The tilt illusion (TI) is an illusion of orientation where a vertical line appears to be rotated away from slanted inducing lines. The apparent divergence of the 2 lines is called angle expansion. We hypothesize the TI is the sum of 2 mechanisms, one producing angle expansion which is limited to local cues, and the other angle contraction which is affected by global cues. Participants made judgments about the horizontal alignment of 2 dots presented in the gap between 2 fields of slanted lines. We varied the slope of the lines and the gap size. As predicted, by reducing local cues by introducing a blank gap between the fields of slanted lines, we found a TI characterized by angle contraction.


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Faculty supervisor: Jack Holmes, University of Wisconsin-Stevens Point