Ever wondered why you can’t teach an old dog new tricks?

It’s the same reason why you can’t pick up a new language nearly as fast as your kid, or why your grandparents suck at typing. (Sorry Grandma.) It’s also the reason why sports stars or piano virtuosos are usually folks who have been practicing since they were barely out of the womb.

Young brains are flexible

That reason is what we neuroscientists call a “critical period”: a developmental stage when we are especially sensitive to certain experiences, like hearing a new language or practicing the piano. During the critical period, these experiences literally rewire your brain and determine how it will function for the rest of your life.

Once that critical period closes, certain parts of the brain lose much of their capacity to change. The same experiences become far less effective in stimulating the rewiring of neurons or other changes in the brain. That’s why so many things are much easier to learn when you’re a kid.

Obviously we adults can still learn plenty of stuff (I’ve recently written about learning here and here FYI). But compared to kids, most of the time we kinda suck at it. A five-year-old would kick my butt at learning a new language, sport, musical instrument, or video game. This despite the fact that I have 26 more years of experiencing the world, using my senses, controlling my body, and generally living life. Plus I have a PhD!

If you think about it, it’s kind of ridiculous.

Sure, you might be better than the five-year-old at learning calculus, which requires abstract, high-level cognition as well as years of education in basic mathematics. But kids are better at more basic forms of learning, especially motor-related activities such as sports. For these tasks, adult learning generally takes more time and conscious effort than learning as a child.

Sound is in the ear of the beholder

Some things are virtually impossible to learn as an adult. A classic example is the ability to distinguish certain sounds in different languages. For instance, the English language has different sounds for “r” and “l”, but Japanese does not. As a result, adult Japanese speakers are unable to distinguish “r” and “l”—even folks who have learned fluent English as adults and speak it every day. Their adult brains simply can’t adapt to hear the difference.

And lest you think we English speakers are somehow superior, there are plenty of sounds in other languages that monolingual English speakers can’t distinguish either—for example, the two types of “t” sounds in Hindi. (You can listen to them in this video and test whether you can tell the difference. Fast forward to 0:45 if you’re short on time.)

Obviously it’s not like Japanese and English and Hindi speakers are born different or something. In fact, all babies are born with the ability to distinguish all sounds in all languages. That’s right, even though babies may look like clueless, helpless entities that spend all day crying and pooping, their brains are actually smarter than yours in some ways. This is what leading language researcher Patricia Kuhl has called “the linguistic genius of babies”.

As we grow up, we lose the ability to distinguish sounds that aren’t relevant for our native language. This reflects a critical period for language learning: a time window during which our brains rewire so that we get better at differentiating the sounds relevant for our native language, but lose the ability to do so for irrelevant sounds. The critical period for becoming fluent in a language without an accent (which means you can properly distinguish and produce all the relevant sounds) seems to last until age 7 or 8.

 

Shaping social behavior

Critical periods don’t just apply to skills that you actively learn; they can also shape social behavior. The best example of this idea is the story of the mid-twentieth century biologist Konrad Lorenz and his confused goslings (of the avian, not Ryan, variety).

Young goslings normally follow around their mother, who is typically the first living thing that they see after they hatch from their eggs. But when Lorenz removed some goose eggs from their mother and hatched them in an incubator, the first living thing the goslings saw was Lorenz—and they immediately started following him around instead. They showed no recognition of their real mother.

Lorenz’s experiment showed that goslings have a super narrow critical period for figuring out who their mom is. Within hours of hatching, they “imprint” on the first moving thing that they see, meaning they assume that’s their mother. The goslings’ brains rapidly rewire so that they can learn to recognize the features of their mother (or bearded biologist, as the case may be), and this recognition can never be unlearned.

Here’s another example of how critical periods in young animals can influence their lifelong behavior. Female rats who are good mothers, meaning they like to groom and lick their pups, raise pups that are better at learning, more resistant to stress, and who grow up to be good mothers themselves. In contrast, bad rat mothers raise pups who grow up to be more anxious and worse at learning.

It’s not just genetic, because good rat mothers given “adopted” babies will raise them to be good, well-adjusted pups. Once a pup is raised in a certain way, it can be really hard or even impossible to change its behavior as an adult. There is certainly evidence that we humans, too, are profoundly influenced by our upbringing; the extent to which we can overcome early life experiences is obviously the subject of much debate.

