivity of pi'0, but have comparatively little effect on pi 0. The pi'0 field sensitivities derived from these measurements have been fitted with linear combinations of the scotopic luminosity function, V' lambda, the M-cone spectral sensitivity function, M lambda, and the L-cone function, L lambda. These fits demonstrate a clear cone influence on pi'0, but they cannot tell us unequivocally whether the influence is from the M-cones, from the L-cones or from both. Accordingly, we made similar measurements in dichromats, who lack one of the two longer wavelength cone types. These measurements revealed an L-cone influence on pi'0 in the deuteranope and an M-cone influence in the protanope. This suggests that both cone types can affect the sensitivity of pi'0. The finding that the steady-state cone signals reduce the sensitivity of pi'0 but have little effect on pi 0 could suggest that pi'0 signals travel through a faster cone pathway (with its own gain control at which both rod and cone signals can reduce rod threshold), while pi 0 signals travel through a separate rod pathway. However, it could simply reflect the fact that pi'0 predominates at higher luminances than pi 0 where the cone excitation level is inevitably greater. To examine the influence of the cones on pi 0 more closely, we: (i) produced transient cone excitation by alternating rod-equated 480 and 679 nm fields; and (ii) extended our steady-state measurements to include deep-red backgrounds of 650 and 680 nm. Both experiments revealed a small, but measurable influence of the cones on pi 0.

Color Vision Defects↗
PubMed1995

Saccades to large coloured targets stepping in open fields.

Subjects tracked a 2.3 deg target that stepped 5 deg, in a randomly chosen direction, each time it was foveated. Targets were coloured patches that were fairly close to white; in some cases precise matches ensured equiluminosity with the background. Viewing conditions provided good colour rendering and neutral colour adaptation. Pale colours were surprisingly well tracked. Multiple regressions showed that the colour and spatial characteristics of the target are important determinants of a primary saccade's latency. Significant factors included target size, achromatic contrast, tritanopic purity difference, and chromatic saturation. Colour-normal subjects always responded more slowly to yellow or blue targets which a deuteranomalous subject tracked quite well. Severely blurring the target had a consistent minor effect.

Adult↗
PubMed1995

Genetic basis of photopigment variations in human dichromats.

The spectral sensitivities of the X-encoded pigments in dichromats were studied using the electroretinogram. The action spectra measured for these subjects correspond to four distinctly different X-encoded visual pigments, two different middle-wave pigments spectrally separated by 7 nm and two different long-wave pigments separated by 5 nm. Amino acid sequences were deduced from examination of the genes encoding the pigments. Pairwise comparisons of the opsin structures and pigment spectra confirm and clarify earlier conclusions. Substitutions in exon 5 of the genes produce the spectral difference that separates human X-encoded pigments into middle- and long-wave classes. Polymorphisms in exons 2-4 produce subtypes of pigments that fall within those major classes. Substitution of half of a middle-wave gene with long-wave sequence (exons 1-3) does not shift the middle-wave spectrum. Combined substitutions at positions 230, 233 and 180 produce a 7 nm shift in the in the middle-wave pigment spectrum. Two subtypes of long-wave pigments that differ in the presence of serine or alanine at position 180 and occur in dichromats, deuteranomalous trichromats, and color normals are spectrally separated by 5-7 nm.

Adolescent↗
PubMed1994

The role of delineation and spatial frequency in the perception of the colours of the spectrum.

The observations of the spectrum made by Newton, Young, Wollaston and Helmholtz are approximated and accounted for. Increasing the number of delineations allows progressively more bands differing in colour to be perceived, in addition to the three blocks of colour seen in the undelineated spectrum. The rate at which further delineation permits more colours to be observed decreases, however, so that up to 30 colours can be perceived in the subdivided spectrum. The wavelength discrimination measurements agree well with previous data. Enhanced colour discrimination is shown to require luminance contrast transients containing only the first few Fourier harmonics.

Adult↗
PubMed1994

Spatial integration in human vision with bichromatically-mixed adaptation field.

