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New Study Shows How Our Brain Responds To Unexpected Outcomes

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A new MIT study shows that your brain sends a burst of noradrenaline when it wants you to pay attention to something important.

This neuromodulator, which is produced by the locus coeruleus, a deep brain region, has extensive effects across the brain. The MIT team discovered that one important job of noradrenaline, also known as norepinephrine, is to help the brain learn from unexpected events in a mouse study.

According to Mriganka Sur, the Newton Professor of Neuroscience and lead author, this project demonstrates that the locus coeruleus encodes unexpected events and that paying attention to these unexpected events is essential for the brain to assess its environment.

In addition to its involvement in communicating surprise, the researchers revealed that noradrenaline aids in the stimulation of behavior that leads to a reward, especially in instances where a reward is unknown.

Changing behavior

Along with dopamine, serotonin, and acetylcholine, noradrenaline is one of the numerous neuromodulators that affect the brain. Neuromodulators, unlike neurotransmitters, which allow cell-to-cell contact, are released across huge swaths of the brain, allowing them to have broader effects.

“Neuromodulatory substances are thought to perfuse large areas of the brain and thereby alter the excitatory or inhibitory drive that neurons are receiving in a more point-to-point fashion,” explains the lead author. 

“This suggests they must have very crucial brain-wide functions that are important for survival and for brain state regulation.”

While scientists have learned a lot about dopamine’s involvement in motivation and reward-seeking, little is known about other neuromodulators, such as noradrenaline. Although it has been related to increased arousal and alertness, too much noradrenaline can cause anxiety.

Previous research has revealed that the locus coeruleus, the brain’s principal generator of noradrenaline, takes input from a variety of brain regions and spreads its messages far and wide. The MIT team set out to investigate its involvement in a type of learning known as reinforcement learning or learning by trial and error, in the new study.

The mice were trained to press a lever when they heard a high-frequency tone but not when they heard a low-frequency tone for this investigation. The mice got water if they responded appropriately to the high-frequency tone, but an unpleasant puff of air if they pressed the lever when they heard a low-frequency tone.

The mice also learned to exert more force on the lever when stronger tones were heard. When the volume was decreased, they were unsure of whether or not they should push. The mice became considerably more hesitant to press the lever when they heard low volume tones when the researchers reduced locus coeruleus activity, suggesting that noradrenaline encourages taking a chance on earning a reward in instances where the payoff is uncertain.

“The animal is pushing because it wants a reward, and the locus coeruleus provides critical signals to say, push now, because the reward will come,” adds Sur.

The researchers also discovered that the neurons that produce this noradrenaline signal appear to transfer the majority of their output to the motor cortex, adding to the evidence that this signal motivates animals to act.

Signaling surprise

While the initial burst of noradrenaline appears to motivate the mice to act, the researchers discovered that a second surge happens frequently after the trial is completed. These surges were minor when the mice received an expected reward. When the trial’s conclusion was unexpected, however, the bursts were substantially greater. The locus coeruleus, for example, released a strong burst of noradrenaline when a mouse received a blast of air instead of the reward it expected.

That mouse would be far less inclined to push the lever in subsequent trials if it wasn’t sure it would get a reward.

“The animal is constantly adjusting its behavior,” the author adds.

“Even though it has already learned the task, it’s adjusting its behavior based on what it has just done.”

When the mice were given an unexpected treat, they also displayed bursts of noradrenaline. Noradrenaline appeared to diffuse to numerous sections of the brain during these bursts, including the prefrontal cortex, which is where planning and other higher cognitive activities take place.

“The surprise-encoding function of the locus coeruleus seem to be much more widespread in the brain, and that may make sense because everything we do is moderated by surprise,” Sur adds.

The researchers are currently looking at the possibility of synergy between noradrenaline and other neuromodulators, including dopamine, which response to unexpected rewards as well. They also seek to learn more about how the prefrontal cortex retains the locus coeruleus’s short-term memory in order to assist the animals to do better in future trials.

Image Credit: Getty

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