Stereopair
of the Cloisters, New College,
Oxford. The right photograph was taken,
the camera was shifted about 3 inches to
the left, and the left photograph was taken.
Oxford. The right photograph was taken,
the camera was shifted about 3 inches to
the left, and the left photograph was taken.
7
THE CORPUS CALLOSUM
AND STEREOPSIS
The corpus callosum, a huge band of myelinated fibers, connects the two cere-
bral hemispheres. Stereopsis is one mechanism for seeing depth and judging
distance. Although these two features of the brain and vision are not closely
related, a small minority of corpus-callosum fibers do play a small role in
Stereopsis. The reason for including the two subjects in one chapter is conven-
ience: what I will have to say in both cases relies heavily on the special crossing
and lack-of-crossing of optic nerve fibers that occurs at the optic chiasm (see
illustration on p. 14), and it is easiest to think about both subjects with those
anatomical peculiarities in mind.
THE CORPUS CALLOSUM
The corpus callosum (Latin for "tough body") is by far the largest
bundle of nerve fibers in the entire nervous system. Its population has been
estimated at 200 million axons—the true number is probably higher, as this
estimate was based on light microscopy rather than on electron microscopy—
a number to be contrasted to 1.5 million for each optic nerve and 32,000 for the
auditory nerve. Its cross-sectional area is about 700 square millimeters, com-
pared with a few square millimeters for the optic nerve. It joins the two cere-
bral hemispheres, along with a relatively tiny fascicle of fibers called the ante-
rior commissure, as shown in the two illustrations on the following pages. The
word commissure signifies a set of fibers connecting two homologous neural
structures on opposite sides of the brain or spinal cord; thus the corpus callo-
sum is sometimes called the great cerebral commissure.
Until about 1950 the function of the corpus callosum was a complete mys-
tery. On rare occasions, the corpus callosum in humans is absent at birth, in a
condition called agenesis of the corpus callosum. Occasionally it may be com-
pletely or partially cut by the neurosurgeon, either to treat epilepsy (thus pre-
venting epileptic discharges that begin in one hemisphere from spreading to
the other) or to make it possible to reach a very deep tumor, such as one in the
pituitary gland, from above. In none of these cases had neurologists and psy-
chiatrists found any deficiency; someone had even suggested (perhaps not seri-
ously) that the sole function of the corpus callosum was to hold the two cere-
bral hemispheres together. Until the 1950s we knew little about the detailed
connections of the corpus callosum. It clearly connected the two cerebral
hemispheres, and on the basis of rather crude neurophysiology it was thought
to join precisely corresponding cortical areas on the two sides. Even cells in the
striate cortex were assumed to send axons into the corpus callosum to termi-
nate in the exactly corresponding part of the striate cortex on the opposite side.
In 1955 Ronald Myers, a graduate student studying under psychologist
Roger Sperry at the University of Chicago, did the first experiment that re-
vealed a function for this immense bundle of fibers. Myers trained cats in a box
containing two side-by-side screens onto which he could project images, for
example a circle onto one screen and a square onto the other. He taught a cat to
press its nose against the screen with the circle, in preference to the one with
the square, by rewarding correct responses with food and punishing mistakes
mildly by sounding an unpleasantly loud buzzer and pulling the cat back from
the screen gently but firmly. By this method the cat could be brought to a
fairly consistent performance in a few thousand trials. (Cats learn slowly; a
pigeon will learn a similar task in tens to hundreds of trials, and we humans
can learn simply by being told. This seems a bit odd, given that a cat's brain is
many times the size of a pigeon's. So much for the sizes of brains.)
Not surprisingly, Myers' cats could master such a task just as fast if one eye
was closed by a mask. Again not surprisingly, if a task such as choosing a
triangle or a square was learned with the left eye alone and then tested with the
right eye alone, performance was just as good. This seems not particularly
impressive, since we too can easily do such a task. The reason it is easy must be
related to the anatomy. Each hemisphere receives input from both eyes, and as
we saw in Chapter 4, a large proportion of cells in area 17 receive input from
both eyes. Myers now made things more interesting by surgically cutting the
optic chiasm in half, by a fore-and-aft cut in the midline, thus severing the
crossing fibers but leaving the uncrossed ones intact—a procedure that takes
some surgical skill. Thus the left eye was attached only to the left hemisphere
and the right eye to the right hemisphere. The idea now was to teach the cat
through the left eye and test it with the right eye: if it performed correctly, the
information necessarily would have crossed from the left hemisphere to the
right through the only route known, the corpus callosum. Myers did the ex-
periment: he cut the chiasm longitudinally, trained the cat through one eye,
and tested it through the other—and the cat still succeeded. Finally, he re-
peated the experiment in an animal whose chiasm and corpus callosum had
both been surgically divided. The cat now failed. Thus he established, at long
last, that the callosum actually could do something—although we would
hardly suppose that its sole purpose was to allow the few people or animals
with divided optic chiasms to perform with one eye after learning a task with
the other.
