Chromatic and luminance losses with multiple sclerosis and optic neuritis measured using dynamic random luminance contrast noise

We measured thresholds for detecting changes in colour and in luminance contrast in observers with multiple sclerosis (MS) and/or optic neuritis (ON) to determine whether reduced sensitivity occurs principally in red-green or blue-yellow second-stage chromatic channels or in an achromatic channel. Colour thresholds for the observers with MS/ON were higher in the red-green direction than in the blue-yellow direction, indicating greater levels of red-green loss than blue-yellow loss. Achromatic thresholds were raised less than either red-green or blue-yellow thresholds, showing less luminance-contrast loss than chromatic loss. With the MS/ON observers, blue-yellow and red-green thresholds were positively correlated but increasing impairment was associated with more rapid changes in red-green thresholds than blue-yellow thresholds. These findings indicate that demyelinating disease selectively reduces sensitivity to colour vision over luminance vision and red-green colours over blue-yellow colours.     

A clinical test to estimate chromatic thresholdsa

A quick and relatively inexpensive colour vision test is described which estimates chromatic thresholds for representative hues of the Munsell colour space. Results suggest that the test is not as sensitive as several pseudo-isochromatic plate tests for colour vision screening. However, the test results correlate well with the anomaloscope range which indicates that the test is useful in estimating the severity of a red-green defect. Results from the screening trial also showed that there is a substantial learning or attention artefact for colour-normal observers. Approximately 36 per cent of the colour-normal subjects made at least one mistake on the test, whereas only nine per cent repeated at least one of their errors. Because these errors made by colour-normals were age-related and were clustered near the blue-yellow axis, detection of subtle blue-yellow defects in older patients will be difficult.     

Recovery of vision and pupil responses in optic neuritis and multiple sclerosis

The recovery of visual performance and pupil responses were investigated in patients with demyelinating optic neuritis (ON) and multiple sclerosis (MS). The pupil constriction amplitude and the time delay (latency) of the pupil response were measured in 14 patients with a history of unilateral ON in response to either achromatic (luminance) or chromatic (isoluminant) stimulus modulation. Five of these subjects were diagnosed later with MS. In addition, we measured detection thresholds for achromatic stimuli using standard visual field perimetry and chromatic thresholds using a new colour assessment and diagnosis (CAD) test that isolates the use of colour signals. The results show that, despite significant improvements in visual function following the acute phase (as assessed using visual acuity and fields), significant pupil response deficits remain. The findings also demonstrate that accurate measurements of pupil responses and chromatic thresholds can reveal deficits that remain undetected with more conventional techniques. These preliminary findings suggest that the techniques described here can provide useful information about remitting and relapsing demyelinative phases, often observed during MS and ON.     

Orientation selectivity in luminance and color vision assessed using 2-d band-pass filtered spatial noise

We evaluated orientation discrimination in color and luminance vision using an external noise paradigm. Stimuli were spatiotemporal patches of 2D orientation noise isolating the achromatic, red-green and blue-yellow mechanisms, and matched in multiples of contrast detection threshold. We found a monotonic increase of orientation discrimination thresholds with the stimuli orientation bandwidths that is similar for both color and luminance contrasts. This dependence was fitted with two suitable models. A variance summation model suggests that internal orientation noise is significantly greater for the chromatic than for the achromatic mechanisms, while the efficiencies are similar. A gain control model of orientation tuning suggests that both chromatic and achromatic mechanisms are characterized by broadly tuned orientation detectors and that the relative chromatic deficit in orientation discrimination may only result from a slightly broader orientation tuning for the chromatic mechanisms. The moderate deficiency in chromatic orientation discrimination may account for the small differences found in shape perception between color and luminance vision.     

Chromatic discrimination losses in multiple sclerosis patients with and without optic neuritis using the Cambridge Colour Test

We assessed chromatic discrimination in multiple sclerosis (MS) patients both with (ON) and without (no ON) a history of optic neuritis using the Cambridge color test (CCT). Our goal was to determine the magnitude and chromatic axes of any color vision losses in both patient groups, and to evaluate age-related changes in chromatic discrimination in both patient groups compared to normals. Using the CCT, we measured chromatic discrimination along the protan, deutan and tritan axes in 35 patients with MS (17 ON eyes) and 74 age matched controls. Color thresholds for both patient groups were significantly higher than controls' along the protan and tritan axes (p < 0.001). In addition, the ON and no-ON groups differed significantly along all three-color axes (p < 0.001). MS patients presented a progressive color discrimination impairment with age (along the deutan and tritan axes) that was almost two times faster than controls, even in the absence of ON. These findings suggest that demyelinating diseases reduce sensitivity to color vision in both red-green and blue-yellow axes, implying impairment in both parvocellular and koniocellular visual pathways. The CCT is a useful tool to help characterize vision losses in MS, and the relationship between these losses and degree of optic nerve involvement.     

