Increment threshold and purity discrimination spectral sensitivities of X-chromosome-linked color-defective observers

Mechanisms contributing to increment threshold and decrement threshold spectral sensitivities

The shape of the human spectral sensitivity function depends on how it is measured. In the increment threshold (IT) technique, sensitivity is typically measured as the inverse of threshold for detection of increments of monochromatic light presented for relatively long durations on achromatic pedestals. Spectral sensitivity functions derived from IT techniques have long been used to reveal contribution from opponent color channels. Although IT functions have been studied extensively, little attention has been given to functions derived from decrement thresholds (DT), partly due to technical challenges of producing appropriate stimuli. Comparison of IT and DT spectral sensitivities may be of interest because there are known asymmetries in the visual system between on- and off-pathways and between increment and decrement responses within these pathways. Consequently, spectral sensitivity functions obtained using DT measures may reveal a different complement of contributing mechanisms than those that produce IT functions. We report here that IT and DT derived spectral sensitivities were essentially identical over much of the visible spectrum. However, decrement sensitivity was slightly greater than increment sensitivity in the shorter wavelengths at modest light levels. This difference was not present at higher light levels, implicating rod pathways as a possible source of the difference. In sum, it appears that under conditions shown to reveal strong contribution from opponent mechanisms, decrement functions are either 1) determined by a similar complement of spectrally opponent mechanisms as those that define increment spectral sensitivities or 2) that the present conditions are insensitive to underlying asymmetries.

The influence of intrinsically-photosensitive retinal ganglion cells on the spectral sensitivity and response dynamics of the human pupillary light reflex

Historically, it was assumed that the light-evoked neural signals driving the human pupillary light reflex (PLR) originated exclusively from rod and cone photoreceptors. However, a novel melanopsin-containing photoreceptive cell class has recently been discovered in the mammalian retina. These intrinsically-photosensitive retinal ganglion cells (ipRGCs) project to the pretectum, the retinorecipient area of the brain responsible for the PLR. This study was therefore designed to examine the relative contribution of rod, cone and the melanopsin photoresponses of ipRGCs to the human PLR. We establish that the melanopsin photoresponse of ipRGCs contributes significantly to the maintenance of half maximal pupilloconstriction in response to light stimuli of 30s or longer, even at low photopic irradiances. Furthermore, we show that the melanopsin photoresponse contributes significantly to three-quarter maximal pupilloconstriction in response to light stimuli as short as 2s. We also demonstrate that cone photoresponses driving pupilloconstriction adapt considerably and contribute little after 30s, but rod photoresponses adapt less and contribute significantly to the maintenance of pupilloconstriction in response to steady-state light stimuli at irradiance levels which are below the threshold of the melanopsin photoresponse.

S-cone discrimination for stimuli with spatial and temporal chromatic contrast

In the natural environment, color discriminations are made within a rich context of spatial and temporal variation. In classical laboratory methods for studying chromatic discrimination, there is typically a border between the test and adapting fields that introduces a spatial chromatic contrast signal. Typically, the roles of spatial and temporal contrast on chromatic discrimination are not assessed in the laboratory approach. In this study, S-cone discrimination was measured using stimulus paradigms that controlled the level of spatio-temporal S-cone contrast between the tests and adapting fields. The results indicate that S-cone discrimination of chromaticity differences between a pedestal and adapting surround is equivalent for stimuli containing spatial, temporal or spatial-and-temporal chromatic contrast between the test field and the surround. For a stimulus condition that did not contain spatial or temporal contrast, the visual system adapted to the pedestal instead of the surround. The data are interpreted in terms of a model consistent with primate koniocellular pathway physiology. The paradigms provide an approach for studying the effects of spatial and temporal contrast on discrimination in natural scenes.

Rod contributions to color perception: linear with rod contrast

At mesopic light levels, an incremental change in rod activation causes changes in color appearance. In this study, we investigated how rod mediated changes in color perception varied as a function of the magnitude of the rod contrast. Rod-mediated changes in color appearance were assessed by matching them with cone-mediated color changes. A two-channel four-primary colorimeter allowed independent control of the rods and each of the L-, M- and S-cone photoreceptor types. At all light levels, rod contributions to inferred PC, KC and MC pathway mediated vision were linearly related to the rod incremental contrast. This linear relationship could be described by a model based on primate ganglion cell responses with the assumption that rod signals were conveyed via rod-cone gap junctions at mesopic light levels.

