The Effects of Underlying Bar Patterns on the Cormack Illusion
Introduction
Cormack, Blake, and Hiris (1992) found that when peripherally viewing the direction of a bar moving diagonally against a lined background, subjects misperceived the direciton of the bar by up to 90 degrees. However, when viewed foveally, the illusion did not occur. The purpose of this experiment was to see whether or not manipulating the rectangular box so it appears underneath the lines had an effect when viewed peripherally.
Methods
For this experiment, we used twenty-four (M=10, F=14) Alma College undergraduate students who all participated on a voluntary basis.
A visual display of a manipulated Cormack Illusion was used on a Macintosh computer. It consisted of a 4.5x3 inch box with diagonal black and white lines which contained a rectangular box underlying the lines, creating a translucent effect. Using simple animation, the rectangular box was moved from the bottom left of the box to the upper right corner. This animation was created using the Superpaint program which was converted to be displayed using the Quicktime Player (See Figure1). Using the same process, the original Cormack Illusion stimulus was created for control, consisting of alternating black and white lines with a rectangular box overlying the lines, creating an opaque effect (See Figure 2). Another visual display located on a separate computer was constructed using the Eye Lines software. This consisted of a single line placed at a 45 degree angle which could be moved to create a degree of difference.
Subjects were asked to peripherally view the translucent version of the Cormack Illusion by focusing on a sticker placed on the wall in line of view over the computer screen. After viewing the moving rectangular bar, the subjects were asked to move the 45 degree angled line to match the angle of direction they saw the bar moving. Subjects were then asked to peripherally view the opaque version of the Coramck Illusion and again move the line to fit the angle of direction. The subjects alternated between the translucent and opaque versions of the illusion for a total of eight trials.
Figure 1. Translucent Version
Figure 2. Opaque Version
Results
It was hypothesized that the translucent version of the Cormack Illusion would produce a stronger effect of the illusion as compared to the opaque version. Although we found that the data was not significant (t=1.263, P<.2), it was found that the translucent version had a slightly larger effect (M=3.605, SD=10.149) in comparison to the opaque version (M=2.171, SD=8.709)(See Figure 3). Note that the figure shows a slightly larger variability in the translucent condition (V=103.005) than the opaque condition (V=75.854). Therefore, some subjects perceived a more stronger effect of the translucent version than other subjects.
Figure 1. Average Illusion Magnitude of the Translucent and Opaque Versions of the Cormack Illusion
Discussion
We concluded that the translucent version of the illusion produced a slightly stronger effect because one perceives more motion in the diagonal lines due to the fact that the lines do not end at the box.
There was a difference between the two versions of the illusion, however, it was difficult to see because of the great amount of variability throughout the subjects. It is possible that there are some people who are not able to perceive this illusion as strongly as others.
References
Beagley, W. (1990). Eye Lines. [Computer Program].
Beagley, W. (2000). [Personal Communication].
Cormack, Blake, & Hiris (1992). Misdirected Visual Motion in the Peripheral Visual Field. Vision Research (32). 73-80.
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