Eye, Brain, and Vision

You can see another example of retinal rivalry if you attempt to fuse two
patches of different colors, say red and green, instead of vertical and horizontal
lines as just described. As I will show in the next chapter, simply mixing red
and green light produces the sensation of yellow. On the contrary, when the
two colors are presented to separate eyes the result is usually intense rivalry,
with red predominating one moment and green the next, and again a tendency
for red and green regions to break up into patches that come and go. The
rivalry however disappears and one sees yellow if the brightnesses of the
patches are carefully adjusted so as to be equal. It seems that color rivalry is
produced by differences in brightness rather than differences in hue.

STEREOBLINDNESS
Anyone who is blind in one eye will obviously have no stereopsis.
But in the population of people with otherwise normal vision, a surprisingly
sizable minority seem to lack stereopsis. If I show stereopairs like the ones on
page 37 to a class of 100 students, using polaroids and polarized light, four or
five students generally fail to see depth, usually to their surprise, because
otherwise they seem'to have managed perfectly well. This may seem strange if
you have tried the experiment of driving with an eye closed, but it seems that
in the absence of stereopsis the other cues to depth—parallax, perspective,
depth from movement, occlusion—can in time do very well at compensating.
We will see in Chapter 9 that if strabismus, a condition in which the two eyes
point in different directions, occurs during infancy, it can lead to the break-
down in connections responsible for binocular interaction in the cortex and,
with it, the loss of stereopsis. Since strabismus is common, mild forms of it
that were never noticed may account for some cases of stereoblindness. In
other cases, people may have a genetic defect in stereopsis, just as they can be
genetically color-blind.
Having paired the two topics, corpus callosum and stereopsis, I shouldn't
miss the chance to capitalize on what they have in common. You can set
yourself the following puzzle: What defect in stereopsis might you expect in
someone whose corpus callosum has been severed? The answer is revealed in
the illustration on this page.
If you look at point P and consider a point Q, closer than P and falling in the
acute angle FPF, the retinal images QL and QR of Q will fall on opposite sides
of the two foveas: QL will project to your left hemisphere and QR will project
to your right hemisphere. This information in the two hemispheres has to
connect if the brain is to figure out that Q is closer than P—in other words, if
it is to perform stereopsis. The only way it can get together is by the corpus
callosum. If that path is destroyed, you will be stereoblind in the shaded area.
In 1970 Donald Mitchell and Colin Blakemore, at the University of Califor-
nia, Berkeley, tested a subject who had had his corpus callosum cut to relieve
epilepsy, and indeed, they found precisely this deficit.
A second, closely related problem is to predict what deficit in stereopsis
would result from a midline section of your optic chiasm, such as Ronald
Meyers made in cats. This problem is in some ways the opposite of the previ-
ous one. From the figure on this page you can see that each eye will be blind in
its nasal retina, or its temporal visual field, and so you obviously will have no
stereopsis in the lightly shaded areas, where normally it is present. Out be-
yond this area only one eye can see at a time, so stereopsis is absent even
normally; now, however, you will be blind in that region, as indicated by the
darker shading. In the area beyond the fixation point, where the blind tempo-
ral fields overlap, you will also be blind. Closer than the fixation point, how-
ever, your intact visual fields overlap, and stereopsis should be present, pro-
vided your corpus callosum is normal. Again, Colin Blakemore found a
patient who had fractured his skull as a boy in a bicycle accident and as a
consequence apparently sustained a perfect midline section of his optic chiasm.
When tested, he proved to have exactly this curious combination of defects
and abilities.