Lateralization: The Split-Brain Experiments and Findings

One of the most interesting aspects of the brain deals with the differences between the left and right cerebral hemispheres. Although the brain may look similar in its entirety, both the left and right hemispheres perform different functions in daily life. Instead of the brain acting as a whole, it actually acts independently based on various areas of the brain. For example, we now know that the hippocampus or hippocampi (sense there are two located in the medial temporal lobe of the brain) plays a role in long-term memory, while the amygdale, another section in the medial temporal lobe, is responsible for emotional response, and memory processing, among other things. Although both are in close proximity, and are part of the same brain, they each play different roles. Similarly, the left and right hemispheres of the brain also play different roles.

After an unsuccessful speech on the possibility of lateralization of function in 1836 by Marc Dax, Paul Broca sparked interest in the topic after discovering that two of his patients with aphasia*, examed postmortem, were found to have lesions on the left-hemisphere of the brain. After more research into the subject matter, this small area near the back left side of the frontal lobe became known as Broca's area, and was the first real proof that the brain functioned, not as a whole, but as separate pieces.

So, if a brain functions based on hemispheres or separate areas, what would happen if a person lost half their brain? Would they still be able to function? Would they be able to survive? These are the exact questions that Myers and Sperry sought to answer in 1953 in an experiment using cats. Before we continue on with the experiment, it is important to know the function and location of the corpus callosum. The corpus callosum is a large band of approximately 200 million axons (Pinel, 2006) that connects the left hemisphere of the brain to the right hemisphere. Due to the Myers Sperry experiment, it was found that the corpus callosum's purpose was to transfer learned information between the two hemispheres. Curing the experiment, the corpus callosum was cut in order to see what would happen, an experiment that had been performed before but found that the monkeys and other animals used acted the same after the cut. However, Myers and Sperry found that the reason for this was because each hemisphere can act independently should the corpus callosum be cut. Essentially, the cats came to have two brains, learning with each side in order to cope with the loss of the corpus callosum.
Although this is true, we also now know that one hemisphere of the brain is most commonly dominant in language and comprehension. Typically, the left hemisphere is dominant in language comprehension and movement. Additionally, there are several tests which assess hemisphere dominance, these are the sodium amytal test, the dichotic listening test, and functional brain imaging.

During the sodium amytal test, a small dose of sodium amytal is injected into the carotid artery. The sodium amytal serves to anesthetize the hemisphere of the same side for several minutes allowing for the ability of the opposite hemisphere to be assessed. Assessment is conducted by asking the patient to recite lists of information such as the alphabet or days of the week. In order to test the opposite side of the brain, sodium amytal is injected in the second artery on the other side of the neck and the test is conducted again. Pinel (2006) explains that when the dominant speech hemisphere is anesthetized, "the patient is rendered completely mute for a minute or two; then once the ability to talk returns, there are errors of serial order and naming. In contrast, when the minor speech hemisphere...is anesthetized, mutism often does not occur at all, and errors are few. The left hemisphere is generally the speech dominant one, with the right hemisphere as the minor speech hemisphere. However, this is only what is common and does not apply to everyone all the time.

The next test, the dichotic listening test, consists of 3 pairs of spoken digits being read through earphones in which one set is read to the right ear (for example: 1, 4, 6) and the other is read into the left (for example: 3, 9, 2). If an individual has language dominance on the right side, their recitation of the listed digits were more accurate from the left ear than the right. Conversely, a more accurate right ear recall indicates left hemisphere language dominance. According to Kimura (1961), opposite hemisphere - ear connections are stronger when two different sounds are competing for access to the same cortical centers.

Functional brain imaging is the last of the split - brain tests. The functional brain imaging test involves a PET or fMRI monitor of brain activity while the test subject performs an activity (i.e. reading, drawing, watch a film, writing, etc). According to Pinel (2006), greater activity in the left hemisphere is generally revealed on language tests showing the typical dominance of the left hemisphere in language.

Further tests on speech laterality and handedness yielded that right-handed people are left hemisphere dominant in language, and most left-handed individuals are as well. An interesting find since one often assumes that right and left handed individuals have completely different functioning brains, however, according to the results reported in Russell and Espir, this is not the case. Russell and Espir (1961) instead reported that "the left hemisphere is dominant for language-related abilities in almost all dextrals (right-handers) and in the majority of sinestrals (left-handers)." The same study indicated that sinestrals are more subject to variation in respect to language lateralization. Similar studies on gender have found that females more often use both hemispheres than men during language related tasks (Allyn & Bacon, 2006).

