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Monday, October 12, 2009

Monday Musings - The Notorious Frankfurter Illusion

Let's see how many of us are awake enough this Monday morning to understand this optical illusion!

If you have two roughly equivalent eyes you will see a ‘sausage’ floating in front of you in mid air, by following these steps:

1. Hold your hands in front of you, at 20–30 cm distance from you, at eye level.
2. Point your index fingers against each other, leaving about 2 cm distance between them.
3. Now look “through” your fingers, into the distance behind them.
4. The sausage should appear now, and you can change its length by varying the distance between the finger tips.
5. For most observers, the sausage will look blurred, at least initially.
6. If you try to look at the sausage, it will disappear, it is only present if you look at something more distant than your fingers.
7. It helps if the background is rather homogenous and has a color very different from your fingers.

Explanation (information taken from website Michael's Optical Illusions & Visual Phenomena.)

Basically, this ‘sausage’ is caused by two mechanisms, (1) physiological double images and (2) interocular rivalry and suppression.

When you look at your fingers, the gaze direction of your two eyes is angled towards each other, so that their lines of sight meet at the target. When you then look into the distance, your eyes shift slightly outward, making their lines of sight nearly parallel. For close objects the image in the two eyes is consequently no longer at the right position, the images are no longer merged and can appear double for your “inner eye”. This is quite normal and occurs all the time, usually these double images are suppressed. So, if the two images overlap, why then doesn't the compound image look like the neighboring figure on the right?


At the end of the image of each finger, there is a rivalry between the image from the two eyes when the brain tries to combine them. In one eye the finger ends, in the other it continues. So what does your brain do in such rivalry situations? If the two images are rather similar, the percept can oscillate between the alternatives. Here, however, we have a high contrast step in one eye, namely the end of the finger, where it is replaced by the background. In rivalry the eye with the higher contrast wins, at least locally; this is here meant by the term ‘suppression’. In the figure on the left this high contrast step is symbolised by the yellow halo.

Links:
Wikpedia on binocular rivalry
84 Optical Illusions & Visual Phenomena

1 comment:

Bagman and Butler said...

Oh yes! I didn't know it had a name but I do this exercise quite often during business meetings. I have also learned that by slightly crossing your eyes while looking at two people, you can superimpose one head on the other and get a kind of morphed affect. This is best done, however, in business meetings that have power points so the lights are dim and your boss can't catch you looking cross-eyed at him.