Comparison of color and luminance vision on a global shape discrimination task

We compared the performances of the blue-yellow, red-green and luminance systems on a shape discrimination task. Stimuli were radial frequency patterns (radially modulated fourth derivative of a Gaussian) with a peak spatial frequency of 0.75 cpd. Stimuli isolated the chromatic (red-green and blue-yellow) and achromatic post-receptoral mechanisms. We showed that in all cases performance, measured as a radial modulation threshold for discrimination between a circular and non-circular stimulus, improves with contrast. Performance was compared across radial frequencies with contrast matched in multiples of stimulus detection threshold. We find that blue-yellow color system performs the worse on this shape discrimination task, followed by the red-green, with the achromatic system performing best. The average difference is a factor of 2 between achromatic and blue-yellow performance, and a factor of 1.7 between red-green and achromatic. Despite these performance losses, chromatic shape discrimination can still reach hyperacuity performance levels. In a secondary experiment we contrast modulate the radial contour to eliminate either the "corners" or "sides" of an RF4 (square) pattern. We find that for the achromatic system, the sides are more important for the task than the corners. However, for the chromatic system, removal of sides or corners produces similar performance deficits. We conclude that color vision has a selective although relatively mild deficit for two-dimensional form perception.     

Motion Coherence Across Different Chromatic Axes

It has been reported that equiluminant plaid patterns constructed from component gratings modulated along different axes of a cardinal colour space fail to create a coherent impression of two-dimensional motion Krauskopf and Farell (1990). Nature, 348, 328–331. In this paper we assess whether this lack of interaction between cardinal axes is a general finding or is instead dependent upon specific stimulus parameters. Type I and Type II plaids were made from sinusoidal components (1 cpd) each modulated along axes in a cardinal colour space and presented at equivalent perceived contrasts. The spatial angular difference between the two components was varied from 5 to 90 deg whilst keeping the Intersection of Constraints (I.O.C.) solution of the pattern constant. Observers were required to indicate the perceived direction of motion of the pattern in a single interval direction-identification task. We find that: (i) When plaids were made from components modulated along the same cardinal axis, coherent “pattern” motion was perceived at all angular differences. As the angular difference between the components decreased in a Type II plaid, the perceived direction of motion moved closer to the I.O.C. solution and away from that predicted by the vector sum. (ii) A plaid made from components modulated along red-green and blue-yellow cardinal axes (cross-cardinal axis) did not cohere at high angular differences (>30 deg) but had a perceived direction of the fastest moving component. At lower angular differences, however, pattern motion was detected and approached the I.O.C. solution in much the same way as a same-cardinal axis Type II plaid. (iii) A plaid made from a luminance grating and a cardinal chromatic grating (red-green or blue-yellow) failed to cohere under all conditions, demonstrating that there is no interaction between luminance and chromatic cardinal axes. These results indicate that there are conditions under which red-green and blue-yellow cardinal components interact for the purposes of motion detection. Copyright © 1996 Elsevier Science Ltd.     

New method based on random luminance masking for measuring isochromatic zones using high resolution colour displays

A new method of measuring normal hue discrimination ellipses and dichromatic zones using a high resolution colour monitor is described. The test involves the detection of chromatic bars on a grey background (x = 0.305, y = 0.323) having a luminance of 34 cd m-2. Elements of the background matrix of square checks are varied randomly in luminance in space and time to provide random luminance masking (RLM) which compensates for differences in the relative luminous efficiency of different observers. The measurement technique provides a rapid and comprehensive colour vision test. Typical results are presented for normal trichromats, protanopes and deuteranopes without RLM and with the RLM set of 25%. The size of the discrimination ellipse in normal observers is the same in both viewing conditions, but the use of the RLM technique reveals the extent of the isochromatic zones in colour deficient observers.     

Evidence for mild blue-yellow colour vision deficits immediately following fluorescein angiography.

