This paper reflects the research and thoughts of a student at the time the paper was written for a course at Bryn Mawr College. Like other materials on Serendip, it is not intended to be "authoritative" but rather to help others further develop their own explorations. Web links were active as of the time the paper was posted but are not updated.
1998 First Web Reports
In Left Vs. Right Brain Modes, a direct comparison is presented in several categories. The left hemisphere is described as verbal, analytical, logical. The right hemisphere is nonverbal (responding to touch and music), intuitive, and sensory. In some ways the descriptions fit with the "left- handed artist" stereotype, such as the left hemisphere being factual, and the right brain responsive to color and shape. However, in other categories the "left handed artist" seems contradictory. For example, the left hemisphere (right-handedness) understands symbols and representations, while the right brain (left-handedness) is precise. As expected, the left hemisphere is identified with practicality and rationality, while the right hemisphere is emotional and originative.
This site also contains a list of behaviors in which each hemisphere is involved. It's still a bit surprising to match up those behaviors with handedness -it almost seems like having a palm reading!
So why does handedness occur in a ratio other than 50-50? Neuro Pearls, a web publication of the Dept. of Neurology at UT Southwestern, has an article titled "Handedness and Cerebral Dominance" by S. Clifford Schold, Jr, MD. He states that only 1 out of 10 people consider themselves left-handed or ambidextrous, and that for these people language function can be in either the left or the right hemisphere. There may be a genetic factor to handedness, as well as occurrence of brain injury. Not so common now is the pressure of society to be right-handed, and this explains people who are right-handed but still have speech function dominance from the right cerebral hemisphere. The presence of the language function in either hemisphere can be determined using the Wada test, in which a barbiturate is injected into an internal carotid artery and the effects upon language are observed.
It is still unknown why handedness and language dominance are related in the brain, or why language is not bilateral.
The American Speech-Language-Hearing Association (ASHA)s website contains the article "Language: Left Brain? Right Brain? Both?" which describes the history of discovering the two hemispheres have different duties to the body, especially in terms of handedness and speech. Damage to the left side of the brain in most of Dr. Marc Dax's patients caused a loss of speech (1836). After his death, many more similar reports were recorded.
Much later, the right hemisphere was found to control essential functions as well. Since the effects of right hemisphere damage are more subtle, as seen in the first website, this conclusion took much longer to reach. For example, a loss of speech would not be apparent, but instead irrelevant and rambling speech would be observed.
It is concluded from this site that both hemispheres are needed for a normal use of language, and to categorize the two hemispheres (as was done in the first website) would be gross oversimplification.
Paul Pietsch's SPLITTING THE HUMAN BRAIN describes a form of surgery once used, and its proper alternative today. To relieve severe epileptic seizures, Pietsch notes the separation of the two hemispheres as a most drastic but sometimes effective operation. A dangerous side effect other than the risk to life was the Split Brain or Jekyll-Hyde Effect which would occur if the two halves have no way of communicating between each other.
Pietsch suggests the use of Vogels new operation, called anterior cerebral commissurotomy, in which the cerebral retractor exposes the corpus callosum and severs the anterior commissure, but leaves the splenium intact. It was once believed this sequence of events was fatal, but modern research proves otherwise.
Again, a person who underwent this operation showed a temporary loss of speech, but as the years went by, this loss of speech became less apparent among patients. This shows that to some extent, the intact splenium does have a role in keeping the two hemispheres interacting properly. However, a surprise was discovered - the inability for a patient to utter the name of an object while it was in his left hand, though transferring the object ot the right hand restored the naming ability. There are yet more connections which are affected by the splitting of the brain!
The experiments then described which were an attempt to determine which hemisphere was controlling what are very interesting and make sense when assigning particular actvities to each particular hemisphere. It was thus concluded that the right hemisphere was not quite inferior to the left hemisphere and that it dealt with concepts such as logic as well as the left hemisphere dealt with words. And now that the two hemispheres were separated, each would have to rely on its own qualities to learn from the outside environment, and each did as well as the other.
It is still evident however, that between the two hemispheres, the dominant, or left one, will win in terms of which one gets control of certain body functions (such as voice). It is still an unanswered question as to how this occurs, even with a severed corpus callosum.
As a final note, it is to be recognized that the above tests were done with famed Roger Sperry, who shared the 1981 Noble Prize in Physiology and Medicine from his work with split-brain experiments.
All these sites together prove that both hemispheres contribute in
different ways to the normal functioning of speech, recognition,
handedness, and other behaviors. More recent research is showing that
although the left hemisphere of the brain may be more dominant, the right
hemisphere is just as crucial to a human being, and the interaction
between these two hemispheres is the root of all cooperation in the
central nervous system. Understanding these two hemispheres not only can
quench our thirst for knowledge and understanding of mapping for brain
functions, but can also give rise to new innovations for cures or remedies
for neurological disorders such as epilepsy.
| Forum | Back to Biology | Back to Serendip |