In previous posts, we started to describe the brain’s anatomy in greater depth. We mentioned that the brain is divided into two hemispheres which are interconnected by fibers (mainly the corpus callosum). Additionally, we explain that brains are wired contralaterally, information presented on the left visual field is actually processed by the right hemisphere and vice versa. For example, the left hemisphere controls the right hand whereas the right hemisphere controls the left one. In this post, we will talk about patients in which the connections between hemispheres are cut off: they now possess a so-called “split-brain”.
Although complete split-brain surgery is not common nowadays, it was mainly used in some patients with epilepsy, where there is a dysregulation of electrical activity in the brain that propagates to connecting areas and causes seizures. During surgery, doctors might only cut off the corpus callosum or all the fibers that connect the two hemispheres [1].
Over 50 years ago, a series of famous experiments were conducted with split-brain patients [2]. To understand these experiments, it is important to know that in most people, the left hemisphere is mainly in charge of speaking.
A specific example happened with patient “Joe” [cited by 1]. He was in front of a screen and the word “Texas” appeared on the left visual field. This visual information was processed by the right hemisphere and, thus, Joe drew a cowboy hat with his left hand. However, the left hemisphere was not aware of seeing the word Texas because Joe had a “split-brain”. Considering that the left hemisphere is typically in charge of speaking, when Joe was asked why he made that drawing, he did not know why his left hand had truly drawn a cowboy hat.
Although these separations seem severe, there might be some degree of unity in a split brain, especially in the matter of controlling actions [1]. In some cases, a few functions may overlap in both hemispheres, or surgery may not have cut off all connections between the two of them [1]. Moreover, with time, patients may learn to adapt on how to detect cues from each hemisphere in order to coordinate tasks, like dancing or playing sports [3]. In fact, people with split-brains can still walk even though it requires both halves of their body to work together.
These remarkable experiments helped us understand more about the processing of perceptual information in the brain. Currently, studying split-brain patients in more detail is an important field of research because they may hold a key to partially understanding consciousness [1]. Stay tuned for future posts where we will review some of the main characteristics of brain matter and neurons!
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Written by: Nicole
Edited by: Natasha
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References:
de Haan, EHF., et al. (2020). “Split-Brain: What We Know Now and Why This is Important for Understanding Consciousness”. Neuropsychology review, 30(2), 224–233. Available at: https://doi.org/10.1007/s11065-020-09439-3
Gazzaniga, MS., et al. (1962). “Some functional effects of sectioning the cerebral commissures in man”. Proceedings of the National Academy of Sciences of the United States of America, 48(10), 1765–1769. Available at: https://doi.org/10.1073/pnas.48.10.1765
Volz, LJ., and Gazzaniga, MS. (2017). “Interaction in isolation: 50 years of insights from split-brain research”. Brain : a journal of neurology, 140(7), 2051–2060. Available at: https://doi.org/10.1093/brain/awx139
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