Finding the Balance Between Stability and Neuroplasticity

On Thursday, May 31, 2018, I had the pleasure of attending a talk at the 2018 World Science Festival in New York, “The Nuts and Bolts of Better Brains: Harnessing the Power of Neuroplasticity.” The moderator, Neurosurgeon/Neuroscientist, Guy Mckhann, began the lecture with a short film that described the concepts of brain stability and neuroplasticity. The film did this by comparing a newborn sea turtle and a newborn human child.

The turtle brain, McKhann explained in the film, is delivered ready to go. The little creature digs out of her hole in the sand with no assistance from a parent, then crawls across the beach toward the water. She enters the ocean without fear and begins swimming, with no instruction whatsoever. She does this for about 9,000 miles as she grows into an adult, until eventually, she closes the circle back to the beach where she was born, to then lay eggs of her own.

This, Mackhann says, is an example of a stable brain; no learning needed. It’s prepackaged by nature with all the experience it needs to survive and reproduce. The human child, on the other hand, can’t even really see until the brain learns to use the eyes in a way that most of us take for granted from our earliest memory. This is the concept of neuroplasticity; the ability of the brain to grow new networks of neurons to form memories, knowledge, and experiences that help humans survive.

As we learn and grow, the brain is constantly inventorying and evaluating what we use the most and what we use the least; tallying up what it should keep and what can go. And as the inventory grows, the brain optimizes itself by pruning the neuron structures that we don’t use. This process of pruning stabilizes the brain, taking away plasticity in favor of preserving the memories, knowledge, and experiences that help children grow into independently functioning adults.

It’s also why, for instance, if we don’t learn a foreign language as children—while the brain is still plastic and changing—chances are that as adults, we will never be able to learn another language. The same principle applies to music. As Mckhann put it, “if the child doesn’t start learning the violin by the age of five, chances are that he’ll never perform at Carnegie Hall.”

So trading plasticity for stability makes a lot of sense in the long term. Sometimes however, the system works against us. There are times when it would be a huge advantage to be able to turn plasticity back on for a while. Neuroscientist, Carla Shatz, who directs Stanford University’s Bio-X program, illustrated this point by describing the way infant brains learn to use the eyes.

The eyes, Dr. Shatz explained, operate like two independent cameras, each delivering a separate 2-dimensional image. With healthy eyes, the brain’s visual cortex receives the image from each eye, and a network pattern of binocular neurons form to combine the two images into a unified stereoscopic view that gives us depth perception. The image below on the left is from the visual cortex of a subject with healthy eyes. You can see that the binocular neurons form well defined dark and light strips; dark strips for one eye and light strips for the other. The image on the right is formed by a subject with problematic eyes. You can see that the striping is not well defined; likely because one or both eyes are not functioning well.


If eye disease happens in an older person with a stable brain, then the binocular neural pattern persists despite the eye disease. If doctors are able to resolve the issue with the eyes, then vision returns to normal. But if a child has eye problems that aren’t fixed rapidly—within a few months and definitely within a year—then the binocular neuron formations may not form, or may be pruned. If that happens, then vision will not return even if the child’s eyes return to full health. It’s this type of situation where it would be useful to reactivate plasticity in the patient’s brain so that brain could fix or rebuild missing neuron structures.

So why can’t it? Dr. Shatz assured the audience and her colleagues on stage that the genes that disable neuroplasticity are well known. She and her Stanford team regularly remove them to restore plasticity in laboratory mice. Those elderly mice go on to perform cognitive tests as if they were young again.

Neuroscientist Alvaro Pascual-Leone, Director of the Berenson-Allen Center for Noninvasive Brain Stimulation and chief of the Division of Cognitive Neurology at Beth Israel Deaconess Medical Center, performs similar feats using non-invasive Cortical Magnetic Stimulation. But, he cautions, it may be a double-edged sword.

Dr. Pascual-Leone says that returning neuroplasticity to a stable brain could potentially subject existing neural networks that handle memory and experiences to pruning. Lab mice perform great with freshly re-plasticized brains, but they’re mice, they can’t tell us what they may have lost in the process. In fact, some neuroscientists like Dr. Pascual-Leone are now even considering the possibility that autism may be a condition that arises when neuroplasticity never switches off. That situation would expose the brain’s neural networks to frequent pruning that might interfere with normal development and operation.

Like many areas of medical science, there’s currently no certainty of ever finding a magic bullet for traumatic or aging-related brain diseases, but the results are tantalizing enough to keep everyone hopeful and inspired to keep exploring. Hopefully, one day soon, these brilliant scientific minds will find that fine line between stability and neuroplasticity, so that we can finally learn that language we skipped in school, or play that instrument we always wanted to master. I, for one, would love to finally learn to play the piano when I finally have a clear calendar in my 90s.

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