The Effects of Vertigo on the Zollner Illusion

    Numerous experiments have been done in an attempt to determine if displaced visual fields have an effect on visual illusions. Most recently, Prinzmetal and Beck (2001) found that several visual illusions' strengths were increased, including the Zollner, Poggendorff and Ponzo illusions. They found, however, that the Muller-Lyer and a size constancy illusion were not affected by body orientation because they are not related to orientation perception. Tilting the body, or even only the head, had an increased effect on the strength of the rod and frame test (Witkin and Asch, 1948 and 1992). Shimamura and Prinzmetal (1999) explain the tilt-induced illusions as attributed to distorted frames of reference. However, Wallace and Moulden (1973) found that body tilt had no effect. This present experiment attempts to shed further light on displaced visual fields and the effect on visual illusions by exploring a different kind of distortion of orientation: vertigo induced by spinning (dizziness). Our hypothesis was that the strength of the Zollner illusion would be increased due to the displaced visual field as a result of the vertigo.

METHODS
    Participants were twenty-four undergraduate students. A personal computer equipped with Eyelines Software (Beagley, 1990) was used. A typical office chair with rotating seat was used to induce the vertigo. Participants in the experimental group were asked to sit in the chair with their eyes closed, and then were spun for thirty seconds. The participants were then asked to adjust the parallels in the Zollner until they appeared to be parallel. The control group participants were not made dizzy and were asked to adjust the parallels as usual. The presented Zollner looked as follows:

RESULTS
    The control group's angle of adjustment was considerably lower and more accurate than the vertigo experimental group (see Figure 1). The Zollner illusion's strength was increased with the vertigo group, as hypothesized.

Figure 1. The degree of angle adjustment was considerably higher in the vertigo experimental group than in the control group for the Zollner illusion.

DISCUSSION
    As the results show, vertigo displaces the visual field and increases the strength of the Zollner illusion. Vertigo does change visual perception of parallels and intersecting lines, similar to past findings of body tilt and body orientation. The balance of and inside the body affects the strengths of the Zollner and other such visual illusions. There may be something inside the brain that relies on the orientation and balance of a body in order to perceive correctly, or more accurately. Throwing off this balance and changing it affects the ability to accurately perceive things, not only limited to visual illusions.

REFERENCES

Asch, S.E., & Witkin, H.A. (1948). Studies in space orientation: I. Perception of the upright with displaced visual fields. Journal of Experimental Psychology, 38, 325-337.

Asch, S.E., & Witkin, H.A. (1948). Studies in space orientation: II. Perception of the upright with displaced visual fields and with body tilted. Journal of Experimental Psychology, 38, 455-477.

Beagley, W.K. (1990). Eye Lines Software (computer program).

Prinzmetal, W., & Beck, D.M. (2001). The tilt-constancy theory of visual illusions. Journal of Experimental Psychology: Human Perception and Performance, 27, 206-217.

Shimamura, A.P., & Prinzmetal, W. (1999). The Mystery Spot illusion and its relation to other visual illusions. Psychological Science, 10, 510-507.

Wallace, G.K., & Moulden, B. (1973). The effect of body tilt on the Zollner illusion. Quarterly Journal of Experimental Psychology, 25, 10-21.

Witkin, H.A., & Asch, S.E. (1948). Studies in space orientation: III. Perception of the upright in the absence of a visual field. Journal of Experimental Psychology, 38, 603-614.

Witkin, H.A., & Asch, S.E. (1948). Studies in space orientation. IV. Further experiments on perception of the upright with displaced visual fields. Journal of Experimental Psychology, 38, 762-782.

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