Chromatic Contrast Sensitivity Research Articles

Contrast Sensitivity in Benign Intracranial Hypertension

A study was undertaken to assess contrast sensitivity impairment in patients with benign intracranial hypertension (BIH) and to determine if contrast sensitivity provides more information about visual dysfunction than Snellen visual acuity. A computer-based system was used to measure spatial contrast sensitivity for chromatic and luminance stimuli at 1,4, and 16 cycles per degree (c/deg). The results from 26 eyes of 15 patients with BIH were compared with those of a control group matched for age and visual acuity. The visual acuity in the BIH group was normal (6/6 or better) in 22 eyes (85%) and slightly reduced (6/9) in the remaining four eyes (15%). Contrast sensitivity was significantly reduced in the BIH group at all spatial frequencies both for chromatic and luminance measures. In addition, there was a significantly greater loss of luminance compared to chromatic contrast sensitivity at low spatial frequency (1 c/deg). Contrast sensitivity measures are more sensitive than Snellen visual acuity in ass...

Spatial-frequency tuning of sustained nonoriented units of the red-green channel.

The existence of nonoriented cells but not of sustained orthogonal masking for achromatic stimuli led to an investigation of the spatial-frequency (SF) tuning of sustained nonoriented color units. For this purpose the Red-Green channel was isolated by the minimum flicker and hue cancellation techniques. Chromatic contrast sensitivity functions (CSF's), threshold elevation (TE) curves, and contrast nonlinearities (TE-versus-mask-contrast curves) were measured with spatially localized vertical color tests and sinusoidal orthogonal color masks by the method of constant stimuli under Gaussian temporal presentation. Results show that (1) color CSF's are a low-pass function of SF, whereas TE curves are a bandpass function of mask SF, and (2) a minimum of six SF-tuned color mechanisms (one low pass and five bandpass functions of SF, with peak SF's of 0.13, 0.5, 2, 4, and 8 cycles per degree and bandwidths of 3.9, 4.4, 2.9, 2.1, 1.1, and 1.2 octaves), similar to oblique-masking color mechanisms, are extracted by the multiple-mechanisms model. These data imply that (1) most of the SF tuning of the broadly oriented color units is already present in the circularly symmetric units, and (2) the latter may be an input to the former.

Spatial Integration of Objectively Equiluminous Chromatic Gratings

Contrast sensitivity functions for achromatic and chromatic gratings tend to be band-pass and low-pass in shape, respectively. Our aim was to test whether spatial integration contributes to the shape difference found at low spatial frequencies. We measured binocular chromatic contrast sensitivity as a function of grating area for objectively equiluminous red - green and blue - yellow chromatic gratings. Chromatic contrast refers to the Michelson contrast of either of the two chromatic component gratings presented in counterphase against the combined background. Grating area ( A) varied from 1 to 256 square cycles ( Af2) at spatial frequencies ( f) of 0.125 – 4.0 cycles deg−1. We used only horizontal gratings at low and medium spatial frequencies to minimise the transverse and longitudinal chromatic aberrations due to ocular optics. At all spatial frequencies studied, chromatic contrast sensitivity increased with grating area. Ac was found to be constant at low spatial frequencies (0.125 – 0.5 cycles deg−1) but decreased in inverse proportion to increasing spatial frequency at 1 – 4 cycles deg−1. Thus, spatial integration depends similarly on spatial frequency for achromatic (Luntinen et al, 1995 Vision Research35 2339 – 2346) and chromatic gratings, and differences in spatial integration do not contribute to the shape difference of the respective contrast sensitivity functions.

