Hoover Chan

Color appearance in the peripheral retina: effects of stimulus size

Hue and saturation scaling were used to measure the appearance of spectral lights as a function of stimulus size for nine loci across the horizontal retinal meridian. At a given locus, each hue (R, Y, G, and B) grew as a function of stimulus size up to some asymptotic value. The parameter values of Michaelis-Menten growth functions fitted to the hue data were used to derive the sizes of the so-called perceptive fields of the hue mechanisms. The fields for all mechanisms increased with eccentricity, and this increase was greater on the temporal than on the nasal retina. By increasing stimulus size it was possible to achieve fovealike color vision to eccentricities of 20 deg. However, even the largest stimuli failed to produce fully saturated hues at 40 deg. The retinal size scales of the four hue mechanisms were not the same; those for R and B were similar, and these mechanisms had the smallest perceptive fields everywhere. The perceptive fields of the hue mechanisms at all loci were larger than anatomical estimates of the sizes of retinal receptive fields.

Describing color appearance: Hue and saturation scaling

Most of the fully elaborated systems for describing color appearance rely on matching to samples from some standard set. Since this is not satisfactory in all situations, various forms of direct linguistic description have been used, ranging from color naming to continuous numerical scaling of sensations. We have developed and extensively applied a particular variant in which subjects use percentage scales to describe their sensations of the four unique hue sensations (red, yellow, green, blue) and of the apparent saturation of colored lights. In this paper we explore the properties of this procedure, including its statistical properties and reliability both between and within subjects, in different contexts. We conclude that the technique is robust, easy to use, and provides direct access to sensory experience.

Color appearance across the retina: effects of a white surround

Hue and saturation scaling was used to measure the appearance of spectral lights as a function of stimulus size at loci spaced across the horizontal meridian. At a given locus, each hue (red, yellow, green, and blue) grew as a function of stimulus size to some asymptotic value. Growth functions fitted to the hue data were used to derive the sizes of perceptive fields of the hue mechanisms. We had previously investigated the size scales of these fields by using stimuli presented on a dark background: Field sizes for all mechanisms increased with eccentricity, this increase was greater on the temporal than on the nasal retina, and the retinal size scales of the different hue mechanisms were not the same. We now report the results of repeating the study with stimuli embedded in a white surround rather than in darkness. The main effects of the white surround were as follows: Sizes of perceptive fields of all mechanisms were smaller everywhere than with the dark surround. For small fields, stimuli appeared everywhere more saturated with the white surround, but larger stimuli appeared less saturated.

Different spatial tunings for ON and OFF pathway stimulation

We compared the spatial tuning of sensitivity to luminance increments and decrements in three types of localized stimulus presentation with a smooth spatiotemporal envelope. The lirst type consisted of spatiotemporal Gabor grating functions with either a positive or a negative bias in luminance, The spatial tuning showed a substantially narrower bandwidth and greater peak sensitivity for the positive Gabors. A similar description could be applied to the results for detection of spots with a 2‐D difference‐of‐Gaussian profile, although the shape of the tuning function differed in several respects. We also used the biased Gabor modulation in n contrast self‐masking paradigm, where the increment (or decrement) was presented against a steady background with the same spatial configuration, over a range of base contrasts. At medium spatial frequencies the biased masking functions were similar to those typically found for unbiased gratings, in showing a threshold facilitation (dipper) at low contrast, and sub‐Weber masking behaviour at higher contrasts. At low spatial frequencies, however, a pronounced asymmetry appeared. Stimuli with a positive bins again showed typical masking behaviour, but a virtually flat masking function was obtained for negative bias stimuli. We conclude that stimuli without abrupt luminance transients reveal pronounced differences in the spatial tuning of responses to positive and negative stimuli, which probably reflect differences in the neural connectivity of the ON and OFF processing systems.

Color appearance: Properties of the uniform appearance diagram derived from hue and saturation scaling

Color appearance can be specified by a procedure of direct hue and saturation scaling. The scaling data can be represented on a 2-D color space termed a uniform appearance diagram (UAD). The orthogonal and bipolar axes of the UAD are based on the four unique hue sensations, which are blue-yellow and green-red. We have previously shown that the technique is reliable and rapid. We now show that the UAD is sufficiently uniform metrically that it can be used to derive, from a single set of scaling data, a wide range of color functions, such as the spectral loci of the unique hues, wavelength discrimination, and similarities among very different colored stimuli. The advantage of deriving a UAD is that it requires only a modest amount of participant time to generate the relevant data, which can be re-generated quickly to meet changing viewing conditions.

Large and small color differences: predicting them from hue scaling

Color appearance can be specified by a procedure of direct hue scaling. In this procedure, subjects look at a stimulus and then simply state the proportions of their sensations using the four unique hue names red, yellow, green, and blue; to completeness, they also state the apparent saturation. Observers can scale stimuli quickly and reliably, and this is true even if they are relatively inexperienced. Thus stimuli can be rescaled whenever viewing conditions change such that a new specification of appearance is required. The scaled sensory values elicited by a set of stimuli are used to derive the locations of the stimuli on a color diagram that is based on appearance and which we term a Uniform Appearance Diagram (UAD). The orthogonal axes of these space are red-green and yellow-blue; the location of a stimulus specifies its hue and its distance from the origin specifies its apparent saturation. We have investigated the uniformity of this space by using a subjects UAD, for a particular set of viewing conditions, to predict both small and large color differences under comparable viewing conditions. For small-scale differences we compared wavelength discrimination functions derived from UADs with those obtained by direct adjustment of a bipartite field. For large-scale differences, subjects rated the degree of similarity of pairs of different wavelengths; these ratings were compared with the distances separating the same pairs of wavelengths on a UAD. In both cases, the agreements were very good, implying that UADs are metrically uniform. Thus, UADs could be used to adjust the hues in a pseudo-color display so that all transitions would be equally perceptible or would differ by specified amounts.

Binocular Summation on Fused Annular Backgrounds

A paradigm has been proposed to test the hypothesis that binocular fusion results in simple summation, S, of monocular photopic luminances: S = ΔLBM(L)/ΔLB(2L) = 1, where ΔLBM and ΔLB are threshold luminances for a monocular and a binocular increment, appearing on a fused background of luminance L or 2L, respectively. This prediction was tested psychophysically, with background size as parameter. Thresholds were measured for a brief (20 ms) foveal flash centered on a luminous disk encircled by a massive black annulus serving as a fusion lock. The flash was presented binocularly, or monocularly, at 30 and 60 cd m−2 background luminances. The diameter of the disk was varied from 3 min to 40 min visual angle. The size of the flash was constant (6 min diameter), except for the smallest disk (3 min diameter). All thresholds varied as a function of background size (the known ‘sensitization effect’). The summation index S did not vary with background size. A mean value S = 0.9 was found; this is consistent with the notion that summation of the order of 90% occurs during binocular fusion. We surmise that even a weak monocular test flash makes fusion less than perfect.