The present study showed that the spatial integration area was expanded even if the background energy was increased, provided that the background consisted of the two colors which were opponent to each other (red/green). This expansion was not found for the subjects with dichromatic color vision even though the same condition was utilized. It was suggested that polarization and its release (the change of adaptation level) of the chromatically-opponent red/green system were involved in control of spatial integration area.

Adaptation, Ocular↗
PubMed1994

Luminance noise and the rapid determination of discrimination ellipses in colour deficiency.

A computer-controlled test of colour vision is described, in which luminance noise and masking contours are used to ensure that the subject's responses depend on chromatic signals. The test avoids the need--common to most computer-controlled tests--to define equiluminance for the individual subject before the colour test itself can be administered. The test achieves a good separation of protan and deutan subjects and reveals the large range of chromatic sensibilities among anomalous trichromats. As a population, dichromats had higher thresholds on the tritan axis of the test than did normals. In an extension of the test, full discrimination ellipses were measured for normal and colour-deficient observers. The nature of anomalous trichromacy is discussed and the possibility is raised that hybrid genes, resulting from genetic recombination, may code for incorrectly labelled or functionally impaired molecules.

Adolescent↗
PubMed1994

Brightness matching and colour discrimination in young diabetics without retinopathy.

This study used the methods of the Farnsworth-Munsell 100-hue test (FM-100), heterochromatic brightness matching (HBM) and wavelength discrimination to test the sensitivity and colour vision of 20 juvenile diabetics with no (16) or very mild (4) retinopathy. Their results were compared to an age-matched control group. The FM-100 results showed a significant increase in error scores throughout the spectrum in comparison to the controls. This deterioration in colour vision was confirmed in the results for the wavelength discrimination task, tested between 440 and 640 nm, where the just noticeable difference in colour was, in general, larger for the diabetic group than the control group. Only at 460 nm were the results of the diabetics similar to those of the controls. The diabetic group were also less sensitive than the control group in the HBM task between 480 and 600 nm. The results show that a deficit in sensitivity and colour vision occurs in diabetics before the onset of a clinically visible retinopathy.

Adolescent↗
PubMed1994

Spectral sensitivity of dichromats: role of postreceptoral processes.

Increment spectral sensitivity functions were determined for dichromatic subjects; these functions were interpreted in the context of parvo and magno pathways. Increments of either 200 or 10 msec in duration were presented on a spatially coincident, 1000 td white background. When compared to the control trichromatic data, the 200 msec functions of dichromats manifest a reduction in sensitivity at middle and long wavelengths. These data are consistent with the notion that 200 msec threshold increments reveal the sensitivity of the parvo pathway; the sensitivity of this pathway is reduced in dichromacy due to pigment replacement with a resultant loss of spectral opponency. The 10 msec functions of dichromats do not show a comparable reduction in sensitivity. Therefore, it is concluded that 10 msec increments reveal the sensitivity of a pathway whose spectral-opponent status is not substantially altered in dichromacy, presumably the magno pathway.

Adult↗
PubMed1994

Surface color naming in dichromats.

In previous experiments, Montag and Boynton [(1987) Vision Research, 27, 2153-2162] found that many dichromats can categorize colors using color naming in fair agreement with color-normal subjects. The contribution of rods to color vision was suspected as underlying this ability. Here we follow up on these experiments by having dichromats name colors under various conditions. When the stimuli are limited to a brief presentation time (60 msec) the dichromats' categorization in the three dimensions of the OSA color space is impaired. Using high light levels so that the rods are saturated does not impair performance. The dichromats named colors during the period of the cone plateau following a rod bleach. Contrary to Montag and Boynton (1987) there was no deficit. These results suggest that an anomalous third cone pigment is responsible for the categorization in three dimensions. It is concluded that the receptors containing the anomalous pigment require greater temporal and spatial summation in order to contribute to the dichromats' color categorization.

Color Perception↗
PubMed1996

Deuteranomaly studied with four perceptual criteria.

In the theoretical section of the present paper, we develop our view of the roles played by the perceptual criteria: indistinguishably equal, neither blue nor yellow, neither green nor red, and heterochromatically equally bright. These criteria constitute a vectorial opponent-colour space, a concept used throughout the paper. Within this framework, two new theorems on psychophysical opponent-colour channels are stated. In the experimental section, the perceptual criteria are applied to the colour vision of an (extreme) deuteranomalous male. A quantitative perceptual description of his deuteranomaly is developed, the main constituents of which are measured deuteranomalous colour matching functions, the deuteranomalous opponent-colour functions derived from them and taking quantitative account of the Abney effect.

