Brain Breakup: What Happens When the Left and Right Hemispheres Stop Talking
Maxx Martinez
Illustrations by Emily Holtz
As you read these words, the over 100 billion neurons that make up your brain are communicating through a complex network of more than 60 trillion neuronal connections [1]. Neurons and the connections between them enable you to do everything from identifying words out of combinations of letters to deciphering their meanings [2]. Of the trillions of connections that exist, one that holds crucial importance is the corpus callosum: a bundle of nerves that links the left and right hemispheres of the brain [3, 4]. Through linking the neural hemispheres, the corpus callosum allows for information to be exchanged across the brain and used to execute coordinated thought and movement [3, 4]. When the corpus callosum is severed, the degree to which neural hemispheres can communicate is significantly impacted [5, 6]. Severing the structure is beneficial to alleviate dangerous symptoms of epilepsy, and is done through a surgical procedure known as a corpus callosotomy [5, 6]. Cutting the corpus callosum causes a variety of unique symptoms, ranging from uncontrolled hand movements to a reduced understanding of one’s own emotions [5, 7, 8]. Despite the procedure’s observable symptoms, the full range of consequences that disrupting the connections within the corpus callosum has on consciousness and thought processes remain largely unknown [5, 7, 8]. Although there is a lack of certainty surrounding the cognitive effects of cutting the corpus callosum, the procedure provides an important perspective on current theories of consciousness [5, 9]. The corpus callosotomy offers a unique opportunity to observe how the brain’s hemispheres function when disconnected from each other, shedding light on how consciousness works in the brain [5].
The Talking Phase: Bridging the Gap Between the Hemispheres
To understand the effects that splitting the corpus callosum has on a person and their consciousness, it is essential to first explore some basic information about the corpus callosum and its role in brain function [5, 10]. Many brain functions are lateralized, meaning they are controlled primarily by either the left or right hemisphere [11, 12, 13]. Speech is an example of a lateralized function, as its production is localized to Broca’s area, a region in the brain's left hemisphere [13, 14, 15, 16]. Speech, language processing, and right-hand control are also lateralized functions of the left hemisphere [13, 16, 17, 18]. The brain’s right hemisphere, in contrast, is the primary location for spatial reasoning and awareness, musical ability, and left-hand control [13, 15, 16] . Despite the lateralization of the brain, many activities require input from both hemispheres and often rely on the corpus callosum — a bridge composed of millions of nerve fibers that link the hemispheres together with a near-seamless exchange of information — to achieve cross-hemispheric communication [4, 19, 20]. Cross-hemispheric communication via the corpus callosum allows for the execution of coordinated full-body movement and whole-brain processing of sensory input [4, 19].
Due to the brain’s lateralization, each hemisphere controls the opposite side of the body; the left hemisphere controls the right side and the right hemisphere controls the left side [14, 21]. Cross-body hemispheric control encompasses everything from arm and leg movement to the intake of visual information from the periphery of each eye [22, 23]. The corpus callosum enables both hemispheres to share and utilize information, leading to a cohesive experience of vision and movement [24, 25, 26]. The interconnectedness of the brain’s hemispheres suggests that neither is truly dominant, as each relies on the other for many functions [11, 12]. The reliance between the brain’s hemispheres debunks the common myth that individuals can be either left- or right-brained, as both the left and right hemispheres must work in unison to execute cognitive and motor functions [11, 12]. Although the brain is lateralized, certain neurological processes arise from activity located in structures that are bilateral, meaning the structures extend across, or independently exist on, both hemispheres [27]. Brain structures that present bilaterally are able to execute their functions in either hemisphere of the brain and often communicate across hemispheres through other neural pathways to an increased degree following a corpus callosotomy [27]. Two examples of regions that are bilateral are the superior temporal gyrus, which is associated with auditory processing, and the posterior cerebellum, which is associated with tasks such as balance and walking [28, 29, 30].
“No Contact:” Is Splitting Up for the Best?
Given the importance of the corpus callosum, it is reasonable to wonder why one would sever the structure, and how the procedure may affect one’s quality of life and mental processes. In some extreme cases of epilepsy, where seizures arise from uncontrolled neuronal firing, seizures localized in one hemisphere can spread to the other hemisphere via the corpus callosum, resulting in an atonic seizure [6]. An atonic seizure occurs when the regular neuronal firing that contributes to muscle tone — or the involuntary tension within muscles activated during activities such as standing — becomes inconsistent, resulting in a loss of normal motor functioning. When individuals lose muscle tone, they lose their ability to stand unassisted, potentially resulting in broken bones and concussions from uncontrolled falls [6]. In rare cases of extreme epilepsy, such as those resulting in atonic seizures where medications fail to reduce seizure severity and frequency, a procedure known as a corpus callosotomy can be performed [10]. A corpus callosotomy severs the corpus callosum and prevents seizures from spreading to the other side of the brain, restricting dysfunctional firing to one area and therefore reducing seizure severity and risk [6]. Following a corpus callosotomy, several people report no longer experiencing atonic seizures, and several others report no longer experiencing seizures of any kind [6].
