Diseases and Disorders

Split-Brain Syndrome: An Overview

Shruthika Padhy


Abstract

Split-Brain Syndrome is a rare syndrome that results from the disconnection of the two hemispheres of the brain via the cleavage of the corpus callosum, the largest connective pathway between the two hemispheres. Due to the lack of interhemispheric communication exhibited by split-brain patients, split-brain research has led to major breakthroughs in hemispheric specialization and callosal function, which have had significant implications regarding a multitude of disorders, such as schizophrenia and Tourette’s Syndrome.

 

Introduction

What if the two hemispheres of one’s brain were severed and left to function independently of each other? This query describes the experience of patients with split-brain syndrome, which occurs when the corpus callosum is meticulously sliced during a palliative surgical procedure called corpus callosotomy, which is commonly used to relieve epilepsy [1]. Thus, split-brain syndrome is also called callosal disconnection syndrome. Since the corpus callosum, a bundle of neuronal nerve fibers along the longitudinal fissure, allows communication between the two hemispheres, their uncoupling disrupts interhemispheric communication. Split-brain patients are a critical avenue to make significant progress in research on the lateralization of the human brain. In fact, in 1981, Roger Sperry received a Nobel Prize in Physiology or Medicine for his work as one of the first people to investigate split-brain [2].

 

Etiology and Diagnosis

Aimed at treating atonic seizures (brief seizures characterized by a sudden loss in muscle tone), a corpus callosotomy is the primary cause of split-brain syndrome. On the former point, it is crucial to note that the corpus callosotomy is not an option pursued for all patients with atonic seizures [3], but only for those who possess uncontrolled refractory (drug-resistant) seizures [4]. In recent times, advancements in pharmacology have largely replaced the corpus callosotomy; therefore usage of the procedure has decreased significantly over the past two decades. Contrary to popular belief,  the corpus callosum is not necessarily wholly split — rather, lesioning or partial severing can also induce split-brain syndrome. In particular, lesioning is sometimes related to that resulting from Marchiafava-Bignami Disease (MBD) [5]. Linked to chronic alcoholism, MBD is a disease defined by progressive demyelination and necrosis of the corpus callosum [6]. Even less prevalent are cases of split-brain provoked by the agenesis of the corpus callosum - interhemispheric communication is generally left unaffected by cases in which the corpus callosum is not present at early stages of human development [7]. This is due to the plasticity hypothesis, which states that neural plasticity offsets the absence of the corpus callosum in young children [7].

 

Sperry’s Experiments

To further study the effect of severing the corpus callosum, Robert Sperry carried out a set of experiments on cats, monkeys, and humans. It should be noted that due to the contralateral organization present in all vertebrates, when Sperry presented an object to the right visual field of the subject, he was testing the function of the left hemisphere, and vice versa. In a 1968 paper, Sperry detailed the results of his experiments {8}. For the sake of brevity, only one of his many experiments is described below. To test the patients’ language capacities, Sperry would say the name of an object and ask the blindfolded patient to retrieve the object with only one hand. By responding to a verbal stimulus and blindly selecting the corresponding item with the left hand, patients showed language comprehension abilities in the minor (right) hemisphere [8][9]. This disproved the theory that only the left hemisphere is associated with language. However, when the volunteer was prompted to name the object, the test failed, leading Sperry to conclude that the right hemisphere had some language function, but no articulation capability [8][9].

While the experiments brought many of the side-effects of split-brain syndrome to light, other effects manifested themselves in ordinary life. Postsurgery, split-brain patients reported some difficulty in performing common tasks like grocery shopping. For instance, one volunteer named Vicki couldn’t reach for an item without her two hands fighting each other, describing them as “repelling magnets,” a glaring consequence of the two hemispheres no longer being able to work in conjunction with each other [2]. Like Vicki, many others delineate their struggles in executing simple tasks, but after practice are able to regain near-normal function. In several other aspects, however, split-brain patients revealed no apparent indications of disability [12]. For example, there were no significant changes measured in the intelligence quotient following surgery [3].

