Robert Cormack, Randolph Blake, and Eric Hiris (1991) discovered a motion illusion that occurs only in the peripheral visual field. The Cormack illusion consists of a 3 x 3 inch grid in the center of a screen. The grid is made up of alternating diagonal black and white lines. The stimulus is a solid black bar that moves at a 45 degree angle from the lower left corner of the grid to the upper right corner of the grid. When this is viewed in the periphery, the bar appears to move directly vertical or to the right.

We decided to test the validity of the idea that periphery is required to view this illusion. We believe that the illusion can only be viewed in the periphery because it is composed primarily of rods. In order to test this, we ran the Cormack illusion in a completely dark adapted situation. In this case the rods were both enhanced and isolated. The idea was that if the rods were the primary reason for seeing the illusion in the periphery, the illusion would be present when viewed fovially as well, as it is believed that a small amount of rods are also located in the foveal field of vision.

In order to test our hypothesis, we asked 28 undergraduate students to volunteer in our experiment. Fourteen of the participants were dark adapted for eight to ten minutes and then brought into a dark room that contained a computer with the Cormack illusion on the screen. The screen was set at a level of brightness where only a dark adapted person could see the Cormack grid. The experimenters ran the illusion as the participants traced the movement of the bar on a dull gray piece of paper. The other 14 participants viewed the Cormack illusion in its normal state. That is, the lights were at normal brightness and the grid and bar were completely visible. These participants traced the movement of the bar on a piece of white paper as the experimenters ran the illusion. All 28 participants experienced four trials, with each trial having a different focal point.

The results of our experiment did not support our hypothesis. In each of the four light and dark trials, there was some illusion present for all of the participants, but not enough to validate our argument. If you would like more information on the work that we have done, our E-mail addresses are: 98heintz@alma.edu, 97lane@alma.edu, 99jenkins@alma.edu

References

Cormack, R.,Blake, R., & Hiris, E. (1991). Misdirected visual motion in the peripheral visual field. Vision Research, 32, 73-80.

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