Flexibility versus stability

At this point you might be wondering, why do critical periods exist in the first place? I mean, if kids are so good at learning things because their brains are more malleable, why wouldn’t we have evolved brains that retain that flexibility into adulthood?

And if adult brains weren’t so resistant to change, we also wouldn’t have problems like Lorenz’s confused goslings: they’d be able to undo their mistake, to relearn who their real mother is, instead of following around some random dude just because he was the first thing they saw. Similarly, rats who happened to be raised by bad mothers would get a second chance later in life.

So what’s up with adult brains being so inflexible?

Well, we can’t know for sure why the brain evolved the way that it did, but we can speculate. (We scientsts are great at speculating, which is generally a euphemism for “mostly BS-ing with a bit of logical reasoning thrown in”.)

Sure, young brains are flexible. They’re good at learning because they can be rewired by experience. But is that always a good thing?

Not necessarily.

First, rewiring the brain is biologically costly. It uses up a lot of energy and resources, like proteins and cell membranes and other crap you learned about in 9^th grade biology.

Second, and probably more importantly, rewiring the brain makes it unstable. It’s like rewiring a computer—the computer can’t keep functioning smoothly if you’re constantly changing stuff, pulling out some connections and adding others. (Okay, so I don’t really know how computers work, but you catch my drift.)

 

The brains of babies and young animals haven’t formed all the right connections yet, so they need to keep changing things up and testing out what works best. And they have parents to help them out when their immature brains are too feeble or unstable to accomplish real tasks, like finding food or defending themselves.

Adults don’t generally have parents to protect them, but we do have brains with fully functioning neural circuits. So it makes more sense for us to keep our brains mostly wired up the way that they already are. It’s worked this far, so why change things now at the risk of screwing stuff up?

In fact, there’s evidence that bad things can happen if an adult brain is too flexible: some studies suggest that people with schizophrenia have brain circuitry that is less stable and more susceptible to rewiring than it’s supposed be. So far this is just a correlation, but it suggests the possibility that having too flexible of a brain as an adult might lead to behavioral instability such as schizophrenic psychoses.

So basically, scientists think that the opening and closing of critical periods reflects a tradeoff between flexibility and stability. A young animal doesn’t know anything about the world, so it needs to be super flexible so that experience can tune its brain to its environment. Since most animals spend their whole lives in a similar environment as they were born in, stabilizing brain circuitry during adulthood makes perfect sense. And adult brains are still capable of many types of learning—they do retain plenty of flexibility—just not as much as young brains.

Next week: how critical periods open and close

Okay, so now you know what critical periods are and why they might exist. But I haven’t even gotten to the part that most scientists in this field are actually studying, which is HOW critical periods open and close. Since this post has gotten super long, I’ll save this discussion for next week.

If you think about it, it’s pretty amazing: your brain knows how to flip a switch to open and close a critical period, to make itself more or less susceptible to change. And what would be even more amazing is if we could figure out how the brain does this and harness that power to alter our own brains at will—to rectify developmental problems or simply make our adult brains better at learning. Stay tuned!

 

References:

Hensch TK. Critical period regulation. Annu Rev Neurosci 27:549-579 (2004).

Johnson JS, Newport EL. Critical period effects in second language learning: the influence of maturational state on the acquisition of English as a second language. Cogn Psychol 21:60-99 (1989).

Kuhl PK. Brain mechanisms in early language acquisition. Neuron 67:713-727 (2010).

Liu D, Diorio J, Day JC, Francis DD, Meaney MJ. Maternal care, hippocampal synaptogenesis and cognitive development in rats. Nat Neurosci 3:799-806 (2000).

Meaney MJ. Maternal care, gene expression, and the transmission of individual differences in stress reactivity across generations. Annu Rev Neurosci 24:1161-1192 (2001).

Purves D, Augustine GJ, Fitzpatrick D, et al., editors. Neuroscience. 2nd edition. Sunderland (MA): Sinauer Associates (2001).

Takesian AE, Hensch TK. Balancing plasticity/stability across brain development. Prog Brain Res 207:3-34 (2013).