The development of chromatic and achromatic contrast sensitivity in infancy as tested with the sweep VEP

Swept-contrast visual evoked potential (VEP) techniques were used to measure the development of contrast sensitivity functions (CSFs) for achromatic and red/green isoluminant chromatic gratings. Subjects were infants of 8, 14, 20 and 32 weeks of age, and adults. Stimuli were 20 deg, 0.3–4 cyc/deg sinusoidal gratings, counterphased at 6 Hz and modulated through white. Achromatic and chromatic CSFs for all ages could be fit simultaneously with a double exponential equation of a common, lowpass shape. Both achromatic and chromatic CSFs exhibited developmental shifts in both sensitivity and spatial scale. From 8 weeks to adulthood, sensitivity increased by 0.64 log units for achromatic gratings and by 0.91 log units for chromatic gratings, yielding an 0.27 log unit larger sensitivity change for chromatic than for achromatic stimuli. Spatial scale shifts were closely similar across achromatic and chromatic CSFs, and were consistent with the factor of about four predicted on the basis of changes in foveal receptor packing density and eye size. The question of uniform vs differential loss of sensitivity for chromatic vs achromatic stimuli at fixed spatial frequencies is discussed.

Contrast sensitivity in benign intracranial hypertension

A study was undertaken to assess contrast sensitivity impairment in patients with benign intracranial hypertension (BIH) and to determine if contrast sensitivity provides more information about visual dysfunction than Snellen visual acuity. A computer-based system was used to measure spatial contrast sensitivity for chromatic and luminance stimuli at 1,4, and 16 cycles per degree (c/deg). The results from 26 eyes of 15 patients with BIH were compared with those of a control group matched for age and visual acuity. The visual acuity in the BIH group was normal (6/6 or better) in 22 eyes (85%) and slightly reduced (6/9) in the remaining four eyes (15%). Contrast sensitivity was significantly reduced in the BIH group at all spatial frequencies both for chromatic and luminance measures. In addition, there was a significantly greater loss of luminance compared to chromatic contrast sensitivity at low spatial frequency (1 c/deg). Contrast sensitivity measures are more sensitive than Snellen visual acuity in assessing BIH-related visual loss and may be of value in the initial assessment and follow-up of these patients.

Behavioural and electrophysiological chromatic and achromatic contrast sensitivity in an achromatopsic patient.

In cases of incomplete achromatopsia it is unclear whether residual visual function is mediated by intact striate cortex or results from incomplete lesions to extrastriate cortical visual areas. A patient with complete cerebral achromatopsia was tested to establish the nature of his residual vision and to determine the integrity of striate cortex function. Behavioural contrast sensitivity, using the method of adjustment, and averaged visually evoked cortical potentials were measured to sinusoidally modulated chromatic and achromatic gratings in an achromatopsic patient and a normal observer. Eye movements were measured in the patient using a Skalar infrared monitoring system. The patient's chromatic contrast sensitivity was normal, indicating that despite his dense colour blindness his occipital cortex still processed information about spatial variations in hue. His sensitivity to achromatic gratings was depressed particularly at high spatial frequencies, possibly because of his jerk nystagmus. These behavioural results were reinforced by the nature of visually evoked responses to chromatic and achromatic gratings, in which total colour blindness coexisted with an almost normal cortical potential to isoluminant chromatic gratings. The results show that information about chromatic contrast is present in some cortical areas, and coded in a colour-opponent fashion, in the absence of any perceptual experience of colour.

Basic visual capacities and shape discrimination after lesions of extrastriate area V4 in macaques.

Ibotenic acid lesions were made in four macaque monkeys in a region of cortical area V4 that corresponds to the lower quadrant of one hemifield. For visual testing, fixation locus was monitored with scleral search coils and controlled behaviorally to place test stimuli either in the lesioned quadrant or in a control location in the opposite hemifield. Some basic visual capacities were slightly altered by the lesions; there was a two-fold reduction of luminance contrast sensitivity as well as red-green chromatic contrast sensitivity, both tested with stationary gratings. On the other hand, little or no loss was found when contrast sensitivity for detection or direction discrimination was tested with 10-Hz drifting gratings nor was there a reliable change in visual acuity. Hue and luminance matching were tested with a spatially more complex matching-to-sample task, but monkeys could not learn this task in the visual field locus of a V4 lesion. If previously trained at this locus, performance was not affected by the lesion. In contrast to the small effects on basic visual capabilities, performance on two form discrimination tasks was devastated by V4 lesions. The first involved discriminating the orientation of colinear groups of dots on a background of randomly placed dots. The second involved discriminating the orientation of a group of three line segments surrounded by differently oriented line segments. Some selectivity of the deficits for form discrimination was shown by the lack of an effect of the lesions on a global motion discrimination. These results show that while V4 lesions cause only slight disruptions of basic visual capacities, they profoundly disrupt form discriminations.

