Split-brain functioning refers to how the two cerebral hemispheres of the brain are involved to different degrees in certain psychological and behavioral functions. In the normal, brain the two hemispheres work together in a coordinated manner and these differences in functioning complement one another. The division of psychological and behavioral functions between the two cerebral hemispheres can be referred to as functional lateralization, asymmetry, or brain laterality.
For centuries it had been suspected that the two hemispheres of the brain had specialized functions. Increased interest in certain divisions of function between the two brain hemispheres can be traced to the 1860s when Pierre Paul Broca (1824-1880), a French physician, reported that on autopsy a number of patients with speech impairments had lesions in the left frontal lobe of their brains. Systematic research on split-brain functioning, however, did not begin until the late 1960s, when Roger Sperry and his colleagues discovered certain regularly occurring differences in the functioning of the two brain hemispheres in split-brain patients (see below), and research has remained strong ever since that time.
Before discussing split-brain functioning in detail, knowledge of some very basic brain anatomy is necessary. The brain is that part of the central nervous system which is encased within the skull. The brain is an incredibly complex organ made up of billions of cells that work together to support life. Although the brain is usually thought of as a single structure, it is actually divided into two halves, which are called cerebral hemispheres. The two hemispheres, separated by a large fissure, are connected by several groups of nerve fibers that transfer information between the hemispheres. The most prominent connecting nerve mass is the corpus callosum. Control of basic physical movements and sensory functions is divided equally between the two hemispheres. Control of these functions by the brain is almost completely crossed, in that the right hemisphere controls the left side of the body, and the left hemisphere controls the right side of the body. For example the right foot, hand, and leg are controlled by the left hemisphere.
The oldest approach to gathering information about asymmetry between the two hemispheres of the brain is observation of behavioral changes or impairments in individuals with a brain injury that is clearly confined to one hemisphere of the brain. Lesions or areas of injury can now be identified using various techniques that allow visualization of the living brain. These brain-imaging techniques allow visualization of various properties of the brain such as cerebral blood flow patterns, and glucose utilization by different parts of the brain, as well as damage and unusual tissue masses. These brain-imaging techniques include computed tomography, magnetic resonance imaging, and x rays. These techniques aid in making inferences about the role of particular areas of the brain in contributing to certain behaviors.
The effects of brain injury must be interpreted with care as the brain tends to adapt to damage, and thus alter how it operates. Observed changes in behavior may more accurately reflect compensation of the remaining unimpaired tissue. Unimpaired tissue may also have a negative reaction, functioning worse than it did previous to the injury and increasing observed behavioral impairment. In sum, behavioral impairment due to injury to a particular area of the brain does not necessarily indicate that the injured area had controlled the impaired behavior. Finally, naturally occurring damage or lesions to the brain may occur across a number of different brain areas, and their effects on an observed behavior may be complex and unspecifiable.
Split-brain surgery is another method used to study lateralized functions of the brain and it has yielded a great amount of information. In split-brain surgery, the corpus callosum is severed. This procedure is used to stop the spread of seizure activity between the hemispheres in those with severe epilepsy. Patients who have had their corpus callosum severed show no changes in most of their daily behaviors. Their intellectual functioning and overall personality seem unaffected.
Scientists, by presenting sensory material to only one hemisphere, allowed observation of the functioning of the two hemispheres in isolation. Presentation to only one hemisphere can be accomplished by presenting stimuli to only one side of a sensory system, such as one eye, ear, or hand, while preventing perception by the sensory organ on the other side of the body. For example, a stimulus might be shown to only one eye, or an object might be put in a patient’s hand, making sure the patient could not see it. The patient is then asked about various aspects of the stimulus to assess how information is transferred between the hemispheres, and which aspects of information are available to the particular hemisphere being assessed. It should be noted that even when the corpus callosum is severed, the two hemispheres of the brain do communicate, albeit in a more limited fashion, through other connecting nerve fibers.
Injecting sodium amytal into the carotid artery on one side of the neck is another technique that allows observation of the lateralized functioning of the two hemispheres. The sodium amytal acts to temporarily anesthetize the brain hemisphere on the side into which it was injected. A patient then would be asked to perform certain tasks, and those tasks which the patient cannot perform or shows impaired performance in are then thought to be controlled to some extent by the anesthetized hemisphere.
Examinations of brains during autopsy have been used to locate brain injuries that are associated with specific behavioral impairment observed while the individual was alive. Researchers may also electrically stimulate certain areas of the brain to see which behavioral functions are affected. This does not hurt the patient as the brain does not have pain receptors. Various brain activities such as metabolic rate and blood flow may also be measured during behaviors associated with sensory processes.
