MÜLLER-LYER ILLUSION

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INSTRUCTIONS

In FIGURE 1 below there is a horizontal line with inward turned wings. We call it the standard stimulus. WITHOUT CLICKING, place your mouse pointer OVER any of the red buttons at the bottom of the box, and a comparison stimulus appears. It consists of a horizontal line with outward turned wings which looks like this: . Each button reveals a comparison stimulus differing in length from all of its neighbours, and all differ in length from the standard stimulus, -- except one, which is exactly as long as the comparison stimulus above it.

Your task is as follows: WITHOUT CLICKING, first place your mouse pointer over each button in turn to view the comparison stimulus it controls, and systematically judge its length against that of the standard stimulus. When you are finally sure you have identified which comparison stimulus is exactly the same length as the standard stimulus, CLICK that button. If the two stimuli do NOT match, an advisory will appear which suggests you try again; otherwise it confirms your choice is correct.

FIGURE 1

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COMMENTARY

The difficulty encountered with this classic illusion illustrates the pre-eminence of the unconscious elements of perception over willful endeavour: Two lines of equal length appear otherwise, even if the observer has measured the actual length of the standard and comparison lines.

The illusion can be enhanced or diminished by elongating or shortening the wings at the ends of the lines, suggesting the wings provide depth cues normally found in real-world contexts. The size-distance theory suggests the outstretched wings at the ends of the longer-appearing line cause it to be perceived as more distant than the upper line whose in-turned wings make it appear closer. For example, if the two lines of the illusion are placed vertically as in Figure 2 below, the 'A' line could be the inside corner of a room and the 'B' line could be the outside corner of a building. However, when the two lines appear on a two dimensional plane, are of equal length, and there are no other depth cues, perspective gives way to illusion.

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