Infant luminance and chromatic contrast sensitivity: Optokinetic nystagmus data on 3-month-olds

Infant color vision is poor, and most psychophysical experiments agree that infant color vision emerges between ages 3 weeks and 3 months. Presumably, the color vision of infants is poor during the months immediately after it has emerged. We have tested two alternative explanations for the poor color vision of infants: (1) there is a special critical immaturity within the color pathways of infants; (2) infants have poor infant color vision because they are insensitive to contrast. Luminance and chromatic contrast thresholds were measured on 3-month-olds using optokinetic nystagmus (OKN), and adult luminance and chromatic contrast thresholds were measured using OKN and two forced-choice methods: direction-of-motion discrimination and grating detection. The infant chromatic-to-luminance contrast threshold ratio shows that infants are as sensitive or even more sensitive than adults to color, depending on the testing method used on adults. This result suggests that the general contrast insensitivity hypothesis is correct. Conservative “worst-case” quantitative analysis strongly suggests that this result is not the consequence of a luminance artifact.

Visual evoked potentials in three-dimensional color space: Correlates of spatio-chromatic processing

Visual evoked potentials (VEPs) were measured for sinusoidal gratings with spatio-chromatic modulation defined in a three-dimensional color space. The spatio-chromatic modulation of the gratings can be decomposed into contributions from an achromatic luminance varying component, an isoluminant component which modulates only the activities of L cones and M cones, and an isoluminant component corresponding to modulation of only S-cone activity. The emphasis of this report is the nature of VEPs resulting from isoluminant spatio-chromatic modulation. The VEP response was characterized along a number of spatial, temporal, and chromatic stimulus dimensions: contrast, spatial frequency, chromaticity in the isoluminant plane, chrominance/luminance ratio, orientation, and temporal frequency. Isoluminant VEPs resulting from stimuli modulating L and M cones are compared with those from S-cone modulation. When appropriate spatiotemporal conditions are employed, both types produce robust VEPs; however, the S-pathway VEPs show considerably longer latencies than do those from LM-pathway activation. The VEP results are compared to psychophysical and single unit electrophysiological observations. VEP latencies exhibit the lowpass character of psychophysical chromatic contrast sensitivity functions but VEP amplitudes show bandpass tuning along both the S and LM axes. An oblique effect, i.e. shorter latencies for horizontal and vertical gratings than for diagonal, is observed in the isoluminant VEP. S-pathway VEPs are used to demonstrate an electophysiological correlate of transient tritanopia. Normative amplitude and latency data for VEPs from selectivity stimulated chromatic mechanisms provide a baseline for clinical electrodiagnostic applications.

Development of infant contrast sensitivity to chromatic stimuli

We have monitored the development of contrast sensitivity to equiluminant red-green chromatic patterns by monitoring visual evoked potentials (VEPs) in 13 infants. The results confirm our previous report [Morrone, Burr and Fiorentini, Proceedings of the Royal Society B, 242 (1990a)] that, before 7–8 weeks of age, there was no response to purely chromatic stimuli, while at the same age Iuminance stimuli of 20% contrast produced reliable responses. At all ages (even before the onset of a chromatic response) the colour mixture to yield equiluminance was similar to that of adults, suggesting that the relative proportion and efficacy of medium- and long-wave cones is similar for infants as for adults. For both luminance and chromatic stimuli, amplitude increased roughly linearly with log-contrast, so sensitivity thresholds could be predicted by linear extrapolation to the abscissa. Detailed contrast sensitivity curves were measured for four infants at various ages. The results show that luminance and chromatic contrast sensitivity develop independently at different rates, probably reflecting differential development of postreceptoral neural mechanisms.

