Figure 1. Ponzo Illusion
Numerous studies have found that orientation has a significant effect on perception. Sulzer and Zener’s study of perception of parallel lines illustrated the strength of the orientation effect. Each subject was shown two lines and asked to adjust a rotating line to parallel with a fixed line. Accuracy declined as the image was rotated toward 45 degrees, but then improved again as the image was shown closer to ninety degrees (1953). A similar study of orientation found that when asked to resolve line gratings, accuracy was poorest at oblique angles close to forty-five degrees (Emsley, 1925 as cited in Appelle, 1972). One of the most convincing studies on this effect was done by Craig and Lichtenstein (1953). After adaptation to lines of various orientation, subjects reported the least amount of disappearance when shown lines adjacent to a forty-five degree angle.
Effects of orientation have been extensively studied with illusions as well. The earliest studies examining the Jastrow illusion found that when exposed to lines in various orientations ranging from zero to ninety degrees showed greater ability to correctly perceive illusory differences when stimuli was presented as vertical or horizontal (1893).
The main goal of this study was to test if orientation had simlar effects on the Ponzo illusion. Specifically, we tested to see if orientation had any effect, and if so, if rotating toward a forty-five degree angle made results more inacurate.
METHOD
Subjects: 30 Alma College students (19 females, 11 males), aged 17-22, volunteered to participate in the study.
Materials: Subjects were shown the Ponzo illusion using the Eye Lines (Beagley, 1990) computer program on Macintosh computers. The original, vertical Ponzo illusion (fig. 1) served as control.
Procedure: Four versions of the Ponzo illusion were shown 3 times, for a total of 12 screens, shown in random, computer generated order. The illusion was turned 15 degrees to the right for each experimental condition, showing the illusion at 0, 15, 30, and 45 degrees, respectively. Subjects were asked to adjust a horizontal line to match the length of another fixed, parallel line by using the mouse. Before starting the study, subjects were instructed by the experimenters not to tilt their heads to the right or to the left. They were then asked move the mouse pointer to the bottom of the screen and start the test.
RESULTS
Initial analysis of the data showed that the mean error for each of the three trials of each Ponzo screen showed increased for the 15 degree (M = 5.537) and 30 degree conditions (M=8.21) , but, surprisingly not for the 45 degree condition (M=5.534). However, further analysis showed that the variability for each condition increased substantially as the angle increased. Of most interest is the standard deviation of each of the trials. There were small increases in standard deviation for zero degrees (sd=3.36), 15 degrees (sd=4.57), and 30 degrees (sd=5.62). However, the standard deviation for the 45 degree condition increased dramatically (sd=8.23). The data is shown in Figure 2. These data support our hypothesis that the 45 degree condition would show the most errors. Also, we performed an analysis of variance (ANOVA) to determine the signficance of the data. In accordance with our hypothesis, there was a significant interaction, F(3,90)=3.418, p<.05.
DISCUSSION
The results of this study are consistent with previous findings. As the orientation of the Ponzo illusion was increased toward 45 degrees, the amount of illusion strengthened. Even though the mean at 45 degrees was similar to the mean of the control, the variance of error was much higher, which suggests subjects performed less well than on other conditions.
REFERENCES
Beagley, W. K. (1990). Eye Lines [Computer Program]. Alma, MI: Alma College.
Craig, E. A., &Lichtenstein, M. Visibility-invisibility cycles as a function of stimulus-orientation. American Journal of Psychology, 1953, 66, 554-563 as cited in Appelle, S. Perception and discrimination as a function of stimulus orientation: The "oblique effect" in man and animals. Psychological Bulletin, 1972, 78, 266-278.
Emsley, H. H. Irregular astigmatism of the eye: Effect of correcting lenses. Transactions of the Optical Society, 1925, 27,28-41 as cited in Appelle, S. Perception and discrimination as a function of stimulus orientation: The "oblique effect" in man and animals. Psychological Bulletin, 1972, 78, 266-278.
Jastrow, J. On the judgment of angles and positions of lines. American Journal of Psychology, 1893, 5, 214-141 as cited in Appelle, S. Perception and discrimination as a function of stimulus orientation: The "oblique effect" in man and animals. Psychological Bulletin, 1972, 78, 266-278.
Sulzer, R. L., & Zener, K. A quantitative analysis of relations between stimulus determinants and sensitivity of the visual perception of parallelness. American Journal of Psychology 1953, 8, 444 as cited in Appelle, S. Perception and discrimination as a function of stimulus orientation: The "oblique effect" in man and animals. Psychological Bulletin, 1972, 78, 266-278.
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