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PubMed1984

Temporal modulation sensitivities of red- and green-sensitive cone systems in dichromats.

The temporal modulation sensitivities of protanopes, deuteranopes and normal observers were measured with sinusoidal grating patterns (of spatial frequency 2.5 c/deg) whose contrast was modulated sinusoidally in time. Contrast sensitivity functions obtained from the three types of observers are almost identical. If we accept that in each class of dichromat only one cone type (red-sensitive, R, or green-sensitive, G) supports threshold, and that post-receptoral filters in both classes of observers are the same, the results show that R and G cones have the same temporal properties.

Adult↗
PubMed1984

The influence of cones on rod saturation with flashed backgrounds.

The increment threshold for a middle-wavelength test flash was measured at the onset of a concentric long-wavelength background flash under conditions that have previously been shown to result in rod system saturation. The influence of the cone system on rod saturation under these conditions was assessed using the Stiles-Crawford effect in normal subjects and by measuring rod thresholds in protanopes, who are deficient in long-wavelength cones. When the background flash is made less effective for cones through the Stiles-Crawford effect, the onset of rod saturation occurs at a higher luminance of background flash than normal. Similarly, protanopes do not show the characteristics of rod saturation until a much higher-than-normal luminance of background flash. The results suggest that rod system saturation with flashed backgrounds is strongly influenced by cones.

Adaptation, Ocular↗
PubMed1984

Dark-adaptation of the human rod system.

Following substantial bleaching, dark-adaptation thresholds of a complete rod monochromat and of a subject with normal colour vision were measured using a Wright colorimeter. When precautions were taken to ensure that the fixation point fell on the same retinal area during the threshold measurements as during the bleaching period, the dark-adaptation threshold curves of the rod monochromat followed exactly the same course as those of the normal subject subsequent to the cone-rod break of the long-term, normal dark-adaptation curve; irrespective of the intensity and the duration of the bleaching and the wavelength of the test stimulation. In contrast to the normal subject, however, the dark-adaptation curves of the rod monochromat showed no evidence of any cone function at photopic intensities. Furthermore, as opposed to previous measurements which show a simple linear relationship between fraction of bleached rhodopsin and log threshold, the present results show that there is a close linearity between log fraction of bleached rhodopsin and log threshold. This linear relationship is obtained despite varying extents of bleaching and subsequent dark-adaptation periods.

Adaptation, Ocular↗
PubMed1984

Temporal integration of the pi 1/pi 3 pathway in normal and dichromatic vision.

Stiles' pi 1 and pi 3 mechanisms are thought to reflect adaptation events at two sites in a single pathway, the first site controlled by the short-wavelength cones alone, the second site controlled by opposing signals from these cones vs the other cone classes. We examined this pathway's temporal integration under conditions that yield the full gamut of possible adaptation states at the two sites. Critical duration of the pi 1/pi 3 pathway was always about 200 msec. In addition, we examined the pi 1 and pi 3 mechanisms of dichromatic vision. Our results suggest that protanopic and deuteranopic vision are characterized by a pi 1/pi 3 pathway similar to that in normal color vision.

Color Perception↗
PubMed1985

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.

Color Perception↗
PubMed1985

Cone antagonism along visual pathways of red/green dichromats.

We have measured thresholds for bichromatic test-mixtures in red/green dichromats. Our results provide strong evidence for cone-antagonistic coding along the dichromats' detecting pathways. The threshold behavior of the isolated dichromatic pathways is consistent with that predicted by Opponent Colors Theory. We compare these threshold data with the dichromats' judgments of "blue/yellow" hue equilibria, which we obtained in supra-threshold color-cancellation experiments. The results are consistent with the hypothesis that both sets of data are mediated by the same dichromatic cone antagonistic pathway.

Adaptation, Ocular↗
PubMed1985

Role of the blue mechanism in wavelength discrimination.

The role of blue cones as well as the pathways they supply (collectively called the "blue mechanism") is evaluated by comparing ordinary wavelength discrimination functions with those obtained using two methods designed to inhibit the blue mechanism selectively. These methods use a just-noticeable-border criterion (JNB), instead of the usual one of just-noticeable-difference, and a yellow preadapting field to induce transient tritanopia. Without transient tritanopia, the data obtained using the just-noticeable-border criterion reveal a small contribution of the blue mechanism to wavelength discrimination. Transient tritanopia, with JNB, produces an additional selective loss of wavelength discrimination in a spectral region flanking 460 nm, which yields a function resembling those for tritanopes previously examined.

Color Perception↗
PubMed1985

Cone mechanisms underlying the color discrimination of deutan color deficients.

An alternation method of color matching was used to obtain a series of extended Rayleigh matches from several deutan color deficients with varying degrees of color discrimination. With large stimulus fields there were differences in the matches made by observers with good color discrimination and the matches made by observers with poor color discrimination. The matches made by observers with poor discrimination could not be modeled with normal cone action spectra. When the field size was reduced the matches of all observers were quite similar and could be modeled with two cone action spectra that were normal in shape and separated by approximately 5 nm. Results suggest that individual differences in ability to discriminate color among deutan observers are not solely related to differences in the cone action spectra.

Adolescent↗
PubMed1986

Background size and saturation of the rod system.

To determine the effect of background size on saturation of the human rod system, we measured threshold-vs-intensity (tvi) functions for a 24'-diameter, middle-wavelength test stimulus against long-wavelength background fields that were either 2 degrees or 8 degrees in diameter. When thresholds were measured against a series of background intensities presented during a single experimental session (a standard tvi procedure), rod increment thresholds were higher against the smaller background and eventually disappeared above cone increment thresholds at moderate background intensities. This result suggests that background diameter does influence rod system saturation. However, when adaptation to the smaller background was limited to a 5 min exposure to a single background intensity per experimental session, then thresholds against that background were rod- rather than cone-mediated and were comparable to rod thresholds against an 8 degrees background. Therefore, under these stimulus conditions, background size apparently does not influence rod system saturation unless adaptation to the smaller background is prolonged. Based on previous studies, we suggest that the rod-desensitizing effect of small backgrounds under conditions of prolonged adaptation is due to a post-retinal mechanism.