Further research has been done in the area of split-brain to better describe it in terms of human beings. While research on animals revealed the capabilities of the brain, it didn't assure that human brains functioned in the same manner. In order to again test the theories behind the split-brain, Sperry and Gazzaniga were put in charge of evaluating epileptic patients who'd recently undergone commissurotomy (transaction of the corpus callosum while still leaving smaller commisures in tact). The idea behind transecting the corpus callosum was that epileptic discharges spread from one hemisphere to the next, and that transecting the corpus callosum would limit the discharge from spreading. When the procedure was conducted, it surprised even the researchers. It was found that those patients who underwent the commissurotomy no longer experienced the major convulsions that typify epilepsy. As Sperry and Gazzaniga evaluated the amazing results of the procedure, they also began to develop new tests by which to figure out just how the split-brain works in humans. In the end they came to the conclusion that the human brain was different from the animals they tested in that the right and left hemispheres had different abilities with the left hemisphere capable of speech, and right incapable (Allyn & Bacon, 2006).

Today, knowledge of split-brain has grown significantly. We now know that it is the left hemisphere of the brain that tell us what we are seeing, while the right hemisphere show us what we have seen. The Sperry and Gazzaniga found in their research that individuals presented with a picture to the right visual field, which corresponds with the left hemisphere, were able to tell what the object was. In fact Pinel (2006) explains that "A split-brain patient asked to name an object flashed in the left visual field is likely to claim that nothing appeared on the screen." Remember, this is because the right brain shows objects, while the left tells what an object is. In contrast to the right visual field, individuals presented with a picture in the left visual field, corresponding with the right brain, were unable to name the object. The same is true with objects placed in the left or right hand. Patients who had an object placed in their left hand, corresponding with the right hemisphere, were unable to identify the object although they can sense something is there. Oddly enough, since the right hemisphere can identify the object (although the individual cannot say what it is) they are able to pick out another just like it from a line up (Pinel, 2006)!

The concept simultaneous learning was also addressed in the split brain. It may sound odd, but split-brain does make it possible to learn two different things at the same time. Not only that, but once hemisphere will correct the other should the answer it gives on the simultaneously asked question. For example, test subjects were shown pictures of two different objects Pinel (2006) chooses to use a pencil in the left visual field and an orange in the right. When the patient was then asked to withdraw the objects from two separate bags at the same time and before taking them out tell the instructor what each hand had in it, the patient replied that there were two oranges. However, upon pulling the objects out of the bags, the patient performed exactly as the images had told him to, and a pencil was found in the left hand and an orange in the right. This is because the left hemisphere is verbal and verbalized that the object was an orange.

Pinel also tells of a similar experiment in which "the helping hand phenomenon is seen. Again the patient is shown a pencil in the left visual field and an orange in the right, but this time asked to pick out each object from a line up in from of them. "As the right hand reached out to pick up the orange under the direction of the left hemisphere, the right hemisphere saw what was happening and thought an error was being made...on some trials, the right hemisphere dealt with this problem in the only way it could: The left hand shot out, grabbed the right hand away from the orange, and redirected it to the pencil.

The split-brain also participates in visual completion, a phenomenon in which a split-brain uses information from surrounding areas in order to complete an image. In a test on the subject matter, patients were shown images of two different faces pasted together. When later asked to explain the faces they saw and point out examples in complete pictures, the patients pointed out complete faces, instead of the half faces they were actually shown.

Although there are other examples and experiments dealing with split-brain, each serve to support the concepts discussed. The brain is not one unit that functions as a whole, but rather a unit made up of many different parts which serve many different purposes. Moreover, the brain can be divided into left and right hemispheres which can function on their own, should their communication lines (i.e. the corpus callosum) be cut.

*Aphasia is a condition in which brain damage prevents or creates difficulty in producing or comprehending language (Pinel, 2006).

References:

Allyn & Bacon. (2006). Chapter 16: Lateralization, Language, and the Split-Bran. PPT. Web Tycho.

Russell, W.R, & Espir. (1961). Traumatic Aphasia - A Study of Aphasia in War Wounds of the Brain. London: Oxford University Press.

Pinel, J, P.J. (2006). Biopsychology, 6th Ed. University of British Columbia, Pearson.

Kimura, D. (1961). Some Effects of Temporal Lobe Damage on Auditory Perception. Canadian Journal of Psychology, 15. 156 - 165.