Rods induce transient tritanopia in blue cone monochromats.

Transient tritanopia is a cone-cone post-receptoral interaction between short-wavelength (S) cones and medium (M) and long (L) wavelength cones. Blue cone monochromats have rods and S cones of normal sensitivity but lack functional M/L cones. All blue cone monochromats tested (n = 8) show significant amounts of transient tritanopia mediated by rods. Attempts to find a similar rod-S cone interaction while silencing the L/M cones in normals yielded only a small amount of S cone sensitivity loss. The results suggest an exaggerated influence of rods on the S cone pathway in the retina of blue cone monochromats.     

An action spectrum for the production of transient tritanopia

The site of yellow/blue chromatic-opponency has been thought to include synergistic input from either or both of the longwave-sensitive cone types to the "yellow" side. It has remained controversial whether one of these cone types provides all the "yellow" input or if both contribute to some extent. By measuring the action spectrum of adapting fields that produce transient tritanopia at their offset it appears that both the red- and green-sensitive cone types contribute in approximately the same ratio that they contribute to luminance. There also appears to be a contribution from an antagonistic red/green mechanism in producing transient tritanopia. These seem consistent with current physiological and psychophysical evidence.     

Filling-in of the foveal blue scotoma.

The blue-blindness (tritanopia) of the human foveola normally goes unnoticed but can be directly visualized by having observers view a flickering, monochromatic, short-wavelength field. The blue scotoma appears as a tiny dark spot in central vision, the visibility of which depends upon the wavelength of the field and the temporal frequency of modulation. Comparisons of fading times as a function of flicker frequency for the blue scotoma, foveal afterimages and optically stabilized images indicate a common time course, consistent with the hypothesis that perceptual filling-in of the foveal blue scotoma reflects the operation of neural processes similar to those involved in fading and regeneration of stabilized images.     

Wavelength discrimination deteriorates with illumination in blue cone monochromats

Two types of incomplete congenital achromats were studied: one type (blue cone monochromats) has a conspicuous short wavelength cone mechanism, and the other type (deutan incomplete achromats) has a conspicuous long wavelength cone mechanism. The photoreceptor mechanisms were inferred from color matches and from test action spectra measured on rod-saturating backgrounds of different wavelengths. Interestingly, the illumination-dependency of color discrimination (for 5 degrees bipartite fields that were centrally fixated) differed between the two patient types, even though rhodopsin photoreceptors were common to both. As illumination level increased, the ability to discriminate wavelength differences deteriorated for the blue cone monochromats, whereas, for the deutan achromats, wavelength discrimination remained relatively constant even near 100,000 scotopic trolands. The performance decrement in the blue cone monochromats was probably not associated with rod saturation, as the field action spectrum to cause a just-noticeable-difference (jnd) decrement in discrimination was poorly fitted by a rhodopsin action spectrum. In addition, the blue cone monochromats had rhodopsin photoreceptors that did not saturate in bright illuminations. The authors hypothesize that the deterioration of wavelength discrimination at high illuminations is not an abnormality of blue cone monochromacy. Rather, it may be a property of the normal color mechanism through which signals from the short wavelength cones pass.     

Transient tritanopia after flicker adaptation

Transient tritanopia, the rise in blue test threshold following extinction of a yellow background, is “abolished” if the adapting background is flickered rectangularly at a low rate (0.25 Hz, duty cycle 0.2), and reduced at rates below 10 Hz. By “abolition”, is meant that the threshold falls to the same extent as after extinction of a II₁-equated “opponently neutral” (494 nm) background. These results, like those of Loomis (1980), require modifications in Pugh and Mollon's (1979) dynamic model for transient tritanopia.     

Transient tritanopia at the level of the ERG b-wave.

A blue stimulus that is just visible in the presence of a yellow adapting field remains invisible for several seconds after the yellow field has been turned off. This “transient tritanopia” is shown to exist already at the level of theb-wave of the electroretinogram. The experiments were done on rhesus monkeys and the results were qualitatively confirmed in human subjects with psychophysical and ERG methods.     

