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Here is the explaination that has been sent out to my neuroscience

graduate programs listserve by a fellow grad student:



1       There is something called the "negative retinal afterimage".

It becomes visible when one given hue stays on the same retinal position

for several seconds (usually we would move our eyes typically 3 times

per second, so this is not disadvantage in normal viewing). The

afterimage builds up as that retinal location adapts to this special

hue, and when looking at a neutral background the complementary colour

is seen.

        

2       This is a good thing, normally, because it helps "colour

constancy", that is we see colours somewhat independent of the ambient

illumination (compare the bluish glacier noon sun with a reddish tint in

the evening living room by the fireside).



3       Ok, so the afterimage is "burnt in", that is that retinal

location is adapted. Now the magenta patch is suddenly switched to grey.

Because of the adaptation, the complementary colour is now seen, which

would be green for magenta, or light grey for a dark grey.



4       The retinal afterimage typically fades away rapidly (over a

few seconds under normal conditions). But here this fade-out does not

reduce the perception of the afterimage, because a new one is uncovered

right after at the next location.



5       In addition, a Gestalt effect, here the "phi

phenomenon" comes into play: the afterimage from the successive retinal

locations is integrated and perceived as one single moving object,

namely the green disk.



6       In summary, the following factors make this illusion rather

compelling:

       * it is rather easy to steadily fixate on the centre

       * most of the time the retinal locations are re-adapted and

         the afterimage is uncovered only briefly

       * a Gestalt effect leads to the perception of a flying green

         disk.





On Fri Nov 4 9:47 , jeff courtman sent:



    ISEN-ASTC-L is a service of the Association of Science-Technology Centers

    Incorporated, a worldwide network of science museums and related institutions.

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    Ok, I'll bite...i think I understand the dots disappearing; kind of like

    the cheshire cat exhibit - you're brain keys on the movement at the edge

    of your peripheral vision....and I assume the green is the complimentary

    of the pink, but????



    Jeff Courtman

    Exhibits & Special Projects Coordinator

    251.208.6851

    www.exploreum.org



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