Lateralization of Function in Cerebral Hemispheres
Lateralization of Function in Cerebral Hemispheres
Jonathan BallOne of the major goals of neuroscience is to be able to understand the relationships between the structures of the nervous system and a persons outward behavior. Often times it is difficult or unethical to directly study the nervous system during a behavior and indirect methods must be used instead. One example of such an indirect method is using a subjects preferred hand to predict which of the two Cerebral Hemispheres is dominant. The are some difficulties with his method of studying lateraliztion of function but if it can be better understood it could have many practical and theoretical implications for the study of neuroscience.
The Cerebral Hemispheres are very similar in appearance, but they differ significantly in their structure. One of the best known differences between the two structures is motor control; the right hemisphere controls the left half of the body and the left hemisphere controls the right half of the body. These motor control differences were discovered mainly through the examination of paralysis caused by stokes or other damage to a specific hemisphere.
In 1861 the theory of lateralization of function was expanded with the discovery by Paul Broca of structure in the right hemisphere that controlled speech, this structure is now known as Broca's area Like many other advances in neuroscience this discovery was made possible by an unfortunate stroke victim, who in this case lost his ability to speak. After his death Broca examined his brain and discovered damage in the right hemisphere at this now famous location. This finding was followed soon after by the discovery of an area, also in the right hemisphere, responsible for understanding of written word. This area become know as Wernicke's area named after the man who discovered it by "studying patients with select comprehension deficits" (1) and comparing these deficits with damages to the brain.
These two discoveries created a watershed of experimentation as other scientists attempted to discover more functional differences between the hemispheres. This body of research has created a very stereotyped understanding of the two hemispheres. The left hemisphere is known as the language hemisphere. It is also believed to control general cognitive functions. Some researchers have gone so far as to ascribe consciousness to the left hemisphere. The right hemisphere is assigned nonverbal process such as special relations and the detection of complicated auditory tones . The right hemisphere is also credited with processing of non-verbal emotional stimuli and with musical abilities.
Even thought the two hemispheres have different functions they do not work independently of each other. They communicate back and forth across the corpus callosum. This is not an equal partnership however, one hemisphere usually dominates over the other, an effect best illustrated by the fact that most people are only good with either their right or left hand. In most cases the left hemisphere is believed to be the dominate hemisphere (4).
These distinctions were by no means absolute, so it soon become necessary to verify whether a given individual fit this profile. This was especially important to brain surgeons who wanted to be able to remove structures of the an epileptics brain without causing major impairments. In order to accomplish these goals many techniques were developed to analyze the functions of the two hemispheres in living people. The first method was called the Wada test. "The Wada test (named for a neurologist, Juhn A. Wada) consists of behavioral testing after the injection of sodium amobarbital (an anesthetic) into the right or left internal carotid artery" . This injection stops of the function of the cerebral hemisphere on which ever side it was injected for about 4-8 minutes (5) which gives the doctor time to perform language test on the functioning hemisphere. Another method of studying hemispheric differences which as been recently been develop in using MRI and PET scans in conjunction with language tests.
Although these techniques of assessing hemisphere function are very informative they are also costly, the Wada test is also inherently dangerous and only done in extreme cases, so a easier method of studying lateralization must be used. To find such a technique many scientist have turned to the use of handedness. The reasoning behind this choice is if a person is right handed it would indicated that their left hemisphere is dominate so it would be safe to infer that the speech functions reside there. If this reasoning proves to be sound then handedness would prove to be convenient method of recognizing the dominant hemisphere in subjects.
Handedness as a reflection of cerebral dominance, beside offering practical benefits, can add some interesting questions to the theoretical discussion of brain=behavior. If the use of a particular hand in a given situation can be seen as a behavior, then it can be used as an index for predicting other apparently unrelated behaviors or attributes. This is possible because two seemingly disconnected events are linked by similar neural functioning. In other words if a person is right handed it may be possible to infer that they have better language skills than left handed people because the right hemisphere of their brain is dominant. If this statement can be made, then one of the basic goals of neuroscience has been reached; the ability to make statement about behavior based on understanding of the structures of the nervous system.
