Additive effect of backgrounds in chromatic induction

Vision Rusrurch Vol. 21, pp. 959 to 961. 1981 Printed in Great Britain

0042-6989/81/060959-03S@2,CO~0 Pergamon Press Ltd

LElTER

ADDITIVE

EFFECT

TO THE EDITORS

OF BACKGROUNDS INDUCTION

IN CHROMATIC

(Receioed I5 July 1980; in revisedform 24 Nouember 1980)

Within the last few years, a controversy has developed concerning the effect of a chromatic background field on the perceived color of a smaller incremental test field. Walraven (1976, 1979) has proposed that chromatic induction effects subtract the color of the background from the area covered by the increment, and that the background’s only effect on the perceived color of the increment is a result of shifting the relative sensitivities of the three cone types. Walraven has christened this subtractive induction effect “discounting the background”. Shevell (1978, 198Oa,b) has concluded that, in addition to shifting relative cone sensitivities, the background may either add or subtract its color from the color of the increment by a variable amount that depends on the parameters of the background. Shevell believes that Walraven’s observations represent only one special case of this general two-process phenomenon. In both Walraven’s and Shevell’s experiments, a variable mixture of 540nm and 66Onm light was superposed on a fixed-luminance 660 nm background. The subject’s task was to adjust the ratio of the 54Onm and 660 nm increments until their combination appeared to be neither reddish nor greenish (i.e. at “red-green equilibrium”). Walraven’s theory predicts that this ratio should be independent of overall incremental luminance. In contrast, Shevell’s theory predicts that relatively less of the 660 nm increment should be required at low incremental luminantes because of the color contribution of the 660 nm background. In addition, Walraven’s theory predicts that the 54Onm increment alone must always appear green, whereas Sheveil’s theory allows it to appear yellow or even red at sufficiently low luminances. Shevell (1978, 1980b) has reported that the 54Onm increment alone can appear yellow, but Walraven (1979) has criticized this result as unreliable because of the (presumed) closeness of the increment to threshold. Walraven’s prediction about the 54Onm increment alone can be generalized as follows: if an incremental stimulus of fixed spectral composition appears green * Pupil size was not measured or control14 so retinal illuminance was not precisely known. If the pupil diameters for the dimmest and brightest backgrounds were assumed to be about 7 and 5 mm, respectively, the corresponding retinal illuminances would be about 10 and 3.50td.

at one contrast level, then it must appear green at all visible contrast levels. The present Letter examines this prediction for conditions which are similar to those of Walravens and Shevell’s experiments. The results generally support Shevell’s two-process theory, but show some surprising differences between subjects which may account for at least part of the previous disagreement. Methods

The stimuli for the present experiment consisted of a foveally fixated circular “green” test flash superposed on a larger concentric “red” background. The subject’s task was to find both the increment threshold of the test flash and the luminance at which it appeared neither red nor green. All stimuli were projected into the hemisphere of a Tiibingen perimeter. No special fixation target was provided, since subjects could easily fixate the center of the background. The background was continuo~ly presented, and had a visual angle of 11”. The background color (dominant wavelength, 644 nm; excitation purity, loOo/,) was produced by a Schott RG2 colored glass filter. The test stimlilus had a diameter of 104’ and a flash duration of either 1.Oset or 0.1 set, and its color (dominant wavelength, 498 run; excitation purity, 800/,) was produced by the narrow-band green glass filter supplied with the perimeter. Each session began with 10 min of dark adaptation. A total of four background levels were then tested, ranging in luminance from 0.3 cd/m2 to 18 cd/m2 in 0.6 log unit steps.* These levels were always tested in order in increasing luminance. The subject was aBowed i-2min to adapt to each new level before testing was begun. For each flash duration, the increment threshold was first measured by a previously described descending limits procedure (Drum, 1980). When possible, the experimenter then found a range of test huninances (3 or 4 levels 0.1 log unit apartj which included the subject’s red-green equilibrium point. The experimenter then presented a pseudo-random sequence of test flashes which included a total of 5 trials at each level, and the subject responded after each triaf with the single hue name which best characterized the test flash. Although subjects were encouraged to respond either “red” or “green” when possible, responses of “yellow”, “white” or “neutral” were