The effects of parvocellular lateral geniculate lesions on the acuity and contrast sensitivity of macaque monkeys

The effects of ablating the visual pathway that passes through the parvocellular (dorsal) LGN were tested in 2 macaque monkeys by measuring acuity and both luminance and chromatic contrast sensitivity. Thresholds were tested monocularly before and after ibotenic acid was used to lesion parvocellular layers 4 and 6 of the contralateral geniculate. The injections were centered at the representation of 6 degrees in the temporal field on the horizontal meridian, and vision was tested with localized stimuli at this location. In addition, in one of the monkeys, a lesion was made in magnocellular layer 1 of the opposite geniculate, and the same thresholds were tested. Physiological and anatomical reconstructions demonstrated complete destruction of the target layers in 1 parvocellular lesions and in the magnocellular lesion, and sparing of the nontarget layers in the tested region. Parvocellular lesions caused a 3-4-fold reduction in visual acuity within the affected part of the visual field, while the magnocellular lesion did not affect acuity. Both luminance and chromatic contrast sensitivity, tested with stationary gratings of 2 c/degree, were severely reduced by parvocellular lesions, but not affected by the magnocellular lesion. However, when luminance contrast sensitivity was tested with 1 c/degree gratings, counterphase modulated at 10 Hz, it was reduced by both parvocellular and magnocellular lesions. This study demonstrates that the parvocellular pathway dominates chromatic vision, acuity, and contrast detection at low temporal and high spatial frequencies, while the magnocellular pathway may mediate contrast detection at higher temporal and lower spatial frequencies.

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.

Development of contrast sensitivity and acuity of the infant colour system

We have monitored the development of infant colour vision by measuring chromatic contrast sensitivity and acuity in eight young infants over a period of 6 months. Steady-state visual evoked potentials (VEPS) were recorded in response to both chromatic (red-green) and luminance (red-black or green-black) patterns that were reversed in contrast over time. For most infants, no response could be obtained to chromatic stimuli of any size or contrast before 5 weeks of age, although luminance stimuli of 20% contrast gave reliable responses at that age. When responses to chromatic stimuli first appeared, they could be obtained only with stimuli of very low spatial frequency, 20 times lower than the acuity for luminance stimuli. Both contrast sensitivity and acuity for chromatic stimuli increased steadily, more rapidly than for luminance stimuli. As the spectral selectivities of infant cones are similar to those of adults, the difference in rate of development of luminance and chromatic contrast sensitivity and acuity stimuli probably reflects neural development of the infant colour system.

Opponent-color receptive-field profiles determined from large-area psychophysical measurements.

By compensating for retinal inhomogeneity and using the opponent-color characteristics of retinal receptive fields, some of the problems associated with previous methods of inferring local spatial-processing properties from sine-wave contrast-sensitivity measurements can be eliminated. Forced-choice contrast detection thresholds were obtained for circular frequency-modulated patterns, over a wide range of spatial and temporal frequencies, with three subjects. Eye-movement artifacts were minimized by image stabilization. The luminance contrast sensitivity (L) for a given isochromatic stimulus and the chromatic contrast sensitivity (C) for the corresponding isoluminance stimulus were measured in rapid succession, under identical conditions, using red and green components with accurately known spectral distributions. Receptive-field center (E) and surround (I) frequency functions were then calculated from the L and C data by solving two linear equations in two unknowns, with coefficients given by the spectral characteristics of the stimuli and the long-wave and middle-wave cone pigments. The results depend on the small differences of large numbers inherent in opponent-color responses, so they are somewhat noisy, but the forms of these (E-I) spatial profiles are similar to those obtained by other methods.