Unfortunately the connection between hand dominance and hemisphere dominance is not as cut and dry as once thought. One of the major confounds with this connection is that between 70 and 95 percent of the population is left hemisphere dominant, this includes many of the people that would be considered left handed . So it is possible that cerebral dominance is not an all or none situation. Another problem is that there is a segment of the population which is ambidextrous. If the normal theory of handedness is followed then these people would have no dominant hemisphere and should possible demonstrated different verbal abilities then right hemisphere dominant individuals. And this is does not seem to be the case because the ambidextrous population and the left handed population that is not right hemisphere dominant, do not show any marked verbal deficits and least none recognizable in normal interactions.
One of the advocates of a reexamination of the study of handedness and lateralization is M.K. Holder who did her doctorate in on the subject. She argues that because "some unknown percentage of humans (maybe 5% to 30%) have anomalous patterns of specialization. These might include: (a) having a right-hemisphere language specialization or (b) having little lateralized specialization" (6) the current assumptions about the relationship between handedness and lateralization are oversimplified. This is especially evident in left handed people who demonstrate use of their "non-dominate hand" in many more activities than right handed people.
Holder believes that one of the major reasons for the flawed connection between handedness and lateralization is due to poor definitions. She argues that the standard assumption that your dominant hand is the one a person uses to write is too limiting and therefore it cannot lead to an adequate understanding of this complex correlation. She also questions other measures of hand dominance such as judging it on a continuum between left and right handedness because there is little information regarding the correlation of certain behaviors with hand dominance.
In order to better define the relationship between handedness and hemispheric dominance Holder has undertaken a project to collect extensive information on the difference between left hander and right handers across many different behavioral and demographic areas (I urge all of you to participate in this extremely interesting study at http://www.indiana.edu/~primate/forms/hands.html). By analyzing the hands which left and right handed people use for different behaviors she hopes to get a clearer picture of just how lateralized function is across the two hemispheres. In addition she maybe able to discover if in fact there is a correlation between a persons dominant hand and seemingly unrelated abilities such as writing. If certain personality traits, such as analytical or emotional can be linked to a particular hemisphere then it maybe possible to make inferences about an individuals personality based on which type of hand dominance they posses.
The basic question at the heart of this debate is the same one faced by almost all branches of neuroscience, can behavior be predicited by the functions for the brain and if so to what extent. In this particular case it is clear that which hemisphere is dominant does produce at least some difference in behavior. What remains to be established is how large a role differences in lateralization of function play in causing differences in behavioral abilities. As with any valuable branch of research once these questions are resolved a new set of questions will be created, such as what is the cause of lateralization, but much must still be done before this question can be proporly tackled.
WWW Sources1) Sleep and Language, from Washington University's Dept. of Anatomy and Neurobiology
2) Large-Scale Features of the Brain, a stop on a Brief Tour of the Brain led by Syracuse
3) V.E. Stone, L. Nisenson and M.S. Gazzaniga. Processing of emotional information in the Two Cerebral Hemispheres. An abstract from Gazzaniga's lab at UC Davis.
4) Piethish, Paul. Splitting the Human Brain. Introduction to the split-brain phenomenon.
5) The Wada Test. A description.
6) Holder, M.K. What does handedness have to do with Lateralization(and who cares)?
03/25/2005, from a Reader on the Web
I'm confused, aren't Broca's area as well as the Wernicke's area are located at the left hemisphere? The 3rd paragrah of your report says they're located on the right hemisphere.
You are right and the paragraph is wrong....Broca's patient (Tan) had damage in the left hemisphere and this area is now referred to as Broca's area which is in the 3rd frontal convolution of the left hemisphere ... Reader on the web, 22 September 2006
The left hemisphere controls speech ... Cathleen McFadden, 21 March 2007