Achromatic Test Research Articles

Visual perception alterations in COVID-19: a preliminary study.

To compare the visual perception (color and chromatic-achromatic contrast vision) of a small cohort of COVID-19 patients at the time of infection and after 6mo with that of a healthy population matched for sex and age. A total of 25 patients (9 females, 16 males, mean age: 54±10y) with COVID-19 hospitalized in the COVID-19 Unit of the University Clinical Hospital of Valladolid were recruited for this preliminary study. Visual perception, as determined by monocular measurement of contrast sensitivity function (CSF) and color vision was assessed in each patient using the Optopad test. The results obtained were then compared with those of a sample of 16 age- and sex-matched healthy controls (5 females, 11 males, mean age: 50±6y) in which the same measurement procedure was repeated. Statistically significant differences between groups were assessed using the Mann-Whitney U test. Measurements were repeated after a minimum follow-up period of 6mo and statistically significant differences between the two time points in each group were assessed using the Wilcoxon signed rank test. Discrimination thresholds (color and chromatic-achromatic contrast vision) and their corresponding sensitivity, calculated as the inverse of the discrimination threshold, were evaluated. Analysis of the data revealed higher contrast threshold results (i.e., worse contrast sensitivity) in the COVID-19 group than in the control group for all spatial frequencies studied in the Optopad-CSF achromatic test and most of the spatial frequencies studied in the Optopad-CSF chromatic test for the red-green and blue-yellow mechanisms. In addition, color threshold results in the COVID-19 group were also significantly higher (i.e., worse color sensitivity) for almost all color mechanisms studied in the Optopad-Color test. At 6mo, most of the differences found between the groups were maintained despite COVID-19 recovery. The present results provide preliminary evidence that visual perception may be impaired in COVID-19, even when the infection has passed. Although further research is needed to determine the precise causes of this finding, analysis of CSF and color vision could provide valuable information on the visual impact of COVID-19.

The effect of blue light filtering lenses on speed perception

Blue-light filtering lenses (BFLs) are marketed to protect the eyes from blue light that may be hazardous to the visual system. Because BFLs attenuate light, they reduce object contrast, which may impact visual behaviours such as the perception of object speed which reduces with contrast. In the present study, we investigated whether speed perception is affected by BFLs. Using a two-interval forced-choice procedure in conjunction with Method of Constant Stimuli, participants (n = 20) judged whether the perceived speed of a moving test stimulus (1.5–4.5°/s) viewed through a BFL was faster than a reference stimulus (2.75°/s) viewed through a clear lens. This procedure was repeated for 3 different BFL brands and chromatic and achromatic stimuli. Psychometric function fits provided an estimate of the speed at which both test and reference stimuli were matched. We find that the perceived speed of both chromatic and achromatic test stimuli was reduced by 6 to 20% when viewed through BFLs, and lenses that attenuated the most blue-light produced the largest reductions in perceived speed. Our findings indicate that BFLs whilst may reduce exposure to hazardous blue light, have unintended consequences to important visual behaviours such as motion perception.

Appearance of Achromatic Colors Under Optimized Light Source Spectrum

Light source spectrum can be optimized for object reflectance to reduce the energy consumption by reducing the amount of light absorbed by surfaces. While the feasibility of this approach to architectural lighting has been demonstrated, this concept has only been tested with objects of highly saturated colors. Here, the color appearance of 24 Macbeth ColorChecker test samples, which includes achromatic surfaces (i.e., gray, black, and white), illuminated by optimized theoretical test spectra and reference light sources (incandescent, equal-energy radiator, and phosphor-coated white LED) were calculated using a color space based on the International Commission on Illumination's (CIE) color appearance model, CIECAM02-UCS. Results show that energy consumption could be reduced to between 39% and 90%, with small shifts in color appearance, when considering all object colors. When lighting is optimized for achromatic test samples, energy consumption and light absorption were reduced more than when optimized for colorful surfaces, with imperceptible shifts in color appearance.