Spatial and temporal chromatic contrast: Effects on chromatic discrimination for stimuli varying in L- and M-cone excitation

Discrimination for equiluminant chromatic stimuli that vary in L- and M-cone excitation depends on the chromaticity difference between the test field and the surrounding area. The current study investigated the effect of the proximity in space and time of a surround to the test field on chromatic contrast discrimination. The experimental paradigm isolated spatial, temporal, and spatial-and-temporal chromatic contrast effects on discrimination. Chromatic contrast discrimination thresholds were assessed by a four-alternative spatial forced-choice procedure. Stimuli were either metameric to the equal energy spectrum, or varied in L-cone activation along a line of constant S-cone activation. A model based on primate parvocellular pathway physiology described the data. Spatial and temporal contrast produced equivalent reductions in chromatic discriminability as the chromatic difference between the test and surround increased. For all test chromaticities, discrimination was best in the absence of chromatic contrast. Chromatic contrast discrimination is determined by either the spatial or temporal contrast component of the signal.

Chromatic parameters derived from increment spectral sensitivity functions

We propose a mathematical model to derive the chromatic parameters from increment spectral sensitivity functions. This model was applied to determine the effective red, green, blue, and yellow mechanism contribution to the detection of the spectral stimuli of five normal trichromatic subjects. Detection thresholds were measured for a 300 ms, 1.2 degrees circular test flash presented on a 100 cd/m2 white background for spectral wavelengths between 410 and 660 nm. The model analysis confirmed that in the red-green wavelength area, the detection of our chosen stimuli was mediated by two distinct (L-M) antagonistic mechanisms: a red-green and a yellow, from the blue-yellow system. We inferred that the red-green mechanism receptive fields consisted of a single L- or M-cone center with a homogeneous or heterogeneous surround devoid of S-cone projections. For the receptive fields of the yellow half of the blue-yellow mechanism, we propose a similar configuration but with S-cone projections present in the surround. This proposal is not concordant with what is currently understood regarding retinal physiology. However, two L-M antagonistic mechanisms in the red-green wavelengths as proposed by our results predict what would appear as an intuitive yellow mechanism with a maximal sensitivity at the 578 nm wavelength, where the red-green mechanism sensitivity is null.

Induction effects for heterochromatic brightness matching, heterochromatic flicker photometry, and minimally distinct border: implications for the neural mechanisms underlying induction

Brightness induction refers to the finding that the apparent brightness of a stimulus changes when surrounded by a black versus a white stimulus. In the current study, we investigated the effects of black/white surrounding stimuli on settings made between red and green stimuli on three different tasks: heterochromatic brightness matching (HBM), heterochromatic flicker photometry (HFP), and minimally distinct border (MDB). For HBM, subjects varied the relative luminance between the red and green stimuli so that the brightness of the two colors appeared equal. For the two other tasks, matches were made based on minimizing red/green flicker (HFP) or the saliency of a red/green border (MDB). For all three tasks, the presence of black/white surrounding stimuli significantly altered red/green settings, demonstrating the existence of induction effects. These results are discussed in terms of which underlying color pathways (L+ M versus L-M) may contribute to induction effects for the different tasks.

