Chromatic perceptive field sizes measured at 10 degrees eccentricity along the horizontal and vertical meridians

Unique yellow shifts for small and brief stimuli in the central retina

The spectral locus of unique yellow was determined for flashes of different sizes (<11 arcmin) and durations (<500 ms) presented in and near the fovea. An adaptive optics scanning laser ophthalmoscope was used to minimize the effects of higher-order aberrations during simultaneous stimulus delivery and retinal imaging. In certain subjects, parafoveal cones were classified as L, M, or S, which permitted the comparison of unique yellow measurements with variations in local L/M ratios within and between observers. Unique yellow shifted to longer wavelengths as stimulus size or duration was reduced. This effect is most pronounced for changes in size and more apparent in the fovea than in the parafovea. The observed variations in unique yellow are not entirely predicted from variations in L/M ratio and therefore implicate neural processes beyond photoreception.

Binocular vs. monocular hue perception

Hue perception has been shown to differ for the same stimulus when presented to the temporal and the nasal areas of the retina. The present study investigated perceptual differences in stimuli viewed binocularly or monocularly in the peripheral retina to determine how hue information combines across the two retinas for a stimulus falling on the temporal retina of one eye and the nasal retina of the other. A hue-scaling procedure was utilized to ascertain hue perception for three color- and binocular-normal observers viewing monochromatic stimuli (450-670nm, 20nm steps) ranging in size from 1.0° to 3.7°. Peripherally-presented binocular stimuli fell upon the nasal retina of one eye and the temporal retina of the other. Hue-scaling results indicated that peripheral binocular hue and saturation perceptions for smaller stimuli were more similar to those of stimuli falling on the temporal retina in the monocular condition. Hue-scaling data were also used to determine perceptive field sizes for the four elemental hues. Binocular perceptive field sizes were more similar to those obtained for stimuli falling on the temporal retina in the monocular conditions. Eye dominance did not appear to have an effect on hue perception. The results seem to indicate that visual information from the temporal retina is weighted more heavily when information from the two eyes is combined cortically. This finding may relate to differences in V1 cortical activation for stimuli presented to the nasal retina versus the temporal retina.

Small field tritanopia in the peripheral retina

If stimuli are made sufficiently small, color-normal individuals report a loss in hue perception, in particular a decrease in the perception of green, in both the fovea and peripheral retina. This effect is referred to as small field tritanopia. It is not clear, however, how rod input may alter the dynamics of small field tritanopia in the peripheral retina. This paper looks at peripheral hue-naming data obtained for small stimuli at mesopic and photopic retinal illuminances under conditions that minimize (bleach) and maximize (no bleach) rod contribution. The data show that attenuation in the perception of green occurs with larger stimuli in the no-bleach condition than in the bleach condition. As retinal illuminance increases, the stimulus size that elicits small field tritanopia decreases, but the stimulus size is still larger under the no-bleach condition. Small field tritanopia in both the bleach and no-bleach conditions may be related to short-wavelength-sensitive (S) cone activity and its potential role in the mediation of the perception of green. The differences in stimulus size for small field tritanopia may be explained by rod input into the magnocellular and koniocellular pathways, which compromises the strength of the chromatic signals and creates a differential loss in the perception of green as compared to the other elemental hues.

Vision under mesopic and scotopic illumination

Evidence has accumulated that rod activation under mesopic and scotopic light levels alters visual perception and performance. Here we review the most recent developments in the measurement of rod and cone contributions to mesopic color perception and temporal processing, with a focus on data measured using a four-primary photostimulator method that independently controls rod and cone excitations. We discuss the findings in the context of rod inputs to the three primary retinogeniculate pathways to understand rod contributions to mesopic vision. Additionally, we present evidence that hue perception is possible under scotopic, pure rod-mediated conditions that involves cortical mechanisms.

