The Neuroscience of Illusion
(Top Posts - Science - 060208)

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May 28, 2008

How tricking the eye reveals
the inner workings of the brain

Brightness and Color Illusions
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In this illusion, created by Edward Adelson
at MIT, squares A and B are the same shade
of gray.


(If you don't believe it, print it out and then
cut out the two squares and place them side
by side.)

This illusion occurs because our brain does
not directly perceive the true colors and
brightness of objects in the world, but instead
compares the color and brightness of a given
item with others in its vicinity.

For instance, the same gray square will look
lighter when surrounded by black than when
it is surrounded by white.

- - -
May 28, 2008

How tricking the eye reveals
the inner workings of the brain

Shape Distortion Illusion
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This illusion is known as the Cafe Wall
illusion, and it was first discovered by
Richard Gregory's laboratory in a cafe
in Bristol, in the U.K.


The black and white tiles are perfectly
straight, but look tilted. It is a shape
distortion illusion: an object will appear
to take on shapes that are different from
its actual shape.

Like brightness and color illusions, shape
distortion effects are also produced by the
interaction between the actual shape of
the object and the shapes of nearby figures.

For the brain, perception is very often
dependent on context.

- - -
May 28, 2008

How tricking the eye reveals
the inner workings of the brain

Illusory Motion
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Some stationary patterns generate the illu-
sory perception of motion. The illusory
effect is usually stronger if you move your
eyes around the figure.


For instance, in this illusion, invented by
the scientist Akiyoshi Kitaoka, the "snakes"
appear to rotate. But nothing is really mov-
ing, other than your eyes!

If you hold your gaze steady on one of the
black dots on the center of each "snake,"
the motion will slow down or even stop.

Because holding the eyes still stops the
illusory motion, we speculate that eye
movements are required to see it. Vision
scientists have shown that illusory motion
activates brain areas that are similar to
those activated by real motion.

- - -
May 28, 2008

How tricking the eye reveals
the inner workings of the brain

Ambiguous Figures
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This bunch of violets contains the faces of
Napoleon, his wife, and their child. Can you
find them among the flowers?


During Napoleon's exile, his supporters used
to distribute reproductions of this 1815 en-
gravings. In such illusions, the brain interprets
same picture in two different ways, with each
interpretation mutually exclusive of the other.

You can see one of two possible images, but
never both of them at the same time. These
so-called ambiguous figures are especially
powerful tools to dissociate the subjective
perception from the physical world.

The physical object never changes, yet our
perception alternates between two (or more)
possible interpretations. For this reason, am-
biguous illusions are used by many labora-
tories in the search for the neural correlates
of consciousness.

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