Electrical impulses in brain encode rules for behaviour
A new study from researchers at MIT and Boston University (BU) has shed light on how neural ensembles form thoughts and support the flexibility to change one's mind.
The research team, led by Earl Miller, the Picower Professor of Neuroscience at MIT, identified groups of neurons that encode specific behavioural rules by oscillating in synchrony with each other.
The results suggest that the nature of conscious thought may be rhythmic, according to the researchers.
"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," said 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," he noted.
The researchers identified two neural ensembles in the brains of monkeys trained to respond to objects based on either their colour or orientation. This task requires cognitive flexibility - the ability to switch between two distinct sets of rules for behaviour.
As the animals switched between tasks, the researchers measured the brain waves produced in different locations throughout the prefrontal cortex, where most planning and thought takes place. Those waves are generated by rhythmic fluctuations of neurons' electrical activity.
When the animals responded to objects based on orientation, the researchers found that certain neurons oscillated at high frequencies that produce so-called beta waves. When colour was the required rule, a different ensemble of neurons oscillated in the beta frequency. Some neurons overlapped, belonging to more than one group, but each ensemble had its own distinctive pattern.
Interestingly, the researchers also saw oscillations in the low-frequency alpha range among neurons that make up the orientation rule ensemble, but only when the colour rule was being applied. The researchers believe that the alpha waves, which have been associated with suppression of brain activity, help to quiet the neurons that trigger the orientation rule.
"What this suggests is that orientation was dominant, and colour 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 stated.
The researchers are now trying to figure out how these neural ensembles coordinate their activity as the brain switches back and forth between different rules, or thoughts.
Some neuroscientists have theorized that deeper brain structures, such as the thalamus, handle this coordination, but no one knows for sure, Miller said.
"It's one of the biggest mysteries of cognition, what controls your thoughts," he asserted.
This work could also help unravel the neural basis of consciousness.
The findings were published in the Nov. 21 issue of Neuron.