A little-reported 2021 case study published in Neurology Clinical Practice shows how resilient the human brain can be. A 29-year-old woman, CB, with no neurological or psychiatric history had a stroke, possibly due to medication issues. The damage was serious enough that a decision was made, with her consent, to remove almost all of the right side of her brain (hemispherectomy). As the study authors put it, “only a small disconnected right occipital pole was retained.”

What impact would that have on her mind? The right hemisphere of the brain is thought by neuroscientists to play a specific role in “nonverbal” cognitive abilities.

From Simply Psychology, we learn,

Left hemisphere function

The left hemisphere controls the right-hand side of the body and receives information from the right visual field, controlling speech, language, and recognition of words, letters, and numbers.

Right hemisphere function

The right hemisphere controls the left-hand side of the body and receives information from the left visual field, controlling creativity, context, and recognition of faces, places, and objects.

According to the left-brain, right-brain dominance theory, the left side of the brain is considered to be adept at tasks that are considered logical, rational, and calculating.

By contrast, the right side of the brain is best at artistic, creative, and spontaneous tasks (Corballis, 2014; Joseph, 1988).

Eagle Gamma, Left Brain vs. Right Brain: Hemisphere Function, Simply Psychology, October 20, 2023.

This approach to the human brain was tailor-made for pop psychology and advice books, of course, including Drawing on the Right Side of the Brain (2012), Left Brains for the Right Stuff, (2015) and Left Brain Speaks, the Right Brain Laughs (2016).

But Simply Psychology also warns, “ … this is an oversimplification, as both hemispheres cooperate in most activities.”

So what happens when there is only one hemisphere left, hence no possibility of further co-operation?

What did the post-op psychological tests show?

Sometimes a child has half of the brain removed. That may be the choice when the more afflicted half is nearly destroyed by epilepsy and the only alternative is to let epilepsy go on to destroy the other half. When the child shows normal behavior afterward, researchers assume that, in a plastic young brain, the left hemisphere has been doing the defective right hemisphere’s jobs for some time anyway, Thus the trauma is just not as severe as would be expected if an adult had the right hemisphere removed after many years of function.

CB was an adult with a career in retail management. So what happened afterward?

At 34, years after the operation, she was living on her own with a child and continued to have some post-stroke physical problems. And mentally? We’ll let the study authors tell it:

Neuropsychological test performances (data collected 5.3 and 7.8 years after the onset of the stroke; figure)4 were notable for normal performance on most tests, including on most nonverbal tests, such as the Block Design test, the Matrix Reasoning test, the Visual Puzzles test, the Judgment of Line Orientation test, the Benton Visual Retention test, the Spatial Span test, and the Faces I and II tests. In addition, she performed within normal limits on several tests of executive functioning, including the Wisconsin Card Sorting test, part B of the Trail Making test, the Color-Word Interference test, and the Similarities test (a test of abstract verbal reasoning and comprehension of implied meanings). Regarding attention, she performed within normal limits on the Line Cancellation test, a test of hemispatial inattention (neglect), and there was no evidence of hemispatial inattention on other neuropsychological tests. Focused and complex attention were within normal limits on parts A and B of the Trail Making Test, respectively.

Bowren M Jr, Tranel D, Boes AD. Preserved Cognition After Right Hemispherectomy. Neurol Clin Pract. 2021 Dec;11(6):e906-e908. doi: 10.1212/CPJ.0000000000001015. PMID: 34992977; PMCID: PMC8723955.

In other words, according to the tests, CB had generally normal mental ability despite being left with only half a brain.

The researchers point out that one limitation of their study is that the same tests have not been done on adults who have had the left side of the brain removed. It would be very interesting but maybe hard to find. Adults who have had either side removed as adults are much rarer than children in a similar situation.

Overall, the finding underlines something that Michael Egnor and Denyse O’Leary talk about in their upcoming book, The Immortal Mind (Worthy June 2025): the essential unity of the human mind. The brain can be halved but the mind can’t. The human mind seeks to work with whatever brain is available to continue to function in this frame of reality.

Hat tip: Head Truth

You may also wish to read: Some people think and speak with only half a brain. A new study sheds light on how they do it.