You may think you’re doing nothing at night, but to your brain, sleep means finally having some spare time to take stock of the day’s events. Freed from the distractions of recording new experiences, a deeply sleeping brain can organize and strengthen memories, especially emotional ones.
For Katherina Hauner, a neuroscientist at Northwestern University Feinberg School of Medicine in Chicago who studies fear, that makes sleep a fascinating frontier. Hauner’s latest research, just published in Nature Neuroscience, explores the connections between fear, memory, and sleep.
The subjects were all healthy adults. While awake, they looked at pictures of faces with neutral expressions and learned to associate these with a mild electric shock, so that eventually these face pictures elicited a fear response in the brain. This is called fear conditioning.
Just to clarify, these were not painful shocks! They were simply startling, like you might get from opening a car door.
And we included an additional stimulus during this fear conditioning: smell. Each face was associated with a neutral smell, like mint or lemon, so both the faces and the smells became associated with the fear response.
Then, when subjects were asleep, we exposed them continuously to one of the smells again—one smell per person, chosen at random. The idea was to initiate the process of fear extinction.