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Brain Bisection Essay

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Updated: Dec 6th, 2019

There is one human brain but appears to be in two halves. The first one is called the left hemisphere while the second one is called the right hemisphere depending on the orientation of the person. It has been assumed that there is only one brain and therefore there is only one person.

But in the medical field, doctors made the discovery that it is possible to split the brain so to speak by cutting the main nerve that connects the two hemispheres. Researchers were able to discover that the left hemisphere is distinct from the right hemisphere.

After medical operations of this nature, researchers conducted experiments to find out the impact of such a medical procedure. Afterwards, the scientific community had to figure out if there are indeed two personalities within one human body.

The split-brain phenomenon is not a by-product of illegal human experiments. It is the unintended result of a medical procedure for epileptic patients. This is made possible by a medical procedure called cerebral commissurotomy or split brain surgery (Moor, 1982, p.91). In a typical surgery of this nature, the nerve that connects the two cerebral hemispheres is severed (Moor, 1982, p.91).

At first glance there seems to be two kinds of brain. A series of experiments were conducted to determine if there are negative effects to split brain surgery. The initial discovery is that the two hemispheres can function independent of the other.

The main question that surfaced after the discovery can be summarized in the following query: “Are there really in the brain thus divided, two separate conscious minds, in effect two co-conscious selves sharing the one cranium (Sperry, 1984, p. 662). There are two major implications of this theory.

First of all “a split-brain patient has two separate spheres of consciousness (Moor, 1982, p.92). Secondly, “two separate spheres of consciousness are two different selves” (Moor, 1982, p.92). It is important to know the answer to this question because a split-brain phenomenon that leads to split personality then the medical practice like the split-brain method must be continued. But there are other reasons why it is important to find out if there are two individuals inside one body.

Before going any further it is important to clarify the basic design of the human brain. According to one commentary “By and large, the left cerebral hemisphere is associated with the right side of the body and the right hemisphere with the left side” (Perry, 1975, p. 229).

Both hemispheres are linked to the spinal column and peripheral nerves through a common brain stem, but they also communicate directly with one another, by a large transverse band of nerve fibres called the corpus callosum, plus some smaller pathways” (Perry, 1975, p. 229).

The normal connection did not bring out what researchers were able to discover later on. But when the nerve was cut for medical purposes scientists were able to discover something that can radically alter how people see the function of the brain.

The Experiments

In one interesting study that supports the idea of brain bisection, a brain-splitting activity was performed as part of a medical procedure to treat a person’s epilepsy. In the said brain-splitting procedure the connection between the two halves of the brain was cut without injuring the brain and at the same time allowing the two halves to function correctly.

In the said procedure the optic chiasma was left intact and therefore there was no other means to connect to the two halves except through the eyes. It was therefore necessary for the researchers to create steps that would make it impossible for the right eye to see what the left eye has been shown in the subsequent experiment. The results stunned the scientific community.

The researchers flashed signals individually to the right and left eye. The signal was limited in duration and distance so that the left brain cannot access the signal intended for the right side of the brain. This procedure is known as tachistoscopic stimulation (Perry, 1975).

In one particular instance two different words were flashed to the two half fields for example the word toothbrush was flashed to right side and the word pencil was flashed to the left side. Interestingly, the right hand picked up the pencil and discarded it.

But the right hand continues to search for the toothbrush and after locating it announced that the task has been completed. In a similar fashion, the left hand rejected the toothbrush. But when it picked up the pencil, the subject also signalled to the researcher that the task has been completed. A more poignant example is given below:

A pipe is placed out of sight in the patient’s left hand, and he is then asked to write with his left hand what he was holding. Very laboriously and heavily the left hand writes the letters P and I. Then suddenly the writing speeds up and becomes lighter, the I is converted to an E, and the word is completed as PENCIL.

Evidently the left hemisphere has made a guess based on the appearance of the first two letters, and has interfered, with ipsilateral control. But then the right hemisphere takes over control of the hand again, heavily crosses out the letters ENCIL, and draws a crude picture of a pipe (Perry, 1975, p. 232).

In another experiment, a split-brain patient was shown two colours. The left hemisphere was exposed to the colour red while the right hemisphere of the brain was shown the colour blue (Preyer & Siebelt, p.138). In addition, the left hand was given a pen to right the answers to subsequent questions.

The same thing was done to the right hand. When the facilitator asked how many colours can the subject see, the reply was that there is only one colour. The subject had to answer the question by writing it down with either left or right hand. When asked to write down the colour that was observed earlier, the right hand wrote down blue while the left hand wrote down red.

It seems difficult to go against the idea that in a split-brain scenario the patient has developed two personalities based on distinct behaviour linked to the two hemispheres. It seems clear that the right hand did not know what the left hand was doing. It is critical to discover if there is indeed two persons battling supremacy in one body. However, the most logical position is given below:

An intact brain contains two cerebral hemispheres each of which possesses perceptual, memory, and control systems adequate to run the body without the assistance of the other. They cooperate in directing it with the aid of a constant two-way internal communication system.

Memories, perceptions, desires and so forth therefore have duplicated physical bases on both sides of the brain… the cooperation of the undetached hemispheres in controlling the body is more efficient and direct than the cooperation of a pair of detached hemisphere, but it is cooperation nonetheless (Perry, 1975, p. 243).

There are numerous studies that can bolster this argument. One major implication of the conclusion provided by Perr (1975) is that it is better for the two hemispheres to work together. The best way to corroborate that assertion is to focus on studies that revealed the distinct differences of the left and right hemisphere. An example is given below:

In this regard, we have found out that the right hemisphere of the patient has a sense of self, for it knows the name it collectively shares with the left. The right hemisphere has feelings, for it can describe its mood. The right hemisphere has a sense of the future, for it knows what day tomorrow is. The right hemisphere has aspirations and goals for the future, for it can describe its occupational choice (LeDoux, Wilson, & Gazzaniga, 1977, p. 419).