959

Letter to the Editors

960

I set

LOG BACKGROUND

LUMINANCE

(cd/m*

1

I t

S-FA

LOG

I

I

0

I

BACKGROUND

LUMINANCE

I 0 ^

1 I

(cd/m’)

Fig. I. Increment thresholds (circles) and red-green equilibrium luminances (triangles) for a 498 nm incremental test flash on a 644nm background. as functions of background luminance. Test flash durations are 1.0 set (top graphs) and 0.1 set (bottom graphs). Data for subjects BD, WH and FA are in the left. middle and right graphs, respectively. All data points indicate geometrical averages for three sessions. All SE greater than 0.1 log unit are indicated by vertical lines.

also accepted. For computation purposes, “red” responses were arbitrarily assigned the number 1, “green” responses - 1 and all other responses 0. The red-green equilibrium point was then taken to be the luminance with a response sum of 0. No equilibrium point was claimed unless it was clearly bracketed by “red” responses below and “green” responses above. Results

Five subjects, all with normal color vision, participated in the experiment. Figure 1 shows the average results of three sessions for three of these subjects. All three subjects reported that the test flash clearly changed from reddish to greenish, at luminances anywhere from 0.6 log unit to 1.5 log units above the increment threshold. In spite of some individual differences, the effect was clearly present at all adapting levels tested, and was nearly identical for the two test flash durations. Between-session variability was small, and the hue responses for individual equilibrium measurements typically went from Consistently red to consistently green within a 0.3 log unit luminance range. All three subjects whose data appear in Fig. 1 were highly practiced psychophysical observers. However, only one of them (the author) was aware of the hypothesis being tested before the start of the experi-

ment. The other two subjects were told of the hypothesis after their first session, but this had no detectable effect on the results of subsequent sessions The results of two additional subjects are not shown because these subjects were unable to do the experiment. In repeated attempts to measure the redgreen equilibrium point, they reported that the hue of the test flash was always either greenish or so weak as to be uninterpretable. Both of these subjects were also highly practiced psychophysical observers, and were aware beforehand of the hypothesis being tested. Discussion

The results of Fig. 1 clearly refute the generality of Walraven’s “‘discounting the background” principle. The fact that a stimulus which appears green in isolation can appear reddish when superposed on a red background directly demonstrates that the background hue can mix with that of the increment. The reports of the two subjects who could not do the present experiment agree very well with Walraven’s (1979) description of his subjects’ responses when confronted with similar conditions. This agreement suggests that individual differences may be responsible for some of the discrepancy between Walraven’s and Sheveil’s data. Such differences do not imply, however, that some subjects discount the back-

Letter to the Editors and others do not. An equally plausible (and more parsimonious) explanation would be that both hue mixing and lateral induction processes are present for all subjects, but that the balance between them differs from subject to subject.

961 REFERENCES

ground

Acknowledgements-I am indebted to William Huppert and Fareed Armaly for technical assistance and to Robert Massof for providing me with a spectral transmission curve for the green filter used in the experiment. This research was supported by NIH Grant No. EY01672 and by a grant from Research to Prevent Blindness. Inc. Department of Ophthalmology, George Washington University, Washington, DC 20037, U.S.A.

BRUCEDRUM

Drum B. (1980) Cone threshold changes with dark adaptation.

vs. retinal eccentricity: Invest

Ophthal.

visual Sci.

19,432-435.

Shevell S. K. (1978) The dual role of chromatic backgrounds in color perception. Vision Res. 18. 1649-1661. Shevell S. K. (198Oa)Further evidence for the two mechanism model of chromatic adaptation. Invest Ophthal. visual Sci. 19 (ARVO Suppl.), 134. Shevell S. K. (1980b) Unambiguous evidence for the additive effect in chromatic adaptation. Visiort Res. 20, 637-639. Wahaven J. (1976) Discounting the background-the missing link in the explanation of chromatic induction. Vision

Res. 16, 289-295.

Walraven J. (1979) No additive effect of backgrounds chromatic induction. Vision Res. 19. 1061-1063.

in