Brain Lateralization: The Split Brain

It’s Professor Dave, let’s split the brain. As we recall from our study of human anatomy,
we can divide the brain into two cerebral hemispheres. We call these the left brain and the right
brain, and they are entirely separate structures apart from the cerebral commissures that connect them. These hemispheres are not the same, and certain
brain functions are localized largely in one hemisphere versus the other. This does not support the myth of the left-brained
or right-brained individual, as was debunked in this fun tutorial, in case you are interested
in pop-psychology. But there remains much to be said about brain
lateralization, including the shocking realization that the two hemispheres can function independently,
and have almost completely separate identities. Let’s get a closer look at the split brain now. Much of our understanding of the split brain
came about due to experiments in the 1950s where the cerebral hemispheres were isolated
from one another by severing the corpus callosum, which is the largest cerebral commissure,
with around two hundred million axons connecting the two hemispheres. This was done for medical reasons with patients
suffering from severe epilepsy. What was quite surprising in all cases, is
that the subject behaves rather normally after this major procedure. Each hemisphere operates independently, and
no major function is lost. However, the hemispheres are unable to communicate
with each other, so it is no longer possible for one hemisphere to know something that
the other hemisphere has learned. The easiest way to test this is to feed different
visual information to each hemisphere of the brain. This is quite simple, given the way information
travels from the eyes to the visual cortex. The left eye sends input to both hemispheres,
but the right visual hemifield of the left eye goes to the left hemisphere, while the
left hemifield goes to the right hemisphere, and precisely the same goes for the right eye. So by selectively showing images to one eye
and not the other, a wide variety of data can be gathered. Speech is localized in the left brain, so
when showing a patient an image and asking them what they see, their verbal response
will depend on whether the image is being shown to the left brain or right brain. Sometimes, when information is given to both
sides of the brain, the left brain may respond verbally while the right brain responds some
other way, like pointing to a word or touching something, and the two answers will be completely
opposing or otherwise incompatible. This and many other experiments seem to suggest
that there are completely separate streams of consciousness going on in the two hemispheres,
each maintaining its own sense of self, at times even displaying different beliefs and
personalities. Now let’s get more specific about the roles
of each hemisphere. To be clear, nothing is totally and completely
lateralized. It is only that one hemisphere is favored
over the other in controlling certain functions. For example, regarding vision, the left hemisphere
is better at recognizing words and letters, while the right is better at recognizing faces,
emotions, and geometrical patterns. The left is better at interpreting spoken
language, while the right is better at analyzing other types of sounds, like music. The left is better at processing verbal memory
and finding meaning in memories, while the right is better at processing nonverbal memory
and recalling perceptual aspects of memories. The left is better for speech, reading, writing,
and arithmetic, while the right is better for spatial reasoning, rotating objects in
the mind, and discerning direction or distance. As we said, this is not to be exaggerated. Many of these actions involve multiple cognitive
activities and thus can’t be limited exclusively to one hemisphere. Furthermore, some people have the specializations
of the hemispheres reversed, so that language is on the right, and interestingly enough
these people are typically left-handed. As to why lateralization exists, some studies
focus on anatomical differences between the hemispheres, such as differences in size between
areas of the cortex in either hemisphere, which do indeed exist, but these have not
been conclusive. Most attempts to explain lateralization are
centered around the most highly lateralized of all brain functions, language, so let’s
more forward and get a closer look at that next.

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