The”Two-Brain” Theory: Lateralization in the Separate Hemispheres

 

The human brain is asymmetrical in function, but is symmetrical in most structural aspects. Lateralization of the human brain is normally used to describe the asymmetrical functionality of the brain. Though the two sides of the body and brain are virtually mirror images of each other, the functionality of the two sides is partially unique, at the very least. For example, ninety percent of people are “right-handed”. The coordination in their right hand is far superior to the coordination in their left hand. Consequently, one arm or hand is almost always stronger than the other.

As far as the two hemispheres in the brain go, there is little similarity outside of structure; though the mind does strive for symmetry in some aspects. Individuals with obsessive compulsive disorder often have a fixation on things being symmetrical; for example, they will only walk on left-squares of the pavement or must have pictures on the wall perfectly aligned with one another. When this pattern is broken, the patient will experience severe anxiety (Hugdahl, 2005). Because the two hemispheres are functionally lateralized, nerve fibers in the corpus callosum serve as a method of communication between the two sides. Lesions in, or disconnection of, the nerve fibers can cause a person to behave as if he or she has “two separate consciousnesses”. Without this nerve connection, one side of the brain will not know what the other side is doing; hence, the “two-brain” theory was formulated (Cardoso, 1998).

It has been common knowledge for many years that the hemispheres of the brain are responsible for the functionality of the opposite sides of the body. It was not until a later date that scientists had an in-depth knowledge of the specialized functions unique to either the left or the right hemisphere. For instance, the left hemisphere is involved in language and rational tasks, while the right hemisphere is responsible for spatial relations and emotional responses (Cardoso & Hugdahl, 1998, 2005). By 1864, French neurologist Paul Broca discovered that when an individual has a lesion in the middle frontal gyrus of the left hemisphere, he or she will no longer be able to verbally communicate, but can understand when spoken to. This disorder is known as “Broca’s [expressive] aphasia.” Wernicke’s [impressive] aphasia, on the other hand, occurs when a lesion is in the left upper posterior area of the temporal lobe. The patient will have the ability to speak, but can not understand verbal dialogue (Hugdahl, 2005).

The study of epileptics has shed much light on brain lateralization, thanks to the initial research of Robert Sperry (Cardoso, 1998). In 1960-1970, Sperry conducted simple experiments on patients who had undergone surgery for epilepsy. By severing the corpus callosum, seizures can no longer spread from one side of the brain to the other (Hugdahl, 2005). The results of his study were so significant, that he received a noble prize for his efforts (Cardoso, 1998). Sperry placed individuals in front of a screen that had words projected on the left and right sides. He instructed the patients to focus their sight on a central point of the screen, so that any projection on the left would be interrupted by the right hemisphere (due to the ‘x’ shaped crossing of fibers from the eyes to the visual cortex). Amazingly, the patients would report they saw nothing when words were on the left side of the screen; whereas, when words were on the right side of the screen, they not only saw, but could also correctly identify them. Even in the first instance, the patients were able to pick up an item (placed outside their field of view) which corresponded to the words on the screen while using their left hand. How could someone be unable to see a word with their left eye, but be able to pick up the corresponding item with their left hand? The answer stems from the fact that the left hemisphere is responsible for verbal functions. Because the person saw the words with their left eye, the information was sent to be processed in the right hemisphere and then forwarded to the left-hand; since the information did not go through the left hemisphere, the person was not able to verbally express what they saw (Hugdahl, 2005).

Another intriguing finding, which adds to the “two-brain” controversy, is the asymmetry in structure and function of the two sides of the planum temporale. This structure, located in the upper posterior plane of the temporal lobes, is 30-35% larger in the left hemisphere than in the right. The left area of the planum temporale has broader spaced columns and more heavily myelinated axons, which indicate better connectivity per neuron and increased transmission speed, respectively. What makes this structure more intriguing is the fact that it is asymmetric in primates and humans only. Some researchers could hypothesize that humans and primates developed the essential structures for verbal processes, but that something went wrong in the primate’s evolutionary path. On the contrary, individuals opposed to the aforementioned theory could conclude that because chimpanzees do not communicate verbally, but have an asymmetric planum temporale, structure differences do not correlate with language functions (Hugdahl, 2005).

I have trouble making a solid stance on either side of the spectrum; my personal opinion is somewhat in the middle. On one hand, it seems as if the two sides of the brain are just that- two sides in one brain. Many studies have supported the conclusion that both sides are responsible for individual specialized functions. As commonly seen in stroke victims, when one side of the brain is damaged, many crucial human processes are no longer functional, as well as physical operations in an entire side of the body. Alternatively, language studies performed on individuals that had a complete hemispherectomy (or the removal of one hemisphere) differ greatly in children and adults. Even though adults seem to suffer from aphasia after a hemispherectomy, children under five years of age rarely suffer from this disorder. This could imply that language lateralization, though somewhat noticeable in children, does not have a profound effect until after puberty (Krashen, 2002); and that we are born with two separate brains that are capable of functioning independent from one another. Do the two parts of the brain “grow” to become dependent on one another as a person ages? As I stated previously, I am torn between both concepts. If one theory is ever agreed upon universally, I will be very shocked. There are very valid arguments in both cases and even though I might not be in complete agreement with either one, I know without a doubt I want to keep both of my hemispheres in tact!

References:

1. Cardoso, Silvia. (1998). What is Mind? Brain & Mind, No. 4. Retrieved February 20, 2006 from http://www.cerebromente.org.br/n04/editori4_i.htm

2. Hugdahl, Kenneth. (2005). Symmetry and asymmetry in the human brain [Electronic Source]. European Review, Vol. 13, No. 2, 119-133.

3. Krashen, Stephen. (2002). The Neurological Correlates of Language Acquisition: Current Research. In Second Language Acquisition and Second Language Learning, 72-82. Retrieved February 22, 2006 from http://www.sdkrashen.com/SL_Acquisition_and_Learning/076.html