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.