Follow-up studies in pattern VECP in demyelinating diseases in children

Spectral sensitivity functions and the transient decrease of sensitivity to short wavelengths after the offset of yellow light (transient tritanopia) were measured by increment threshold techniques in patients suffering from hereditary macular degenerations. Color vision defects were determined by arrangement tests and the anomaloscope. Central areolar choroidal dystrophy was found to produce a mild protan defect and to reduce foveal spectral sensitivity throughout the visible spectrum by a factor of 100; it also abolishes transient tritanopia. Electroretinogram (ERG) was normal, electrooculogram (EOG) subnormal. Stargardt's disease, despite numerous fluorescent macular spots, does not abolish transient tritanopia nor does it reduce spectral sensitivity, although scotopic matches were performed on the Nagel anomaloscope. Only in severe, advanced cases was transient tritanopia reduced and spectral sensitivity found to follow the absorption spectrum of rods. Routine ERGs and EOGs were normal. Vitelliform macular degeneration, despite the ophthalmoscopically pronounced dystrophic macula, produced only very small changes in spectral sensitivity and transient tritanopia, although a widened matching range on the Nagel anomaloscope and electrophysiological abnormalities were found. Apparently damage of the retinal circuit which connects long and short wavelength-sensitive cones, caused by hereditary conditions, is different from that caused by retinotoxic drugs.     

Transient tritanopia: Its abolition at high intensities

Transient tritanopia, the increase in the threshold of a blue test following extinction of a long-wavelength background, is reduced or abolished at high intensities of the background. The dependence of this effect on background wavelength supports a bleaching/cancellation hypothesis put forward by Pugh and Mollon (Vision Res.19, 293–312).     

Blue cone monochromasia: diagnosis, genetic counseling and optical aids

Cone and rod functions of three blue cone monochromats (age 13-20, male) from three different families were investigated. In contrast to rod monochromats, they lack prominent nystagmus. Color matches as determined using the Nagel anomaloscope came close to those of rod monochromats but the green primary appeared slightly brighter to them. In color discrimination tests (Farnsworth-Munsell 100-hue and Panel D-15 desaturated), maximum confusion followed protan rather than scotopic axes. Measurements of spectral sensitivity revealed action spectra exclusively of blue sensitive cones, even under conditions that should isolate green- or red-sensitive cones. After 20 minutes of dark adaptation, rods determined the spectral sensitivity function. Transient tritanopia, which in normals results from the interaction between cones sensitive to short and long wavelengths, was completely absent in blue cone monochromats. Visual acuity (Snellen charts and contrast gratings) revealed values between 20/200 and 20/60. Recognition of high-spatial low-contrast gratings was improved by blue cut-off filters (Schott BG 28) and considerably worsened by yellow cut-off filters (Schott OG 510). Since alteration of visual acuity induced by cut-off filters was not found in rod monochromats, this two-filter test is a means of differentiating quickly between rod achromats and blue cone monochromats. As the mode of inheritance is autosomal recessive in rod achromats and x-linked recessive in blue cone monochromats, differential diagnosis is important for correct genetic counseling.     

Effect of foveal tritanopia on reaction times to chromatic stimuli

To investigate the effect of foveal inhomogeneities on sensitivity to chromatic stimuli, we measured simple reaction times (RTs) and detection thresholds to temporally and spatially blurred isoluminant stimuli at retinal eccentricities from 0 deg to 8 deg. Three color-normal subjects participated. Contrast gain was derived from the slope of the RT versus contrast function. With a Gaussian spatial distribution (S.D. = 0.5 deg) and modulation between white (CIE x, y, L = 0.31, 0.316, 12.5 cd x m(-2)) and blue (MBDKL 90 deg), gain was maximal at about 2-deg eccentricity and declined by approximately 1 log unit towards the center and the periphery. The red (0 deg) and green (180 deg) cardinal axes showed maximum gain in the center, whilst the yellow (270 deg) data were intermediate. Although the spatial extent of the Gaussian spot was much larger than the S-cone free zone, we wished to determine whether foveal tritanopia was responsible for the marked drop in sensitivity to the 90-deg stimulus. To align the color vector along a tritan line, we used a smaller disk (0.3 deg) with a blurred edge and measured detection threshold, rotating the vector until minimum central sensitivity was obtained. Other workers have used transient tritanopia or minimally distinct border to similar effect. By repeating this at different locations in color space, a group of vectors were obtained. These converged near to the S-cone co-punctal point, evidence that they lay along tritan confusion lines. These threshold findings were then confirmed using the RT-derived contrast gain function. The tritan vectors were less pronounced as stimulus size increased. With the vector optimized to produce foveal tritanopia, the RT gain versus eccentricity functions for the 90-deg and 270-deg stimuli both fell markedly in the center and periphery, and sensitivity peaked at about 3-deg eccentricity. There are some similarities between these findings and the underlying photoreceptor distributions. As a result, there is a greater difference in gain between red-green and blue-yellow systems in the center than in the near periphery. We conclude that the RT versus contrast function is a sensitive index of foveal opponency.     

