Spatial masking does not reveal mechanisms selective to combined luminance and red-green color

Electrophysiological evidence for higher-level chromatic mechanisms in humans

Color vision in humans starts with three types of cones (short [S], medium [M], and long [L] wavelengths) in the retina and three retinal and subcortical cardinal mechanisms, which linearly combine cone signals into the luminance channel (L + M), the red-green channel (L - M), and the yellow-blue channel (S-(L + M)). Chromatic mechanisms at the cortical level, however, are less well characterized. The present study investigated such higher-order chromatic mechanisms by recording electroencephalograms (EEGs) on human observers in a noise masking paradigm. Observers viewed colored stimuli that consisted of a target embedded in noise. Color directions of the target and noise varied independently and systematically in an isoluminant plane of color space. The target was flickering on-off at 3 Hz, eliciting steady-state visual evoked potential (SSVEP) responses. As a result, the masking strength could be estimated from the SSVEP amplitude in the presence of 6 Hz noise. Masking was strongest (i.e. target eliciting smallest SSVEPs) when the target and noise were along the same color direction, and was weakest (i.e. target eliciting highest SSVEPs) when the target and noise were along orthogonal directions. This pattern of results was observed both when the target color varied along the cardinal and intermediate directions, which is evidence for higher-order chromatic mechanisms tuned to intermediate axes. The SSVEP result can be well predicted by a model with multiple broadly tuned chromatic mechanisms. In contrast, a model with only cardinal mechanisms failed to account for the data. These results provide strong electrophysiological evidence for multiple chromatic mechanisms in the early visual cortex of humans.

Model of parafoveal chromatic and luminance temporal contrast sensitivity of humans and monkeys

Rhesus monkeys are a valuable model for studies of primate visual contrast sensitivity. Their visual systems are similar to that of humans, and they can be trained to perform detection tasks at threshold during neurophysiological recording. However, the stimulus dependence of rhesus monkey contrast sensitivity has not been well characterized. Temporal frequency, color, and retinal eccentricity affect the contrast sensitivity of humans in reasonably well-understood ways. To ask whether these factors affect monkey sensitivity similarly, we measured detection thresholds of two monkeys using a two-alternative, forced-choice task and compared them to thresholds of two human subjects who performed the same task. Stimuli were drifting Gabor patterns that varied in temporal frequency (1-60 Hz), L- and M-cone modulation ratio, and retinal eccentricity (2°-14° from the fovea). Thresholds were fit by a model that assumed a pair of linear detection mechanisms: a luminance contrast detector and a red-green contrast detector. Analysis of model fits indicated that the sensitivity of these mechanisms varied across the visual field, but their temporal and spectral tuning did not. Human and monkey temporal contrast sensitivity was similar across the conditions tested, but monkeys were twofold less sensitive to low-frequency, luminance modulations.

Nonlinear two-stage model for color discrimination

We modified a two-stage model for color discrimination proposed in a previous study [Color Res. Appl.25, 105 (2000)]; in order to extend the model to wider conditions, we considered the conditions with luminance modulations in addition to color modulations. Using the modified model, we successfully predicted color discrimination data with test color changes along both the chromatic and luminance axes under a variety of background colors. Both qualitative and quantitative assessments in modeling showed that nonlinearity is required in both the cone and the cone-opponent stages to interpret adaptation effects of both color and luminance on color discrimination. This fact suggests that the nonlinear properties at each stage have different roles in color perception.

Higher order color mechanisms: a critical review

A large number of studies, using a wide variety of experimental techniques, have investigated the "higher-order" color mechanisms proposed by Krauskopf and colleagues in 1986. Results reviewed here come from studies of chromatic discrimination at threshold, habituation, classification images, spatial alignment and orientation effects, and noise masking. The bulk of the evidence has been taken to support the existence of multiple, linear color mechanisms in addition to (or after) the three putative low-level cardinal mechanisms. But there remain disconcerting inconsistencies in the results of noise masking experiments, and the results of chromatic discrimination experiments clearly show that there are a very limited number of labeled-line mechanisms near threshold. No consensus on higher order mechanisms has been reached even after more than 20 years of study.

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 detection and discrimination analyzed by a Bayesian classifier

Detection and threshold-level discrimination of Gabor patches were studied under the conditions of noise masking, in an attempt to isolate 'higher-order' or nonclassical color mechanisms. Detection contours in the equiluminant plane of cone contrast space were measured by varying test chromaticity in the presence of chromatic masking noise. Three equiluminant noise directions were used, in separate experiments. In the discrimination experiment, observers had to discriminate between pairs of stimuli that were fixed at their masked threshold contrasts. A Bayesian color classifier model was used to analyze the discrimination data, with no free parameters. There was no evidence of nonclassical color mechanisms in either the detection or discrimination data.