Chromatic and achromatic vision of macaques: role of the P pathway

Chromatic and achromatic contrast sensitivity were measured in a human observer, 2 normal macaque monkeys, and 3 monkeys with severe toxicant-induced damage to the parvocellular projecting retinogeniculate pathway (P cell-deficient monkeys). Damage to the P pathway was produced by the oral administration of acrylamide monomer (Eskin and Merigan, 1986). Contrast sensitivity was measured in all subjects with isochromatic luminance gratings, as well as isoluminant chromatic gratings, modulated along several directions of a color space that represents color-opponent and luminance contrast (Krauskopf et al., 1986). The chromatic and achromatic sensitivity of the control monkeys was virtually identical to that of the human observer. Chromatic sensitivity of the P cell-deficient monkeys, measured at a low spatial frequency (0.3 c/deg), along a constant-blue color axis, was 0.9-1.5 log units lower than that of controls. Similar losses were seen along a tritanopic confusion axis and along 2 intermediate axes of color direction. Chromatic thresholds measured at higher spatial frequency (2.0 c/deg) were similarly reduced. Counterphase-modulated chromatic gratings were used to test color sensitivity over a range of temporal frequencies up to 15 Hz, and the loss of color vision was substantial over the entire range of frequencies. The luminance contrast sensitivity of the P cell-deficient monkeys for stationary gratings decreased after exposure by 0.5-0.8 log units. These results indicate that the chromatic and achromatic spatial vision of macaques is very similar to that of humans. They also suggest that the P pathway plays an important role in macaque chromatic sensitivity at all spatial frequencies, as well as achromatic sensitivity at high spatial and lower temporal frequencies.

Global spatiochromatic mechanism accounting for luminance variations in contrast sensitivity functions.

Psychophysical luminance and chromatic spatial contrast sensitivity functions (CSF's) exhibit a number of variations that depend on the average luminance of the stimulus grating. The contrast sensitivity at each spatial frequency and the maximum resolvable spatial frequency decrease with decreasing average luminance. Most prominently, luminance CSF's have peaks that shift toward lower frequencies, broaden, and disappear as luminance decreases. Chromatic CSF's are flat at low frequencies and do not exhibit luminance-related variations in shape. In this paper, we account for the luminance-dependent variations of CSF's by incorporating them into an earlier model that accounts for the basic high-luminance shapes of psychophysical luminance and chromatic spatial CSF's. These variations are modeled by increasing the spatial extent of a global center mechanism while decreasing the inhibitory effect of a global surround on that center. The results support the hypothesis that the global center mechanism changes size as a means of maintaining the dynamic range and controlling the signal-to-noise ratio as luminance varies.

Inference of global spatiochromatic mechanisms from contrast sensitivity functions.

A single spatiochromatic signal can describe both chromatic and luminance cosine gratings by varying the spatial phase of that signal. This signal representation permits chromatic and luminance spatial contrast sensitivity functions to be described as linear combinations of two global spatiochromatic mechanisms. If the spatiochromatic mechanisms are assumed to have Gaussian spatial profiles, we obtain one mechanism with broad spatial frequency tuning and one with narrow tuning. We find that the relative spatial shapes and extents of these mechanisms are in agreement with the center-surround organization of receptive fields of single retinal and geniculate cells. Furthermore, the chromatic responses of the derived mechanisms are in accord with the color-opponent nature of spatially antagonistic receptive fields.

Chromatic contrast and visual sensitivity: evidence for disparate mechanisms.

The hue, saturation, and brightness of a field of white light were varied by the simultaneous chromatic stimulation of neighboring retinal areas. Visual sensitivity was measured by determining the threshold luminance of chromatic test stimuli presented as increments upon these background fields. The spatial position of the test stimulus was varied within the central field of induced color. Chromatic-contrast effects extending over relatively large retinal areas may be maintained by a mechanism operating at a different level of the visual system than that at which increment sensitivity is determined. The data are discussed in relation to the explanation of chromatic contrast based upon selective lateral adaptation and the concept of opponent-response induction.