THE "EYES WIDE SHUT" ILLUSION
NOW PUBLISHED IN PERCEPTION, 2018
This is an easy non-computer demonstration, using a standard enlarging (make-up/shaving) mirror. Viewers sit about 70cm from the mirror and, closing one eye, look at the closed eye in the mirror; (almost all of the mirror should be taken up by the eye, eyelid, eyebrows; otherwise adjust your distance). Now, without moving, switch eyes, i.e. close the open eye and open the closed eye. Surprisingly, you now see the other closed eye! After switching back & forth a few times, open both eyes. Now you see an open eye. Which eye is it? To find out, close one eye. Strangely, whichever eye you close, that's the eye you see!
Each person looks at his or her own eyes. What can be more beautiful? Switching back and forth between eyes is fun and strange. The eye seen with both eyes open has a strange and beautifully hazy quality to it. This is due to mirror distortion, but also to the core of the illusion: actually you are fusing images of the two eyes!
We expect to see one eye or the other, so that by closing one eye, we will find out if this is the eye we are looking at – and the mirror-image eye should also close – or if this is the eye we are looking with – and the mirror image eye should not close, but should be shut off: We can't see with a shut eye! Surprisingly, whichever eye we close, that's the one we are looking at, not the eye we are looking with! How can this be possible?
With only a standard household mirror, we get a wonderful illusion. Everyone can test and experiment with this illusion at home, show it to friends and family, and discuss its origins. Since the explanation depends on a number of elements, it is extremely educational for exploring the visual system.
Significance to understanding the visual system:
The illusion rests on the following elements:
Binocular fusion: when the two eyes see different pictures, we obtain either binocular rivalry or binocular fusion. (In binocular rivalry, the visual system alternates every few seconds between two views. In binocular fusion, both views are maintained and united into a single percept.) To check whether you are really fusing two views, note where the nose is with the two eyes open!
Mirror images: when looking in a mirror, we do not reverse up-down nor left-right, only forward-backward (See Gregory, Mirrors in Mind). But we are used to seeing other faces (and writing) reversed. (We cross arms to shake right hands.) This adds to the confusion of which eye we are seeing. When viewing with one eye open, note on which side of the closed eye is the nose!
Obviously it would be hard to know the difference between the eyes if they were left-right symmetric. Are they? Not quite; the corners are different, the nose is only on one side, etc. But, close enough to fool us! That's why it is hard to know whether we are looking at the right or left eye – or actually at both, without doing the shut-eye test.
The Eyes Wide Shut Illusion as published in Perception 47(9), 985–990 (2018)
For the fuller unpublished version