Effects of Retinal Fatigue on the Perceived Luminosity of Red and Green Color Samples
Stephanie Proudfoot and Lesley Scott
09saprou@alma.edu or 06lgscot@alma.edu
Alma College
Psychologists know that fatiguing the eyes to a chromatic color, such as red, allows the observer to see its green afterimage against a white background. The following are three experiments done on topics related to color, fatigue, and luminosity during the twentieth century.
Sir W. de W. Abney experimented with fatiguing a participants right eye to white light, then having the participant match a chromatic color seen in the right eye to a color on a spectrum seen in the left eye. He found that all hues become darker, but do not change hue. Then the participants right eye was fatigued with bright red light, which caused great difficulty in matching green rays in the spectrum (1912).
Much later, D. H. Kelly tested the observers ability to detect changes in contrasts of chromatic and achromatic colors, using an unstabilized flicker grating to detect minimum contrast thresholds without adaptation and a stabilized version to induce adaptation. Kelly found that in the stabilized model, participants could only discriminate between very high contrast achromatic colors, and there was no discrimination between the red and green chromatic colors. This was because opponent-color pathways do not respond to spatial patterns (such as the chromatic and achromatic grating) without variation (1981).
Slightly more recently, an experiment was done which tested the brains responses to change in luminance of red or green. The results showed that adaptation causes colors to be perceived as increasingly achromatic, due to desaturation of the color resulting from prolonged exposure to it (W. M. Paulus et al. 1986).
In this experiment, we concentrated on fatiguing the eye to red and observing how the red would appear in comparison with green, according to measurements of luminosity. The purpose of this experiment was to determine the extent to which isoluminant red and green color samples differ in perceived luminosity after adaptation to a red color sample of medium luminosity. Our hypothesis was that after fatiguing to medium red, a light red would appear lighter, a dark red would appear darker, and a medium red would appear to be the same luminosity as it was prior to fatiguing the eyes.
Methods
Participants:
21 college students took part in this experiment. They were both male and female, ranging in age from 18 to 22. They reported having no type of colorblindness. They also did not know the full purpose of this experiment until it had been completed, but were informed of the procedure.
Apparatus and Stimuli:
The experiment was set up and run with a slide show program on Windows computers, and consisted of seven slides shown on the screen in the same order. The first three slides and the last three slides were set up in the same way, with the top three-quarters of the screen displaying red and the bottom quarter displaying a selection of greens that differed only in luminosity (see figure 1). The greens were numbered from 1 to 15, with 1 being darkest and 15, lightest. To record answers, participants were given a sheet of paper numbered 1 to 15 for each of the test slides, and were told to circle the number of their choice. The reds varied from slide to slide, with slides 1, 2, and 3, and 5, 6, and 7, having light, medium, and dark luminosities, respectively. Slide #4 displayed only the medium luminosity red, without the selection of greens at the bottom. This served as the fatigue color.
Procedure:
All participants were clearly informed about the procedure of this experiment. They were brought into the computer lab one at a time and all used the same computer. For the first three slides, they compared the brightness of the red sample to the green samples at the bottom, and selected the green they thought best matched the red, according to perceived luminosity. These controls gave us a measure of error for the experiment. Then the participants were told to look at the center of slide #4 for 60 seconds, while the experimenter timed them. This was to cause fatigue to red of medium luminosity. Then they repeated the first part of the experiment for slides 5, 6, and 7, selecting from a range of greens the one they thought was isoluminant with the red sample at the top of the screen. The data from the greens that were selected was collected by hand, compiled into a table, and organized into a graph.
Results
Overall, participants tended to rate the post-fatigue light red as being slightly lighter than what it was before, and the post-fatigue dark red as being slightly darker, with no average change in the medium red. These changes were not very large, but the adjustments were made in the directions we expected them to be made (figure 2). There were also some deviations in the direction of change, but not enough to affect the average direction of change.
As shown in figure 2, light red increased in average perceived brightness by two-thirds of a step and dark red decreased in average perceived brightness by less than one-third of a step. The averages of the trials using a medium-intensity red equaled each other.
Several participants reported seeing gray instead of red on the post-fatigue light red slide. This may have skewed data for that slide because participants had trouble deciding what intensity the gray actually was. In fact, the gray often appeared the same intensity, or even darker than the original light red.
Discussion
These results show that although fatiguing the eyes to a medium-intensity red color sample causes a slight change in brightness perception, it does not produce enough of a change to be significant. The fact that there were deviations in the direction of change also suggests that fatigue does not have a large effect on brightness perception.
The most surprising finding in this experiment was that the post-fatigue light red actually appeared gray. Since the gray was reported to be about the same brightness as the pre-fatigue light red, this suggests that while hue and saturation are affected by fatigue, luminosity is not. As I stated before, this may have skewed the data because participants found it more difficult to match the gray with the green than the red with the green.
Along this same line is another factor which could have caused a distortion in the data: time. Most of the participants took so long deciding which green they should select for the gray sample that the strongest effects from fatigue may have worn off by the time they got to the last slide. This probably caused the difference in amount of adjustment between the light reds and the dark reds, where participants adjusted the post-fatigue light red more than twice the amount that they adjusted the post-fatigue dark red.
These findings support the article by W. M. Paulus, et al. They showed that fatigue causes desaturation of color, which was what caused the light red sample to appear gray. D. H. Kellys article about discrimination between chromatic colors was also supported by this experiment, because since the greens were not involved in the fatiguing process, they were subsequently unaffected by the medium-intensity red fatigue.
In conclusion, this means that although fatigue greatly affects perception of hue and saturation, it has relatively little effect on perception of luminosity. Fatigue to chromatic colors also has no effect on their color opposites.
More Posters
Web Posters from other courses and projects
Alma College Psychology Department