Research on hemispheric lateralization in neurologically normal individuals has also been carried out primarily by studying visual field asymmetries and by using a dichotic listening procedure wherein subjects are presented with two different verbal messages, one to each ear, at the same time. Because the corpus callosum is intact in these individuals, researchers have had to tailor their stimulus presentation methods, for instance, by only very briefly flashing visual stimuli, in order to compare the abilities of the hemispheres in these individuals. These modifications seem to successfully tap into hemispheric differences, and results from this body of research generally support findings of hemispheric specialization in split-brain patients and in those with other neurological impairment. This research is important in that generalizing findings from split-brain patients and those with neurological impairment to those who are neurologically unimpaired is problematic.
While reports of physical differences between the two halves of the brain had been reported intermittently since the late 1800s, these differences were generally considered relatively minor and too small to explain observed differences in functioning of the left and right hemispheres. In 1968, however, research was reported that found strong and clear anatomical differences between the two hemispheres in areas believed to be of great importance for speech and language.
This research found a longer temporal plane in the left hemisphere than in the right in 65 of 100 brains examined at autopsy. Eleven brains had a longer temporal plane in the right hemisphere, and the remaining 24 showed no difference. On the average, the temporal plane was one-third longer in the left hemisphere than in the right. A number of studies have supported these findings, and on average, approximately 70% of the brains studied showed longer or larger temporal planes in the left hemisphere than in the right.
The temporal plane lies in a region of the brain called Wernicke’s area. This area was named after German neurologist Karl Wernicke (1848–1904) because he is credited with being the first to observe that injuries in this region often leads to various symptoms of aphasia. Aphasia is a general term describing any partial or complete loss of linguistic abilities that is caused by a lesion in the brain. Wernicke’s area seems to play a strong role in various language functions.
Another anatomical difference in the hemispheres involves the corpus callosum, which has been found to be larger in left-handers and those who are ambidextrous (showing no strong hand preference) than in those who consistently prefer their right hand. Some researchers believe it may be larger in these individuals because mental functions seem to be spread more equally across their hemispheres and this may necessitate a greater amount of interaction and thus connection between the hemispheres.
In addition to these larger physiological differences between the two brain hemispheres, there seem to be more microscopic differences, such as the dispersion of different types of brain cells. Examination of brains at autopsy has revealed some consistent differences, in the number and size of certain neurons between the two hemispheres. A region of the temporal lobe that is part of the auditory association cortex, which is involved in higher-level processing of auditory information and especially speech sounds, is larger on the left side of the brain. And an area lying mainly on the angular gyrus between the temporal and parietal lobes was also found to be larger on the left side. Lesions to this area have been associated with problems in naming objects and in word-finding tasks. Interestingly, enlargement of these areas in the left hemisphere is associated with having a larger left-temporal plane, so that larger anatomical asymmetries seem to be related to more microscopic asymmetries.
Thus it can be seen that there are some relatively consistent anatomical differences between the two hemispheres of the brain. Whether these physical differences are causally related to observed behavioral differences between the two hemispheres, however, is still unclear. This is partially due to the fact that much of this information has come from the study of brains post-mortem so that there is often little knowledge of the types of behavioral asymmetries these individuals may have exhibited before death.
One of the most apparent asymmetries related to the human brain is hand use preference. Differences in abilities between the hands reflect asymmetries in the cerebral hemispheres” functions. Studies show that about 90% of people across cultures are right-handed, while non-human animals tend to be divided pretty evenly in terms of limb preference. The question of why human beings show an overwhelming favoring of the right hand compared to other animals has been the subject of much theorizing and assumes greater importance when one understands that an individual’s handedness has been found to correspond in complex ways to how various functions are distributed between the left and right hemispheres.
Handedness is generally defined as the almost exclusive use of one hand for such activities as writing and other one-handed behaviors. There is much individual variation however in the frequency, strength, and efficiency of differential hand use, and an individual’s handedness can be assessed in a number of ways. While asking an individual which hand they tend to favor might seem the simplest approach, this does not tell the researcher about an individual’s possible common variations in hand preference across different activities which the individual may neglect to report. For instance, while someone may write with their right hand, they may throw balls, or use scissors with their left hand.
The most common method used to assess handedness is questionnaires that ask the individual which hand they use across a number of different behaviors. Findings from these questionnaires indicate that those who show a preference for the right hand use it more consistently across tasks than those who show a left hand preference. Those who preferred their left hand did not prefer it as consistently as right handers, instead they tend to also use their right hand in a number of behaviors. Some researchers believe direct behavioral observation of the individual performing a number of activities is the most precise method of determining handedness.
As stated earlier, an individual’s handedness seems related to how certain functions are distributed between the left and right hemispheres, and while there are numerous exceptions to every general rule about the asymmetry of certain functions, there are some commonly lateralized functions. The most obvious of these is language.
