The
human mind can rapidly absorb and analyze new information as it flits
from thought to thought. These quickly changing brain states may be
encoded by synchronization of brain waves across different brain
regions, according to a new study from MIT neuroscientists.
The researchers found that as monkeys learn to categorize different
patterns of dots, two brain areas involved in learning — the prefrontal
cortex and the striatum — synchronize their brain waves to form new
communication circuits.
“We’re seeing direct evidence for the interactions between these two
systems during learning, which hasn’t been seen before.
Category-learning results in new functional circuits between these two
areas, and these functional circuits are rhythm-based, which is key
because that’s a relatively new concept in systems neuroscience,” says
Earl Miller, the Picower Professor of Neuroscience at MIT and senior
author of the study, which appears in the June 12 issue of
Neuron.
There are millions of neurons in the brain, each producing its own
electrical signals. These combined signals generate oscillations known
as brain waves, which can be measured by electroencephalography (EEG).
The research team focused on EEG patterns from the prefrontal cortex
—the seat of the brain’s executive control system — and the striatum,
which controls habit formation.
The phenomenon of brain-wave synchronization likely precedes the
changes in synapses, or connections between neurons, believed to
underlie learning and long-term memory formation, Miller says. That
process, known as synaptic plasticity, is too time-consuming to account
for the human mind’s flexibility, he believes.
“If you can change your thoughts from moment to moment, you can’t be
doing it by constantly making new connections and breaking them apart in
your brain. Plasticity doesn’t happen on that kind of time scale,” says
Miller, who is a member of MIT’s Picower Institute for Learning and
Memory. “There’s got to be some way of dynamically establishing circuits
to correspond to the thoughts we’re having in this moment, and then if
we change our minds a moment later, those circuits break apart somehow.
We think synchronized brain waves may be the way the brain does it.”
The paper’s lead author is former Picower Institute postdoc Evan
Antzoulatos, who is now at the University of California at Davis.
Humming together
Miller’s lab has previously shown
that during category-learning, neurons in the striatum become active
early, followed by slower activation of neurons in the prefrontal
cortex. “The striatum learns very simple things really quickly, and then
its output trains the prefrontal cortex to gradually pick up on the
bigger picture,” Miller says. “The striatum learns the pieces of the
puzzle, and then the prefrontal cortex puts the pieces of the puzzle
together.”
In the new study, the researchers wanted to investigate whether this
activity pattern actually reflects communication between the prefrontal
cortex and striatum, or if each region is working independently. To do
this, they measured EEG signals as monkeys learned to assign patterns of
dots into one of two categories.
At first, the animals were shown just two different examples, or
“exemplars,” from each category. After each round, the number of
exemplars was doubled. In the early stages, the animals could simply
memorize which exemplars belonged to each category. However, the number
of exemplars eventually became too large for the animals to memorize all
of them, and they began to learn the general traits that characterized
each category.
By the end of the experiment, when the researchers were showing 256
novel exemplars, the monkeys were able to categorize all of them
correctly.
As the monkeys shifted from rote memorization to learning the
categories, the researchers saw a corresponding shift in EEG patterns.
Brain waves known as “beta bands,” produced independently by the
prefrontal cortex and the striatum, began to synchronize with each
other. This suggests that a communication circuit is forming between the
two regions, Miller says.
“There is some unknown mechanism that allows these resonance patterns
to form, and these circuits start humming together,” he says. “
That
humming may then foster subsequent long-term plasticity changes in the
brain, so real anatomical circuits can form. But the first thing that
happens is they start humming together.”
A little later, as an animal nailed down the two categories, two
separate circuits formed between the striatum and prefrontal cortex,
each corresponding to one of the categories.
“This is the first paper that provides data suggesting that coupling
in the beta-band between prefrontal cortex and striatum may play a key
role in category-formation. In addition to revealing a novel mechanism
involved in category-learning, the results also contribute to better
understanding of the significance of coupled beta-band oscillations in
the brain,” says Andreas Engel, a professor of physiology at the
University Medical Center Hamburg-Eppendorf in Germany.
“Expanding your knowledge”
Previous studies have shown that during cognitively demanding tasks,
there is increased synchrony between the frontal cortex and visual
cortex, but Miller’s lab is the first to show specific patterns of
synchrony linked to specific thoughts.
Miller and Antzoulatos also showed that once the prefrontal cortex
learns the categories and sends them to the striatum, they undergo
further modification as new information comes in, allowing more
expansive learning to take place. This iteration can occur over and
over.
“That’s how you get the open-ended nature of human thought. You keep
expanding your knowledge,” Miller says. “The prefrontal cortex learning
the categories isn’t the end of the game. The cortex is learning these
new categories and then forming circuits that can send the categories
down to the striatum as if it’s just brand-new material for the brain to
elaborate on.”
In follow-up studies, the researchers are now looking at how the
brain learns more abstract categories, and how activity in the striatum
and prefrontal cortex might reflect that type of abstraction.
The research was funded by the National Institute of Mental Health.