Color responses and their adaptation in human superior colliculus and lateral geniculate nucleus

We use an fMRI adaptation paradigm to explore the selectivity of human responses in the lateral geniculate nucleus (LGN) and superior colliculus (SC) to red–green color and achromatic contrast. We measured responses to red–green (RG) and achromatic (ACH) high contrast sinewave counter-phasing rings with and without adaptation, within a block design. The signal for the RG test stimulus was reduced following both RG and ACH adaptation, whereas the signal for the ACH test was unaffected by either adaptor. These results provide compelling evidence that the human LGN and SC have significant capacity for color adaptation. Since in the LGN red–green responses are mediated by P cells, these findings are in contrast to earlier neurophysiological data from non-human primates that have shown weak or no contrast adaptation in the P pathway. Cross-adaptation of the red–green color response by achromatic contrast suggests unselective response adaptation and points to a dual role for P cells in responding to both color and achromatic contrast. We further show that subcortical adaptation is not restricted to the geniculostriate system, but is also present in the superior colliculus (SC), an oculomotor region that until recently, has been thought to be color-blind. Our data show that the human SC not only responds to red–green color contrast, but like the LGN, shows reliable but unselective adaptation.

A McCollough Effect Generated at Binocular Site

Following exposures to alternating gratings with unique combination of orientation and colors, an achromatic grating would appear tinted with its perceived color contingent on the grating's orientation. This orientation-contingent color after effect is called the McCollough effect. The lack of interocular transfer of the McCollough effect suggests that the McCollough effect is primarily established in monocular channels. Here we explored the possibility that the McCollough effect can be induced at a binocular site. During adaptation, a red vertical grating and a green horizontal grating are dichoptically presented to the two eyes. In the ‘binocular rivalry’ condition, these two gratings were constantly presented throughout the adaptation duration and subjects experienced the rivalry between the two gratings. In the ‘physical alternation’ condition, the two dichoptic gratings physically alternated during adaptation, perceptually similar to binocular rivalry. Interestingly, following dichoptic adaptation either in the rivalry condition or in the physical alternation condition, a binocularly viewed achromatic test grating appeared colored depending on its orientation: a vertical grating appeared greenish and a horizontal grating pinkish. In other words, we observed a McCollough effect following dichoptic adaptation, which can only be explained by a binocular site of orientation-contingent color adaptation.

Scanning laser polarimetry of the retinal nerve fiber layer in perimetrically unaffected eyes of glaucoma patients

To compare scanning laser polarimetry (SLP) measurements of retinal nerve fiber layer (RNFL) thickness in perimetrically unaffected eyes of glaucoma patients with those in their fellow eyes with field loss and eyes of healthy subjects. Observational case-control study. Twenty-three glaucoma patients with a reproducible visual field (VF) defect in one eye (mean mean deviation [MD], -5.71 decibels [dB]) and a normal VF in the other one (i.e., < or =1 VF test point below the 5% probability level [mean MD, -0.01 dB]) and 73 control eyes of as many age-matched healthy subjects (mean MD, 0.39 dB). The MDs and pattern standard deviations of the glaucoma patients' eyes with normal VFs and the control eyes did not statistically significantly differ (independent samples t test, P = 0.15 and P = 0.61, respectively). All subjects were measured in both eyes with the GDxVCC, a commercially available instrument featuring SLP with automated variable corneal compensation. Standard automated perimetry was assessed by means of the Humphrey Field Analyzer (24-2 Full Threshold or Swedish interactive threshold algorithm Standard achromatic test program). The standard GDxVCC parameters TSNIT (temporal, superior, nasal, inferior, temporal) Average, Superior Average, Inferior Average, TSNIT Std. Dev., and Nerve Fiber Indicator (NFI) were determined. We also assessed the thickness values in 6 parapapillary sectors. In addition, we calculated the proportion of eyes per group with an NFI of > or =40. GDxVCC measurements showed more RNFL thinning in the perimetrically unaffected eyes of glaucoma patients than in the healthy control eyes. The RNFL in the perimetrically unaffected eyes of glaucoma patients was thicker than that in their fellow eyes with field loss. The NFI had a value of > or =40 in 11 of 23 (47.8%) perimetrically unaffected eyes of glaucoma patients, 19 of 23 (82.6%) eyes with VF loss of glaucoma patients, and 3 of 73 (4.1%) healthy control eyes. With the GDxVCC, thinning of the RNFL may be detected in perimetrically unaffected eyes of glaucoma patients with field loss in their fellow eyes.