 

Split-Brain and Consciousness

The classical view on Split-Brain Syndrome established by pioneer Sperry asserts that the disconnection of the two hemispheres leads to split consciousness. In his studies conducted in the 1950s and 1960s, Sperry determined that the divided hemispheres “are unable to share information about stimulus identity, shape, and higher-order associations” [10]. This essentially means that the detachment of the cortical connections between the two hemispheres must result in the formation of two independent conscious perceivers: one being the right hemisphere responding to stimuli in the left visual field, and the second being the left hemisphere responding to stimuli in the right visual field [8][11].    

While previous studies suggested that split-brain led to split consciousness, an article published in 2017 refuted this view and contended that “a split brain produces one conscious agent who experiences two parallel, unintegrated streams of information” [13]. In other words, this study implied that though split-brain patients display split visual perception, they do not demonstrate split consciousness. To support this theory, psychologist Yair Pinto proposes five hallmarks of split-brain syndrome: response×visual field interaction, hemispheric specialization, inability to compare stimuli across the midline, split attention, and post hoc confabulation [13]. First, response×visual field interaction is the most critical piece of evidence supporting Sperry’s theory that refers to the phenomenon of patients only responding adequately to stimuli in the right visual field with the right hand, and vice versa. The second hallmark, hemispheric specialization, states that certain hemispheres are more competent at certain tasks. Third, the inability to compare stimuli across the midline describes the previously mentioned split visual perception due to the absence of communication. Fourth, split attention suggests that space-based attention and object-based attention are established in separate hemispheres [13][15][16]. Finally, the last hallmark, post hoc confabulation, alludes to the incapacity of patients to verbally explain the actions of their left hands, whose actions they were unaware of because the right hemisphere (which controls the left hand and visual field) cannot communicate with the left hemisphere (which is responsible for the verbal explanation).

Of these 5 hallmarks, Pinto argues that because hemispheric specialization, post hoc confabulation, and split attention are not exclusive to split-brain patients and are present in healthy adults, the less drastic theory of unintegrated perception and conscious unity should be favored [13]. Under this model, split visual perception accounts for the inability to compare stimuli across the midline. Nonetheless, the first hallmark, response×visual field interaction, still remains to be dealt with. To address this, Pinto conducted a quantitative study in lieu of Sperry’s entirely qualitative studies. If Sperry’s theory of two independent conscious perceivers is assumed to be true, patients will be expected to only be able to accurately respond to stimuli in the left visual field with the left hand, and the right visual field with the right hand and through a verbal manner [14]. However, Pinto’s quantitative experiment yielded contradictory results {11}{13}. According to a 2017 report on the study, patients showed “well above chance-level recognition of location, orientation, and identity of stimuli throughout the entire visual field, irrespective of response type (left hand, right hand, or verbally)” {11}{13}. Therefore, Pinto proceeded to state that due to the absence of response×visual field interaction in some split-brain patients, it could not be used as evidence to support split consciousness. With the rebuttal of the hallmarks of split-brain syndrome, Pinto put forward his theory of split visual perception and unified consciousness.

This proposition of unified consciousness adds to the idea that the severity of split-brain syndrome may have been overestimated. According to this model, even with the disconnection of the corpus callosum, where nearly all interhemispheric communication takes place, conscious unity can still be established. However, critics of Pinto’s model insist that his sample size of two patients is too minuscule to be significant [14].

 

Conclusion

Over time, the usage of the corpus callosotomy to relieve drop seizures has eroded partly due to the difficulty and drastic nature of the procedure. As a result, split-brain patients are few and far between and the field of split-brain research is disappearing. Despite this, split-brain research will continue to leave an impact through its significant contributions in determining hemispheric specialization, corpus callosum function, and their relations with a variety of other neural disorders.


References


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Shruthika Padhy

Shruthika Padhy


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