You reach over to the stove to grab a pot. You didn’t realize that the vodka was still burning. Oh! This painful accident probably taught you a lesson. Adaptively learn from unexpected events so as not to repeat our mistakes. Our brains can be wired to pay extra attention when we are surprised. In the recent Nature Researchers at the Massachusetts Institute of Technology found evidence that the hormone norepinephrine changes brain activity— and the subsequent behavior of the animal — in these strange moments.
Norepinephrine is one of several chemicals that can flood the brain with powerful signals. Past research shows that norepinephrine is activated when we feel excited, anxious, or alert, and that it promotes learning. But new research shows that it plays an important role in responding to the unexpected.
The MIT team used a technique called optogenetics to study norepinephrine in mice. Scientists have added special light-sensitive proteins to neurons that act as a “switch” for cells when pulses of laser light hit them. They focused on modifying an area of the brain called the locus coeruleus, which houses the cells responsible for the release of norepinephrine. Using lasers, the researchers were able to stop these cells from producing the hormone under certain circumstances. They combined this method with photo tag, a method in which proteins flash with light, allowing scientists to observe the activity of locus coeruleus cells and then determine how much norepinephrine is produced.
The researchers then developed a trial-and-error learning task for the rodents. Mice could press levers when they heard a sound. Two sounds were heard. After high-frequency tones of about 12 kHz, lever-pressing mice were rewarded with water that they could drink. For low-frequency tones, about four kilohertz, mice that hit the lever were in for a slightly unpleasant surprise: they were blown with uncomfortable air. Over time, the mice learned to press the lever only when they heard the high-frequency sounds because they received water. They avoided the lever when they heard low-frequency tones.
When the researchers examined what was happening in the locus coeruleus during these events, they saw that norepinephrine production spiked at two different times: before the mice pressed the lever and when the rodents received either water or a breath of air.
The researchers suspect that the first surge of norepinephrine indicates that it plays a role in the animal’s decision to seek rewards. One of their experiments confirms this assumption. When the scientists turned down the volume of the tones, making it harder to distinguish the high-frequency notes that produced water from the low-frequency ones that blew air, the mice appeared confused. Some were reluctant to press the lever, but some did it anyway. The researchers then used their optogenetic switch to block the release of norepinephrine, which caused the mice to hesitate even more. The norepinephrine release seemed to increase the chances that the mice would take risks when they were unsure of the outcome. In addition, the researchers tracked the release of norepinephrine before the mice pressed the lever and found that it was directed to the motor cortex of the brain, an area involved in sending nerve impulses that stimulate muscle movement. In other words, the surge in brain chemistry helped make the mice press the lever.
As for the second norepinephrine surge, the researchers found that the chemical was released when the mice received either air or water — and this brain response was strongest when the outcome was unexpected. For example, after the mice learned to associate higher tones with receiving water, the researchers decided to shake things up a bit by sometimes releasing air instead. Not only did these changes change the mice’s behavior — for example, an unexpected gasp of air made the mice press the lever more cautiously during the next trial — the scientists found that the locus coeruleus released more norepinephrine when the animals received these unexpected results. The researchers also found that blocking the release of norepinephrine made the mice less accurate in learning when to press a lever to get a water reward. This finding helps argue that norepinephrine is particularly useful in helping us learn from the unexpected.
In the future, researchers may investigate whether similar processes exist in humans. Scientists can also study how norepinephrine works with other chemical transmitters, such as dopamine, that play a role in our response to rewards. A new study suggests that there may be a much more complex neurochemical story behind the “element of surprise” than anyone ever expected.
Are you a scientist specializing in neuroscience, cognition, or psychology? Have you read a recent peer-reviewed paper that you’d like to write about for Mind Matters? Send suggestions to Scientific American’s Mind Matters editor Daisy Juhasz at firstname.lastname@example.org