Unconscious Effort

People can categorize words while asleep, a study shows.

By | September 16, 2014


The human brain can process information and even direct responses to cues while a person is sleeping, according to a study published last week (September 11) in Current Biology.

Researchers from École Normale Supérieure in Paris first asked awake people to classify words—such as “dog” and “stamp”—as either animals or objects by pushing buttons with their left or right hands. After this task became automatic, the same people were asked to repeat the test with a different list of words as they fell asleep.  Electroencephalogram (EEG) recordings of brain activity on each side of their heads showed that sleeping participants still prepared to press the correct buttons, although they could not actually press them, and their response times were slower. A separate experiment that asked participants to categorize real words and “pseudowords” while sleeping and awake had similar results.

Although previous work demonstrated that people can process external stimuli while asleep, “what we've been able to show here is that you can go all the way up to making decisions, to preparing actions,” study author Sid Kouider of École Normale Supérieure told The Christian Science Monitor. The participants’ sleeping brains appeared to activate neural motor circuits involved in pressing a button.

“This explains some everyday life experiences such as our sensitivity to our name in our sleep, or to the specific sound of our alarm clock, compared to equally loud but less relevant sounds,” Kouider said in a press release. “Far from falling [into] a limbo when we fall asleep, parts of our brain can routinely process what is going on in our surroundings and apply a relevant scheme of response.”

The researchers measured responses during non-REM sleep; future experiments may reveal whether such brain activity persists during other sleep stages.

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