Some children have half of their brains removed (hemispherectomy) to control massive seizures that would otherwise destroy the child’s whole brain. Specialists were surprised that the children functioned fairly normally — certainly compared to what would have been expected. A recent study of post-hemispherectomy patients has provided dramatic evidence of rewiring:

An unprecedented study of brain plasticity and visual perception found that people who, as children, had undergone surgery removing half of their brain correctly recognized differences between pairs of words or faces more than 80% of the time. Considering the volume of removed brain tissue, the surprising accuracy highlights the brain’s capacity — and its limitations — to rewire itself and adapt to dramatic surgery or traumatic injury.

The findings, published by University of Pittsburgh researchers today in the Proceedings of the National Academy of Sciences (PNAS), is the first-ever attempt to characterize neuroplasticity in humans and understand whether a single brain hemisphere can perform functions typically split between the two sides of the brain.

University of Pittsburgh, “Word and face recognition can be adequately supported with half a brain” at ScienceDaily (October 25, 2022) The paper requires a fee or subscription.

There seem to be two visions in conflict about how brain hemispheres work. On the one hand, we have the classical left brain–right brain division theory, according to which the two parts of the brain are like two components of a machine. One does one thing and the other does the other.

On the other hand, there are proponents of neuroplasticity, according to which the brain is constantly rewiring, wherever and whenever possible, to continue to perform functions. This data strongly supports neuroplasticity:

Neuroplasticity is a process that allows the brain to change its activity and rewire itself, either structurally or functionally, in response to changes in the environment. And even though brain plasticity peaks early in development, our brains continue to change well into adulthood.

University of Pittsburgh, “Word and face recognition can be adequately supported with half a brain” at ScienceDaily (October 25, 2022)

It appears that a more correct account would read like this: The brain becomes increasingly specialized through life. It is very plastic earlier, less so later. For example, in the researchers’ telling, “The left hemisphere matures into the primary place for reading printed words, and the right hemisphere matures into the primary place for recognizing faces.” (Presumably, in a non-literate culture, the primary place for “reading printed words” would be doing something else, relevant to the person’s life… )

In any event, adults who have had a stroke late in life may have more difficulty in adapting than children who have had a hemispherectomy. But the issue is not the amount or type of loss so much as the ability to adapt to it. Here’s the researchers’ experiment in a nutshell:

To assess word recognition capacity, researchers presented their participants pairs of words, each differing by only one letter, such as “soap” and “soup” or “tank” and “tack.” To test how well the children recognized different faces, scientists showed them pairs of photos of people. Either stimulus appeared on the screen for only a fraction of a second, and the participants had to decide whether the pair of words or the pair of faces were the same or different.

Astoundingly, the single remaining hemisphere supported both of those functions. The capacity for word and face recognition between control subjects and people with hemispherectomies differed, but the differences were less than 10%, and the average accuracy exceeded 80%.

University of Pittsburgh, “Word and face recognition can be adequately supported with half a brain” at ScienceDaily (October 25, 2022)

Findings like these show that the mind–brain complex in the human being is not at all like a computer or other machine, as is often claimed or assumed. As neurosurgeon Michael Egnor has pointed out, computers are not like that: “My computer is a network and has redundancy. But if I cut it in half with a chainsaw it sure as hell won’t work.”

We are only beginning to scratch the surface…

You may also wish to read: A neurosurgeon on why some people function with only half a brain. The study results are reassuring and they point to two larger truths. First, the mind is not the brain and second, people tend to see the mind–brain relationship in terms of the cultural conceptions that matter to them.