When I learned about brain physiology in the early 1990s, we were taught that people can't grow new neurons (brain cells), and that neurons stop growing new connections around age 25, both of which make new learning harder for adults. Popular wisdom says the same: You can't teach an old dog new tricks. Now that I have passed the 50-year mark myself, I'm glad to report this was a false conclusion.
My daughter had an elementary school classmate with epilepsy, who needed to have half of his brain removed. One approach in these cases is to sever the corpus callosum, a thick set of neural fibers that connects the left and right hemispheres, which gives people two semi-autonomous brains. But this boy’s seizures were so bad that the usual approach didn’t do the trick, and his doctors were worried that he would die without a more radical approach. So in about the 5thgrade he had another operation in which his left hemisphere was completely removed. Physicians told him that he might not be able to use his right hand again, that he might not be able to speak (language resides in the left hemisphere), and that he would have to re-learn many basic skills. Certainly there was a re-learning curve, but it happened much faster than you might expect. The surgery occurred in autumn, and by spring he was performing in a talent show at the school. He has continued to make an amazing recovery, and went on to win a chess championship last year: https://www.denverpost.com/2021/10/27/brain-surgeries-17-year-old-chess/ He and his brother also now have a Twitch channel called "MasterMind Chess," with a tag line that refers to his surgery: "2 National Masters...1 1/2 minds!" Here's the link to their videos: https://www.twitch.tv/mastermindchess
In fact, this now-high-schooler's experience is not all that unusual. A 2019 study reported results for six patients who had similarly been treated with hemispherectomy (i.e., either their left or right hemisphere had been surgically removed -- three patients on each side of the brain). The authors found a normal amount of interconnection within each of the usually identified brain regions (language, motor, etc.) based on fMRI scans. But they also found greater than normal connectivity between regions of the brain that are generally understood to have different functions, suggesting that part of neuroplasticity might depend on activity across multiple brain areas instead of a one-to-one replacement of specific specialized ones that are no longer there. In another study, 23 patients with hemispherectomy performed just as well on standard IQ tests as people who had not had this type of surgery, and in 73% of those cases they actually performed better on the IQ test than their own pre-surgical results. That might be a side effect of the problems that typically lead to hemispherectomy -- i.e., severe epilepsy and seizures -- which probably prevent people from reaching their true potential. It's remarkable, however, that these patients had greater cognitive abilities with less brain tissue.
Here's another report of a patient, known by the initials E.G., who was born without a significant chunk of her left cortical hemisphere. It was a particularly important chunk too, usually containing Wernicke's area which is thought to be responsible for language comprehension and Broca's area which is believed to be responsible for speech production. Based on a traditional, structural understanding of the brain one might have expected E.G. to be completely unable to communicate, at least using language. The patient's brain instead developed comparable areas in the right hemisphere of her cortex, which were in a parallel location and functioned about the same as another person's left hemisphere typically would. Her frontal lobe (where we have located the Narrative system) was connected to her right temporal lobe in the same way that most people's frontal lobes are connected to their left temporal lobe, so E.G.'s Narrative system worked just the way everyone else's does despite the structural differences in her brain. In fact, E.G. was so functional in everyday life that no one knew she was missing a major segment of her cortex until she happened to have a brain scan for unrelated issues at the time she was 25 years old. She is now over 50 and still going strong -- in fact, she ended up being the subject of the research article linked above only because she read about the researchers' prior work in the newspaper and reached out to them with an offer to study her "interesting brain."
The journal article about this patient's brain included a statement from E.G. herself (it would be fantastic to see more of these in medical journals). Here's what she says:
Learning about my brain differences is interesting to me. Though Dr. Fedorenko's studies answer some questions about how my brain is wired the same as or differently than a typical brain, it does not tell others who I am. Please do not call my brain abnormal, that creeps me out. My brain is atypical. If not for accidently finding these differences, no one would pick me out of a crowd as likely to have these, or any other differences that make me unique.
In the past, several well-meaning but misguided healthcare professionals have told me that I should not have more than a 5th grade vocabulary, that I should have seizures, or that I should have other deficits and limitations. I do not. They seemed disappointed, even angry, that I did not have the limitations they unilaterally pronounced that I should have, without the benefit of any further investigation.
I successfully completed college and graduate school, have a vocabulary in the 98th percentile, and learned a foreign language (Russian) well enough that I dreamed in it.
