Biology 202
2001 Third Web Report
On Serendip

It is thought that this has to do more with your brain and "neurological wiring" than habit. But what does this mean?

You have probably heard of hemispheric lateralization- hemispheric referring to the division of the brain into left and right halves, and lateralization describing the brain's tendency to make the two halves specialize in controlling different functions (1). Furthermore, the right side of your body is controlled by your left brain while the left side of your body is controlled by your right side of the brain. The connection between the hemispheres is the corpus callosum, a thick band of nerves. One of the most common beliefs is that the left hemisphere controls symbolic processing and rational thinking whereas the right hemisphere is more artistic, intuitive and creative (2). Many myths have emerged from this idea (5). We often hear students complain about being "right-brained in a left brained world" and others claiming that they do not perform well in a particular subject because they are either right-brained or left-brained. I remember being told that you should listen to lectures with your "dominant" side of my brain. In fact the idea has become so infused in today's society that we see several prevalent views of the left brain/right brain theory. A couple years back there was a car advertisement for Saab, which claimed the car as a "car for the left side of your brain." With so many simplified and popularized views of brain lateralization the task of sorting the fantasy from reality is daunting and not so easy. The big question remains standing- does hemispheric asymmetries exist and if so to what extent can we categorize functions as right or left brain, looking specifically at language?

At first glance the human brain appears symmetrical, leading one to wonder how it is that behavioral asymmetries emerge from biological identical hemispheres. Yet there are many cognitive and behavioral asymmetries in humans documented, many of which can be attributed to one hemisphere of the brain or the other (1) (5).

Yet before we explore hemisphere specialization it is interesting to note that the left and right hemisphere have shown to function independently. The notion of the left and right brain as functionally asymmetrical originally came from observations regarding the behavior of humans (2). The most obvious reason being that if the human brain where completely symmetrical, so that the right brain and left brain were functionally equivalent, then injury to homologous areas of the two hemispheres should have equivalent effects. Both Broca and Wernicke were led to postulate the importance of specific areas of the left hemisphere for certain aspects of language as a result of observing the correlation between injury to particular regions of the left hemisphere and specific language disorders. Broca for example based his observations on a patient named Tan. Though he could understand everything said to him, Tan was unable to respond. His speech was impaired and he could only respond in the monosyllabic word, "tan," thus deriving his name. Over time Tan's right side of his body became weaker and he eventually became paralyzed on that half. Broca predicted that patients like Tan with a disorder in speech articulation would have a lesion in the third left frontal convolution of the brain. When Tan patient died, a postmortem confirmed Broca's prediction. This area of the brain became known as "Broca's area" and the disease as Broca's aphasia. Though Broca's area is not the only area specialized for speech it illustrates that hemispheric lateralization does exist (4) (7).

Another speech disorder attributed to Wernicke's area was discovered in 1874. Here the patient articulated normally, even fluently, but the speech tended to be nonsensical, without coherency and full of lexical selection and grammatical errors. Both Broca's and Wernicke's areas were localized in what is called the language hemisphere, which is the left hemisphere for virtually all right-handed persons and the majority of left-handed persons. An indication that left hemisphere is dominant for aspects of language come from the so-called split brain experiments, which were explored in particularly vivid demonstrations of hemispheric asymmetry in the 1960s and early 1970s ().

In a normal brain, the left and right hemispheres are richly interconnected, with corpus callosum being the largest tract of interconnecting fibers. In the early 1960s Ronald Meyers and Roger Sperry did pioneering work with split brain patients. What made the split brain patients so interesting is that the left and right hemispheres are disconnected and thus no longer communicate with each other. Consequently these patients were used to determine how each hemisphere functions in isolation (3).

In order to examine the competence of a single hemisphere it is necessary to present the stimulus information to only one hemisphere at a time and to give one hemisphere the opportunity to respond. In one of the experiments a cat had its optic chiasm and corpus callosum severed, two independent learning centers were established - one in each hemisphere of the cat's brain. If the cat had its right eye open and its left eye covered and learned to make a simple conditioned response, it was unable to make the same response when the right eye was covered and the left eye was open. It was as if the learning was unable to be communicated to the other side of the brain; thus, it was obvious that information available to one side remained off-limits to the other. Sperry concluded that the brain had, "Two separate realms of conscious awareness; two sensing, perceiving, thinking and remembering systems" (3).

In another experiment split brain experiment, a word such as "fork" was flashed on the right side of the screen in a way that only the right hemisphere of a patient could receive the information. The patient would not be able to say what the word was. However, when asked write what he saw, his left hand would begin to write the word "fork" even though the patient himself had no idea what he had just written. The patent could sense that he wrote something but would have no idea what. Since there was no connection between the hemispheres on could conclude that information presented to the right half of the brain cannot convey this information to the left. If we follow the assumption from Broca's and Wernickes' speech studies that the language hemisphere is the left, then the results are all the more interesting (6).

Split brain studies indicate that the each hemisphere functions independently. On a similar note, structural asymmetries have been found in the brain. Differences between the length of the left and right sylvian (lateral) fissures were described as well as studies showing that the planum temporale, the dorsal surface of the temporal lobe, is typically larger in the left hemisphere than the right. Furthermore, because the left planum temporale (the dorsal surface of the temporal lobe) is significantly larger than the right in 65 percent of the population it was commonly accepted that a difference in the size of cortical regions could account for the left hemisphere's specialization for language (6). Despite these findings the categorization of functions to one hemisphere over the other is tricky. Many of the studies are based on patients with neurological diseases (i.e. split brain) and makes one wonder if these differences are as pronounced in normal patients.

In fact, in the intact brain, it is rarely the case that one hemisphere can perform a task normally whereas the other hemisphere is completely unable to perform the task at all. Bilingual individuals are shown to have both hemispheres specialized for language. In this case both hemispheres often have considerable ability to perform the same task, even though they may go about it in a different ways(6). Thus hemispheric difference tends to be smaller and more subtle than popularized views have suggested. Can listening to a lecture from my left ear so it would reach my language hemisphere give me a better understanding of the lecture and the language the lecture was in? Probably not. Thus, though some brain specialization exists we have one brain not two. The brain works as a unity. That is, the two hemispheres are part of a larger, anatomically extensive system that encompasses both hemispheres, the connections between them numerous subcortical structures, and more. All too often this fact is forgotten in the enthusiasm about hemispheric asymmetry and hemispheric "specialization." Thus, the two hemispheres interact in the normal brain to produce unity of thought and action.

WWW Sources

2) Splitting the Human Brain , Breif overview of Split brain surgery and experiments

3) Two Brains or One , Describes early experiments by Sperry

4) On Articulated Language , An essay on language disorders

5) Left Brain, Right Brain , A mainstream viewpoint on the Left/Right Brain phenomenon

6) Hemispheric Specialization , A cognet article on Hemisphere lateralization with information on language lateralization

7) The Brain and Language , An overview of Broca and Wernicke's areas of the brain

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