Researches on a unilaterally blue-blinded rhesus monkey

Psychophysical measures of hue (wavelength) discrimination and spectral sensitivity were collected over a 3-year-period on a rhesus monkey whose right eye had been exposed to intense blue light 10 years prior and had shown a pronounced loss of blue sensitivity in an increment-threshold, spectral-sensitivity task. Hue discrimination, to a somewhat greater degree than spectral sensitivity, revealed large differences between the normal and blue-exposed eye. The difference limens were in some cases 100 nm for the blue-exposed eye compared to 10-15 nm for the normal eye. The hue-discrimination functions from the blue-exposed eye were similar in form to those from human tritanopes (blue-blind humans), and those from the monkey's normal eye were similar to those from normal humans. Detailed functions, where the variable wavelength was shorter as opposed to longer than the reference wavelength, were shown separately for each of the monkey's eyes; those from the blue-exposed eye were very similar to analogous functions from the one case where they have been shown separately for a human tritanope.     

Visual short-term effects of Viagra: double-blind study in healthy young subjects

PURPOSE: To investigate short-term visual effects of a single 100-mg dose of Viagra (sildenafil citrate) in healthy men. DESIGN: Randomized, double-blind, placebo-controlled clinical trial of drug effects on normal volunteers conducted by a single center. METHODS: Twenty men, aged 20 to 40 years, were treated with either a placebo or 100 mg sildenafil. Visual function tests included electroretinogram (ERG) recordings, on-/off- and 3.3 Hz-flicker-ERG recordings, anomaloscope matches, and measurements of cone contrast sensitivities and transient tritanopia. RESULTS: Most visual tests did not differ between the sildenafil and placebo groups. However, statistically significant increases in sensitivity during transient tritanopia were observed as well as significant prolongations in the implicit times of scotopic a-wave, photopic b-wave, and 3.3 Hz-flicker a-wave and b-wave ERG recordings. The magnitude of the differences correlated with peak sidenafil plasma concentration. Although rod amplitudes of the ERG recordings tended to be higher and cone amplitudes lower in the sildenafil group after drug ingestion, the differences were nonsignificant. There were no reports of visual side effects, and all electrophysiologic and psychophysical measurements returned to the normal range within 24 hours. CONCLUSIONS: A single oral dose of 100-mg sildenafil given to healthy young men led to small but statistically significant transient changes of outer and inner retinal function, as detected by ERG and psychophysical methods. Although the acute effects were fully reversible within 24 hours, it would be worthwhile to compare them with those induced by other PDE5 and PDE6 inhibitors.     

Just noticeable inhomogeneity criterion for determining wavelength discrimination functions

The visual criterion of just noticeable inhomogeneity is described for determining wavelength discrimination functions. It involved determining the wavelength differences between reference and test fields required to produce a just noticeable inhomogeneity which cannot be eliminated by a brightness adjustment. The fields formed a checkerboard pattern the element size of which variable. Tritanopic delta lambda functions were obtained by using the small field insensitivity of the fovea. Just noticeable border, data obtained from bipartite field studies, were replicated with this checkerboard field.     

Recovery after severe ethambutol intoxication - psychophysical and electrophysiological correlations

Six patients with severe ocular side effects caused by therapeutical doses of the tuberculostatic drug ethambutol were investigated during the course of recovery with psychophysical and electrophysiological methods. Three patients developed an optic atrophy with permanently reduced vision as a likely consequence of additional risk factors such as diabetes, alcohol abuse, and reduced kidney function. The severity of the neuritis of the optic nerve was not related to the total intake of ethambutol. The likelihood of a permanent ocular damage increased sharply if the visual acuity had dropped below a value of 1/10. Permanently prolonged latency of the P-100 component was found in visual evoked potentials even in cases with good recovery from ethambutol-induced damage. The recovery of color vision could be monitored very well with the Farnsworth-Munsell 100-Hue Test which revealed a diffuse impairment of color discrimination with a slight prevalence of the red-green axis. In addition to the known disturbances of the red-green antagonistic neurons, it could be demonstrated by measuring transient tritanopia and spectral sensitivity functions that ethambutol also affects the blue-yellow antagonism at the retinal level.     

Effects of ageing on postreceptoral short-wavelength gain control: Transient tritanopia increases with age

We investigated the effect of ageing on the neural gain control in the short-wavelength opponent channel. In order to tackle specifically postreceptoral changes, we determined the effect of ageing on transient tritanopia, a paradoxical and transient reduction of short-wavelength sensitivity after the presentation of a long-wavelength adapting light. The results demonstrate an unexpected and significant increase of transient tritanopia with age, which cannot be explained by a general decline of short-wave sensitivity or the selective reduction of retinal illumination. Instead, our data imply that ageing affects also short-wavelength gain control at the site of chromatic opponency or beyond. Age-related changes of adaptation processes should therefore be considered an important factor influencing the visual performances of the elderly.     

