Your sleeping brain is listening

A soundly sleeping brain monitors its surroundings. It also can respond to them without waking, a new study finds. For instance, even while snoozing, a person’s brain can sort words correctly into different categories.

This is not the first study to show that the sleeping mind can do useful work. Two years ago, researchers in Israel showed the sleeping brain can learn.

Sid Kouider of the National Center for Scientific Research (or CNRS) in Paris, France, led the new study. His team recorded brain signals from people — all wide awake — as they classified spoken words as either animals or objects. To do that, the participants might push a button with their right hand when they heard an animal name. Then they’d use their left hand to push a different button when the word represented some other type of object.

As each volunteer responded, the researchers tracked their brain activity. They did this by measuring the participants’ brain waves. Such recordings are known as EEGs. And those EEGs showed that when an individual was about to hit the right button, the brain responded differently than when it was going to hit a left button.

Then each participant nodded off. The researchers again made EEGs recording brain activity. As this happened, the researchers played recordings of a different set of words. These showed that the volunteers’ brains continued sorting the words into their proper categories. When a sleeper heard “horse,” the EEG looked as if that person was preparing to hit a button with her (or his) right hand, not the left.

Kouider’s team reported its findings September 11 in Current Biology.

The new data demonstrate one way in which the brain monitors the outside world during sleep. Those results also may help explain how meaningful sounds, such as a baby crying or a spoken name, may more easily wake someone than other sounds do.

Power Word

EEG or electroencephalogram   A record of electrical activity in the brain. Electrodes attached to the surface of the head chart a series of brain waves.