A new study, published in a recent issue of Neuron, sheds light on how neural ensembles form thoughts and support the flexibility to change one's mind. Earl Miller, the Picower Professor of Neuroscience at MIT, led the study which has identified groups of neurons that encode specific behavioral rules by oscillating in synchrony with each other. The nature of conscious thought, the results suggest, may be rhythmic.
"As we talk, thoughts float in and out of our heads. Those are all ensembles forming and then reconfiguring to something else. It's been a mystery how the brain does this," says Miller, who is also a member of MIT's Picower Institute for Learning and Memory. "That's the fundamental problem that we're talking about - the very nature of thought itself."
Using monkeys trained to respond to objects based on either their color or orientation, the team identified two neural ensembles. Such a task requires cognitive flexibility, which is the ability to switch between two distinct sets of rules for behavior.
"Effectively what they're doing is focusing on some parts of information in the world and ignoring others. Which behavior they're doing depends on the context," says Tim Buschman, an MIT postdoc.
The team measured the brain waves produced in different locations throughout the prefrontal cortex as the animals switched between tasks. The prefrontal cortex is where most planning and thought takes place, and the waves are generated by rhythmic fluctuations of neurons' electrical activity.
They found that when the animals responded to objects based on orientation, certain neurons oscillated at high frequencies that produce so-called beta waves. A different ensemble of neurons oscillated in the beta frequency when color was the required rule. They found that there was some overlap, with certain neurons belonging to more than one group, but each ensemble had a distinctive pattern.
The research team found an interesting oscillation in low-frequency alpha range among neurons that make up the orientation rule ensemble, but only when the color rule was being applied. Alpha waves have long been associated with suppression of brain activity, leading the team to suggest that the alpha waves helped to quiet the neurons that trigger the orientation rule.
"What this suggests is that orientation was dominant, and color was weaker. The brain was throwing this blast of alpha at the orientation ensemble to shut it up, so the animal could use the weaker ensemble," Miller says.
The team, which included Daniel Bullock from Boston University, is now attempting to figure out how these neural ensembles coordinate their activity as the brain switches between different rules or thoughts. No one knows for sure, but some neuroscientists theorize that deeper brain structures, such as the thalamus, handle this coordination.
"It's one of the biggest mysteries of cognition, what controls your thoughts," Miller says.
The findings of this study could also help unravel the neural basis of consciousness.
"The most fundamental characteristic of consciousness is its limited capacity. You only can hold a very few thoughts in mind simultaneously," Miller says. These oscillations may explain why that is: Previous studies have shown that when an animal is holding two thoughts in mind, two different ensembles oscillate in beta frequencies, out of phase with one another.
"That immediately suggests why there's a limited capacity to consciousness: Only so many balls can be kept in the air at the same time, only a limited amount of information can fit into one oscillatory cycle," Miller says.
Studies have shown that patients with schizophrenia have reduced beta oscillations, suggesting that disruptions of these oscillations may be involved in such neurological disorders.