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That is a very well done visual illusion.  This basic 
effect is well-known in visual science, but the technique 
for constructing this demonstration is new, and makes a 
useful contribution to the world of illusions.  Note that 
its creator, John Sadowski, has written a nice how-to 
tutorial, so anyone with Photoshop can now make their own 
version from their own colored photo (perhaps of their own 
science center?): 
http://www.johnsadowski.com/color_illusion_tutorial.html

Like many of the best illusions, this one actually 
combines multiple visual effects, including "chromatic 
adaptation", "isoluminance", and "filling-in", which 
synergize to make for an especially strong illusion.  This 
does make it more complicated to explain than just a 
simple afterimage, but I'll give it a shot:

The primary effect, as already noted, is an afterimage due 
to "chromatic adaptation", in which color-sensitive 
neurons in your eyes adjust their sensitivities and 
neutral points based on the colors they're exposed to. 
 That's responsible for the complementary colors seen in 
this illusion.  But the illusion is stronger than just a 
simple colored afterimage, such as "Bird in a Cage", 
because it also taps into the way colors are contained and 
accentuated by luminance edges.  (You can see just the 
simple afterimage part of this illusion by staring at the 
colored image, then switching your gaze to a blank white 
screen instead of the B&W image.)

Notice that the original colored image looks quite flat 
and out-of-focus.  It's actually a surprisingly detailed 
photograph, as you can see for yourself just by looking at 
it through a red filter.  We normally use the luminance 
contrast in an image to see its details and depth, but 
this colored image was processed in Photoshop to remove 
its luminance contrast and make it approximately 
"isoluminant".  (Viewing it through a red filter serves to 
restore luminance contrast.)

Also, notice that the second B&W image in the illusion is 
exactly superimposed on the colored image.  So as long as 
your eyes stay reasonably fixed on the dot, the afterimage 
colors will be contained within the appropriate achromatic 
edges, which stabilizes them.  In general, colors "fill 
in" within the edges in a picture - that's why it's okay 
to color only approximately within the lines.  (Aside: an 
interesting related effect is the division of the 
spectrum, in which superimposing thin black lines on a 
continuous spectrum -as from sun through a prism- seems to 
divide it up into discrete colored segments.)  When your 
eyes move off the dot, the colors no longer align with the 
edges, in effect smearing out the afterimage - this is 
also why we don't constantly see distracting afterimages 
in everyday life.

To give historical credit, this type of illusion was 
described and analyzed in a classic 1962 paper by 
neuroscientist Nigel Daw, titled "Why After-Images are not 
Seen in Normal Circumstances" (Nature 196: 1143 - 1145, 
1962).  And if you want to see a great new variation, 
check out this demonstration created last year in a Dutch 
vision lab, which vividly demonstrates how much afterimage 
colors depend on the edges that contain them: 
http://www.nici.kun.nl/~robvl/Filling%20in%20the%20afterimage.html

-Richard the Color Scientist


On Sun, 6 Jul 2008 19:03:51 -0700
  Chuck Howarth <[log in to unmask]> wrote:
> ISEN-ASTC-L is a service of the Association of 
>Science-Technology Centers
> Incorporated, a worldwide network of science museums and 
>related institutions.
> *****************************************************************************
> 
> An after image?  Yes, but...  It seems to me that there 
>is something  else going on here.  The standard 
>after-image effects last for  several seconds and slowly 
>fade away.  The effect Dennis found seems  different: 
> the image appears colored, and continues to appear 
> colored for quite a while as long as you continue to 
>look at the  dot.  But turn your gaze to the side, and it 
>reverts to black and  white.  If you then look back at 
>the dot, the color does not  reappear, as it would with 
>most other after-image effects.   So I  think you guys 
>got some more explaining to do.....
> 
> Chuck Howarth
> Gyroscope, Inc.
> 283 Fourth Street, Suite 201
> Oakland, CA  94607
> [log in to unmask]
> 510-986-0111
>  
> 
> On Jul 6, 2008, at 5:43 PM, Eric Yuan wrote:
> 
>>
>> I'm curious whether the afterimage effect plays a role 
>>in enhancing  
>> vision,
>> such as detecting motion, or widening our effective 
>>field of vision  
>> when we
>> scan our surroundings?  Maybe straightening out our 
>>vision during a  
>> bumpy
>> car ride?
>>
>> -Eric
>>
>>
>> -----Original Message-----
>> From: Informal Science Education Network
>> [mailto:[log in to unmask]] On Behalf Of 
>>Paul Koenig
>> Sent: Sunday, July 06, 2008 8:11 PM
>> To: [log in to unmask]
>> Subject: Re: [ISEN] Very neat illusion
>>
>>
>> It's an afterimage in the eye. I know the Exploratorium 
>>has a few
>> exhibits/activities based on the same thing. Here's what 
>>I found after
>> a quick search for more explanation:
>> http://www.exploratorium.edu/snacks/bird_in_cage/index.html
>> http://www.exploratorium.edu/snacks/afterimage/index.html
>>
>> It's an effect I know all too well after spending a lot 
>>of time with
>> anaglyph glasses.
>>
>> - Paul Koenig
>>
>> On Jul 6, 2008, at 4:50 PM, [log in to unmask] 
>>wrote:
>>
>>> Dear all,
>>>
>>> My son sent me an optical illusion unlike any I've seen 
>>>before.  I
>>> thought you would enjoy it.  Does anyone know what is 
>>>happening in  
>>> the
>>> eye/brain to make this work?
>>>
>>> http://www.johnsadowski.com/big_spanish_castle.php
>>>
>>> Dennis
>>>
>>> "Work hard to find something that fascinates you. When 
>>>you find it  
>>> you
>>> will know your lifework" -- Richard Feynman
>>>
>>> Dennis Schatz
>>>
>>> Senior Vice President for Strategic Programs
>>>
>>> Pacific Science Center
>>>
>>> 200 Second Ave. No.
>>>
>>> Seattle, WA 98109
>>>
>>> Phone - 206-443-2867
>>>
>>> Fax - 206-443-3631
>>>
>>> www.dennisschatz.org <http://www.dennisschatz.org>

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