In detail: How learning changes brain
Learning is about connections: when the pathways between neurons get stronger, information flow is faster and smoother. We get better at triple toe-loops on the ice, or crooning a romantic ballad for “American Idyll.”
Nice notion, but what, exactly, is changing? Here, we get help from a singing bird that may never get beyond YouTube: the zebra finch.
In a study in Nature, Richard Mooney, a professor of neurobiology at Duke University, used a laser-powered microscope to peer into the brains of male zebra finches 60 days after hatching. Mooney and colleagues studied the birds’ first exposure to the song of their species, and correlated the amount of learning with changes in tiny spines on the dendrites of nerve cells, or neurons, in a motor circuit for singing in the bird’s brain.
Dendrites are branch-like structures on neurons that detect chemical signals, known as neurotransmitters, released from other neurons. When a neurotransmitter diffuses across a tiny gap, called a synapse, it is often received on tiny “dendritic spines” that, in their billions, define the neural wiring and thus the computational properties of the brain.
More and larger dendritic spines make stronger synapses, and that makes stronger and more reliable brain circuits.
Learning: All a matter of spines
In the study, Mooney, Todd Roberts, Katherine Tschida and Marguerita Klein labeled certain neurons so they would glow when viewed under a microscope; they then watched these neurons in living finches and measured what percentage of the dendritic spines appeared or disappeared over a two-hour interval.
The next day, these juvenile birds were allowed to hear the song of an adult male “tutor” of their species. Afterwards, the researchers looked at the spines again, noting that some had appeared, disappeared or changed size. In the following weeks, these birds never again heard their characteristic song, although some went on to learn it.
The birds with the highest spine turnover before hearing the tutor had a significant increase in spine size and stability immediately after tutoring. Eventually, these birds also did the best job of copying the tutor’s song.
“We made an average measure of the turnover rate, and found some juveniles with high turnover and others with low turnover, and asked what their learning outcomes were, many weeks later,” says Mooney. “Those with high turnover before hearing a tutor song eventually learned more, and those with low turnover essentially learned nothing from their tutor. Apparently, if you have more dynamic spines, you are better equipped to encode and learn from experience. That’s a pretty important message, and this is the first time that relationship has been established in the context of learning a new behavior.”
The pre-test differences in spines proved to have long-lasting influence, Mooney told us. “We are seeing changes in the brain that occur really quickly, within hours, even though the process of learning that is unfolding will take many weeks to complete. It’s not like the animal hears the tutor, and has mastered the song by the time we see the change.”
And what makes a dumb bird? A fixity in the number of dendritic spines, Mooney says. The fact that spines tend to become fixed in mature birds could explain why birds cannot learn to sing if they wait too long to hear another bird’s song.
The same phenomenon could explain why it’s difficult or impossible for adult people to become fluent in a new language.
The study not only provides support for the notion that certain neural pathways grow stronger during learning, but it also explains how this could happen. A better distribution of dendritic spines eases the flow of nerve signals across synapses, which helps the bird remember how to sing like another zebra finch.
In essence, the mechanism that Mooney was watching comes down to a neural hybrid of use-it-or-lose-it and trial and error. “The spines are asking, ‘Am I needed? If no, I go. If yes, I stay,’” says Mooney. “They are waiting for a signal, and when it comes through, they are almost like a piece of photographic film. They respond and the synapse is permanently altered.”
Watching – and learning
Humans have many forms of learning, Mooney admits, “But bird song is imitative learning, and imitation is not only the sincerest form of flattery; it’s also basis of much of our culture,” such as speech, art and music. “Finding an animal model in which you can study cultural transmission of behavior is a really powerful way to explore how the brain responds to modeling of behavior by another animal.”
Smart brains are flexible brains, Mooney found. “We were able to see differences in juvenile brains in animals that were the same age, but some could learn and some could not. This suggests that if the brain is in a highly stable condition, you can get stuck.”
– David J. Tenenbaum
Terry Devitt, editor; Steve Furay, project assistant; S.V. Medaris, designer/illustrator; David J. Tenenbaum, feature writer; Amy Toburen, content development executive
- Rapid spine stabilization and synaptic enhancement at the onset of behavioural learning, Todd F. Roberts et al, Nature, 18 Feb. 2010. ↩