Higher order color mechanisms

Evidence supporting the existence of higher order color mechanisms, that is, ones beyond the previously identified second stage mechanisms is presented. This evidence includes a reanalysis of the data of Krauskopf et al. [Vision Res. 20, 1123-1131 (1982)] on the desensitizing effects of viewing chromatically modulated fields, new experiments on a generalized version of the "transient tritanopia" experiment of Mollon and Polden [Phil. Trans. R. Soc. Lond. 278, 207-240 (1977)] and results on the relationship between discrimination and detection of brief color changes.     

Color naming and hue discrimination in congenital tritanopia and tritanomaly

The color naming performance of one congenital tritanope and two congenital tritanomals was investigated. They were required to apply one or more of the names blue, green, yellow, red and white to spectral stimuli (400–650 nm) exposed for 364 msec. They employed all four color names with the use of blue and green being most different from normal. The tritans did not report the spectrum as markedly desaturated, even in the yellow. Index of Nameable Color Difference (I.N.C.D.) curves derived from the color-naming data closely resemble the tritan wavelength discrimination function measured elsewhere by traditional wavelength difference methods.     

Detecting color vision in a malingerer

A patient describing himself as totally color blind was ordered by the judicial system to have his color vision investigated in order to establish his suitability for military service. Basic clinical (Farnsworth Panel D-15, Moreland and Rayleigh anomaloscope equations), electroretinographic (ERG) and psychophysical techniques (spectral sensitivities) were applied to determine the extent of his color discrimination performance and cone function. These standard procedures were complemented by a test for cone interaction (transient tritanopia) and by newly developed cone-isolating flicker large-field ERG recordings. The patient's data consistently indicate the function as well as the functional interaction of the middle-wavelength-sensitive (M-) and the short-wavelength-sensitive (S-) cones. But the function of the long-wavelength-sensitive (L-) cones was completely absent. Hence the patient was correctly demonstrated to be a protanope. This study establishes that standard classical procedures, in combination with newly developed and easy to apply psychophysical and ERG ones, which can be reliably used to assess true color discrimination performance, in difficult cases of malingering.     

A blue sensitive mechanism in the pigeon retina: lambda max 400 nm

Electroretinographic studies of the pigeon visual system have revealed a blue mechanism with λ_max 400 nm. Spectral sensitivity after strong selective-chromatic-adaptation is well fit by a Dartnall nomogram combined with an appropriate oil-droplet. The relationship of these data to pigeon luminosity and wavelength discrimination are discussed.     

How well does color perimetry isolate responses from individual cone mechanisms?

PURPOSE: Over the past two decades there has been renewed interest in the use of color perimetry as a means of detecting early functional defects resulting from glaucomatous optic neuropathy and other forms of ophthalmic and neurologic pathology. The most popular form of color perimetry employs a colored background that selectively desensitizes two of the cone mechanisms, while the wavelength of the test target is selected to favor detection by the remaining, relatively unadapted, cone mechanism. While there are data to support the assertion that blue on yellow perimetry adequately isolates the short wavelength sensitive (SWS) cone mechanism, the only estimates of isolation of the other two cone mechanisms are for the region of the visual field corresponding to the fovea. The purpose of this experiment was thus to determine the amount of cone mechanism isolation that is afforded by automated perimetry when using yellow, blue, and magenta adapting backgrounds. METHODS: To estimate cone mechanism isolation, we determined spectral sensitivity for a range of narrow-band stimuli using a standard 30-2 program on a modified Humphrey perimeter. Targets were presented against three different backgrounds of different luminance; yellow at 2.1 log cd.m-2, magenta at 1.3 log cd.m-2, and blue at 1 log cd.m-2. Sensitivity values for each background at each stimulus position were plotted as a function of wavelength, normalized, and then fitted with cone sensitivity templates to determine the relative sensitivity of the three cone mechanisms. RESULTS: The maximum relative isolation of an individual cone mechanism was achieved with a yellow background, where there was an average relative isolation of 0.94 for the SWS cone mechanism; the blue background provided a relative isolation of 0.89 for the long wavelength sensitive (LWS) cone mechanism. The magenta background proved poor at isolating the medium wavelength sensitive (MWS) cone mechanism, where a relative isolation of only 0.51 was obtained. CONCLUSION: Although color perimetry is capable of isolating individual cone mechanisms, the magnitude of isolation in normal observers may be small under certain circumstances. Therefore, when the technique is used to examine pathologic states, it may be necessary to employ at least two target wavelengths to determine the cone mechanism that is performing target detection. Furthermore, we suggest that MWS cone mechanism isolation may be improved through the combination of the so-called silent substitution technique with that of selective adaptation.