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Random brain action plays crucial role in decision making

The brain’s prefrontal cortex – the seat of decision making – has no input into the timing of random actions, new research shows.

Neuroscientists at the Champalimaud Centre for the Unknown (CCU) in Lisbon, Portugal, reveal the unexpected finding in a report that aims to unpack how humans and other animals decide how and when to act.

Neuroscientists have long accepted that even in strictly controlled laboratory conditions, the exact moment when a subject will decide to act is impossible to predict.

The combination of reason and randomness that drives how and when action-based decisions are made is thought to carry an evolutionary fitness dividend.

If an animal repeated the exact same flight-or-fight response every time a certain set of circumstances arose, it chances of survival would be fatally low, because a predator would learn to anticipate the action. A random element in either timing or execution is therefore beneficial.

Different brain regions produce predictability and randomness in decision-making.
Different brain regions produce predictability and randomness in decision-making. Credit: GIL COSTA / CHAMPALIMAUD CENTRE FOR THE UNKNOWN

Using rats, the CCU team, led by Masayoshi Murakami, set out to discover which parts of the brain influence response randomness.

Previous studies have identified two areas active in the co-ordination of decision-making and consequent movement: the medial prefrontal cortex (mPFC); and part of the motor cortex known as M2.

In a two-phase experiment, rats were taught to associate a particular tone with a reward. Waiting to act until a second tone sounded, after a randomly generated interval, produced a greater reward.

The interval was designed to test the rodents’ patience, inviting them to act impulsively before the second signal.

Murakami and his colleagues monitored the neural activity of the rats’ mPFC and M2 regions to measure their involvement in random action.

The two different regions within the brain seem to play very different roles in the generation of action timing, says co-author Zach Mainen: “The medial prefrontal cortex appears to keep track of the ideal waiting time based on experience. The secondary motor cortex also keeps track of the ideal timing but in addition shows variability that renders individual decisions unpredictable.”

He describes the finding as “most surprising”, one that suggests a “not-well-appreciated ‘separation of powers’ within the brain”.

The experimental results, he adds, find at least metaphorical resonance on a much larger scale: “A similar interplay between optimisation and generation of variability underlies the theory of evolution. Here, we have begun to see how this plays out in the brain.”


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