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Brown is a contrast color that depends on complex combinations of chromatic and achromatic signals. We measured brown perception with variations in chromaticity and luminance in center-surround configurations. In Experiment 1, the dominant wavelength and saturation in terms of S -cone stimulation were tested with five observers in a fixed surround luminance ( 60 c d / m 2 ). A paired-comparison task required the observer to select the better exemplar of brown in one of two, simultaneously presented, stimuli (1.0° center diameter; annulus of 9.48° outer-diameter). In Experiment 2, the same task was tested with five observers in which surround luminance was varied (from 13.1 to 99.6 c d / m 2 ) for two center chromaticities. The results were a set of win–loss ratios for each stimulus combination and converted to Z -scores. An ANOVA did not reveal a significant main effect of the observer factor but revealed a significant interaction with red/green ( a ∗ ) [but not with the dominant wavelength and the S -cone stimulation (or b ∗ )]. Experiment 2 revealed observer variation in interactions with surround luminance and S -cone stimulation. The averaged data plotted in 1976 L ∗ a ∗ b ∗ color space indicate that high Z -score values widely distribute in the area of a ∗ from 5 to 28 and b ∗ over 6. The balance of the strength between yellowness and blackness differs among observers owing to the amount of induced blackness required for the best brown.

Fundus photography, entoptic visualization and psychophysical evidence all indicate that the peak density of the macular pigment (MP) is in the fovea and it decreases in density with increasing retinal eccentricity.

Mixtures of monochromatic lights that appear achromatic were measured for fifty normal trichromats ranging in age from 11 to 78 years. Stimuli were presented to one eye as a 1-sec flash (10 sec interstimulus interval) in Maxwellian view. The achromatic locus was found by varying the intensity ratio of a mixture of unique blue and unique yellow while maintaining constant overall retinal illuminance. Measurements were obtained at three intensity levels (10, 100, 1,000 td) for each observer. Additional verification of the position of the achromatic locus in color space was obtained for twenty-three subjects using a mixture composed of 600 nm and its spectral complement. There were no significant changes in the achromatic loci as a function of age. The mean achromatic locus in CIE chromaticity space was x,y = 0.31,0.31, or u'v' = 0.21,0.46. These results imply that the three classes of cones undergo parallel losses in sensitivity (specified at the cornea) as a function of age.

Previous studies have shown that diabetics make tritan-like errors in color-matching experiments1,2 and demonstrate elevated increment thresholds for test lights detected by an S-cone mechanism3,4. While these experiments indicate the presence of a defect in visual pathways that receive an input from S-cones, they are unable to identify which components within these pathways are responsible for abnormal visual functions associated with diabetes. The purpose of this study is determine if diabetes introduces an anomaly either prior to and/or at the sites of cone interaction, or after the pooling of cone signals within chromatically-opponent pathways.

Three color-normal observers described the appearance pf colors in conditions of temporal induction. One of four inducing fields (unique blue, unique green, unique yellow, unique white) was foveally viewed for 5 s followed immediately by a reference stimulus of the same size (0.75°) and spatial location as the inducing field. After presentation of this temporal sequence, the observers described the reference stimulus by assigning percentages to the terms red, green, yellow, blue, black, and white. The reference stimulus was 3 or 5 trolands and appeared achromatic to the observer when viewed without the inducing field. The inducing field ranged in intensity from −0.4 to 3.2 log td. At the low illuminance levels the reference stimulus changed its appearance; both chromatic and achromatic color induction occurred. As the illuminance of the inducing field increased, the reference stimulus gradually became blacker and the chromatic components less prominent. The retinal illuminance level at which the reference stimulus appeared 100% black was the same for the four different inducing fields.

The additivity of induced blackness was tested in conditions of temporal contrast. Two adult observers foveally viewed an inducing field composed of either one of seven variable wavelengths (430-670 nm, 40-nm steps), one of two addend wavelengths (510 and 630 nm), or a combination of one variable wavelength and one addend wavelength for 5 s followed immediately by a broadband (5500-K) reference stimulus of the same size (0.75°) and spatial location as the inducing field. The observers increased the radiance of the inducing field until the reference stimulus appeared completely black. Additivity tests were also obtained with the same wavelength combinations for each observer using heterochromatic flicker photometry (HFP) and direct brightness matching. The additivity results from blackness induction resembled those from HFP, whereas the results from brightness matching showed clear additivity failures of the cancellation type. These findings were also corroborated by additivity tests performed on spatially induced blackness. These data indicate that the perception of blackness is mediated by neural mechanisms that additively combine the input of middle- and long-wave photoreceptors.

Spectral unique hues (blue, green, yellow) were determined for 50 observers ranging in age from 13 to 74 years. Each unique hue was measured at three luminance levels (0.5 log unit steps). There were no significant changes in the spectral locations of red-green equilibrium hues (unique blue and yellow) as a function of luminance level or age. In contrast, significant shifts in unique green loci occurred both as a function of age and luminance. Unique green loci shifted toward shorter wavelengths with advancing age. These results are consistent with the hypothesis that with advancing age there is a parallel decline in the input of all three cone types to the red-green chromatic channel, and either a selective decline in shortwave sensitive cone input to the yellow-blue chromatic channel or a change in the way in which cone signals are combined within the yellow-blue channel.