Color appearance seen through a colored filter of various sizes

It will provide us an effective method to study the color perception of the elderly if we can employ for young subjects a pair of glasses with color property chosen to simulate that of the elderly. One problem has to be solved before using such glasses based on the concept of the recognized visual space of illumination, that is, the size of the glasses. When a piece of a white paper is seen through a yellow filter in a room, it naturally appears yellowish. But if the observer uses the filter to cover his/her eyes entirely the white paper returns to its original white; this is known as color constancy. Between these two extreme cases, the color is expected to change from yellow to white. Chromaticness and hue of an achromatic test chart of a small or large size were determined by the elementary color naming method when the chart was seen monocularly through a red, yellow, green or blue filter, respectively, placed at various distances from the eyes. Chromaticness was large at around 40% when the visual field through the filter was equal to or smaller than the test chart, but it rapidly decreased when the visual field extended over the test chart so that objects other than the test chart were included in the visual field. It decreased to almost zero when the filter was about 5 cm or less away from the eye. It was concluded that we do not necessarily have to use goggles to cover the entire visual field but can use a normal pair of glasses made of colored filters for young subjects to study the color perception of the elderly with understanding that the study is only for the color perception experienced while wearing the glasses.

Is the McCollough Effect Coded in Disparity-Sensitive Units?

Previous studies have demonstrated that the McCullough effect (ME) is coded by both binocular and monocular units [see for example Vidyasagar, 1976 Nature (London)261 39]. At ECVP95, McLoughlin and Savoy (1995 Perception24 Supplement, 94) presented a contiguity-learning based model capable of accounting for a large dataset of both binocular and monocular ME variants. One issue not directly addressed by this model concerns the site of binocular ME adaptation. Specifically, are the binocular ME ‘units’ disparity selective? The methods used were developed from those of McLoughlin and Savoy. Subjects were exposed to alternating coloured gratings at various crossed and uncrossed disparities (relative to fixation points). A colour cancellation paradigm was used to record the strength of the ME for a series of binocularly disparate composite achromatic test gratings both before and after adaptation. These test gratings had a small random amount of colour added to them so that the subjects could not assume that the patterns were achromatic before making adjustments. Subjects waited 5 min in the dark after adaptation before being tested. This removes any possible confounds associated with retinal afterimages. Preliminary results suggest, in agreement with previous studies, that the main component of the ME is monocular. However, the binocular component appears to be sensitive to disparity. A second series of experiments wherein two opponent MEs were induced, one at crossed and the other at uncrossed disparities, supports this finding. As in the Vidyasagar variant, the magnitude of the ME is extremely weak in this case.

The relation of lightness and stereoscopic depth in a simple viewing situation

The effect of stereoscopic depth on perceived lightness was studied using a simple, achromatic stimulus arrangement. In Experiment 1, depth/lightness interactions were sought between a single test field and a single induction field. In Experiment 2, depth/lightness interactions were looked for between a single test field and two induction fields. Stimuli were presented on a computer screen and viewed with a stereoscope. The subjects reported perceived lightness of the achromatic test field by rating its apparent blackness along a dimension of 0%-100%. In Experiment 1, they reported lightness judgments of the test field across 13 perceived depth levels and 8 contrast levels. In Experiment 2, they gave lightness judgments of the test field across 7 perceived depth levels and 16 contrast levels. We were particularly interested in observing the generality of Gilchrist's coplanar ratio hypothesis. The results showed that when stereopsis and contrast levels are the available cues, depth and lightness percepts are independent, and it is retinal ratios, not coplanar ratios, that dictate lightness perception. We conclude that before the relative depth location of an object is determined, its lightness value is known through sensory-level processes.

Artifacts in fusion and stereopsis testing based on red/green dichoptic image separation.

A red/green anaglyph stereotest (TNO) was administered to a group of patients who had anisometropic amblyopia, and the Worth 4-Dot test for fusion to two groups of strabismus patients. Both tests were administered twice, with the red/green lens position of the glasses reversed between eyes for the second administration. Twelve of 15 patients with anisometropic amblyopia exhibited a stereo acuity difference of 2:1 or more, and 18 of 89 strabismic patients changed Worth 4-Dot fusion or suppression status, between the two positions of the glasses. A group of seven strabismic patients who showed the reversal effect on the Worth 4-Dot test had the results of one glass' position discrepant with those of a geometrically identical achromatic test. It appears that the red/green format can introduce artifacts in binocular vision testing. Testing twice, with the red/green glasses reversed between trials, will prevent misinterpretation of binocular status and may provide diagnostically useful information.

