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
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.
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,
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 breakdown in connections responsible for binocular interaction in the cortex
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
In 1970 Donald Mitchell and Colin Blakemore, at the University of California, 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
previous 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 beyond 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
temporal fields overlap, you will also be blind. Closer than the fixation
point, however, your intact visual fields overlap, and stereopsis should be present,
provided 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
When tested, he proved to have exactly this curious combination of defects and abilities.