The epicentre of thought, emotion, consciousness, and debatably soul, remains largely at mystery to modern science. Split-brain syndrome, a side effect of an epilepsy surgery called a corpus callosotomy, provides insight into the location and nature of consciousness, and raises more questions about what is going on in our heads.
Epilepsy is a condition defined as “two unprovoked seizures occurring more than 24 hours apart or a single unprovoked seizure if recurrence risk is high” (Thijs, et al., 2019). Seizure is then classified generally as an imbalance of excitatory and inhibitory activity within a neuronal network, which when sustained, can interfere with other neuronal networks and function (Thijs, et al., 2019). This imbalance leads to electrical abnormality in nerve cells, and depending on location and severity, can manifest as loss of awareness or consciousness, unusual thoughts and sensations, and bodily convulsions. Generally, the wider the spread of electrical abnormality, the more severe the seizure (Hopkins, 2021).
Surgical treatment of epilepsy focuses on preventing the spread of abnormal brain activity, often isolating the area of the brain where it begins. In the case of a focal resection, the area of seizure origin is removed, but if this area is essential to function, other methods must be used (Kiriakopoulos, et al., 2024). One such method is a corpus callosotomy, where the main line of communication between the right and left hemispheres of the brain – the corpus callosum pictured in Figure 1 – is severed. This effectively prevents seizure propagation between hemispheres, but in certain cases presents a strange and eerie symptom, known as disconnection syndrome or split-brain syndrome (Asadi-Pooya, et al., 2008; Pinto, de Haan and Lamme, 2017).
Figure 1: The corpus collosum, highlighted in red, is the primary connection between the left and right hemispheres of the brain (Queensland, 2017).
In all humans, each hemisphere is more dominant or exclusive to certain functions. The area responsible for language processing is located in the left hemisphere, with field of vision and motor function of each side of the body evenly split between the two hemispheres. In split brain syndrome patients, the hemispheres seem to act independently of each other, almost as two separate entities or consciousnesses. If an object is shown in the field of vision associated with the non-language-dominant hemisphere, the subject will verbally report seeing nothing, yet will draw an image of the object with the stimulated hemisphere’s associated hand. If the subject is then shown the actions of their hand, they cannot explain the intention or memory of the action (Pinto, de Haan and Lamme, 2017). Similarly, verbal command will only elicit response in the hand controlled by the language-dominant hemisphere. This independent behaviour is further observed in alien hand syndrome, where the non-dominant hand of a patient acts against the will of the patient, often to the extremes of throwing objects or striking the patients head and body. These patients often have to restrain their non-dominant hand with their dominant one (Asadi-Pooya, et al., 2008).
These symptoms, although not universally observed in all split-brain syndrome patients, have given rise to two theories of consciousness. One suspects that consciousness resides in the language dominant hemisphere, and the other hemisphere functions as a teammate that communicates and processes information for the language hemisphere. This is supported by the exclusive reported awareness a patient has of the actions of their language hemisphere. The second theory is that consciousness is split into two between the hemispheres, enabling the autonomous action of each hemisphere in split brain syndrome patients, shown in alien hand syndrome (Pinto, de Haan and Lamme, 2017).
Split brain syndrome has blurred the already fuzzy definition of consciousness, perhaps implying the existence of multiple consciousnesses in one person. This implication adds yet another variable to the question: what and where in your head makes you, you?
References
Asadi-Pooya, A.A., Sharan, A., Nei, M. and Sperling, M.R., 2008. Corpus callosotomy. Epilepsy & Behavior: E&B, 13(2), pp.271–278. https://doi.org/10.1016/j.yebeh.2008.04.020.
Hopkins, 2021. Types of Seizures. [online] Available at: <https://www.hopkinsmedicine.org/health/conditions-and-diseases/epilepsy/types-of-seizures> [Accessed 28 January 2024].
Kiriakopoulos et al., 2024. Types of Epilepsy Surgery. [online] Epilepsy Foundation. Available at: <https://www.epilepsy.com/treatment/surgery/types> [Accessed 28 January 2024].
Pinto, Y., de Haan, E.H.F. and Lamme, V.A.F., 2017. The Split-Brain Phenomenon Revisited: A Single Conscious Agent with Split Perception. Trends in Cognitive Sciences, 21(11), pp.835–851. https://doi.org/10.1016/j.tics.2017.09.003.
Queensland, 2017. Corpus callosum. [online] Available at: <https://qbi.uq.edu.au/brain/brain-anatomy/corpus-callosum> [Accessed 30 January 2024].
Thijs, R.D., Surges, R., O’Brien, T.J. and Sander, J.W., 2019. Epilepsy in adults. Lancet (London, England), 393(10172), pp.689–701. https://doi.org/10.1016/S0140-6736(18)32596-0.