Evidence that lightness contrast has an effect on the magnitude of the Ponzo illusion has been shown in Jaeger, Treiber and Pollack's (1980) study. They claim that the light intensity of the converging lines, or wedge, of the Ponzo illusion affects the overestimation of the enclosed line length. In their study, a Ponzo illusion with intermediate wedge lines caused the greatest amount of overestimation when the wedge lines were darker in color (black) than when the wedge lines were lighter in color (light gray). An effect of similar nature has also been shown in the Muller Lyer illusion (Weintraub, Tong, and Smith). It should be noted that in the previously mentioned Ponzo study the illusions only included one enclosed line between the wedge lines, and that subjects were asked to report the line as shorter or longer than comparison lines. In the current study, subjects were presented Ponzo illusions on a computer. These illusions contained two horizontal lines within the wedge and the subjects were asked to adjust the bottom line until it appeared to be the same length as the top line. The wedges were presented in three degrees of brightness: light, medium, and dark. The enclosed horizontal lines remained constantly black. The hypothesis was that subjects would be able to more closely adjust the length of the bottom horizontal line to match the length of the top horizontal line within a light gray than within a black wedge, because previous studies have shown that a gray wedge has a weaker effect on the magnitude of the illusion. Another effect that dealt with attention was also tested in this study. Half of the subjects were told to only focus on the horizontal lines in each illusion, and the other half were given no attention-related commands. The hypothesis is that the subjects who were told to only focus on the horizontal lines will be closer in their adjustment abilities than those who were not given such a command, due to attentional differences.
Subjects. 20 Alma College students.
Stimuli. The experiment was conducted on Macintosh computers using the Eye Lines software designed by Dr. W. Beagley. The stimuli consisted of three random cycles of three types of Ponzo illusions. All Ponzo illusions were structurally identical and consisted of a wedge of two converging lines and two parallel lines enclosed within each wedge. Each wedge line was 147 millimeters long with the left line of the wedge angled at 60 degrees and the right line of the wedge angled at 120 degrees. Each horizontal line was 58.8 millimeters long and was angled at zero degrees. Three different variations of Ponzo illusions were presented to each subject. One wedge had very light gray lines, the second had medium (darker) gray lines, and the third had dark black lines. Intermixed with the presentation of illusions were control lines without any wedge. These were presented to help separate the effect that the illusion had on the subjects.
Procedure. In each presentation of the illusion or control lines, subjects were asked to use the computer mouse and adjust the bottom line until it appeared to be the same length as the top line. All measurements were recorded by the software as the subjects completed the procedure. Half of the subjects were told prior to the adjusting task to focus only on the horizontal lines in each stimulus presentation. The other half of the subjects were not given any attention-related command.
The effect of the attentional variables on the subjects can be seen below in Figures 1 and 2.
The above results indicate that the attentional command hypothesis was not supported and, in fact, shows a somewhat opposite effect. The large difference between groups on control line adjustment should not be noted due to a few outlying numbers in the data affecting the means. The lightness effect was also not significant in this study, although it may have had some effect on the subjects' adjustments. This effect could be tested in the future using the same computer program without the attentional command. The fact that the attentional command did not have a noticeable effect on the subjects' overall judgment is probably due to the subjects' inability to block the Ponzo wedge out of their field of vision, even if told to focus only on the horizontal lines. Future studies could involve the use of alternative attentional commands to see if specific commands have an effect on perception and judgment of line length.
Jaeger, T., Treiber, F., Pollack, R. Effect of lightness contrast on Ponzo illusions. Bulletin of the Psychonomic Society, 1980, 15, 1-4.
Weintraub, D.J., Tong, L., Smith, A.J. Muller-Lyer versus size/reflectance illusion: Is the age related decrement caused by a declining sesitivity to brightness contours? Developmental Psychology, 1973, 8, 6-15.