Full/Half/Split Brains: People with Unique Brains

Our Senses: An Immersive Experience - Rob DeSalle 2018

Full/Half/Split Brains: People with Unique Brains

“What it comes down to is that modern society discriminates against the right hemisphere.” —Roger Wolcott Sperry, neurobiologist

Can one survive without a full brain? I have discussed instances where a part of the brain has been removed surgically to stave off epileptic fits, and it does not result in complete loss of neural function. The removal of part of the brain improves the health of the patient and allows the brain to function in some cases. But removal of parts of the brain can also be disastrous. Henry Molaison (H.M.) comes to mind immediately; in that instance, the removal of the inner part of his brain to stop epileptic fits resulted in the loss of his short-term memory and some sensory perception. Although anencephaly (the lack of development of the brain) will be fatal to a fetus suffering from the syndrome, there are less drastic fetal brain development syndromes. Hemimegalencephaly results in a fetus with disproportionate development of the two hemispheres of the brain. In extreme cases, one hemisphere can be greatly reduced so that the person with this syndrome literally has half a brain. But people with this syndrome can live relatively normal lives.

Removal of one of the hemispheres of the brain in an operation called a hemispherectomy is also sometimes a necessary step to stop extreme cases of epilepsy. Case studies and long-term follow-up research of children who underwent hemispherectomies and analyses of individuals born lacking a hemisphere of the brain indicate that the visual system is impacted severely. Ahsan Moosa and his colleagues conducted a follow-up study of a large cohort of children with hemispherectomies (box 12.1). The average time from the operation in their study was about six years, so the children had a considerable time for the plasticity of their brains to kick in. The study concluded that vision, though affected by the initial operation, was not significantly affected in 75 percent of the kids at follow-up. This result suggests that there is a considerable degree of corrective plasticity of the visual system in the brain. Several case studies support this result; researchers observe that regions of the remaining half of the brain in hemispherectomies and hemimegalencephalics that are dedicated to vision enlarge with age. Other senses like olfaction and balance are also affected, but as with stroke, these other senses are not focused on in any detail. Speech and reading are also affected by hemispherectomies and hemimegalencephaly (fig. 12.1).


This operation is an extreme version of the one performed on Henry Molaison. Of course, doctors would not do this operation if it didn’t work, and it is usually performed on children younger than two years who do not respond to repeated drug treatment for epilepsy. Children of this age are preferred for this operation because of the plasticity with which the brain develops in early childhood. Massive disruptions of the wiring of the brain by removal of half of it can be compensated for by neural rewiring as the child develops. Such children lead very normal if not exceptional lives. Either the right or left hemisphere of the brain can be targeted. Some hemispherectomies—ones called anatomical—involve complete removal of the hemisphere. An alternative to this drastic operation is a functional hemispherectomy, where specific parts of the brain such as the temporal lobe are removed and the corpus callosum is severed. The corpus callosum plays a big role in split-brain phenomena. Accidental and operative disruptions of the two hemispheres offer researchers and clinicians a good approach to the clinico-anatomical correlation method to examine sidedness of the brain.

Injuries to the brain (from stroke, surgery, or accident) reveal how each side of the brain affects function on the opposite side of the body. Understanding some of the nuances of this sidedness is evident in the impact of stroke to vision and to motor skills. If the left side of the brain is damaged, function of the right side is affected. For instance, if we go back to the modern analysis of Leborgne’s brain, all of the lesions in brain connections that caused his problems were localized to the left side. Massive damage to the left side of his brain left him using a single word (“tan”) when he tried to speak, a function of the right side of the brain.


Figure 12.1. Left, image of a brain from the top of the head down of a person missing the left hemisphere; the white area is nonbrain tissue and fluid. Right, image of a brain from the rear of the skull of a person with a hemispherectomy.

Anomalies in speech are collectively called aphasias. A transcription from a video recording of an older gentleman, Jack, who suffered damage to Broca’s area is poignant, because it is obvious that Jack really knows what he wants to say but simply has to struggle to get the words out, whether they address the question or not. Note the lack of fluid speech and struggle with words that is characteristic of individuals with this kind of aphasia.

INTERVIEWER: What did you do about the ache?

JACK: Uh Home. (pause), Uh Doctor. (pause), And legs. (pause), Walking. (pause), No good.

People who have aphasia caused by damage to Wernicke’s area (Chapter 11), which is usually located on the left side of the brain, can speak fluently but cannot string the right words together. Unlike Broca’s aphasia, where fluency is nearly obliterated, Wernicke’s aphasia results in the inability to recognize what is coming out of the mouth. Here is an example of a gentleman named Byron, who has Wernicke’s aphasia.

INTERVIEWER: Hi Byron. How are you?

