Behind the adrenaline rush: Here’s what happens in the brain when we experience surprise

A burst of adrenaline could help us learn when we experience surprises, according to a new study. Noradrenaline is released when we need to focus and pay attention to something important, say scientists.

Scientists at the Massachusetts Institute of Technology also report that noradrenaline, also called norepinephrine, helps stimulate behavior that leads to a reward. This is especially true in situations where it is unclear whether a reward will be offered.

Noradrenaline is one of several neuromodulators that influence the brain, along with dopamine, serotonin, and acetylcholine. Unlike neurotransmitters, which allow cells to communicate, neuromodulators are released over large swathes of the brain which means they have more general effects. The adrenaline is produced by a structure deep in the brain called the locus coeruleus and can affect the brain in a variety of ways.

“What this work shows is that the locus coeruleus encodes unexpected events, and paying attention to those surprising events is crucial for the brain to take stock of its environment,” says study senior author Mriganka Sur, a neuroscience professor in MIT’s Department of Brain and Cognitive Sciences, in a statement. “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. This suggests they must have very crucial brain-wide functions that are important for survival and for brain state regulation.”

Scientists have long known about the effects of dopamine on the brain and its role in motivation and reward pursuit, but less is known about noradrenaline. It has been linked to arousal and boosting alertness, but too much of it can lead to anxiety.

Most of the brain’s noradrenaline is produced by the two locus coeruleus nuclei, one in each brain hemisphere. The neurons of the locus coeruleus are labeled with green fluorescent protein. (Credit: Gabi Drummond)

Previous studies of the locus coeruleus have shown that it receives input from many parts of the brain and also sends its signals far and wide. For the new study, the team set out to look at its role in reinforcement learning, or learning by trial and error.

The researchers trained mice to push a lever when they heard a high-frequency tone and not to when they heard a low-frequency tone. When the mice responded correctly to the high-frequency tone, they were given water, but if they pushed the lever when they heard a low-frequency tone, they received an unpleasant puff of air.

The creatures also learned to push the lever harder when the tones were louder. When the volume was lower they became more confused about whether they should push or not.

When the researchers inhibited activity of the locus coeruleus, the mice became much more hesitant to push the lever when they heard low volume tones. This suggests noradrenaline made them more likely to take a chance on getting a reward in situations where the payoff is uncertain.

This suggests the locus coeruleus creates signals as if to say, “push now, because the reward will come.” The neurons that create this noradrenaline signal appear to send most of their output to the motor cortex, which gives them more evidence that this signal stimulates the animals to take action.

A second burst of adrenaline was found to take place after the animals had pushed the button. When the mice received the reward they expected this second burst was small. However, when the outcome was different to what they expected the reward the brain sent out a large burst of noradrenaline.

In later trials, that mouse would be much less likely to push the lever when it was uncertain it would receive a reward. Bursts of noradrenaline appeared to spread to many parts of the brain including the prefrontal cortex where planning and other higher cognitive functions tend to occur.

“The animal is constantly adjusting its behavior,” says Sur. “Even though it has already learned the task, it’s adjusting its behavior based on what it has just done. 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.”

The team now want to explore the relationship between noradrenaline and dopamine.

The findings are published in the journal Nature.

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