Computerized color-vision test based upon postreceptoral channel sensitivities

An automated, computerized color-vision test was designed to diagnose congenital red–green color-vision defects. The observer viewed a yellow appearing CRT screen. The principle was to measure increment thresholds for three different chromaticities, the background yellow, a red, and a green chromaticity. Spatial and temporal parameters were chosen to favor parvocellular pathway mediation of thresholds. Thresholds for the three test stimuli were estimated by four-alternative forced-choice (4AFC), randomly interleaved staircases. Four 1.5-deg, 4.2 cd/m2square pedestals were arranged as a 2 × 2 matrix around the center of the display with 15-minute separations. A trial incremented all four squares by 1.0 cd/m2for 133 ms. One randomly chosen square included an extra increment of a test chromaticity. The observer identified the different appearing square using the cursor. Administration time was ∼5 minutes. Normal trichromats showed clear Sloan notch as defined by log (ΔY/ΔR), whereas red–green color defectives generally showed little or no Sloan notch, indicating that their thresholds were mediated by their luminance system, not by the chromatic system. Data from 107 normal trichromats showed a mean Sloan notch of 0.654 (SD = 0.123). Among 16 color-vision defectives tested (2 protanopes, 1 protanomal, 6 deuteranopes, & 7 deuteranomals), the Sloan notch was between −0.062 and 0.353 for deutans and was <−0.10 for protans. A sufficient number of color-defective observers have not yet been tested to determine whether the test can reliably discriminate between protans and deutans. Nevertheless, the current data show that the test can work as a quick diagnostic procedure (functional trichromatism or dichromatism) of red–green color-vision defect.

Color vision sensitivity in normally dichromatic species and humans

Spectral-sensitivity functions for large, long-duration increments presented on a photopic white background indicate that wavelength-opponent mechanisms mediate detection in both normal and dichromatic humans. Normal humans exhibit high color-vision sensitivity as they discriminate the color of spectral flashes at detection-threshold intensities. However, dichromatic humans require stimuli up to about 0.4 log units above detection intensity to see certain colors. This low color-vision sensitivity in human dichromats may be an abnormal condition involving a defect in postreceptoral color processing. To test this hypothesis, we determined color-discrimination thresholds in normally dichromatic species: chipmunk, 13-lined ground squirrel, and tree shrew. For comparison, we also tested humans with normal and abnormal (deutan) color vision with the same apparatus and methods. Animals were trained to perform spatial two-choice discrimination tasks for food reward. Detection thresholds were determined for increments of white, 460 nm, 540 nm, 560 nm, 580 nm, 500 nm/long-pass, and 500 nm/short-pass on white backgrounds of 1.25 cd/m2, 46 cd/m2, and 130 cd/m2. Animals were also trained to respond to the colored increments when paired with the white increment when both were at equally detectable intensities Color-discrimination thresholds were determined by dimming stimulus pairs (coloredvs. white) until the subjects could no longer make the discriminations. Results indicated that the normally dichromatic species could discriminate colored stimuli from white at a mean intensity of 0.1 (±0.1) log units above detection threshold. The ability of normally dichromatic species to discriminate color near detection-threshold intensity is consistent with increment spectral-sensitivity functions that indicate detection by wavelength-opponent mechanisms. In keeping with previous studies, normal human trichromats discriminated color near detection-threshold intensities but humans with deutan color vision required suprathreshold intensities to discriminate the color of middle and long wavelengths. This high color-vision sensitivity of normally dichromatic species suggests that the low color-vision sensitivity in dichromatic humans is an abnormal condition and indicates a possible defect in their postreceptoral color-vision processing.

Molecular basis of an inherited form of incomplete achromatopsia

Mutations in the genes encoding the CNGA3 and CNGB3 subunits of the cyclic nucleotide-gated (CNG) channel of cone photoreceptors have been associated with autosomal recessive achromatopsia. Here we analyze the molecular basis of achromatopsia in two siblings with residual cone function. Psychophysical and electroretinographic analyses show that the light sensitivity of the cone system is lowered, and the signal transfer from cones to secondary neurons is perturbed. Both siblings carry two mutant CNGA3 alleles that give rise to channel subunits with different single-amino acid substitutions. Heterologous expression revealed that only one mutant forms functional channels, albeit with grossly altered properties, including changes in Ca2+ blockage and permeation. Surprisingly, coexpression of this mutant subunit with CNGB3 rescues the channel phenotype, except for the Ca2+ interaction. We argue that these alterations are responsible for the perturbations in light sensitivity and synaptic transmission.

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.