Detection of chromoluminance patterns on chromoluminance pedestals I: threshold measurements

Measurement of the detection thresholds of patterns on pedestals of various kinds has the potential of providing insight into the mechanisms that mediate pattern vision. This study is concerned with chromoluminance patterns, that is, patterns that vary over space in luminance, chromaticity, or both. Contrast thresholds for 1 c/deg Gabor patterns (targets) were measured as a function of the contrast of Gabor pedestal patterns (TvC functions), where the pedestals paired with each target were modulated in a wide range of directions in color space. For most target-pedestal pairs, the TvC function decreased (facilitation) and then increased as pedestal contrast increased. The increase went above the absolute contrast threshold (masking) for all target-pedestal pairs except in cases where facilitation occurred at the upper end of the pedestal contrast range. The specific form of the TvC function varied greatly with the target and pedestal, consistent with a general model of pedestal effects proposed by Foley [Journal of the Optical Society of America A, 1994, 11(6)]. There were two sets of target-pedestal pairs for which facilitation did not occur, but masking did occur: pairs in which the target was a luminance modulation and the pedestals were individually isoluminant and pairs in which the pedestal was blue/yellow and the target was in any of our directions except blue/yellow.

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.

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

Both chromatic and luminance-modulated stimuli are served by multiple spatial-frequency-tuned channels. This experiment investigated the independence versus interdependence of spatial frequency channels that serve the detection of red-green chromatic versus yellow-black luminance-modulated stimuli at low spatial frequencies. Contrast thresholds for both chromatic and luminance-modulated gratings were measured within 12 individual subjects using a repeated-measures design. Spatial frequencies ranged from 0.27 to 2.16 c/deg. A covariance structure analysis of individual differences was applied to the data. We computed statistical sources of individual variability, used them to define covariance channels, and determined the number and frequency tuning of these channels. For luminance-modulated gratings, two covariance channels were found, including one above and one below 1 c/deg [cf. Peterzell, & Teller (1996). Individual differences in contrast sensitivity functions: the coarsest spatial pattern analyzer. Vision Research, 36, 3077-3085]. For chromatic gratings, correlations between thresholds for most spatial frequencies were uniformly high, yielding a single covariance channel covering all but the highest spatial frequency tested. A combined analysis of both data sets recovered the same three covariance channels, and showed that detection thresholds for low-frequency red-green chromatic and luminance-modulated stimuli are served by separate, statistically independent processes.

The achromatic mechanism and mechanisms tuned to chromaticity and luminance in visual search

The purpose of the study was to determine whether visual search can be mediated by an achromatic, or luminance, mechanism in which signals are independent of the chromaticity of the stimuli. Experiments were designed to determine whether variability in the chromaticity of distractor stimuli made it more difficult to search for a target that differed from the distractor stimuli in luminance. Variability in the chromaticity of the distractors had little or no effect on search times when the target stimulus was white. Variability in the chromaticity of the distractors increased search times when the target was a reddish or bluish chromaticity. Results obtained with white targets suggest that these searches are mediated by an achromatic mechanism in which the signals are independent of the chromaticity of the stimuli. Results obtained with reddish and bluish targets suggest that searches for those targets may be mediated by mechanisms tuned to both chromaticity and luminance. Further experiments in which observers searched for targets that differed from distractors in both chromaticity and luminance provided additional support for the second conclusion.

Facilitation between the luminance and red-green detection mechanisms: enhancing contrast differences across edges

Previous studies have shown that detection of a red-green test pattern, such as a spot or grating, may be facilitated two to three times by a suprathreshold luminance pedestal of the same shape. We measured facilitation between the red-green (RG) and luminance (LUM) detection mechanisms using sine and square-wave gratings. Facilitation of RG by luminance pedestals was 3-fold for in phase sine-wave gratings of 0.8 cpd and a remarkable 7-fold for square-wave gratings. The latter facilitation was greatly reduced at intermediate relative phases and was generally reduced at higher spatial frequencies. We show that on a uniform field, the red or green regions of low spatial frequency test patterns are detected approximately independently, but in the presence of the LUM pedestal RG becomes sensitive to the red-green difference across the luminance edges. Under optimal conditions (with the low-frequency, square-wave luminance pedestal) this increased red-green sensitivity corresponds to a wavelength discrimination threshold as small as approximately 0.04 nm. This conversion of RG into an 'edge detector' may explain why facilitation is twice as large for square-wave gratings (bipolar patterns) than spots (unipolar patterns). The reverse facilitation, that of LUM by the red-green pedestal, is weaker and the results suggest that this is because LUM is initially sensitive to the light-dark difference across luminance edges even in the absence of the red-green pedestal.