Eye, Brain, and Vision

COLOR AND
THE SPATIAL VARIABLE
We saw in Chapter 3 that an object's whiteness, blackness, or gray-
ness depends on the light that the object reflects from some source, relative to
the light reflected by the other objects in the scene, and that broad-band cells at
an early stage in the visual pathway—retinal ganglion cells or geniculate cells—
can go far to account for this perception of black and white and gray: they
make just this kind of comparison with their center-surround receptive fields.
This is surely Hering's third, spatially opponent black-white process. That the
spatial variable is also important for color first began to be appreciated a cen-
tury ago. It was tackled analytically only in the last few decades, notably by
psychophysicists such as Leo Hurvich and Dorothea Jameson, Deane Judd,
and Edwin Land. Land, with a consuming interest in light and photography,
was naturally impressed by a camera's failure to compensate for differences in
light sources. If a film is balanced so that a picture of a white shirt looks white
in tungsten light, the same shirt under a blue sky will be light blue; if the film is
manufactured to work in natural light, the shirt under tungsten light will be
pink. To take a good color picture we have to take into account not only light
intensity, but also the spectral content of the light source, whether it is bluish
or reddish. If we have that information, we can set the shutter speed and the
lens opening to take care of the intensity and select the film or filters to take
care of color balance. Unlike the camera, our visual system does all this auto-
matically, and it solves the problem so well that we are generally not aware
that a problem exists. A white shirt thus continues to look white in spite of
large shifts in the spectral content of the light source, as in going from over-
head sun to setting sun, to tungsten light, or to fluorescent light. The same
constancy holds for colored objects, and the phenomenon, as applied to color
and white, is called color constancy. Even though color constancy had been
recognized for many years, Land's demonstrations in the 1950s came as a great
surprise, even to neurophysiologists, physicists, and most psychologists.
What were these demonstrations? In a typical experiment, a patchwork of
rectangular papers of various colors resembling a Mondrian painting is illumi-
nated with three slide projectors, one equipped with a red, the second with a
green, the third with a blue filter. Each projector is powered by a variable
electric source so that its light can be adjusted over a wide range of intensities.
The rest of the room must be completely dark. With all three projectors set at
moderate intensities, the colors look much as they do in daylight. The surpris-
ing thing is that the exact settings do not seem to matter. Suppose we select a
green patch and with a photometer precisely measure the intensity of the light
coming from that patch when only one projector is turned on. We then repeat
the measurement, first with the second projector and then with the third. That
gives us three numbers, representing the light coming to us when we turn on
all three projectors. Now we select a different patch, say orange, and readjust
each projector's intensity in turn so that the readings we now get from the
orange patch are the same as those we got before from the green one. Thus
with the three projectors turned on, the composition of light now coming
from the orange patch is identical to the composition of light that a moment
ago came from the green. What do we expect to see? Naively, we would
expect the orange patch to look green. But it still looks orange—indeed, its
color has not changed at all. We can repeat this experiment with any two
patches. The conclusion is that it doesn't much matter at what intensities the
three projectors are set, as long as some light comes from each. In a vivid
example of color constancy, we see that twisting the intensity dials for the
three projectors to almost any position makes very little difference in the col-
ors of the patches.
Such experiments showed convincingly that the sensation produced in one
part of the visual field depends on the light coming from that place and on the
light coming from everywhere else in the visual field. Otherwise, how could
the same light composition give rise at one time to green and at another to
orange? The principle that applies in the domain of black, white, and gray,
stated so clearly by Hering, thus applies to color as well. For color, we have an
opponency not only locally, in red versus green and yellow versus blue, but
also spatially: center red-greenness versus surround red-greenness, and the
same opponency for yellow-blueness.
In 1985, in Land's laboratory, David Ingle managed to train goldfish to
swim to a patch of some preassigned color in an underwater Mondrian dis-
play. He found that a fish goes to the same color, say blue, regardless of
wavelength content: it selects a blue patch, as we do, even when the light from
it is identical in composition to the light that, in a previous trial and under a
different light source, came from a yellow patch, which the fish had rejected.
Thus the fish, too, selects the patch for its color, not for the wavelength con-
tent of the light it reflects. This means that the phenomenon of color constancy
cannot be regarded as some kind of embellishment recently added by evolu-
tion to the color sense of certain higher mammals like ourselves; finding it in
a fish suggests that it is a primitive, very basic aspect of color vision. It would
be fascinating (and fairly easy) to test and see whether insects with color vision
also have the same capability. I would guess that they do.
Land and his group (among others, John McCann, Nigel Daw, Michael
Burns, and Hollis Perry) have developed several procedures for predicting the
color of an object, given the spectral-energy content of light from each point in
the visual field but given no information on the light source. The computation
amounts to taking, for each of the three separate projectors, the ratio of the
light coming from the spot whose color is to be predicted to the average light
coming from the surround. (How much surround should be included has
varied in different versions of the theory: in Land's most recent version, the
surround effects are assumed to fall off with distance.) The resulting triplet of
three numbers—the ratios taken with each projector—uniquely defines the
color at that spot. Any color can thus be thought of as corresponding to a point
in three-dimensional space whose coordinate axes are the three ratios, taken
with red light, green light, and blue light. To make the formulation as realistic
as possible, the three lights are chosen to match the spectral sensitivities of the
three human cone types.
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 projec-
tor; 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 lumi-
nance 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 favor-
ite 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 cov-
ered 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 dem-
onstrated 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 Yar-
bus, 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 back-
ground, stabilizing the border of the patch on the retina will cause it to disap-