A molecule makes the brain react to surprises

Faced with a surprise, the brain is crossed by a rush of norepinephrine, a molecule with a neuromodulatory action that helps it focus attention and learn from the unexpected event. This is demonstrated by a study conducted on mice by researchers from the Massachusetts Institute of Technology (MIT). The results are published in Nature.

Norepinephrine (as well as dopamine, serotonin and acetylcholine) is a neuromodulator that can affect brain activity. Unlike neurotransmitters, which mediate neuronal communication, neuromodulators are released over large areas of the brain, resulting in much wider effects. Noradrenaline, in particular, is produced by a small region of the brain, called the locus coeruleus, which codes for unexpected events, trying to help the subject react as quickly as possible to take stock of the surrounding environment.

This mechanism has been studied in a specific type of learning, that which occurs by trial and error. The MIT researchers trained the mice to push a lever only when they heard a high-frequency tone: in this case they received water as a reward, otherwise in case of error they were hit by an annoying puff of air. The mice also learned to push the lever harder when the sound volume was louder, while they were more wavering when the volume was low. By inhibiting the activity of the locus coeruleus, the mice became much more reluctant to push the lever in the event of low volume tones, suggesting that noradrenaline prompts them to try to get a reward even when it is uncertain.

. “The animal pushes the lever because it wants a reward and the locus coeruleus provides critical signals to say ‘push now, because the reward will come,'” explains neuroscientist Mriganka Sur. His team also found that neurons that produce norepinephrine send most of it to the motor cortex, further clue that this signal stimulates animals to act.

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