The Muller-Lyer illusion.

depth are used automatically whether or not the figures are seen to be lying on a flat surface. As Gregory predicted, the two-dimensional Muller-Lyer figures appear three-dimensional when presented as luminous models in a darkened room.

It seems likely that the depth cues of two-dimensional drawings would be less effective than those of photographs. Supporting evidence was reported by Leibowitz et al. (1969). They found that the extent of the Ponzo illusion was significantly greater with a photograph than with a drawing.

Gregory's misapplied size-constancy theory is ingenious. However, Gregory's claim that luminous Muller-Lyer figures are seen three-dimensionally by everyone is incorrect. It is puzzling that the Muller-Lyer illusion remains when the fins on the two figures are replaced by other attachments (e.g., circles). Such evidence was interpreted by Matlin and Foley (1997) as supporting the incorrect comparison theory, according to which our perception of visual illusions is influenced by parts of the figure not being judged. Thus, for example, the vertical lines in the Muller-Lyer illusion may seem longer or shorter than their actual length simply because they form part of a large or small object.

Evidence in line with incorrect comparison theory was reported by Coren and Girgus (1972). The size of the Muller-Lyer illusion was greatly reduced when the fins were in a different colour from the vertical lines. Presumably this made it easier to ignore the fins.

DeLucia and Hochberg (1991) obtained convincing evidence that Gregory's theory is incomplete. They used a three-dimensional display consisting of three 2-foot high fins on the floor. It was obvious that all the fins were at the same distance from the viewer, but the typical Muller-Lyer effect was obtained. You can check this out by placing three open books in a line so that the ones on the left and the right are open to the right and the one in the middle is open to the left. The spine of the book in the middle should be the same distance from the spines of the other two books. In spite of this, the distance between the spine of the middle book and the spine of the book on the right should look longer (see Figure 3.4). Many visual illusions are reduced or eliminated when the participants have to take some form of appropriate action with respect to the figure. For example, Gentilucci et al. (1996) carried out a study with the Muller-Lyer illusion (see Figure 3.3). The participants were asked to point to various parts of the illusion. There were small effects of the illusion on hand movements, but these effects were much smaller than those

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