Illusion - Is Seeing Really Believing?/Illusion - Is Seeing Really Believing (1998)(Marshall Media)[Mac-PC].iso/pc/illusion/hub_fie.cxt/00207_Field_frep68.txt

Text File | 1996-12-30 | 4KB | 127 lines

    That color can be so
computed predicts color
constancy because what counts
for each projector are the ratios
of light from one region to light
from the average surround. The
exact intensity settings of the
projectors no longer matter: the
only stipulation is that we have
to have some light from each
projector; otherwise no ratio
can be taken. One consequence
of all this is that to have color
at all, we need to have
variation in the wavelength
content of light across the
visual field. We require color
borders for color, just as we
require luminance borders for
black and white. You can easily
satisfy yourself that this is
true, again using two slide
projectors. With a red filter (red
cellophane works well) in front
of one of the projectors,
illuminate any set of objects. My
favorite is a white or yellow
shirt and a bright red tie. When
so lit, neither the shirt nor the
tie looks convincingly red: both
look pinkish and washed out.
Now you illuminate the same
combination with the second
projector, which is covered
with blue cellophane. The shirt
looks a washed-out, sickly
blue, and the tie looks black:
it's a red tie, and red objects
don't reflect short wavelengths.
Go back to the red projector,
confirming that with it alone,
the tie doesn't look especially
red. Now add in the blue one.
You know that in adding the
blue light, you will not get
anything more back from the
tie--you have just demonstrated
that--but when you turn on the
blue projector, the red tie
suddenly blazes forth with a
good bright red. This will
convince you that what makes
the tie red is not just the light
coming to you from the tie.

    Experiments with stabilized
color borders are consistent
with the idea that differences
across borders are necessary for
color to be seen at all. Alfred
Yarbus, whose name came up in
the context of eye movements
in Chapter 4, showed in 1962
that if you look at a blue patch
surrounded by a red
background, stabilizing the
border of the patch on the
retina will cause it to
disappear: the blue melts away,
and all you see is the red
background. Stabilizing the
borders on the retina
apparently renders them
ineffective, and without them,
we have no color.

    These psychophysical
demonstrations that
differences in the spectral
content of light across the
visual field are necessary to
perceive color suggest that in
our retinas or brains we should
find cells sensitive to such
borders. The argument is
similar to the one we made in
Chapter 4, about the perception
of black or white objects (such
as kidney beans). If at some
stage in our visual path color is
signaled entirely at color-
contrast borders, cells whose
receptive fields are entirely
within areas of uniform color
will be idle. The result is
economy in handling the
information. We thus find
ourselves with two advantages
to having color signaled at
borders: first, color is
unchanged despite changes in
the light source, so that our
vision tells us about properties
of the objects we view,
uncontaminated by
information about the light
source; second, the
information handling is
economical. Now we can ask
why the system evolved the way
it did. Are we to argue that the
need for color constancy led to
the system's evolving and that
an unexpected bonus was the
economy--or the reverse, that
economy was paramount and
the color constancy a bonus?
Some would argue that the
economy argument is more
compelling: evolution can
hardly have anticipated
tungsten or fluorescent lights,
and until the advent of
supersuds, our shirts were not
all that white anyway.