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.