Though a corpus callosotomy successfully alleviates seizure symptoms, there are notable side effects individuals experience following the procedure [6]. Interestingly, many people who receive a corpus callosotomy report feeling and behaving ‘normally’ [9, 31]. Most people who undergo the operation spend only a few days in the hospital, and report increased independence in daily tasks following their recovery [32, 33, 34]. However, despite a reported general sense of normalcy, many experience distinct side effects following a corpus callosotomy [5, 8]. One common and noticeable side effect is referred to as ‘alien hand syndrome,’ a condition in which one limb acts involuntarily, leading to a feeling of estrangement with the affected limb [5, 7, 35]. For example, one hand may pull a candle away as the other tries to light it, or one hand may button a shirt only for the other to undo each button [7, 36]. An often overlooked consequence of a corpus callosotomy is that, following the procedure, individuals lose their ability to correctly link their emotional responses with stimuli processed by the left hemisphere [8]. In one study, a visually frightening scene was shown to the peripheral right eye of participants who had undergone a corpus callosotomy, leading to the scene being processed solely by the left hemisphere of the brain. The sight of this scene made the person feel uneasy, but when asked what was wrong, the person attributed their discomfort to the room they were in, rather than the scene they had observed. When the experiment was repeated, this time presenting the scene to participants’ left periphery — which was processed by their right hemisphere — participants immediately attributed their unease to the scene they were shown. The participants’ inability to properly attribute the emotional response experienced to the scene processed by the left hemisphere may stem from the fact that most emotional comprehension occurs in the right hemisphere. Since the link between the hemispheres is now severed, the left hemisphere no longer receives the right hemisphere's emotional interpretations. As a result, when asked about their feelings, participants who received a corpus callosotomy were unable to access relevant information to answer the question, and could only rely on the logically thinking left brain to ‘guess’ the cause of their discomfort [8].
In another well-known split-brain experiment, the notion that a corpus callosotomy leads to a lack of communication between the hemispheres of the brain is further supported [5, 9, 37] . In the experiment, participants were told to stare at a dot in the middle of a screen. Different images were then flashed on both sides of the dot, and, due to the brain’s lateralization, each image was processed by one hemisphere of the brain. When asked what they saw, participants were able to name the stimulus shown on the right side of the screen, as the image was processed by the left side of the brain, where the speech centers are predominantly located. Because the left hand is controlled by the right side of the brain, which processes the image shown to the left side of the dot, when instructed to draw what they saw using their left hand, participants drew the image shown on the left side. When asked why they drew something other than what they reported seeing, participants were often confused and provided rationales that had nothing to do with the circumstances of the experiment [5, 9, 37]. For example, in one rendition of the study, a person was shown the words ‘bell’ and ‘music,’ to the left and right sides of the dot respectively, and was then asked to point out what they saw using their left hand [9, 38]. Out of a group of images depicting music-related items, including someone hitting a drum and someone playing a trumpet, the person selected a picture of a bell. When asked why they chose that specific image, the participant insisted that they ‘must have heard a bell ringing on [their] way into the lab’ and that this was the last time they had heard music [9, 38]. Although it is possible that the participant heard a bell before engaging in the study, it is far more likely that the person's left hemisphere, which has an essential role in speech control, was trying to develop a rationale for the actions of their left hand [5, 38] . Due to the split in the corpus callosum, the two hemispheres of the brain cannot communicate, and the left brain is therefore unable to access the reason for the right brain’s actions [5, 38].
Who Gets Custody: A Battle for Consciousness
Decades of research on the effects of a corpus callosotomy has sparked debate as to whether severing the corpus callosum splits one’s consciousness into two distinct conscious agents, or whether one’s consciousness remains unified [5, 9, 39, 40]. A unified consciousness is one in which all experiences generated by the body are perceived through one perspective and sense of self [5, 9]. A split consciousness is one in which each hemisphere contains its own conscious agent which operates separately from the other. The belief that consciousness would split arose from the theory that each hemisphere of the brain — the left side represented by speech and right side represented by left-hand movements — could form conflicting conclusions based on the same stimulus, without being aware of the other hemisphere's response [5, 9]. What this theory fails to account for, however, is that the inability to compare visual stimuli from each hemisphere is not consistent across all cases in which people receive a corpus callosotomy; many people who have undergone a corpus callosotomy retain their ability to name an object no matter where in the visual field it is presented, indicating that individual difference may play a large role in whether consciousness is split [5, 9, 40]. Some experiments show evidence supporting the existence of a split consciousness when comparing some stimuli, but a unified consciousness when comparing others [5, 9]. In another experiment, two shapes were presented to participants in a similar manner to the previous study. When asked to identify whether the shapes were the same or different, most participants were unable to do so. However, when the shapes were replaced with tilted lines that were either parallel to or identical to each other, participants were able to consistently identify the relationship between the lines accurately. The ability to successfully compare the tilted lines, but not the shapes, suggests that cross-hemisphere communication takes place at varying degrees depending on the task [5, 9].