Adaptation, Ocular↗
PubMed1986

The use of a system analysis approach to electrodiagnostic (ERG and VEP) assessment.

To evaluate the integrity of a biological system and its constituent functional units, a systematic study of input-output relations adopted from engineering has proven appropriate. With such an approach, sequential analysis can be implemented to probe the various parameter extractions along, for example, the visual system. The a priori assumption in this approach is that the visual world is processed along functionally separate pathways yielding distinct percepts such as contrast and motion. This so-called channel approach has proven useful not only to basic vision research but also for clinical application. The present overview shows that on the basis of the ERG or VEP, a type of functional anatomy can be performed with the biological system of interest remaining intact. Finally, it will be demonstrated that electrophysiological output parameters of the visual system can also serve as a non-invasive entry to investigate general systemic disorders.

Adult↗
PubMed1987

Rod and cone system contributions to oscillatory potentials: an explanation for the conditioning flash effect.

The oscillatory potentials (OPs) of the human electroretinogram (ERG) are smaller in response to the initial flash of a series than to subsequent flashes. To investigate a possible rod system contribution to this "conditioning flash effect," we have examined OPs in normals and rod monochromats. The OPs recorded from rod monochromats were similar to those recorded from normals under test conditions that selectively stimulate rods. However, under conditions that in normals stimulate both rods and cones and that result in maximal amplitude of the OPs, the rod monochromats exhibit markedly reduced OPs. This finding suggests that the initial (conditioning) flash operates by adapting the rod system contribution to the OPs, so that the OPs in response to subsequent flashes result primarily from the cone system. In agreement with this hypothesis, the conditioning flash effect did not occur when flashes were presented against a background which eliminated the rod system response nor during the cone plateau phase of dark adaptation.

Color Vision Defects↗
PubMed1987

Equilibrium hue judgements of dichromats.

It is generally held that protanopes and deuteranopes see only regions of blues and yellows in the visible spectrum, with an achromatic point, called the neutral point, separating these regions. Considerations of a zone model of color vision for the dichromatic observer led us to predict that a reduced form of red/green discrimination would allow equilibrium blue judgements to be made by protanopes. We show that protanopes can make equilibrium blue determinations with as much reliability as they make neutral point settings. Our results indicate that protanopes but not deuteranopes are able to rely on a reduced form of red/green discrimination in the short wavelength part of the spectrum. Protanopes describe wavelengths longer than the neutral point as yellow. Between the neutral point and equilibrium blue, different wavelengths are described as having varying aspects of blue and green; and short of equilibrium blue they appear reddish blue. For dueteranopes, the spectrum longer than the neutral point appears yellow, and short of it, blue. The results of our experiments showing that the protanopic equilibrium blue is invariant with intensity variations, as it is in the trichromat, add support to the idea of a reduced form of red/green discrimination for protanopes. Our results also allow the evaluation of various models of protanopia.

Color Perception↗
PubMed1987

Color discrimination and neural coding in color deficients.

Rayleigh color match ranges obtained from color deficient observers varied considerably as a function of spatial and temporal parameters of stimulus presentation. The results suggest that color discrimination losses in color deficients result from abnormalities in the spatial and temporal properties of neural coding in addition to cone photopigment abnormalities.

Color Perception↗
PubMed1987

Rod influence in dichromatic surface color perception.

Two protanopes, two deuteranopes, and two normal subjects named 424 OSA Uniform Color Scales samples using single-word color terms of their choice under three different experimental conditions. When viewing a stimulus field subtending about 4 deg, the performance of the dichromats revealed a substantial ability to discriminate colors along the red-green axis. When the stimuli were limited to the central fovea, or when rods were excluded with a bleach, dichromats could no longer categorize colors in the red-green dimension. The different conditions did not affect the performance of the normals. The results suggest that rods contribute signals used by dichromats, along with lightness cues, to help discriminate and categorize surface colors.

Adolescent↗
PubMed1987

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.

Animals↗
PubMed1988

Temporal summation in the achromat.

We investigated temporal summation of the rods in a complete achromat, who lacks cone vision. Critical duration (tc) was estimated both at the achromat's preferred area of fixation and at an area 12 deg laterally in the nasal visual field. Comparable tc determinations were made in a normal trichromat. At background luminances of 0.0 and 0.6 scot. td, where the rods mediate detection, the values of tc were similar for the achromat and the normal. At a luminance of 813 scot. tds, however, where the middle-wavelength sensitive cones mediate detection in the trichromat, the tc for the achromat was much longer than that for the trichromat.

Adaptation, Ocular↗
PubMed1989

The luminance-response function of the dark-adapted human electroretinogram.

A hyperbolic equation of the form (R/Rmax) = Ln/(Ln + Kn) has been used to describe the b-wave luminance-response function of the dark-adapted human electroretinogram (ERG). However, this equation accurately describes the function only at low to moderate flash luminances. At high flash luminances, a second amplitude increase or "limb" appears in the function. The results of the present study demonstrate that this limb does not represent a cone system response. First, the spectral sensitivity of the entire luminance-response function is rod-determined in normal subjects. Second, the limb was present in a rod monochromat, in whom cone system ERGs were indistinguishable from noise. Instead, the nonmonotonic nature of the luminance-response function may result from a luminance-dependent algebraic summation of the components that underlie the ERG waveform of the rod system.

Color Vision Defects↗