The Muller-Lyer illusion is an image with a horizontal line with a chevron pointing in opposite directions on each end. When the tails of these chevrons point outward, the line appears to be longer than when they point inward (Welch, 2004). The most predominant theories of why this occurs are the perspective theory and the averaging (or confusion) theory. The perspective theory states that the illusion of length is caused by the misjudgment of size constancy. The tails make the two-dimensional image appear as a three-dimensional figure (Greene, 1997). In other words, the Muller-Lyer length illusion occurs because the tails indicate that the horizontal line is closer or further away Since the images are the same size on the retina, the distance between the eye and the image is misjudged (Welch, 2004).
This experiment tests the effect on the illusion of a second set of chevrons added to the tails-out figure. This would no longer make the figure look like the corner of a room, and therefore the perspective theory should not apply. The illusion should decrease.
Methods:
This experiment was set up using the Eye Lines Mac 3.2 program. The control figure was a line with the length of 130.5 mm. The single-tails-out figure was the same length as the control, with chevrons at both ends. Each tail was at 45 degrees from the horizontal and 26.1 mm long. The double-tails-out figure was exactly the same, only with an addition set of chevrons, 21.75 mm from the end of the horizontal line. While running the experiment, one of these three figures would appear, with an adjustable horizontal line below it, offset 21.75 mm to the right. Fifteen people were tested using these stimuli.
Each participant typed their initials into the program. They then were presented with one of the three figures, and the horizontal adjustable line. The participant lengthened or shortened the line until they thought it was the same length as the horizontal part of the above figure. This was repeated 8 more times, until each of the stimuli had been presented 3 times in a random order.
Results:
Table 1. Experiment Results
| Control Figure | Single Tails-Out Figure | Double Tails-Out Figure | |
| Mean | 5.53 mm | 12.39 mm | 13.02 mm |
| Standard Error | 1.00 mm | 1.65 mm | 1.78 mm |
The single-tailed illusion increased the estimated length of the line from the control. The double-tailed illusion did the same, to a greater extent. The standard error was effected in a very similar manner.
Discussion:
Instead of decreasing the illusion, adding a second set of chevrons increased the illusion by 8%. This may discredit the perspective theory; however it more likely credits it instead. Instead of looking like the corner of a room, the double-tails-out figure resembles a folded piece of paper, which may in fact increase the effect of the illusion, rather than eliminate it. This is supported by the results of this experiment.
Some problems with this experiment include small numbers of participation, as well the need for clarification of the experimental procedure for participants. The distance between the first and second set of chevrons could have also affected the results of this experiment, but there was no measure of that variable in this experiment.
References:
Delucia, Patricia R (1993). "A Quantitative Analysis of Illusion Magnitude Predicted by Several Averaging Theories of the Muller-Lyer Illusion." Perception & Psychophysics 53.5: 498. FirstSearch. Alma Col. 5 Apr. 2006.
Greene, Ernest, and Brian Nelson (1997). "Evaluating Muller-Lyer Effects Using Single Fin-Set Configurations." Perception & Psychophysics 59.2: 293-312. FirstSearch. Alma Col. 5 Apr. 2006.
Nijhawan, Romi (1992). "Three-Dimensional Muller-Lyer Illusion." Perception & Psychophysics 49.4 (1991): 333. FirstSearch. Alma Col. 5 Apr. 2006.
Predebon, John. "Framing Effects and the Reversed Muller-Lyer Illusion." Perception & Psychophysics 52.3: 307. FirstSearch. Alma Col. 5 Apr. 2006.
Welch, Robert B., Robert B. Post, Wayland Lum, and William Prinzmetal (2004). "The Relationship Between Perceived Length and Egocentric Location in Muller-Lyer Figures with One Versus Two Chevrons." Perception & Psychophysics 66.7: 1095-1104. FirstSearch. Alma Col. 5 Apr. 2006.
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