AIMS: We have investigated the short term effects of fluorescein angiography on the blue-yellow, red-green, and luminance contrast sensitivity of patients with early age-related macular degeneration (ARMD). METHODS: Nine ARMD patients with no exudative complications and a visual acuity of 20/60 or better in the tested eye were selected. Cardinal colour directions for the isolation of the red-green, blue-yellow and achromatic (luminance) visual mechanisms were determined for each patient. Contrast sensitivity was measured in each cardinal colour direction immediately before and 20 min after standard 20-flash fluorescein angiography. RESULTS: A significant, albeit mild, reduction for blue-yellow contrast sensitivity following angiography was observed (ANOVA, alpha = 0.05). Fluorescein angiographic exposure had no significant effect on red-green or luminance contrast sensitivity. CONCLUSION: Our results show that fluorescein angiography causes at least a short term deficit selective to blue-yellow contrast sensitivity in our patient group.     

Chromatic, spatial, and temporal losses of sensitivity in multiple sclerosis

Chromatic, spatial, and temporal losses of sensitivity were measured in 15 eyes of 10 patients with recovered optic neuritis. Chromatic sensitivities (for both red-green and blue-yellow) were measured using color-mixture thresholds; the chromatic sensitivity loss was classified as "selective" if it was significantly greater than the achromatic loss. Spatial and temporal sensitivities were measured with contrast sensitivity functions and flicker modulation sensitivity, respectively; these losses were classified as selective if the losses at high (spatial or temporal) frequencies were significantly greater (or significantly less) than losses at low frequencies. All patients had central fixation and were optically corrected carefully. In 1 eye, selective losses of sensitivity for red-green and blue-yellow were combined with a selective loss of sensitivity at high spatial (but not temporal) frequencies. This type of loss may indicate a selective loss of small axons in the optic nerve. The 8 other eyes that showed significant losses were generally nonselective in their chromatic, spatial, and temporal losses; this may indicate a nonselective loss of small and large axons.     

Luminance mechanisms mediate the motion of red-green isoluminant gratings: the role of "temporal chromatic aberration"

In this paper we use a dynamic noise-masking paradigm to explore the nature of the mechanisms mediating the motion perception of drifting isoluminant red-green gratings. We compare contrast thresholds for the detection and direction discrimination of drifting gratings (1.5 cpd), over a range of temporal frequencies (0.5-9 Hz) in the presence of variable luminance or chromatic noise. In the first experiment, we used dynamic luminance noise to show that direction thresholds for red-green grating motion are masked by luminance noise over the entire temporal range tested, whereas detection thresholds are unaffected. This result indicates that the motion of nominally isoluminant red-green gratings is mediated by luminance signals. We suggest that stimulus-based luminance artifacts are not responsible for this effect because there is no masking of the detection thresholds. Instead we propose that chromatic motion thresholds for red-green isoluminant gratings are mediated by dynamic luminance artifacts that have an internal, physiological origin. We have termed these "temporal chromatic aberration". In the second experiment, we used dynamic chromatic noise masking to test for a chromatic contribution to red-green grating motion. We were unable to find conclusive evidence for a contribution of chromatic mechanisms to the chromatic grating motion, although a contribution at very high chromatic contrasts cannot be ruled out. Our results add to a growing body of evidence indicating the presence of dynamic, internal luminance artifacts in the motion of chromatic stimuli and we show that these occur even at very low temporal rates. Our results are compatible with our previous work indicating the absence of a chromatic mechanism for first order (quasi-linear) apparent motion [Vision Res. 40 (2000) 1993]. We conclude that previous conclusions based on the motion of chromatic red-green gratings should be reassessed to determine the contribution of dynamic luminance artifacts.     

A single mechanism for both luminance and chromatic grating vernier tasks: evidence from temporal summation

Vernier thresholds are determined by luminance rather than chromatic contrast when both are present in vernier targets. The role of luminance and chromatic mechanisms in vernier performance under equiluminant conditions remains uncertain. Temporal summation functions for vernier thresholds with luminance and red-green equiluminant gratings were compared to those for detection thresholds with similar stimuli. Vernier thresholds showed similar temporal summation for luminance and chromatic gratings, which is consistent with a single mechanism underlying vernier performance in the two conditions. However, detection thresholds showed a shorter temporal summation duration for luminance gratings than for chromatic gratings, which suggests that two different mechanisms underlie detection thresholds. Analysis of physiological data supports the hypothesis that the frequency-doubled response of ganglion cells in the magnocellular pathway can provide accurate spatiotemporal information for vernier performance at equiluminance.     

