People can learn new associations between scents and sounds in their sleep, according to a new study in the journal Nature Neuroscience.

Researchers had sleepers wear masks into which pleasant and unpleasant odors were pumped: shampoo and perfume, or carrion and rotting fish. The smells were accompanied with different tones.

When the sleeping subjects were exposed to a bad smell, they breathed shallowly. When they were exposed to a good smell, they inhaled deeply. They eventually responded this way to smell-associated tones regardless of whether the smell was present.

The subjects continued to alter their breathing in response to the tones once they woke up, even though they said they had no memory of smelling or hearing anything while they had been asleep.