McCollough aftereffects in strabismus and amblyopia

In strabismic people, the functioning of the deviating eye is impaired. This impairment is binocular in origin since it depends on stimulation of the straight eye; sensitivity and acuity of the deviating eye are usually better when the straight eye is patched than when both eyes are viewing. We investigated interactions between the eyes in strabismic people by using the McCollough color aftereffect as an exploratory procedure. Both normal and strabismic people adapted to black and colored bars and then examined black and white test bars. They reported colors in the achromatic test bars that were complementary to the adapting colors; this is the McCollough aftereffect. Both a monocular McCollough effect and a binocular McCollough effect were induced in people having normal vision. Strabismic people did not show a binocular McCollough effect. Aftereffect strengths were the same in each eye of normal people but were stronger in the impaired eye of strabismic people. A speculation is offered as to why the “non-seeing” eye of strabismic people “sees” the aftereffect better.

Achromatic color categories.

Sternheim and Boynton’s (1966) continuous judgmental color-naming technique was used to assess the elemental nature of achromatic colors. Results from three subjects indicate that the names “black” and “white” are necessary and sufficient to describe achromatic test lights associated with the name “gray.” On the basis of the criteria that establishes a color as elemental, black and white were confirmed as elemental, whereas gray was not. These findings support the assumption of various opponent processing models that there are only two elemental achromatic sensations. In addition, the results support Hering’s (1920) view that black and white differ from the paired-chromatic opponent colors in that they are not mutually exclusive.

Spatial contrast detection for colour patterns under selective chromatic adaptation

This paper reports the results of a fundamental study investigating the filtering characteristics of the colour mechanisms that encode spatial information. The shape of the contrast sensitivity function of the human visual system with red, green and blue stimuli was investigated under selective chromatic adaptation, and at eccentricities of 0°, 13° and 26°. The results are compared with those obtained with an achromatic test, at the same eccentricities. The band-pass character of the contrast sensitivity function is brought to light for the three selective chromatic adaptation states obtained. It is more pronounced in peripheral than in foveal vision, whatever the temporal frequency tested. The authors suggest an interpretation of the shape changes of the contrast sensitivity curves based on the neurological properties of achromatic and chromatic mechanisms. The functional exploration of the neurophysiological and optical properties of the visual system may be useful to display designers and users as far as the system efficiency for using colour as an information code is concerned.

The development of contrast constancy in human infants

We use an fMRI adaptation paradigm to explore the selectivity of human responses in the lateral geniculate nucleus (LGN) and superior colliculus (SC) to red–green color and achromatic contrast. We measured responses to red–green (RG) and achromatic (ACH) high contrast sinewave counter-phasing rings with and without adaptation, within a block design. The signal for the RG test stimulus was reduced following both RG and ACH adaptation, whereas the signal for the ACH test was unaffected by either adaptor. These results provide compelling evidence that the human LGN and SC have significant capacity for color adaptation. Since in the LGN red–green responses are mediated by P cells, these findings are in contrast to earlier neurophysiological data from non-human primates that have shown weak or no contrast adaptation in the P pathway. Cross-adaptation of the red–green color response by achromatic contrast suggests unselective response adaptation and points to a dual role for P cells in responding to both color and achromatic contrast. We further show that subcortical adaptation is not restricted to the geniculostriate system, but is also present in the superior colliculus (SC), an oculomotor region that until recently, has been thought to be color-blind. Our data show that the human SC not only responds to red–green color contrast, but like the LGN, shows reliable but unselective adaptation.

Spatial sensitivity of the human visual system under selective chromatic adaptation: shape of the contrast sensitivity curves, foveal versus peripheral vision

The authors investigated the shape of the contrast sensitivity function of the human visual system for red, green and blue stimuli under selective chromatic adaptation, and, at eccentricities of 0 degrees , 13 degrees and 26 degrees . The results are compared to those obtained for an achromatic test, at the same eccentricities. The stimuli (diameter 8 degrees ) are sinusoidal grating patterns presented steadily or with a phase inversion at temporal frequencies of 2 Hz and 8 Hz. The threshold is determined by an ascending adjustment method. In foveal vision, the bandpass character of the sensitivity function has been brought to light for the three selective chromatic adaptation situations. The bandwidth and optimal frequency are different for the four stimuli: white, red, green and blue test lights, allowing for the chromatic adaptation level reached. In peripheral vision, the bandpass character is strengthened. The red and green curves tend to merge, reflecting either a change in the selective chromatic adaptation with eccentricity or a different evolution of the spatial frequency filtering properties for the three mechanisms with eccentricity. The authors suggest an interpretation of the shape changes of the contrast sensitivity curves, based on the neurological properties of achromatic and chromatic mechanism.