Texture-orientation mechanisms pool colour and luminance contrast

Do texture-sensitive mechanisms operate separately on, or pool, luminance and colour contrast information? We addressed this question by measuring threshold-versus-amplitude functions for orientation-modulated (OM) gratings comprised of gabor elements defined by either colour or luminance contrast. In both the uncrossed (all elements in test and mask defined by either colour or luminance contrast) and crossed (equal mixtures of luminance and colour contrast in both test and mask) conditions, evidence of sub-threshold facilitation between test and mask was obtained. The sub-threshold facilitation in the crossed condition could not be accounted for by luminance artifacts in the ostensibly isoluminant gabors. The results are consistent with a single visual mechanism sensitive to OM textures that pools information from both the luminance and chromatic post-receptoral mechanisms.

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.

Chromatic contrast sensitivity: the role of absolute threshold and gain constant in differences between the fovea and the periphery

A model of foveal achromatic and chromatic sensitivity [Vision Res. 36, 1597 (1996)] was extended to the peripheral visual field. Threshold-versus-illuminance functions were analyzed to determine effects of eccentricity on absolute thresholds and gain constants of chromatic and luminance mechanisms. The resulting peripheral model successfully predicted peripheral contrast sensitivity as a function of wavelength, for both white and 500-nm backgrounds. We conclude that the short-wavelength-sensitive cone opponent mechanism may mediate thresholds in Sloan's notch in the normal periphery and that interpretation of reduced chromatic sensitivity in the periphery requires an explicit model of how eccentricity affects both the gain constant and the absolute threshold.

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.

Forced-choice staircases with fixed step sizes: asymptotic and small-sample properties

Visual detection and discrimination thresholds are often measured using adaptive staircases, and most studies use transformed (or weighted) up/down methods with fixed step sizes--in the spirit of Wetherill and Levitt (Br J Mathemat Statist Psychol 1965;18:1-10) or Kaernbach (Percept Psychophys 1991;49:227-229)--instead of changing step size at each trial in accordance with best-placement rules--in the spirit of Watson and Pelli (Percept Psychophys 1983;47:87-91). It is generally assumed that a fixed-step-size (FSS) staircase converges on the stimulus level at which a correct response occurs with the probabilities derived by Wetherill and Levitt or Kaernbach, but this has never been proved rigorously. This work used simulation techniques to determine the asymptotic and small-sample convergence of FSS staircases as a function of such parameters as the up/down rule, the size of the steps up or down, the starting stimulus level, or the spread of the psychometric function. The results showed that the asymptotic convergence of FSS staircases depends much more on the sizes of the steps than it does on the up/down rule. Yet, if the size delta+ of a step up differs from the size delta- of a step down in a way that the ratio delta-/delta+ is constant at a specific value that changes with up/down rule, then convergence percent-correct is unaffected by the absolute sizes of the steps. For use with the popular one-, two-, three- and four-down/one-up rules, these ratios must respectively be set at 0.2845, 0.5488, 0.7393 and 0.8415, rendering staircases that converge on the 77.85%-, 80.35%-, 83.15%- and 85.84%-correct points. Wetherill and Levitt's transformed up/down rules--which require delta-/delta+ = 1--and the general version of Kaernbach's weighted up/down rule--which allows any delta-/delta+ ratio--fail to reach their presumed targets. The small-sample study showed that, even with the optimal settings, short FSS staircases (up to 20 reversals in length) are subject to some bias, and their precision is less than reasonable, but their characteristics improve when the size delta+ of a step up is larger than half the spread of the psychometric function. Practical recommendations are given for the design of efficient and trustworthy FSS staircases.

Receptor noise as a determinant of colour thresholds

Inferences about mechanisms at one particular stage of a visual pathway may be made from psychophysical thresholds only if the noise at the stage in question dominates that in the others. Spectral sensitivities, measured under bright conditions, for di-, tri-, and tetrachromatic eyes from a range of animals can be modelled by assuming that thresholds are set by colour opponency mechanisms whose performance is limited by photoreceptor noise, the achromatic signal being disregarded. Noise in the opponency channels themselves is therefore not statistically independent, and it is not possible to infer anything more about the channels from psychophysical thresholds. As well as giving insight into mechanisms of vision, the model predicts the performance of colour vision in animals where physiological and anatomical data on the eye are available, but there are no direct measurements of perceptual thresholds. The model, therefore, is widely applicable to comparative studies of eye design and visual ecology.

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.