Although the Muller-Lyer illusion was first discovered in 1888, its effects are still not fully understood(Dewar,1967; Erlebacher & Sekuler,1969). This study was designed to further analyze the illusive effect of the acute angle figure of the Muller-Lyer illusion using the method of adjustment. The research hypothesized that the illusion would be strengthened by the addition of extra acute angles along the length of the line, adding to the subject's confusion and causing them to more drastically misjudge the actual length of the shaft.
Sixteen subjects were chosen randomly from the Alma College population of students and faculty. Eye Lines software was used to construct a test which could be conducted using Macintosh computers. The five test figures shown below were developed for the study.
All figures were 90 millimeters in length, with all oblique angles held constant at 25 degrees from the horizontal. The subjects were individually seated before a computer monitor which simultaneously displayed a test figure, and a second horizontal line whose length could be adjusted using the computer's mouse. Subjects were instructed to adjust the second line until they felt that it visually was the same length as the test figure. Each of the five test figures was presented twice. Once with the adjusteable line initially shorter, and once as longer than the test figure. These were all run in a random fashion for three trials.
All of the figures resulted in a broad range of variation among the subjects. The measurements obtained from the control line were used as a basis of comparison for the rest of the figures. The data indicated that the original acute angle Muller-Lyer illusion figure elicited the strongest effect. The segmented Muller-Lyer figure and the Muller-Lyer figure segmented with angles also produced a strong amount of error, although slightly less that the traditional Muller-Lyer figure. The plain segmented line resulted in less error than any of the various Muller-Lyer figures.
When analyzed, the results were in contradiction to the expected hypothesis that the illusion would be strengthened by the addition of extra acute angles along the length of the line, adding to the subject's confusion and causing them to more drastically misjudge the actual lenght of the shaft. The addition of either Muller-Lyer tails or segments made the line look shorter, but the two effects were not additive. If anything, the addition of segments to the Muller-Lyer weakened the illusion slightly. The results obtained in this study suggest that the typical error when measuring the acute angle Muller-Lyer figure results from the total figure. The acute angle version of the Muller-Lyer illusion figure cannot be made stronger by the addition of segments
References
Beagley, W. K. (1993) Eye Lines: Generating data through image manipulation, issues in interface design, and the teaching of experimental thinking. Behavior Research Methods, Instruments, & Computers, 25, 333-336
Dewar, Robert E. (1967). Stimulus determinants of the magnitude of the Muller-Lyer illusion. Perceptual and Motor Skills, Vol. 24, 708-710
Erlebacher, Albert and Sekular, Robert (1969). Explanations of the Muller-Lyer illusion: Confusion theory examined. Journal of Experimental Psychology, Vol. 80, No. 3, 462-467
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