STEREOBLINDNESS
Severing the corpus callosum 
leads to a loss of stereopsis in 
the shaded part of a subject's 
visual world.

    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 first 
circles 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 
breakdown 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 to the left.

    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 California, 
Berkeley, tested a subject who 
had had his corpus callosum cut 
to relieve epilepsy, and indeed, 
they found precisely this 
deficit.