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The essence of this illusion is that the 2-dimensional silhouette is
ambiguous. It's consistent with several possible 3-dimensional
interpretations, and our perception of such ambiguous stimuli is often
unstable and can spontaneously flip between different interpretations. The
most common interpretations of this illusion are a dancer rotating
clockwise, a dancer rotating counter-clockwise, and a dancer oscillating
from side-to-side.
Probably the best known related illusion is the Necker Cube, an ambiguous
2-D drawing of a wireframe cube which spontaneously reverses in depth
interpretation. A rotating Necker Cube, like the rotating dancer, also
appears to change direction as the depth interpretation changes. Michael
Bach also has a demo and explanation of the Rotating Necker Cube at
http://www.michaelbach.de/ot/sze_Necker/index.html.
Other well-known related illusions include the Ames Window and the Hollow
Mask, which Richard Gregory demonstrates on his website at
http://www.richardgregory.org/experiments/index.htm.
You're right, the Pulfrich Effect only applies when you have a dark filter
over one eye [or similar viewing conditions]. The reversal of the rotating
dancer silhouette happens without filters, so it does not involve the
Pulfrich Effect. (Though as an interesting variant, you can use the
Pulfrich Effect to affect your depth interpretation of the dancer.)
-Richard
At 12:53 PM 11/11/2008, you wrote:
>ISEN-ASTC-L is a service of the Association of Science-Technology Centers
>Incorporated, a worldwide network of science museums and related institutions.
>*****************************************************************************
>
>Richard: thanks for the links - Although you can see a related EEG
>(by following the links), I still don't have a clue as to what is
>going on....guess I need to do a little more research....
>
>I'm still not sure if it has to do with Pulfrich or not - if you go
>to the wikipedia link http://tinyurl.com/2rz4va, all the references
>require a darkened lens over one eye. It would appear the effect is
>physiological and has to do with retinal stimulation correlated to
>visual cortex processing time.
>
>
>
>
>
>
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