BYRON: I’m happy. Are you pretty? You look good.

INTERVIEWER: What are you doing today?

BYRON: We stayed with the water over here at the moment and talked with the people for them over there, they are diving for them at the moment.

Psychologists call Byron’s response “word salad,” because it is a mish mash of words, usually not addressing the original query or a coherent idea. Whereas Jack could intellectually address the interviewer’s question, Byron cannot. There is a reason why two older men are given here as an example. Women tend to be able to lateralize, or use, both sides of their brain for language better than men, and there are fewer examples of women showing these aphasias. Although the phenomena I discuss here may seem to have little to do with the senses, it is quite the opposite. Language, speech, and writing are a kind of synthesis of the senses, and higher-level functioning of the brain, which includes speaking and reading, are complex processes involving the senses. Processing the signals of vision, smell, taste, and other senses are just as complex as language.

Broca and Wernicke pioneered brain region studies in the late nineteenth century that culminated in the work of Roger Wolcott Sperry, who received the Nobel Prize in 1981, nearly twenty years after his seminal work on brain sidedness was accomplished in the 1960s. Although Broca, Wernicke, and others recognized the localization of functions to certain regions of the brain, Sperry was able to conceptualize the different functions of the right and left hemispheres. Sperry and his younger colleague Michael Gazzaniga worked with epileptic patients who required surgery because they did not respond to drug treatment. Split-brain individuals endure intentional surgery to sever communication between the left and right sides of the brain. The idea was that epilepsy in some cases is caused by hyperconnected communication between the brain’s left and right sides. When the corpus callosum (the region of the brain connecting the right and left hemispheres) is surgically split, the neural connection from one side of the brain to the other is severed. Because the neural circuitry between the left and right hemisphere is disrupted, the epileptic fits cease. Although this procedure can stop epileptic fits, it also results in some strange effects in patients.

The left and right sides of the brain in general need to communicate with each other to properly interpret information from the outer world. The exceptions to this rule have been noted, however, for hemispherectomies and hemimegalencephalics. One surprising result of split brains was Sperry’s discovery that the two hemispheres of the brain had different subtasks that are often combined to result in a complex brain function. Interpreting the information from the outer world is a highly complex brain function. After split-brain surgery, the two sides of the brain carry on with their neural tasks, such as gathering visual, olfactory, auditory and other information. But now the two halves do not communicate, so the left half doesn’t know what the right half is experiencing, and vice versa. Because the behavior of the split-brain patients is so distinct, Sperry was able to establish several important rules about left- and right-brain function. First, it’s really not a left versus a right brain but a dominant brain versus its partner, a nondominant brain. In most humans, the dominant side is the left side. Next, Sperry pinned down that the dominant side (almost always the left side) focuses on and solves analytical and verbally based tasks such as language. The nondominant side (usually the right side) has been thought to be dedicated to emotional and several nonverbal functions. Functions such as creativity have also been attributed to this side of the brain, but these attributes are more than likely not hemisphere-centric.

Another consideration is the context of maleness and femaleness. The general thinking has been that the dominant side of the brain was the female side and nondominant the male. The reasoning has been that women are more verbal than men but men are more spatially oriented. But recent studies of male and female brains have proven this line of thinking wrong. The real difference in male and female brains on average results from how the hemispheres of the brain are wired. Madhura Ingalhalikar and her colleagues looked at the brains of nearly a thousand youths, split pretty evenly between males and females. By using the diffusion tensor imaging method, they were able to map the neural connections of these developing brains. Their results indicate that males tend to have more connections in their brain within hemispheres, whereas females tend to have more connections between hemispheres for the cerebrum.

This result probably means that female cerebral halves are cross-talking more to each other than male cerebral halves. There is a telling difference, however, in the connections between the cerebellum between males and females. In males, there is more cross-wiring from the left cerebellum to the right than in women. Remember that the cerebrum harbors higher-order functions and the cerebellum in general controls muscle movement and coordination. Some researchers think that this is the basis for the general observation that males on average live in a more motor-skill-based world than females. On the other hand, as the reasoning goes, women live in a more intuitive, communication-based world. Although gross overgeneralizations should more often than not be ignored, the difference in wiring is intriguing.

With respect to the senses, the dominant side of the brain is better at expressing what the brain perceives. Verbalization is one of the human brain’s favorite things to do, as is evident from our propensity to speak about everything and anything. The nondominant side of the brain is more adept at making sense of or analyzing information. If the information churns up some emotional feelings, that is accomplished in the nondominant side. To conclude that our brains have this dual nature is a major finding about our human condition, and Sperry’s contribution was indeed worthy of the Nobel Prize. His student Michael Gazzaniga, perhaps also worthy of a Nobel for carrying the original work to wonderful logical extremes, has focused nearly fifty years of his career on split brains of individuals who have undergone such operations. Gazzaniga throughout his career has exploited an ingenious way of examining how the left and right brains communicate with each other using visual input in split-brain patients.

