From: Anders Sandberg (asa@nada.kth.se)
Date: Wed Oct 30 2002 - 00:55:00 MST
On Tue, Oct 29, 2002 at 08:25:39PM -0800, Adrian Tymes wrote:
>
> If you mean you just looked at the column on the picture containing the
> two squares, you'd still have seen the shadow gradient. That's what
> causes the illusion - shadows being so common in nature that our image
> processing wetware automatically compensates for them.
Yes. It is the color constancy system that is fooling us. Our
subjective visual model of the world is quite different from
the objective spectral input - our brains are constantly
adjusting color and brightness estimates based on light
conditions, shadows and expectations. A red book still looks
red when it is in shadow or moved around against a colored
background - but that is largely due to us knowing it won't
change color and that it "really" is read; change the
conditions so that our knowledge doesn't apply (like looking
through a small tube so that you don't know what you are
looking at) and the color is changed. A grey sky is acually
almost as white as a sheet of paper, but experienced as dark
due to our knowledge of what a sky should look like. And a
shadowed checker square looks bright due to the contrast from
surrounding squares and adjusted for the effect of the shadow.
http://www.hhmi.org/senses/b140.html
http://psych.athabascau.ca/html/Psych402/Biotutorials/24/cortex.shtml
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