It has been found in previous studies that there are a number of variables that can influence brightness, one of which is size (Freeman, 1967). Nakatani, 1989, found that there is an assimilational illusion that occurs when stimuli are presented one after another. To control for this, he found that a set-fixing phase included in the study prevents assimilaltion. The purpose of this study was to determine the effect of size on the perceived brightness of two adjacent circles of varying size.
Methods
Twenty students volunteered to participate in the experiement. The experiment was designed and run on a Macintosh computer using Eyelines software developed by Dr. Walter Beagley at Alma College (1990). The stimuli consisted of two circles side by side on a solid white background. The set-fixing phase stimuli consisted of two medium sized circles (diameter 1.75 inches) of brightness 53% (percentage of white) for which brightness could not be adjusted. The experimental stimuli contained a circle on the left which remained the same size (diameter 1.75 inches) and bightness (53% ), and a circle on the right which was one of three sizes: small (diameter 0.75 inches), medium (diameter 1.75 inches) or large (diameter 3.25 inches). It was adjusted from a stsrting brightness level of 9.8% black (dark gray), 53% (light gray) or 0% black.
The order of stimulus presentation was random. Each circle size (small, medium and large) was presented twice for each color following the presentation of the set-fixing phase (Twenty-four screens, 12 adjustments total). The adjustment error was recorded for each trial.
The subjects sat two feet in front of the computer screen eye level with the stimuli. He/she was instructed how to adjust brightness of the circle on the right to the match the brightness of the circle on the left using the mouse. An experimenter was present at all times to help with questions.
Results
When looking at both the size and the initial brightness variables together, it becomes evident that there is an overall trend for subjects to adjust the smaller sized circles more, regardless of the initial brightness and the direction of adjustments of the stimulus. The subjects were consistent with this trend as they tended to adjust the medium and large circles less than the small. Therefore, when the circles were initially dark and had to be adjusted brighter, the subjects tended to adjust the small circles too much and actually overshot the standard brightness by a mean of 0.052. The subjects did not adjust the medium and large circles enough and in comparison to the standard circle were just short on the standard brightness by means of 0.023 and 0.0085 consecutively. (see figure one below)
This same effect was observed, but in the opposite direction, when the stimulus started from a light colored object and the subject had to adjust it to be the same brightness as the standard circle. Again, it was observed that the subject adjusted the brightness of the small circle more in the direction of the standard brightness. It was followed next by the medium circle; and finally, by the large circle producing the least adjustment. (see figure two below)
Discussion
Our overall results indicate that, neither the size nor initial brightness of the circles had an independent effect. First, this is indicated by the fact that when the medium and large stimili were adjusted from light to dark, the subjects tended to stop the adjustment to soon. In contrast, the smaller circles were adjusted more closedly to match the actual brightness. The adjustment error was positive which indicates that the subjects tended to see the large circle as matching in brightness even before it had reached the actual color of the standard. This contradicts what was expected according to the theory of lateral inhibition. According to this theory we would expect larger objects to appear lighter due to less lateral inhibition. Smaller objects on the other hand receive more lateral inhibition and therefore we would expect them to appear darker than they actually are. Our results may possilbly be due to "confusion" of the brain as to what it's been asked to do. Instead of processing the visual information to match the two circles in brightness, the brain may instead look at absolute amounts of darkness on the screen. Thus since the background was white there may have been a kind of "trade off" that occurs between the size of the circles and the amount of dark presence.Secondly, when considering both size and brightness together, we observed that generally small circles were manipulated more in either direction of adjustment. This may best be explained by the fact that smaller objects are more difficult to see. Thus the subject compensated by adjusting brightness to a larger extent and in the case of adjusting towards the darker hue actually overshoot the standard. This would indicate that the direction of the brightness adjustment appeared to have more of an impact on the effect of the illusion.
Therefore, varying one aspect alone does not appear to make much difference. However, size and direction of brightness adjustment appear to interact.
References
Beagley, Walter (1990) Eye lines. (Computer Program) Alma Colege, Michigan
Freeman, Robert B., Jr. (1967)The effect of size on perceived brightness. Psych. Bulletin, 67, 165-187.
Nakatani, Katsuya. (1989) Fixed set in the perception of size in relation to lightness. Perceptual and Motor Skills, 68, 415-422.
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