Color Perception Tests↗
PubMed1995

Phototransduction in human cones measured using the alpha-wave of the ERG.

To study human cone phototransduction, the alpha-wave of the ERG was recorded from color normals, dichromats, and patients with retinitis pigmentosa. A model of the activation phase of phototransduction, previously fitted to responses from single rods and the rod alpha-wave, was modified and fitted to the human cone alpha-wave. The modified model fits the cone alpha-wave well and allows questions about human cone phototransduction to be addressed. In particular, we conclude that: (1) the amplification of the activation phase of human cone transduction is comparable to that of the human rods. (2) Steady lights have relatively little effect on the amplification of cone transduction. (3) The normal alpha-wave elicited by red flashes is dominated by the L cones, consistent with a ratio of L:M cones of > 1. (4) Retinitis pigmentosa caused by mutations of the rhodopsin gene can affect cone phototransduction. Finally, a simpler computational expression is shown to approximate the modified model's responses.

Adult↗
PubMed1996

Normal saturation processing provides a model for understanding the effects of disease on color perception.

Saturation discrimination has been reported to be affected early in the course of a disease. Our empirical data show a compressive curvi-linear relationship between opponent/nonopponent channel activity and saturation thresholds in normal trichromatic observers. This relationship can be explained by a model based on the Hurvich and Jameson saturation coefficient (1957), Psychology Reviews, 64, 384-404. The model considers effects of both selective and nonselective channel losses on saturation processing based on the assumption that disease produces elevated thresholds while maintaining normal psychometric response functions. Both the model and data support clinical observations of saturation losses occurring early in disease. However, the results also indicate that saturation may not be the best modality for monitoring long-term progression of such conditions. We suggest that the different processing characteristics for blue-yellow thresholds may yield added information for saturation testing under some circumstances and that saturation processing occurs at a higher cortical level.

Adult↗
PubMed1995

Enhanced S cone syndrome: evidence for an abnormally large number of S cones.

The cellular basis of the hypersensitivity of the S (blue) cone system in patients with enhanced S cone syndrome was examined by analyzing ERGs from three patients. The patients had large alpha-waves in response to the blue and white flashes. These alpha-waves were shown to be driven nearly entirely by the S cones. Although these S cone alpha-waves were 4-6 times the size of the normal L/M cone alpha-wave, they are of the same form, and could be quantitatively described with the same model previously shown to fit cone alpha-waves. We propose that the retina of these patients has many more S cones than the normal retina and that these cones replace some of the normal L/M cones and many of the rods.

Adolescent↗
PubMed1996

Confusion points and constant-luminance planes for trichromats, protanopes and deuteranopes.

The confusion points of dichromats are derived from the constant-luminance planes of trichromats, protanopes and deuteranopes experimentally defined by heterochromatic-flicker photometry: (1) the zero-luminance planes of the observers considered in this experiment intersect almost exactly in a line that crosses the plane of the chromaticity diagram in the tritanopic-confusion point and confirm that the short-wavelength sensitive cones can be considered to have no contribution to luminance; (2) protanopic- and deuteranopic-confusion points are taken as being defined by the intersection of the tangent line to the long-wavelength region of the spectrum locus and the zero-luminance plane for protanopes and deuteranopes, respectively.

Adaptation, Ocular↗
PubMed1996

Rod temporal channels.

Mechanisms underlying rod temporal contrast sensitivity have been considered in terms of a fast retinal signal predominating at mesopic levels and a slower retinal signal predominating at scotopic levels. Here we use a small signal masking method, which has previously been used to delineate the cone-mediated cortical temporal channels, to investigate their rod-mediated cortical counterparts. The results suggest that there are three different rod-mediated cortical temporal channels, one which is lowpass and two which are bandpass. These mechanisms co-exist at all light levels and their relative sensitivity depend on the stimulus spatio-temporal frequency.

Color Vision Defects↗