Evoked potentials specific to spatial patterns of luminance and colour

The human brain generated electrical responses (EPs) to changes in the chromatic contrast of an equiluminant spatially-patterned stimulus even when the eye's longitudinal and transverse chromatic aberrations were simultaneously cancelled. Pattern EPs were strongly affected by ocular chromatic aberration. EP and subjective estimates of longitudinal chromatic aberration agreed closely. Psychophysical contrast threshold correlated with the amplitudes of EPs elicited both by monochromatic patterns of checks and by equiluminant patterns of alternately-coloured checks or bars.Restricting stimulation to the fovea may enhance the effect of intersubject variations of neuroanatomy upon EPs.Physiological signals seem to be segregated into colour channels until the first responses to contrast. The EP data suggest that there is an antagonistically-organized chromati-contrast channel in parallel with colour-coded luminance-contrast channels. EP results suggest that the human visual system handles colour information differently when colour is linked with spatial form than when colour is not linked with form.     

The chromatic selectivity of visual crowding

Purpose:  Precortical vision is mediated by three opponent mechanisms which linearly combine receptoral outputs to form a luminance channel (L + M) and two chromatic channels, red-green (L/M) and blue-yellow (S/L + M). These colour channels are known to undergo considerable cortical reorganisation in order to form multiple, 'higher-order' mechanisms tuned to a variety of axes in colour space. Here we ask the extent to which the basic colour-opponent mechanisms interact in the phenomenon of visual crowding, where nearby targets interfere with the spatial processing of a central test target.
Methods:  The task was to identify the orientation of a central Gabor patch whilst an annular plaid was positioned around the patch. The radius of the annulus was varied in order to produce different amounts of crowding. The chromatic content of the central patch and the annulus could be varied independently along the (L + M), (L/M) and (S/L + M) cardinal axes.
Results:  For all targets, when the target and crowding annulus were of the same chromaticity, the crowding effect increased with decreasing separation of the target and annulus. When the test and crowding stimuli had different chromatic properties, very little crowding was observed, even at the minimum separation of test target and crowding annulus.
Conclusions:  The crowding effect appears to be critically dependent on whether or not the target and crowding stimuli share a common chromatic axis. These results suggest a method by which performance in crowding-limited tasks might be enhanced through appropriate selection of chromatic stimulus content.     

Spatial frequency tuned covariance channels for red-green and luminance-modulated gratings: psychophysical data from human infants

This study concerns the spatial-frequency-tuned channels underlying infants' contrast sensitivity functions (CSFs) for red-green chromatic stimuli, and their relationship to the channels underlying infants' CSFs for luminance-modulated stimuli. Behavioral (forced-choice preferential-looking) techniques and stationary stimuli were used. In experiment 1. contrast thresholds were measured in 4- and 6-month-olds, using isoluminant red-green gratings with spatial frequencies ranging from 0.27 to 1.53 c deg. In experiment 2. contrast thresholds were measured in 4-month-olds. using both red-green and luminance-modulated gratings in the same low spatial frequency range. Covariance analyses of individual differences were performed. Experiment 1 revealed one dominant covariance channel for the detection of red-green gratings, with a second channel contributing to detection of the highest spatial frequencies used. Experiment 2 revealed two to three channels serving color and luminance: but surprisingly these channels were not statistically separable for luminance versus chromatic stimuli. Thus, covariance channels for color and luminance that are independent for adults [Peterzell & Teller (2000). Spatial frequency tuned covariance channels for red-green and luminance-modulated gratings: psychophysical data from human adults. Vision Research, 40, 417-430] are apparently interdependent in infants. These data suggest that for infants, detection thresholds for chromatic and luminance-modulated stimuli may be limited by common mechanisms.     

Visual Ageing: Unspecific Decline of the Responses to Luminance and Colour

We have investigated whether ageing affects selectively the responses to equiluminant patterns of pure colour contrast. In two groups of subjects (mean ages 29 and 72 yr) contrast thresholds were measured psychophysically for the detection and for the discrimination of the direction of motion of drifting gratings. The gratings were modulated either in pure luminance contrast (and uniform colour), or pure chromatic contrast (red-green equiluminant gratings). In subjects of the same age groups, visual evoked potentials (VEP) were recorded in response to gratings with either pure luminance contrast or pure colour contrast sinusoidally reversed in contrast at various temporal frequencies. It was shown that psychophysical contrast sensitivity for equiluminant patterns deteriorates significantly with age, and VEP latency increases. However, these effects of ageing on the responses to patterns of pure colour contrast are substantially the same as those observed in the same subjects for stimuli with pure luminance contrast. The results suggest that ageing causes a small and unspecific decline of the response of the visual system to luminance and colour contrast. Copyright © 1996 Elsevier Science Ltd.     