Supersaturation in the peripheral retina

Foveal and peripheral hue-scaling data were obtained for a 1° foveal stimulus and a 3° stimulus presented at 10° retinal eccentricity under both bleach (reducing rod input) and no-bleach (permitting rod input) conditions. Uniform appearance diagrams (UADs) were generated from the data. Peripheral stimuli appeared more saturated than foveal stimuli (i.e., supersaturated), especially in the green-yellow region of the UADs. This effect was particularly pronounced for the peripheral bleach condition. The range of wavelengths perceived as green-yellow in the peripheral retina was expanded as compared to the fovea, while the range of wavelengths experienced as blue-green was compressed. This indicates that there are shifts in the unique hue loci with retinal location. While several factors can be ruled out as potential causes for these perceptual differences, the underlying mechanism of this supersaturation effect in the peripheral retina is unknown.

Color appearance at ±10° along the vertical and horizontal meridians

Hue-scaling data were collected from three observers using the "4+1" color-naming procedure for circular (0.25°-5°), monochromatic (440-660 nm) stimuli. Stimuli were presented at ±10° along the vertical and horizontal meridians under conditions chosen to include both rod and cone signals (no bleach) and to minimize rod contribution (bleach). All color-naming data were analyzed and compared using uniform appearance diagrams. Smaller stimuli appear more desaturated under both bleach conditions. This effect is particularly detrimental for the perception of green and is influenced by retinal location and exacerbated with rod input. As stimulus size increases and perceptive field sizes are filled for all four elemental hues, the differences in hue perception among the four peripheral locations and the two bleach conditions are attenuated. Results are consistent with predictions based on known differences in the underlying retinal mosaic among the four locations.

Rod hue biases produced on CRT displays

Studies of rod hue biases using monochromatic stimuli have shown that rod stimulation can shift the balance of hues at mesopic light levels. We found that the CRT display produced all three previously identified rod hue biases, which shifted the loci of all four unique hues at low mesopic light levels. Rod hue biases occurred at 2.6 cd/m(2) for some observers but not at 26 cd/m(2). At optimal light levels below 0.5 cd/m(2), rod hue biases varied among observers but generally (1) enhanced green versus red at unique yellow and sometimes at unique blue, (2) enhanced blue versus yellow at both unique green and unique red, and (3) enhanced red versus green at unique blue. Rod hue biases persisted for some observers even for smaller foveal stimuli.

Middle- and long-wavelength discrimination declines with rod photopigment regeneration

Hue-discrimination functions were derived from hue-naming data (480-620 nm, 20 nm steps) obtained in 4 min intervals from 4 min to 28 min postbleach at 10° temporal retinal eccentricity. Hue-naming data were also obtained in the fovea. Hue-discrimination functions derived at the 4, 8, and 12 min intervals were very similar to those derived in the fovea. As time postbleach exceeded 12 min and rod sensitivity increased, the shape of the hue-discrimination functions changed. Most notably, the minimum between 560-580 nm disappeared and the just noticeable differences (JNDs) for the longer wavelength stimuli increased. The long-wavelength suppression in hue discrimination may be due to rod input in the magnocellular pathway interacting and affecting the long-wavelength sensitivity of the parvocellular pathway.

Foveal spatial summation in human cone mechanism

We measured at the fovea the chromatic contrast threshold for stimuli modulated along different chromatic directions in the isoluminant plane of MBDKL colour space, considering the two cardinal axes (L/M) and S/(L + M) and other intermediate non-cardinal directions. This psychophysical determination was conducted as a function of stimulus size. The test stimulus was a foveal isoluminant Gaussian patch with a raised cosinusoidal temporal profile superimposed on a neutral background. The task was performed binocularly. The increment threshold was measured for three observers by a Bayesian adaptive psychometric method (QUEST). The Ricco area of complete spatial summation was estimated from the threshold-versus-area curves. The perceptive fields are smaller for the L/M-cone opponent direction than the S/(L + M)-cone opponent. The perceptive field sizes for the stimuli in non-cardinal chromatic directions and stimuli modulated at the (L/M)-cone opponent direction present similar values. Measurements were made at two luminance levels, 5 and 40 cd m(-2), but the differences found were small. The perceptive field sizes found could be associated with LGN area.