Isoluminant Red-green Gratings Research Articles

High acuity vision corrected for chromatic and monochromatic aberrations is associated with color discrimination without red-green or blue-yellow sensations

From the earliest measurements of contrast sensitivity for chromatic gratings, it has been reported that isoluminant red-green gratings appear achromatic at high spatial frequencies. To better understand this phenomenon, we used a forced-choice paradigm to measure contrast sensitivity functions for red-green and S-cone isolating sinusoidal chromatic gratings. For all red-green grating experiments, monochromatic aberrations were compensated via adaptive optics. Longitudinal and transverse chromatic aberration were compensated by a specialized Badal system and a Vernier alignment task, respectively. We measured responses to red-green gratings under two conditions. In the first condition, which is comparable to the measurement of conventional chromatic contrast sensitivity, we measured contrast thresholds for detecting the grating pattern from uniform-field distractor targets. In the second condition, for a series of spatial frequencies, subjects were required to pick out a red-green grating from isochromatic grating distractors. Subjects were able to detect red-green gratings with a much higher spatial frequency cut-off (>30 c/deg) than for S-cone isolating gratings. Subjects were able to discriminate colored from isochromatic gratings with a high spatial frequency cutoff of only 10–12 c/deg, which was more similar to that found for S-cone gratings. Thus, subjects were able to detect changing red-green wavelength content at high spatial frequencies without perceiving hue sensations. The results are consistent with the unconventional hypothesis that achromatic and hue sensations are separated at the level of two subpopulations of midget ganglion cells, one serving high acuity black-white vision and a second more sparse mosaic serving hue perception at a lower spatial resolution.

High resolution of colour vision, but low contrast sensitivity in a diurnal raptor.

Animals are thought to use achromatic signals to detect small (or distant) objects and chromatic signals for large (or nearby) objects. While the spatial resolution of the achromatic channel has been widely studied, the spatial resolution of the chromatic channel has rarely been estimated. Using an operant conditioning method, we determined (i) the achromatic contrast sensitivity function and (ii) the spatial resolution of the chromatic channel of a diurnal raptor, the Harris's hawk Parabuteo unicinctus The maximal spatial resolution for achromatic gratings was 62.3 c deg-1, but the contrast sensitivity was relatively low (10.8-12.7). The spatial resolution for isoluminant red-green gratings was 21.6 c deg-1-lower than that of the achromatic channel, but the highest found in the animal kingdom to date. Our study reveals that Harris's hawks have high spatial resolving power for both achromatic and chromatic vision, suggesting the importance of colour vision for foraging. By contrast, similar to other bird species, Harris's hawks have low contrast sensitivity possibly suggesting a trade-off with chromatic sensitivity. The result is interesting in the light of the recent finding that double cones-thought to mediate high-resolution vision in birds-are absent in the central fovea of raptors.

Blobs versus bars: Psychophysical evidence supports two types of orientation response in human color vision

The classic hypothesis of Livingstone and Hubel (1984, 1987) proposed two types of color pathways in primate visual cortex based on recordings from single cells: a segregated, modular pathway that signals color but provides little information about shape or form and a second pathway that signals color differences and so defines forms without the need to specify their colors. A major problem has been to reconcile this neurophysiological hypothesis with the behavioral data. A wealth of psychophysical studies has demonstrated that color vision has orientation-tuned responses and little impairment on form related tasks, but these have not revealed any direct evidence for nonoriented mechanisms. Here we use a psychophysical method of subthreshold summation across orthogonal orientations for isoluminant red-green gratings in monocular and dichoptic viewing conditions to differentiate between nonoriented and orientation-tuned responses to color contrast. We reveal nonoriented color responses at low spatial frequencies (0.25-0.375 c/deg) under monocular conditions changing to orientation-tuned responses at higher spatial frequencies (1.5 c/deg) and under binocular conditions. We suggest that two distinct pathways coexist in color vision at the behavioral level, revealed at different spatial scales: one is isotropic, monocular, and best equipped for the representation of surface color, and the other is orientation-tuned, binocular, and selective for shape and form. This advances our understanding of the organization of the neural pathways involved in human color vision and provides a strong link between neurophysiological and behavioral data.

A Higher Order Motion Region in Human Inferior Parietal Lobule: Evidence from fMRI

The proposal that motion is processed by multiple mechanisms in the human brain has received little anatomical support so far. Here, we compared higher- and lower-level motion processing in the human brain using functional magnetic resonance imaging. We observed activation of an inferior parietal lobule (IPL) motion region by isoluminant red-green gratings when saliency of one color was increased and by long-range apparent motion at 7 Hz but not 2 Hz. This higher order motion region represents the entire visual field, while traditional motion regions predominantly process contralateral motion. Our results suggest that there are two motion-processing systems in the human brain: a contralateral lower-level luminance-based system, extending from hMT/V5+ into dorsal IPS and STS, and a bilateral higher-level saliency-based system in IPL.