Ultimately, the answer to whether consciousness is split into multiple agents is dependent on what theory one uses to define consciousness [5]. One theory of consciousness, known as the global neuronal workspace theory, suggests that all subconscious information processing occurs in different subsections of the brain called cortical modules, each of which sends signals to a central ‘headquarters’ [5, 41]. At these cortical ‘headquarters,’ the strongest signals, which are determined by signal frequency and number, are broadcast throughout the brain and become conscious, coordinated thoughts. Under this system of consciousness, it can be argued that those who have undergone a corpus callosotomy do display split consciousness, as the ability for modules to communicate with cortical headquarters across hemispheres is lost when connections between hemispheres are severed. Therefore, each hemisphere’s cortical ‘headquarters’ is only able to broadcast information within its own hemisphere, leading to two separate conscious broadcasts occurring simultaneously. The global neuronal workspace theory indicates that depending on the level of hemispheric separation — or what percent of the corpus callosum is severed and where it is severed — different types and amounts of information may be able to cross to the ‘cortical headquarters’ within the other hemisphere of the brain. Differing levels of hemispheric separation explain why some people are able to communicate across visual fields while others cannot, and why some stimuli, such as the tilted lines discussed in the previous experiment, can almost always be compared across visual fields. Varying degrees of hemispheric separation leads neuroscientists who subscribe to the global neuronal workspace theory to view consciousness as a spectrum of connectedness, rather than completely split or completely unified [5, 41].
Another popular model used for defining consciousness is recurrent processing theory [5, 42]. Recurrent processing theory argues that the brain, and therefore one’s consciousness, can intake information passively, or without one actively paying attention to the input [5, 9, 42]. Information absorbed passively is not always readily available to be recalled consciously, but still factors into decision-making [5, 9, 42]. In one experiment, people who had not undergone a corpus callosotomy were shown different moving objects to the outer periphery of each eye [5, 9]. The act of watching a different moving object with each eye caused a person’s attention to be split between the two objects, and led each hemisphere to independently process the information from one of the eyes. Despite the participant’s divided attention, when asked to recall the moving objects afterward, it seemed clear that their perception of viewing the moving objects was unified [5, 9]. In another experiment, known as a partial report paradigm, participants were instructed to stare at a cross in the middle of a screen, at which point several letters appeared in a circle around the cross for less than a second [5, 43]. Next, a blue dot appeared on the screen in a location that was previously occupied by a letter, at which point the participant was instructed to quickly state the letter that was at that location [5, 43]. Participants consistently performed better when asked to recall a letter that was at a specific location versus when simply asked to list the letters they saw during the experiment [5, 9, 43]. The inability to recall the information until specifically prompted suggests that acquisition of information does not equate to the ability to convey said information to others [5, 43]. In other words, the brain subconsciously takes in information that is made available to the conscious mind after being prompted with specific cues, such as instructions. Recurrent processing theory argues that people who have undergone a corpus callosotomy do not have a split consciousness, but rather have a greater disassociation between their consciousness and what they are able to communicate to others. The theory suggests that due to a lack of understanding of the disjointment, it appears as if there are two existing conscious agents, but in reality, there exists only one that is not fully capable of integrating and reporting the information it has access to. Recurrent processing theory can also explain observed personal differences in people's ability to report information; it suggests that their ability to do so is dependent on the percentage of the corpus callosum that is severed and where the severance occurs. As a result of differences between individual patients, each person may be able to integrate and report varying amounts of information, leading to a range of reportability in lieu of the split versus unified consciousness dichotomy or the spectrum of consciousness previously mentioned [5, 43].
Moving On: Lessons Learned
Whether one views a corpus callosotomy as a split in consciousness or a shift in ability to report known information, it is clear that the procedure alters the brain’s inner workings by restricting information flow between neural hemispheres [5]. Despite the lack of knowledge about the procedure’s full implications, a corpus callosotomy remains a crucial intervention that alleviates otherwise untreatable and debilitating symptoms of epilepsy [10]. While less invasive alternatives to the procedure are being explored, the corpus callosotomy provides valuable insight into the mechanisms underlying consciousness and thought, deepening our understanding of the interconnected nature of the brain’s hemispheres and their impact on consciousness as a whole [5,10].
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