THE CORPUS CALLOSUM
AND STEREOPSIS
The corpus callosum, a huge band of myelinated fibers, connects the two cere-
bral hemispheres. Stereopsis is one mechanism for seeing depth and judging
distance. Although these two features of the brain and vision are not closely
related, a small minority of corpus-callosum fibers do play a small role in
Stereopsis. The reason for including the two subjects in one chapter is conven-
ience: what I will have to say in both cases relies heavily on the special crossing
and lack-of-crossing of optic nerve fibers that occurs at the optic chiasm (see
illustration on p. 14), and it is easiest to think about both subjects with those
anatomical peculiarities in mind.
THE CORPUS CALLOSUM
The corpus callosum (Latin for "tough body") is by far the largest
bundle of nerve fibers in the entire nervous system. Its population has been
estimated at 200 million axons—the true number is probably higher, as this
estimate was based on light microscopy rather than on electron microscopy—
a number to be contrasted to 1.5 million for each optic nerve and 32,000 for the
auditory nerve. Its cross-sectional area is about 700 square millimeters, com-
pared with a few square millimeters for the optic nerve. It joins the two cere-
bral hemispheres, along with a relatively tiny fascicle of fibers called the ante-
rior commissure, as shown in the two illustrations on the following pages. The
word commissure signifies a set of fibers connecting two homologous neural
structures on opposite sides of the brain or spinal cord; thus the corpus callo-
sum is sometimes called the great cerebral commissure.
Until about 1950 the function of the corpus callosum was a complete mys-
tery. On rare occasions, the corpus callosum in humans is absent at birth, in a
condition called agenesis of the corpus callosum. Occasionally it may be com-
pletely or partially cut by the neurosurgeon, either to treat epilepsy (thus pre-
venting epileptic discharges that begin in one hemisphere from spreading to
the other) or to make it possible to reach a very deep tumor, such as one in the
pituitary gland, from above. In none of these cases had neurologists and psy-
chiatrists found any deficiency; someone had even suggested (perhaps not seri-
ously) that the sole function of the corpus callosum was to hold the two cere-
bral hemispheres together. Until the 1950s we knew little about the detailed
connections of the corpus callosum. It clearly connected the two cerebral
hemispheres, and on the basis of rather crude neurophysiology it was thought
to join precisely corresponding cortical areas on the two sides. Even cells in the
striate cortex were assumed to send axons into the corpus callosum to termi-
nate in the exactly corresponding part of the striate cortex on the opposite side.
In 1955 Ronald Myers, a graduate student studying under psychologist
Roger Sperry at the University of Chicago, did the first experiment that re-
vealed a function for this immense bundle of fibers. Myers trained cats in a box
containing two side-by-side screens onto which he could project images, for
example a circle onto one screen and a square onto the other. He taught a cat to
press its nose against the screen with the circle, in preference to the one with
the square, by rewarding correct responses with food and punishing mistakes
mildly by sounding an unpleasantly loud buzzer and pulling the cat back from
the screen gently but firmly. By this method the cat could be brought to a
fairly consistent performance in a few thousand trials. (Cats learn slowly; a
pigeon will learn a similar task in tens to hundreds of trials, and we humans
can learn simply by being told. This seems a bit odd, given that a cat's brain is
many times the size of a pigeon's. So much for the sizes of brains.)
Not surprisingly, Myers' cats could master such a task just as fast if one eye
was closed by a mask. Again not surprisingly, if a task such as choosing a
triangle or a square was learned with the left eye alone and then tested with the
right eye alone, performance was just as good. This seems not particularly
impressive, since we too can easily do such a task. The reason it is easy must be
related to the anatomy. Each hemisphere receives input from both eyes, and as
we saw in Chapter 4, a large proportion of cells in area 17 receive input from
both eyes. Myers now made things more interesting by surgically cutting the
optic chiasm in half, by a fore-and-aft cut in the midline, thus severing the
crossing fibers but leaving the uncrossed ones intact—a procedure that takes
some surgical skill. Thus the left eye was attached only to the left hemisphere
and the right eye to the right hemisphere. The idea now was to teach the cat
through the left eye and test it with the right eye: if it performed correctly, the
information necessarily would have crossed from the left hemisphere to the
right through the only route known, the corpus callosum. Myers did the ex-
periment: he cut the chiasm longitudinally, trained the cat through one eye,
and tested it through the other—and the cat still succeeded. Finally, he re-
peated the experiment in an animal whose chiasm and corpus callosum had
both been surgically divided. The cat now failed. Thus he established, at long
last, that the callosum actually could do something—although we would
hardly suppose that its sole purpose was to allow the few people or animals
with divided optic chiasms to perform with one eye after learning a task with
the other.
The
corpus callosum is a thick, bent plate
of axons near the center of this brain sec-
tion, made by cutting apart the human ce-
rebral hemispheres and looking at the cut
surface.
of axons near the center of this brain sec-
tion, made by cutting apart the human ce-
rebral hemispheres and looking at the cut
surface.
Here
the brain is seen from above. On the
right side an inch or so of the top has been
lopped off. We can see the band of the cor-
pus callosum fanning out after crossing,
and joining every part of the two hemi-
spheres. (The front of the brain is at the
top of the picture.)
right side an inch or so of the top has been
lopped off. We can see the band of the cor-
pus callosum fanning out after crossing,
and joining every part of the two hemi-
spheres. (The front of the brain is at the
top of the picture.)
