Illusion - Is Seeing Really Believing?/Illusion - Is Seeing Really Believing (1998)(Marshall Media)[Mac-PC].iso/pc/illusion/hub_fie.cxt/00068_Field_frep16.txt

Text File | 1996-12-30 | 7KB | 249 lines

THE SIGNIFICANCE OF CENTER-
SURROUND FIELDS
Colors and brightnesses are
relative concepts: they are the
result of a computation done by
our retina and brain on the
visual scene.

    Why should evolution go to
the trouble of building up such
curious entities as center-
surround receptive fields? This
is the same as asking what use
they are to the animal.
Answering such a deep question
is always difficult, but we can
make some reasonable guesses.
The messages that the eye sends
to the brain can have little to
do with the absolute intensity
of light shining on the retina,
because the retinal ganglion
cells do not respond well to
changes in diffuse light. What
the cell does signal is the result
of a comparison of the amount
of light hitting a certain spot
on the retina with the average
amount falling on the
immediate surround.

    We can illustrate this
comparison by the following
experiment. We first find an
on-center cell and map out its
receptive field. Then,
beginning with the screen
uniformly and dimly lit by a
steady background light, we
begin turning on and off a spot
that just fills the field center,
starting with the light so dim
we cannot see it and gradually
turning up the intensity. At a
certain brightness, we begin to
detect a response, and we
notice that this is also the
brightness at which we just
begin to see the spot. If we
measure both the background
and the spot with a light meter,
we find that the spot is about 2
percent brighter than the
background. Now we repeat the
procedure, but we start with the
background light on the screen
five times as bright. We
gradually raise the intensity of
the stimulating light. Again at
some point we begin to detect
responses, and once again, this
is the brightness at which we
can just see the spot of light
against the new background.
When we measure the
stimulating light, we find that
it, too, is five times as bright as
previously, that is, the spot is
again 2 percent brighter than
the background. The
conclusion is that both for us
and for the cell, what counts is
the relative illumination of the
spot and its surround.

    The cell's failure to respond
well to anything but local
intensity differences may seem
strange, because when we look
at a large, uniformly lit spot,
the interior seems as vivid to us
as the borders. Given its
physiology, the ganglion cell
reports information only from
the borders of the spot; we see
the interior as uniform because
no ganglion cells with fields in
the interior are reporting local
intensity differences. The
argument seems convincing
enough, and yet we feel
uncomfortable because,
argument or no argument, the
interior still looks vivid! As we
encounter the same problem
again and again in later
chapters, we have to conclude
that the nervous system often
works in counterintuitive
ways. Rationally, however, we
must concede that seeing the
large spot by using only cells
whose fields are confined to the
borders--instead of tying up the
entire population whose
centers are distributed
throughout the entire spot,
borders plus interior--is the
more efficient system: if you
were an engineer that is
probably exactly how you would
design a machine. I suppose
that if you did design it that
way, the machine, too, would
think the spot was uniformly
lit.

    In one way, the cell's weak
responses or failure to respond
to diffuse light should not come
as a surprise. Anyone who has
tried to take photographs
without a light meter knows
how bad we are at judging
absolute light intensity. We are
lucky if we can judge our
camera setting to the nearest f-
stop, a factor of two; to do even
that we have to use our
experience, noting that the day
is cloudy-bright and that we are
in the open shade an hour
before sunset, for example,
rather than just looking. But
like the ganglion cell, we are
very good at spatial
comparisons--judging which of
two neighboring regions is
brighter or darker. As we have
seen, we can make this
comparison when the
difference is only 2 percent,
just as a monkey's most
sensitive retinal ganglion cells
can.

    This system carries another
major advantage in addition to
efficiency. We see most objects
by reflected light, from sources
such as the sun or a light bulb.
Despite changes in the
intensity of these sources, our
visual system preserves to a
remarkable degree the
appearance of objects. The
retinal ganglion cell works to
make this possible. Consider
the following example: a
newspaper looks roughly the
same--white paper, black
letters--whether we view it in a
dimly lit room or out on a beach
on a sunny day. Suppose, in
each of these two situations, we
measure the light coming to our
eyes from the white paper and
from one of the black letters of
the headline. In the following
table you can read the figures I
got by going from my office out
into the sun in the Harvard
Medical School quadrangle:

        Outdoors     Room
White paper    120    6.0
Black letter    12    0.6

    The figures by themselves
are perfectly plausible. The
light outside is evidently
twenty times as bright as the
light in the room, and the black
letters reflect about one-tenth
the light that white paper does.
But the figures, the first time
you see them, are nevertheless
amazing, for they tell us that
the black letter outdoors sends
twice as much light to our eyes
as white paper under room
lights. Clearly, the appearance
of black and white is not a
function of the amount of light
an object reflects. The
important thing is the amount
of light relative to the amount
reflected by surrounding
objects.

    A black-and-white
television set, turned off, in a
normally lit room, is white or
greyish white. The engineer
supplies electronic
mechanisms for making the
screen brighter but not for
making it darker, and
regardless of how it looks when
turned off, no part of it will
ever send less light when it is
turned on. We nevertheless
know very well that it is
capable of giving us nice rich
blacks. The blackest part of a
television picture is sending to
our eyes at least the same
amount of light as it sends
when the set is turned off. The
conclusion from all this is that
"black" and "white" are more
than physical concepts; they
are biological terms, the result
of a computation done by our
retina and brain on the visual
scene.

    As we will see in Chapter 8,
the entire argument I have
made here concerning black
and white applies also to color.
The color of an object is
determined not just by the light
coming from it, but also--and to
just as important a degree as in
the case of black and white--by
the light coming from the rest
of the scene. As a result, what
we see becomes independent
not only of the intensity of the
light source, but also of its
exact wavelength composition.
And again, this is done in the
interests of preserving the
appearance of a scene despite
marked changes in the
intensity or spectral
composition of the light source.