In another commentary the idea of two persons in one head could not be supported by facts “A more promising response to the argument begins with the observation that in actual case, the alleged two persons themselves do not believe they are two” (Davis, 1997, p.211).

In a controlled environment it is easy to show that the left hemisphere functions independent of the right hemisphere. Thus, there is a distinction that can be made between the two. However, there is not enough evidence to prove that both hemispheres can function without the other.

In order to push the study to the next level it is important for researchers to refine the experiment. It is important to continue conducting research on animals. A split-brain surgery must be made. In one of the studies that would be designed to know more about this phenomenon, researchers must try to eliminate the function of the left hemisphere and find out if the subject can still function in a normal manner. It is not clear how they can proceed but it can be argued that animal subjects may not survive such ordeal. One way to handle the problem is to kill the left hemisphere and observe if the body can be sustained with only half of the total brain capacity.

It is highly doubtable that researchers will be able to conduct experiments of this nature without experiencing a high-level of mortality. The brain is a very sensitive organ and the process of destroying one hemisphere can lead to death. If the subject cannot survive the initial phase of the experiment then it is unlikely that the said experiment can be replicated on humans. However, it is only through a more refined study can scientists truly determine if there are two persons in one cramium

One Brain and One Person

The main argument that shoots down the idea of multiple person or at least two persons in one head cannot be substantiated with facts. Even in patients that underwent special surgery there is no split-personality disorder that was documented to have occurred in the aftermath of the surgery. There is normalcy and the subject can perform normal tasks unhindered. If there are two personalities in one head then one can expect chaos.

The person with this problem cannot function normally in society. There should be evidence of agitated mental state. This should be the expected outcome if there are two forces that are battling inside the person. One of the most important arguments against the “two person” concept is the fact that split-brain surgery is considered an effective treatment modality for patients suffering from epilepsy. If there were complications and other unintended consequences because of the surgery then the medical community should have discontinued its practice.

Another important piece of evidence that would support the unified brain or one person theory is the observations made in the aftermath of a surgery that required the separation of the two hemispheres by cutting the nerves adjoining the two. According to one study, “these patients following surgery appear in ordinary, everyday behavior to be very typical, single-minded, normally unified individuals” (Sperry, 1984, p. 662).

In other studies the intelligence tests on patients that underwent brain bisection and hemidecortication showed no significant difference suggesting that one hemisphere is more dominant than the other (Zaidel, Zaidel, & Sperry, 1981). Another compelling argument is given by a group of researchers who asserted that a person cannot physically fall asleep and then at the same time swim in the ocean (Schechter, 2009, p.107).

Although the great cerebral commissure or the corpus callosum is the primary nerve that connects the two hemispheres, it is also important to point out that there are other additional connecting commissures (Churchland, 1986, p.174).

Researchers must not discount the fact that “since the brain is not divided at all in the midbrain and brain stem, there are presumably communicating routes via these structures” (Churchland, 1986, p.174). It must be made clear also that it is possible that there were no split-brain that occurred after the surgery.

In other words it is possible that the interesting research findings mentioned earlier were just temporary. It is possible that the brain can find a way to reconnect the two even without the main nerve that connects the two hemispheres.

Those who support the theory of divided self points to experimental results that shows that “the mute right brain communicates only with electrochemical means … sensory information from the patient’s left side is processed by the right side of the brain and vice versa” (Franks, 2010, p.3).

This argument can be used to favor the unified brain concept. It can be said that there are two hemispheres that may act independent of each other but having distinct sets of functions. The left hemisphere may be the main part of the brain that is in-charge of speech while the right hemisphere can be the part that is in-charge of non-verbal thought processes.


The split-brain surgery revealed amazing discoveries in the field of psychology and neuroscience. After numerous experiments conducted to determine the brain functions of patients that underwent split brain surgery, it was discovered that the left hemisphere is independent of the right hemisphere.

It seems that these two can function without the help of the other. However, based on observations made on normal people and those that underwent split brain-surgery, the design and function of the brain points to a unified whole. It has been discovered that the body needs these two hemispheres to work together as one.


Churchland, P. (1986). Neurophilosophy: toward a unified science of the mind-brain. MA: MIT Press.

Davis, L. (1997). Cerebral hemispheres. An International Journal for Philosophy in the Analytic Tradition, 87, 207-221.

Franks, D. (2010). Neurosociology: the nexus between neuroscience and social psychology. London: Springer Science.

Garvey, G. (2002). The split-brain human computer user interface. Leonardo, 35(3), 319-325.

LeDoux, J., Wilson, D., & Gazzaniga, M. (1977). A divided mind: observations on the conscious properties of the separated hemispheres. Annals of Neurology, 2, 417-421.

Moor, J. (1982). Split brains and atomic persons. Philosophy of Science, 49, 91-106.

Perry, J. (1975). Personal Identity. CA: University of California Press.

Preyer, G., & Siebelt, F. (2001). Reality and humean supervenience. Oxford: Rowman & Littlefield Publishes, Inc.

Schechter, E. (2009). How many minds: individuating mental tokens in the split-brain subject. MI: ProQuest.

Sperry, R. (1984). Consciousness, personal identity and the divided brain. Neuropsychologia, 22(6), 661-673.

Zaidel, E., Zaidel, D., & Sperry, R. (1981). Left and right intelligence: case studies of raven’s progressive matrices following brain bisection and hemidecortication. Cortex, 17, 167-186.

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