While you sleep, what goes through your mind?

Quite a bit, it turns out. Scientists had long believed that the sleeping mind shuts out all but the most intrusive stimuli, such as alarm clocks and wailing infants. But new research indicates that the brain actually welcomes some sounds inside, where they are identified and categorized, all without the sleeper's awareness.

A study published this week in the journal Current Biology describes two experiments in which the minds of sleeping subjects were able to distinguish between different kinds of words.

In the first experiment, researchers asked participants with EEG electrodes stuck to their heads to listen to spoken words and classify them as animals or objects by pressing a button with either their left or right hands.

The experimenters then let the subjects nod off, telling them to close their eyes and recline as they continued the classification task. Once participants were fully asleep, the researchers switched to an entirely new list of words.

The scientists found that, even though the participants were asleep and would have no memory of the words that they "heard," the EEG readouts indicated that their brains still behaved as though they were commanding their hands to press the right or left buttons.

Although they could not take the final step of pressing the button, the subjects' brains were still able to ingest and classify the auditory information, and prepare to move their hands.

Experimenters also carried out a second experiment where participants distinguished real and "pseudo" words, such as "flipu." In that experiment, too, the subjects' brains seemed to correctly categorize the sounds.

Being awake or asleep seemed to have no effect on accuracy. The main difference is that they were slower: sleeping subjects took two to three times longer to decide in which category a word belonged.

Lead author Sid Kouider, a cognitive neuroscientist at the École Normale Supérieure in France, confirmed findings from earlier experiments that demonstrated humans' ability to take in semantic information even when asleep.

"What we've been able to show here is that you can go all the way up to making decisions, to preparing actions," Kouider told the Monitor.

What else can the brain do while its owner sleeps? According to Kouider, in theory we should be able to do any task that we can accomplish while on 'auto-pilot.’

"Any procedure that can be simplified enough to become automatic, then in principle should remain active while you're asleep," said Kouider.

Kouider and his co-authors write that future research could include looking at other regions of the brain responsible for higher-order functions and determining if a similar strategy can be used to induce responses. Researchers also now have the opportunity to see if this kind of brain activity is possible in sleep stages other than the non-REM sleep stage that Kouider's team focused on.

Kouider's research follows a number of studies examining the sleeping brain and its ability to process external stimuli. In a 2012 study designed by Anat Arzi and Noam Sobel, experimenters found that humans were able to learn new information while asleep by conditioning participants to pair certain odors and tones. Kouider is moving in a similar direction with his own research.

"We're testing whether you can have people falling asleep while they're learning something," said Kouider, "and if they can continue to learn when they're asleep."

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According to Kouider, the more we understand why we spend almost a third of our time fast asleep, the better we can determine if we can optimize our unconscious mind processes for tasks like learning.

"I don't think it's science fiction," said Kouider. "I think that's where we're going."