ISEN-ASTC-L is a service of the Association of Science-Technology Centers
Incorporated, a worldwide network of science museums and related institutions.
*****************************************************************************

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.
>
>
>
>
>
>
>***********************************************************************
>For information about the Association of Science-Technology Centers and 
>the Informal Science Education Network please visit www.astc.org.
>
>Check out the latest case studies and reviews on ExhibitFiles at 
>www.exhibitfiles.org.
>
>The ISEN-ASTC-L email list is powered by LISTSERVR software from L-Soft. 
>To learn more, visit
>http://www.lsoft.com/LISTSERV-powered.html.
>
>To remove your e-mail address from the ISEN-ASTC-L list, send the
>message  SIGNOFF ISEN-ASTC-L in the BODY of a message to
>[log in to unmask]
>

***********************************************************************
For information about the Association of Science-Technology Centers and the Informal Science Education Network please visit www.astc.org.

Check out the latest case studies and reviews on ExhibitFiles at www.exhibitfiles.org.

The ISEN-ASTC-L email list is powered by LISTSERVR software from L-Soft. To learn more, visit
http://www.lsoft.com/LISTSERV-powered.html.

To remove your e-mail address from the ISEN-ASTC-L list, send the
message  SIGNOFF ISEN-ASTC-L in the BODY of a message to
[log in to unmask]