The Effect of Depth Perception on the Magnitude of the Muller-Lyer Illusion

Amanda Fritz, Nicole Lowis, Jennifer Loomis

05aafrit@alma.edu
Alma College

Using the perspective theory we know that the Muller-Lyer Illusion is viewed as having a difference in size because of the tail ends of the line. The tail ends that point in seem to make the line look closer and the tails that are pointed out make the line look farther away (Gregory 1966). There are many things that may also effect the reasons we do not see the illusion as equal, as stated by R.H. Day (1989, 1990) in his conflicting cues theory. This theory states that the length of the lines and the overall length of the figure both affect our perception of the illusion. The point of this experiment is to see if distance will have an effect on the magnitude of the illusion, and if greater distance equals stronger illusion or weaker illusion. The experiment was based on using a box to move the distance of a man made Muller-Lyer Illusion and seeing if depth perception is going to change the magnitude.


Methods

In this experiment the subjects were twenty Alma College students. The equipment used was black foam board that was attached to a wooden box to eliminate reference points, a peep hole made in the box for the subjects to look in. The length of the box was 18 inches, the width of the box was 15 inches, and the depth was 35 inches. The peep hole dimensions were 6 inches by 4 inches. The illusion itself was constructed out of poster board. The tails of the tails-in illusion were adjustable so that the subject could modify the length of the line to match it to the length of the tails-out line. Three slits, ten inches apart, were used to keep the distances fixed from subject to subject. Black dowel rods were used to support the seemingly free-standing illusion.

Subjects were then seated in front of the peep hole and asked to observe the illusion at the one of the three predetermined distances. The subject then informed the experimenter how much the tails-in line needed to be adjusted in order to match the lenght of the tails-out line. The number of centimeters of adjustment was then recorded. The means for each distance were then compared and graphed.


Results
As shown in Figure 1, there was an increase in the mean number of centimeters the illusion was moved at each ten inch increment. When the illusion was ten inches away from the subject, the mean error of the illusion was 0.13 centimeters. However, when the illusion was at the farthest distance (30 inches), the mean error of movement was 0.65 centimeters.


Discussion It can be concluded that depth perception can influence the strength of the Muller-Lyer Illusion. When a participant looked at the illusion, it was stronger when placed at the farthest distance from them. It is concluded that it is harder to correct the Muller-Lyer Illusion at a greater distance, because the lines are closer to equal even when they are not. When it is closer to the participants physically, it was easier to change the lines to be equal.


References
Gregory, R.L. (1966). Eye and Brain. New York: McGraw-Hill.
Day, R.H. (1990). The Bourdon Illusion and Haptic Space. Preception and Psychophysics, 47, 400-404.

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