As our thoughts dart from this to that, our brains absorb and analyze new information at a rapid pace. According to a new study, these quickly changing brain states may be encoded by the synchronization of brain waves across different brain regions. Waves originating from two areas involved in learning couple to form new communication circuits when monkeys learn to categorize different patterns of dots.
Brain synapses, or the connections between neurons, are believed to underlie learning and long-term memory formation (with the help of sleep, as we just learned). But that process is too time-consuming to account for the human mind’s flexibility. “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. [Synaptic] plasticity doesn’t happen on that kind of time scale,” MIT’s Earl Miller explains in a news release.
He adds: “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.” Miller and Evan Antzoulatos from the University of California, Davis, believe the brain does it through a synchrony of brain waves: Neurons that hum together encode new information.
Millions of neurons in the brain produce its own electrical signals; these generate brain waves, and these oscillations are measured by EEGs. The duo focused on EEG patterns from the striatum (red, above), which controls habit formation, and the prefrontal cortex, (blue, above) the seat of the brain’s executive control system. “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 explains. “The striatum learns the pieces of the puzzle, and then the prefrontal cortex puts the pieces of the puzzle together.”
They measured EEG signals as monkeys learned to assign patterns of dots into one of two categories. In the beginning, the monkeys were shown two different examples from each category. After each round, the researchers doubled the number of exemplars. At first, the monkeys memorized which exemplars belonged to which category, but as those numbers doubled, the monkeys couldn’t memorize them all. That’s when they began to learn the general traits characterizing each of the two categories. By the last stages of the experiment, the monkeys were able to categorize all 256 exemplars correctly.
As the monkeys shifted from rote memorization to category-learning, there was a corresponding shift in their EEG patterns. Brain waves independently produced by the prefrontal cortex and the striatum began to synchronize with each other -- suggesting that a communication circuit is forming between the two regions.
“There is some unknown mechanism that allows these resonance patterns to form, and these circuits start humming together,” Miller 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.”
When the monkeys nailed down the two categories, two separate circuits formed between the striatum and prefrontal cortex, each corresponding to one of the categories. Once the prefrontal cortex learns the categories and sends them to the striatum, they undergo further modification as new information comes in -- this occurs over and over.
The work was published in Neuron this week.
[Via MIT News Office]
Image: Jose-Luis Olivares/MIT