Split-brain experiments are very logical in their design. Visually, the left eye collects information and sends it to the right side of the brain. Likewise, the right eye collects information and sends it to be processed by the left side of the brain. Next, we need to recall that our brains have evolved to have right-brain-specific and left-brain-specific functions. If the right eye sees nothing, then the split-brain person will verbalize that he or she sees nothing even if the left eye is viewing Andy Warhol’s Campbell’s soup cans. Strangely though, if asked to draw what he or she sees, the split-brain person will try to reproduce the soup cans. This is because the right eye transmits the “nothing” image to the left brain, which then tries to verbalize what the eyes have just seen—nothing. But the left eye transmits the image of the Campbell’s soup can to the right brain, which can interpret it mechanically as a drawing. On the other hand, if the Warhol soup can is flashed to the right eye, the split-brain patient will answer something like, “I see a Warhol.” By setting up a system whereby the left eyes and right eyes of split-brain patients view different pictures or items and then asking questions about what the eye sees, researchers can discover amazing intricacies about how our brains deal with visual junk.

One of the more famous split-brain experiments concerns flashing the word “face” to the left eye and the word “smile” to the right (fig. 12.2). The split-brain patient is then asked to describe what he or she saw through drawing and then asked to explain the drawing verbally. In one case a patient was asked to draw with his right hand (which is controlled by the left brain) what had been seen. He drew a smiling face. Sounds about right. But when asked to describe in words why the smiling face had been drawn, the patient, who can’t integrate both words verbally, makes up a stunningly clever explanation. To justify drawing the smiling face, the patient answers that he drew a face, and a smiling face is more pleasant than a frowning one. And he concluded by saying, “Who wants a frowning face around?” Strange, but completely explainable by right brain—left brain dynamics. The left brain, having limited information, makes up a logically pleasing story to compensate not only for the lack of information but also for the haunting need to draw the face with a smile because of the lingering specter of seeing the word “smile” with the right eye. Psychologists call this phenomenon of justifying and unifying what is seen from both eyes by the split-brain patient a “unified sense of self and mental life.”

One of the more interesting experiments Gazzaniga and his colleagues performed with split-brain patients concerns the recognition of self. He asked, “Severing the corpus callosum in humans has raised a fundamental question about the nature of the self: does each disconnected half brain have its own sense of self?” A basic understanding of how we perceive the outer world would need some partial answer to this question. By using a face-morphing technique, David Turk and his colleagues made the question “Is it Mike or me?” A split-brain patient’s (JW) face was computer morphed by increments of a tenth with the face of a long-time associate who just happened to be Gazzaniga (MG). In other words, a facial spectrum going from left to right was created with JW’s face at the left end and MG’s at the right. The eight faces in between looked 90 percent like JW and 10 percent like MG, then 80 percent like JW and 20 percent MG, and so on. In other words, if JW was not a split-brain person, then the photos would look, from left to right, 100 percent self, 90 percent self, 80 percent self, and so on.


Figure 12.2. Split-brain experimental design.

Next, Turk and colleagues used the classic approach of exposing the morphed images to the left brain (via the right eye) and to the right brain (via the left eye). At each exposure, JW answered the question, “Is it me?” or “Is it Mike?” The outcome is that the left hemisphere rapidly detects partial images of the self, and kind of linearly. The right brain, on the other hand, can recognize self only with a nearly full picture of self. More precisely, the morphed face needs to have at least 80 percent of self in it to be recognized as self. Because the left or dominant hemisphere recognizes self even with a very little of self there, it suggests a stronger role for the left hemisphere in what Gazzaniga calls “retrieval of self knowledge.” But the experiments do not imply that each half of the brain has an individual sense of self; rather, they show that sense of self comes from specialized functions of both hemispheres together.

By far the most bizarre case of the human brain dealing with visual information and turning it into perception is found in the work of V. S. Ramachandran and colleagues. Ramachandran is most famous for his work on synesthesia and phantom limbs, and he uses the example of Capgras syndrome to demonstrate how needy the brain is with respect to creating explanations for unexplainable sensory input and how this is an important part of perception. Individuals with Capgras syndrome will claim that people close to them are impostors. In the case that Ramachandran studied, the male individual claimed that his mother was an impostor. He would be introduced to her and claim something like, “She looks like my mom, but she isn’t.” The emotional response of this person was measured when he was confronted with his mother, and this analysis indicated that he simply responded neutrally to his mother—every mother’s nightmare and more than likely producing overwhelming guilt in the son.

