Why Do We Lose Learning Ability?

Study examines brain changes during puberty to explain loss of learning ability.

March 21, 2010, 6:15 PM

March 22, 2010— -- Any mom who's struggled to ski as her children whizzed by on the bunny hill, or any dad who's had his kid figure out a new remote in 30 seconds flat can attest: adult brains don't learn as quickly as children's brains.

Last week researchers at the State University of New York (SUNY) figured out a small piece of the puzzle explaining why our learning ability declines. They also -- at least in mouse models -- pointed to a temporary antidote to the learning decline that starts in adolescence.

"Clearly there are major differences between mice and humans in terms of motivation and how we behave, but very simple processes still stay true," said Sheryl Smith, lead author on the paper explaining the findings published Friday in the journal Science. "We know that we all have the same kinds of brain chemicals."

"I hope people recognize that the teenage brain is quite different than a child's brain or an adult brain," she said.

Smith and her team at SUNY found a change in the part of the brain called the hippocampus during puberty. Before puberty, the hippocampus has very few GABA receptors, then the numbers swell during puberty. Smith said the number of GABA receptors diminishes again in adulthood, but never falls to pre-pubescent levels.

A GABA (gamma aminobutyric acid receptor) receptor works in complex lock and key system within the brain. The GABA receptor sits in a brain cell called a neuron, and when it sees a neurotransmitter called GABA, it puts a brake on brain activity.

For some reason, and Smith could only guess why, the presence of the GABA receptors in the teenage brain blocks a specific learning ability.

"It's special kind of learning," said Smith, who gave the example of learning how to spot openings in a moving basketball game, or memorizing a location of a shop when driving around town.

"But it could be something as simple as remembering where you put the keys when you set them down," said Smith. "For a mouse, it might be something as simple-minded as your parents teaching you how to remember to find the nest."

Smith tested the mice's learning ability by putting them on a round, movable surface which had a specific section designed to give an uncomfortable jolt should the mouse set foot there. She found the young mice easily learned where the jolt was coming from and how avoid it in one or two tries, but the mice going through puberty never learned how to avoid the jolt on the moving surface.

Stress Can Help or Hurt Learning

Smith then documented another unique property of the GABA receptors in puberty -- the reaction to a stress steroid called THP. In young mice, and presumably young humans, the THP steroid acts as a calming tranquilizer, which also temporarily impedes our ability to learn.

"THP is released later on … about 20-30 minutes after the stress," said Smith, who gave an example of giving a presentation as a stress. "Its role is to relax you. It acts like a subtle tranquilizer."

"In general, the drugs that relax you impair learning -- In fact doctors who sometimes administer tranquilizers along with a procedure do so so that you don't remember everything," said Smith.

But while adults and children are relaxed by THP (and learn less); adolescents have an opposite reaction because of the increased GABA receptors. The adolescent mice learn more.

Smith discovered the difference by comparing the learning rates of regular adolescent mice, and mice that were genetically engineered to lack the GABA receptors.

But, Smith warned that stressing out your teenager might not be the best way to counteract the GABA receptor change.

"If parents are really overbearing… then over-doing the stress is bad, and they are general going to undo learning," Smith recommended. "The best stress in the teenage years is if you can motivate the child to put the stress on themselves -- that's harder to do."

But Dr. Margaret Blythe, chair of the American Academy of Pediatrics Committee on Adolescence, said human adolescent behavior is so complex that such a small aspect like GABA receptors and stress are unlikely to change behavior.

"I don't even know if I would draw the analogy -- their (human adolescents) social environment is much more complicated," said Blythe, who pointed out a range of hormones social stresses and even learning tasks are far more complicated with hamans than mice.

"We are asking teens to make very complicated decisions as well, that would not be the case for mice," said Blythe.

Scientists Know We Lose Some Learning Abilities With Age

Still, Elissa L. Newport, chair of the Brain & Cognitive Sciences at the University of Rochester in Rochester, N.Y., says studies have strongly shown decline in learning power for humans.

"Loss of learning systems show that after puberty you find a decline in ability to learn. In our data there is a decline in the ability to learn new languages throughout childhood -- and when you get to late puberty, it looks like you've hit the bottom," said Newport.

"For language learning, adults certainly still can learn languages, but they on average are not as successful as children no matter how long they are exposed to the language in question," she added.

But never fear -- scientists say just because you will inevitably reach an adult brain, doesn't mean nature cheated you of a lifetime of learning.

Helen Neville, director of the Brain Development Lab at the University of Oregon, explained that most people's brains aren't finished developing until age 25. From birth until age 25, the body is "pruning away" extra synapses according to an individual's genes and experiences.

"Just about every mammal that's been looked at does this over production and loss, but it happens to human to the most," said Neville. "We've made animal models where this loss doesn't occur, and they're impaired."

So rather than think of our brain as a Jell-O mold that sets by age 25, Neville likes to think of the brain as a work of art.

"A sculptor begins with more clay than they end up with because they take away material to make a shape," said Neville. "That's really the way to think about these changes in the brain."

To learn more about how the human brain changes from birth to adulthood, you can visit changingbrains.org.

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