As the saying goes, old dogs—or mice or monkeys or people—can’t learn new tricks. But why? Neuroscientists have started to unravel the brain changes that are responsible. And as a new paper in the journal Science shows, they can even use these research findings to reverse the process. Old mice, at least, really can go back to learning like young ones.
The new study builds on classic work done by Michael Merzenich at the University of California, San Francisco, and colleagues. In the early 2000s they recorded the electrical activity in brain cells and discovered that young animals’ brains would change systematically when they repeatedly heard something new. For instance, if a baby monkey heard a new sound pattern many times, her neurons (brain cells) would adjust to respond more to that sound pattern. Older monkeys’ neurons didn’t change in the same way.
At least part of the reason for this lies in neurotransmitters, chemicals that help to connect one neuron to another. Young animals have high levels of “cholinergic” neurotransmitters that make the brain more plastic, easier to change. Older animals start to produce inhibitory chemicals that counteract the effect of the cholinergic ones. They actually actively keep the brain from changing.
So an adult brain not only loses its flexibility but suppresses it. This process may reflect the different agendas of adults and children. Children explore; adults exploit.
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From an evolutionary perspective, childhood is an adaptation designed to let animals learn. Baby animals get a protected time when all they have to do is learn, without worrying about actually making things happen or getting things done. Adults are more focused on using what they already know to act effectively and quickly. Inhibitory chemicals may help in this pro cess. Nature often works according to the maxim, “If it ain’t broke, don’t fix it.” In this case, there’s no need to change a brain that’s already working well.
In the new research, Jay Blundon and colleagues at St. Jude Children’s Research Hospital in Memphis, Tenn., tried to restore early-learning abilities to adult mice. As in the earlier experiments, they exposed the mice to a new sound and recorded whether their neurons changed in response. But this time the researchers tried making the adult mice more flexible by keeping the inhibitory brain chemicals from influencing the neurons.
In some studies, they actually changed the mouse genes so that the animals no longer produced the inhibitors in the same way. In others, they injected other chemicals that counteracted the inhibitors. (Caffeine seems to work in this way, by counteracting inhibitory neurotransmitters. That’s why coffee makes us more alert and helps us to learn.)
In all of these cases in the St. Jude study, the adult brains started to look like the baby brains. When the researchers exposed the altered adult mice to a new sound, their neurons responded differently, like babies’ neurons. The mice got better at discriminating among the sounds, too. The researchers also reversed the process, by getting young brains to produce the inhibitory chemicals—and the baby mice started acting like the adults.
The researchers suggest that these results may help in some disorders that come with aging. But should we all try to have childlike brains, perpetually sensitive to anything new? Maybe not, or at least not all of the time. There may be a tension between learning and acting, and adult brain chemistry may help us to focus and ignore distractions.
Babies of any species are surrounded by a new world, and their brain chemistry reflects that. Being a baby is like discovering love on your first visit to Paris after three double espressos. It’s a great way to be in some ways, but you might wake up crying at 3 in the morning. There is something to be said for grown-up stability and serenity.
Corrections & Amplifications
An earlier version of this article incorrectly gave the first name of Jay Blundon of St. Jude Children’s Research Hospital as John. (July 8, 2017)