In those who are right-handed, with very few exceptions, speech functions are primarily located in the left hemisphere. This is also the case for most left-handers, but many more left-handers than right-handers seem to have speech functions located either mostly in the right hemisphere or distributed more evenly between the hemispheres. In general, it seems that left-handers are more likely to have an even distribution of certain behavior functions between the left and right hemispheres than are right-handers.
Injury of the left hemisphere, or presentation of information to the right hemisphere alone, often results in impaired speech, reading abilities, naming of objects, or comprehending spoken language. The left hemisphere in most human beings seems to exert primary control over linguistic abilities, as well as numerical and analytic behaviors. The right hemisphere in most human beings seems to exert primary control over nonverbal activities such as the ability to draw and copy geometric figures, various musical abilities, visual-spatial reasoning and memory, and the recognition of form using vision and touch.
There is also evidence that the two hemispheres of the brain process information differently. It seems that the right hemisphere tends to process information in a more simultaneous manner, synthesizing and bringing diverse pieces of information together. The left hemisphere seems to process information in a logical and sequential manner, proceeding in a more step-by-step manner than the right hemisphere.
In terms of the processing, experiencing, and expression of emotion, there are some very intriguing findings. Based on a number of studies looking at the location of lesions in individuals who showed uncontrollable laughter or crying, it seems the left hemisphere
Aphasia —A general term describing any partial or complete loss of linguistic abilities that is caused by a lesion in the brain.
Brain-imaging techniques —High technology techniques allowing non-intrusive visualization of the brain, these include computed tomography, positron emission tomography, and functional magnetic resonance imaging.
Cerebrum —The upper, main part of the human brain, it is made up of the two hemispheres, and is the most recent brain structure to have evolved. It is involved in higher cognitive functions such as reasoning, language, and planning.
Corpus callosum —The most prominent mass of nerve fibers connecting the two cerebral hemispheres of the brain; it aids in the transfer of information between them.
Hemispheres —The two halves of the cerebrum, the largest and most prominent structure of the brain, which are connected by a number of nerve fiber masses.
Laterality —Used generally to describe the asymmetry of the brain hemispheres in particular cognitive functions.
Lesion —Used commonly to describe a limited area of damage to living tissue matter caused by surgical intervention, disease, or injury.
Parietal lobes —Regions of the cerebral hemispheres that are above the temporal lobes and between the frontal and occipital lobes.
Split-brain surgery —A technique in which the corpus callosum is severed; it is used to stop the spread of seizure activity between the hemispheres in those with severe epilepsy. Research on cerebral asymmetry with split-brain patients has yielded a considerable amount of information.
Temporal lobe —The lobe of the cerebrum that is in front of the occipital lobe and below the lateral fissure.
Temporal plane —An area of the brain lying in a region called Wernicke’s area, so-called because Wernicke observed that injuries in this region often lead to aphasic symptoms.
is highly involved in the expression of positive emotions, while the right hemisphere is highly involved in the expression of negative emotions. Some researchers believe that the two hemispheres of the brain usually serve to mutually inhibit each other so that there is a balance, and uncontrollable emotional outbursts are rare. Based on tests with normal subjects, it seems that the right hemisphere plays a major role in the perception of emotion. In sum, much evidence indicates that the right hemisphere is more involved than the left in processing emotional information and in producing emotional expressions, but the left hemisphere seems to play a unique role in expressing positive emotions. The reader should note that research on brain asymmetry and emotion is relatively new and findings should be interpreted with caution.
Much research is being carried out to assess the roles of genetic and environmental factors in the development of hemispheric asymmetry. Because of the difficulty or impossibility in manipulating either of these factors, and or in designing studies that can accurately separate their influence, there are no firm conclusions at this time. It seems safe to say that environmental and genetic factors interact to determine the division of functions between the hemispheres.
It has become clear that the two hemispheres differ in their capabilities and organization, yet it is still the case that in the normal brain the two hemispheres work together in a coordinated manner, and both hemispheres play a role in almost all behaviors. Indeed, the differences in functioning between the hemispheres that have been found seem to complement one another. Research on how the two hemispheres differ and interact continues unabated, and improvements in technologies used to measure the brain as well as accumulating knowledge promise increasing gains in our knowledge of the human brain.
Annett, Marian. Handedness and Brain Asymmetry: The Right Shift Theory. New York: Psychology Press, 2002.
Kandel E, Schwartz J, Jessel T. Principles of Neural Science, 4th ed. p1182. New York: McGraw-Hill; 2000.
Klar, A.J.S.“An Epigenetic Hypothesis for Human Brain Laterality, Handedness, and Psychosis Development.” Cold Spring Harbor Symposia on Quantitative Biology. <http://www.cshl-symposium.org/doi/abs/10.1101/sqb.2004.69.499?cookieSet=1&journalCode=sqb> (accessed on November 26, 2006).