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.

Temporal summation of magnetic response to chromatic stimulus in the human visual cortex.

The temporal-summation characteristics of the human visual cortex were investigated by recording the magnetic responses to isoluminant red-green gratings. In one condition, exposure duration (ED) of a single-pulse stimulus was varied between 16.7 ms and 200 ms, and in the other, stimulus-onset-asynchrony (SOA) of a double-pulse (presented for 16.7 ms each) stimulus was varied between 16.7 ms and 200 ms. The magnetic responses showed an initial peak at a latency of around 100 ms, the origin of which was estimated to be in the vicinity of the striate cortex. The peak amplitude increased with increasing ED and decreased with increasing SOA, showing a clear sign of temporal summation. The critical ED and SOA estimated from the peak amplitude vs. ED/SOA functions were about 50 ms. These values indicate the upper limit of temporal summation for chromatic stimuli in the human early visual cortex.

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.

Absence of linear subthreshold summation between red-green and luminance mechanisms over a wide range of spatio-temporal conditions.

We have tested the independence of red-green chromatic and luminance mechanisms at detection threshold using a method of subthreshold summation. Stimuli were isoluminant red-green gratings and yellow-black luminance gratings that uniquely activate the red-green color and luminance mechanisms, respectively. Stimuli were Gaussian enveloped 0.25, 1 or 4 cpd sinewave gratings, counter-phase flickered at 0, 5 or 9 Hz. The threshold detection of red-green color contrast was measured in the presence of a subthreshold amount of luminance contrast, and vice versa. The results allow a model of linear summation between the color and luminance mechanisms to be rejected, but are well fitted by a model, assuming that these mechanisms are independent but combine to determine detection by probability summation, with a high summation index (median value = 4). We conclude that there are independent red-green chromatic mechanism and luminance detection mechanisms over this range of spatio-temporal conditions.

Infant eye movement asymmetries: Temporal-nasal asymmetry is reversed at isoluminance in 2-month-olds

Tested monocularly with luminance-modulated stimuli, young infants classically show a directional asymmetry of eye movements: optokinetic nystagmus (OKN) and OKN-like eye movements are more reliably elicited by temporo-nasally- (T-N) than by naso-temporally- (N-T) directed stimuli. In the present experiment, 2-month-old infants were tested with isoluminant red-green gratings. A reverse asymmetry was found: at isoluminance, directionally appropriate eye movements were more readily elicited by N-T- than by T-N-directed stimuli. The results suggest the presence of a reverse asymmetry in the ensemble of direction-selective neurons that controls young infants' immature eye movements in response to isoluminant stimuli.

Orientation and spatial frequency selectivity of adaptation to color and luminance gratings

Prolonged viewing of sinusoidal luminance gratings produces elevated contrast detection thresholds for test gratings that are similar in spatial frequency and orientation to the adaptation stimulus. We have used this technique to investigate orientation and spatial frequency selectivity in the processing of color contrast information. Adaptation to isoluminant red-green gratings produces elevated color contrast thresholds that are selective for grating orientation and spatial frequency. Only small elevations in color contrast thresholds occur after adaptation to luminance gratings, and vice versa. Although the color adaptation effects appear slightly less selective than those for luminance, our results suggest similar spatial processing of color and luminance contrast patterns by early stages of the human visual system.

Influences of level and time course of retinal outflow on the food intake pattern of rabbits

Tested monocularly with luminance-modulated stimuli, young infants classically show a directional asymmetry of eye movements: optokinetic nystagmus (OKN) and OKN-like eye movements are more reliably elicited by temporo-nasally- (T-N) than by naso-temporally- (N-T) directed stimuli. In the present experiment, 2-month-old infants were tested with isoluminant red-green gratings. A reverse asymmetry was found: at isoluminance, directionally appropriate eye movements were more readily elicited by N-T- than by T-N-directed stimuli. The results suggest the presence of a reverse asymmetry in the ensemble of direction-selective neurons that controls young infants' immature eye movements in response to isoluminant stimuli.

The effect of unilateral lesions in the sensory-motor cortex on motor behaviour in the rabbit

Tested monocularly with luminance-modulated stimuli, young infants classically show a directional asymmetry of eye movements: optokinetic nystagmus (OKN) and OKN-like eye movements are more reliably elicited by temporo-nasally- (T-N) than by naso-temporally- (N-T) directed stimuli. In the present experiment, 2-month-old infants were tested with isoluminant red-green gratings. A reverse asymmetry was found: at isoluminance, directionally appropriate eye movements were more readily elicited by N-T- than by T-N-directed stimuli. The results suggest the presence of a reverse asymmetry in the ensemble of direction-selective neurons that controls young infants' immature eye movements in response to isoluminant stimuli.