A chunk of my brain stem is also missing. One neurosurgeon told me that years ago, this sort of deficit was only found in the autopsies of infants who died and that any deficit in the brain stem was thought to always cause death, but that as CT, MRI, and PET scans have gotten better and better, such things are found more commonly. I think it likely that as more and more brains are scanned, other atypical arrangements will be found more frequently, as well. But for now, my brain is special, unique, and interesting, and I am excited that it can help neuroscientists understand the plasticity of the human brain.
An older understanding of the brain suggested that specific areas have specific functions, working together like a group under the direction of its CEO in the frontal lobes. That's probably not true with respect to the "executive system" in the frontal lobes; in contrast, I have argued that the Narrative mind is less like an executive and more like a sports commentator calling out plays on the field as they occur. E.G.'s case suggests that frontal-lobe development actually depends in part on the input coming from language centers in the temporal lobes, a reverse of the usual understanding of the causal arrow. The neuroplasticity findings also defy the idea of left-right hemisphere differences that underlies much contemporary brain mythology. In all of the hemispherectomy cases, people who lost their left-hemisphere language areas were able to develop similar abilities using their right hemispheres, which have been traditionally understood to be "intuitive," nonverbal, and imagery-based.
If neuroplasticity does not involve the simple replacement of one functional brain area with another, how does it work? First, research over the past 30 years has shown that some areas of the brain actually can grow new neurons in adulthood. This is particularly true in the hippocampus (involved in forming and accessing memories), the cerebellum (involved in balance), and the olfactory bulb (involved in the sense of smell; its ability to continue creating new connections might be one reason that people find smells so evocative). Moderate to vigorous physical activity seems to promote the formation of new neurons in these areas, while also improving cognitive performance and reducing dementia risk. As people grow new neurons in areas connected to memory, they can re-learn or even re-experience previous events, which could help to overcome accumulated symptoms of trauma and stress. New learning can also prompt new patterns of behavior through the workings of the Intuitive mind.
A second mechanism for neuroplasticity is through the ongoing generation of myelin, the fatty protein that surrounds the arms of many neurons to a greater or lesser degree. Myelin is the orange area shown in the diagram above. Myelin works like the plastic insulation around a copper wire: It prevents neuro-electric signals from dispersing along the length of the neuron and keeps the full electric charge directed out the far end. More myelination equals more efficient transmission of information between neurons, particularly between neurons in different areas of the brain that might be joined by a long axon -- the "tail" at the end of the neuron cell. Recent research shows that new myelin cells can and do grow in the adult brain, and that only some of these cells are retained, which means that the brain "wires" gain efficiency in some areas but not others. Like the findings about exercise, adult brains exposed to more varied and interesting sensory experiences tend to produce more myelin.
A third line of research shows that adult neural cells can and do grow new dendrites, which are the smaller axon-like arms around the head of the neuron. Dendrites allow neurons to pass electrochemical signals back and forth with the other cells in their immediate neighborhood, and growing new dendrites means that neurons can communicate in different ways or come to serve different purposes based on linkages with different cells. In parallel with neural growth, the brain also shows evidence of neural pruning, a process in which less-used dendrites drop off or are reabsorbed into the cell. Together, these two processes have the potential to dramatically change an individual neuron's function within the brain by un-linking it from some neural partnerships and creating new linkages with others.
All of these mechanisms make some brain areas more connected over time, and others less so. A fundamental rule of brain functioning is Hebbian learning, often stated as "neurons that fire together, wire together." In other words, the more often two neurons fire at the same time, the easier it becomes for them to activate one another in the future. The hemispherectomy studies suggest that increased coordination across traditional brain areas is the reason that these patients can regain lost functioning -- by increasing the amount of wiring between diverse areas of the brain. If you were to try that with wires in your house, you would short out your circuit breakers. But it's important to remember we have relatively little evidence that most mental phenomena are localized in one brain region or another; instead, what we know from studies of brain-damaged patients is mainly that people without a particular brain area have difficulty performing some mental task. In Aristotle's terms, neurons in a specific spot like Wernicke's area are a "necessary cause" for human language abilities, but not necessarily a "sufficient cause" of them. Studies using fMRI show activation, but even when a particular brain region is more highly active, it is certainly not the only part of the brain that's activated at that time. Neural plasticity suggests tat the broad pattern of activation across the whole brain is probably more important than previously understood in producing everyday mental states. Neuroplasticity gives the brain more options for reproducing those whole-brain patterns of activation, even when a specific bundle of neurons is gone.
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