Ramachandran offers the following explanation for this bizarre behavior. The person he examined was an individual who had suffered head trauma. This individual’s limbic system, in particular the amygdala, a region deep inside of the brain responsible for emotions, had been damaged. In addition, more than likely the connections of the temporal cortex to the limbic system had been altered. Another important piece of information here is that a region of the temporal lobe called the fusiform gyrus is responsible for processing facial images and for facial recognition and is also connected to the limbic system, just as many other regions of the brain are. So, the individual sees his mother, recognizes her as such, tries to send this knowledge to his amygdala for emotional processing, which is thwarted, and is left with the only logical explanation possible—this isn’t my mother because I lack emotion for her. His brain, trying to make sense of some messed up stuff he is seeing and thinking, makes up the impostor story. The clincher for this explanation is that if his mother calls him on the telephone and speaks to him, he recognizes her voice and properly sends this information to his amygdala and has the emotional response his mother adores. No more imposter.

Michael Gazzaniga had this to say about split-brain studies when imaging technology began to explode on the research front: “I have no doubt that the interplay between split-brain research and other methodologies such as neuroimaging will continue to shed light on the human mind and brain.” However, the split-brain studies that I have used to introduce the pathways with which our brains use to process sensory information are a thing of the past. Because surgeons performing the operation concluded that the surgeries were not as effective as they should be, they were discontinued. Many of the split-brain patients who resulted from the surgery are now dying of natural causes, and with them is disappearing the ready opportunity to study split-brain phenomena. Hemispherectomies continue for very young children, but because there is so much wonderful rewiring of the brain as these children develop to adulthood, split-brain effects are not pronounced enough for researchers to exploit. Accidents will continue to happen, and these unfortunate events, when they occur to very specific regions of the brain, can produce split-brain phenomena. Researchers have for some time suggested that individuals who are afflicted by a rare birth disorder called agenesis of the corpus callosum (AgCC) might be the saviors of the split-brain paradigm. Only more recently have such cases become important because of the dwindling population of surgically induced split-brain patients. These unfortunate AgCC individuals are born with complete or partial absence of the corpus callosum. Without a corpus callosum, the neural fibers that make up this structure and run latitudinally to connect the brain hemispheres instead develop in a longitudinal pattern within the hemispheres. Partial disorders of the corpus callosum go by different names, but the effect is the same as with the surgeries that cause split-brain phenomena.

There are some distinct similarities of surgical split-brain people and people with AgCC. Although AgCC people have limited connections between the hemispheres of the brain, they seem to have integrated the two hemispheres more than surgically produced split-brain people. The age at which the AgCC patient is examined is also a factor in the connectivity of the two hemispheres, indicating that neural plasticity may in some cases compensate for the lack of connectivity as children with AgCC develop into adolescence and adulthood. One major similarity of AgCC and surgically caused split-brain people is that they are severely impaired in dealing with complex situations. Sadly, patients with AgCC manifest many of the characteristics of autistics. One of the more famous and visible cases of AgCC is connected to the 1988 movie Rain Man, in which actor Dustin Hoffman plays an autistic adult. The person on which his character is based was Laurence Kim Peek, who died in 2009, and was considered a megasavant because of his capacity to remember things. Hoffman’s portrayal of Peek was a wonderful and complex portrayal of a man with autistic characteristics caught up in the world outside the institution where he had long lived.

In 2013, Pratik Mukherjee and colleagues examined several people with AgCC and determined not only that the corpus callosum was affected by this developmental disorder but also that the cingulate gyrus showed abnormalities. It is well known that this region of the brain is critical for processing information and placing it in an emotional context. Without normal connectivity to this region of the brain, the emotional response to sensory information is lost in the missed connections. This study explained a lot of the behavioral attributes of Peek and others with this unfortunate developmental syndrome. How studies of AgCC people will be incorporated into split-brain research is another story, but such people and their curious brain structures caused by developmental problems might be an important inroad to understanding the nuances of how the brain processes complex sensory information that lead to emotional-, logical-, and perception-based responses to the outer world.

One clue is how AgCC patients react to and interpret proverbs. Proverbs are those catchy little one-sentence statements that need to be interpreted in nonliteral contexts for the point of the sentence to be comprehended, such as “You can’t judge a book by its cover.” It turns out that AgCC patients fare pretty poorly on the proverb tests compared to peers who have an intact corpus callosum, meaning that there are some basic differences in how AgCC individuals process complex sensory input.

Overall, the human brain is pretty amazing at coping with the signals from the outer world. Humans have developed some astonishing, clever, unique, and sometimes logic-defying neural mechanisms for coping with their sensory world. And these are especially interesting when the senses interact in crossmodal ways. In fact, more than likely few of our sensory experiences are mediated by a single sense.