Evoked potential evidence of adaptation to spatial Fourier components in human vision.

INVESTIGATIONS of the nature of human spatial vision have yielded considerable evidence consistent with the hypothesis that the human visual system contains channels selectively sensitive to narrow ranges of spatial frequencies1. Unfortunately, much of this evidence, which comes from studies of the detection of gratings, can also be explained in terms of the detection of single edges or lines2. However, two studies3,4 of orientation-contingent colour aftereffects have involved attempts to discriminate between these explanations. Through the use of checkerboard stimuli, which have major Fourier components at 45° to the check edges5, these studies showed that chromatic adaptation occurs at orientations determined by the Fourier components of the adapting pattern and not by the edges present. After subjects had adapted to upright red and oblique green checkerboards, achromatic test gratings appeared pink when vertical or horizontal and green when oblique. Comparable aftereffects were observed when achromatic checkerboards were viewed after adaptation to coloured gratings3. The strongest aftereffects were obtained4 for test gratings with a fundamental spatial frequency a factor of 1.5 above the periodicity of the adapting checkerboard; that is, close to the fundamental Fourier component of the checkerboard. We present here evidence, obtained from evoked potentials, that attenuation of the activity of orientation-specific cells in one region of the human visual cortex occurs largely in cells which are tuned to the orientations of the fundamental Fourier components of an adapting pattern rather than to the orientations of the edges contained in the pattern.

Plasticity of orientation specific chromatic aftereffects

Changes in the orientation of McCollough effects were explored with and without prolonged exposure to prismatic tilt and subsequent viewing of chromatic inducing stimuli. Prolonged exposure to prism adaptation generates a reduction in the optically induced tilt and thus a disparity between the perceived and the retinal orientation of the chromatic inducing stimuli. Saturated chromatic aftereffects, following prism adaptation and subsequent induction of the McCollough effect, were obtained when the orientation of the achromatic test gratings matched the perceived, rather than the retinal, orientation of the inducing chromatic gratings.

Correlation between the chromatic visual evoked response and McCollough aftereffect.

ABSTRACTEighteen subjects received a 10‐min exposure to alternating stripe‐color patterns designed to elicit the orientation‐specific McCollough red and green complementary aftereffect (CAE). They were also exposed to 7 Hz flickering red and green light in order to elicit the chromatic visual evoked response (CVER) from occipital and vertex electrodes. Spectral analysis of the sinusoid‐like occipital CVER was then compared with strength of aftereffect. Although CVER activity at the fundamental frequency did not correlate significantly with CAE. significant correlations were obtained for CVER activity at higher harmonic frequencies. The results showed that 14 and 21 Hz activity of the green CVER correlated positively with green CAE, and 21 and 28 Hz activity of the red CVER correlated positively with red CAE. These findings agree with the hypothesis that color coding of red involves a faster neural system. Moreover, it appears that achromatic test stimuli elicit complementary aftereffects, the subjective strength of which is not related to the CVER of the training color, but rather the effectiveness of the experienced color to evoke CVER harmonic activity during direct stimulation.

Effects of surround/test field luminance ratio on induced colour

Abstract.— According to Kirschmann's third law the induced colour is at its maximum when the inducing and induced fields are of equal luminance. Later studies (Kinney, 1962) show the induced colour to be most pronounced at a luminance ratio (inducing/induced) of about 4/1. In the present study the amount of colour induced into an achromatic test field was determined for one inducing colour, red, by letting observers judge the colour strength of the induced field. The test (or induced) field luminance was varied to give luminance ratios between 0.5/1 and 2/1. The results show that both colour strength and blackness increase as the luminance ratio is increased. The fact that the test field was judged even to have maximum chromatic colour strength and maximum blackness at the same time is discussed in relation to the method used and in relation to earlier studies on the “mode of appearance” of colours and the bidimensionality of achromatic colours.

Movement-induced modulation of orientation-specific color aftereffects

The intensity of the McCollough effect is modified when, following exposure to the inducing chromatic stimuli, the achromatic test gratings are seen oscillating orthogonally to their orientations. Green aftereffect seen on stationary test gratings is enhanced by oscillations, while pink aftereffect present on the stationary gratings fades upon oscillation of the test stimulus. These opponent changes are tentatively accounted for in terms of an interaction between Fechner-Benham type induced color and processes that mediate the orientation-specific chromatic aftereffects.