Temporal phase response of the short-wave cone signal for color and luminance

A chromatic discrimination paradigm was used to measure the temporal phase of the S (short-wave cone) signal relative to the L--M (long-wave cone minus middle-wave cone) signal. Suprathreshold equiluminant red-green flicker that stimulates the L--M mechanism was presented on a steady, intense yellow-green adapting field. Violet flicker that stimulates the S cones was added to the red-green flicker at different temporal phase angles, and the violet modulation depth was varied to achieve a chromatic discrimination threshold. A template was fitted to the data relating thresholds to phase: the location of the template symmetry axis showed that the S signal lagged L--M by about 75-90 degrees at 10 Hz. This is about one half the phase lag obtained for luminance or motion discrimination. The phase discrepancy shows that there are separate luminance and chromatic mechanisms receiving S cone inputs. The hue of the flicker in the present study varied strongly with phase angle, with the positive and negative excursions of the S cone signal producing a reddish-blue and greenish-yellow, respectively, and these colors combined with the reddish and greenish hues produced by the L--M signal. The observed phase shift, and measured color appearance of the combined flicker, account for the colors seen on a radially segmented disk of Munsell hues when rotated: the colors differ strikingly depending on the direction of rotation.     

The spatial tuning of color and luminance peripheral vision measured with notch filtered noise masking.

We have measured the spatial bandwidths of the bandpass red-green chromatic and luminance mechanisms at four locations in the nasal visual field (0, 10, 20 and 30 degrees) using a method of notch filtered noise masking which effectively removes the artifact of off-frequency looking for our stimuli. Detection thresholds were measured for luminance or isoluminant red-green Gaussian enveloped test gratings of 0.5 cpd embedded in 1/f noise. Firstly, thresholds were obtained as a function of increasing noise spectral density and were fitted using a standard noise masking model. These results support the existence across the visual field of independent, red-green chromatic and luminance mechanisms with similar sampling efficiencies. Secondly, we measured thresholds in notch filtered noise as a function of notch width and derived the spatial bandwidth of the detection mechanism. We find both color and luminance mechanisms have similar bandwidths which remain virtually constant across eccentricity. These results indicate strong overall similarities between the early processing of color and luminance vision, and lend support to the role of color as an 'intrinsic image' in spatial vision. The results are discussed in the light of the anchored channel and shifting channel models of peripheral contrast sensitivity and pattern detection.     

Colour vision as a post-receptoral specialization of the central visual field

The experiments address the question whether there is evidence that the central visual field is any more specialized for colour than it is for luminance contrast detection. The decline in contrast sensitivity across the visual field for colour-only (red-green) gratings is compared to that for monochromatic luminance gratings at a range of spatial frequencies in the nasal and temporal fields. Measurements are made of the chromatic spatial summation area and the relevant parts of the chromatic temporal contrast sensitivity function at different eccentricities in order to control for their influence on the decline in contrast sensitivity. Results show that at each spatial frequency colour contrast sensitivity declines with eccentricity approximately twice as steeply as luminance contrast sensitivity. The more rapid decline in colour contrast sensitivity than luminance contrast sensitivity across the visual field reveals that chromatic mechanisms are more confined than luminance mechanisms to the central field.     

When S-cones contribute to chromatic global motion processing

There is common consensus now that color-defined motion can be perceived by the human visual system. For global motion integration tasks based on isoluminant random dot kinematograms conflicting evidence exists, whether observers can (Ruppertsberg et al., 2003) or cannot (Bilodeau & Faubert, 1999) extract a common motion direction for stimuli modulated along the isoluminant red-green axis. Here we report conditions, in which S-cones contribute to chromatic global motion processing. When the display included extra-foveal regions, the individual elements were large ( approximately 0.3 degrees ) and the displacement was large ( approximately 1 degrees ), stimuli modulated along the yellowish-violet axis proved to be effective in a global motion task. The color contrast thresholds for detection for both color axes were well below the contrasts required for global motion integration, and therefore the discrimination-to-detection ratio was >1. We conclude that there is significant S-cone input to chromatic global motion processing and the extraction of global motion is not mediated by the same mechanism as simple detection. Whether the koniocellular or the magnocellular pathway is involved in transmitting S-cone signals is a topic of current debate (Chatterjee & Callaway, 2002).