Your Brain Pays Attention to Unfamiliar Voices, Even While You Sleep

(Inside Science) -- Even when sleeping deeply you are more aware of what is going on around you than you might realize. New research suggests that the human brain is constantly monitoring its surroundings, including processing sounds, to decide if you need to wake up -- it could even let you learn in your sleep.

Manuel Schabus, a neuroscientist at the University of Salzburg in Austria, and his colleagues wanted to see what kind processing the brain could do while asleep. The researchers played a recording while their adult test subjects slept through a night. The recording included familiar and unfamiliar voices speaking different names, including the subject’s own name. Whether the name was the subject’s own or something different had no effect on brain activity. But the familiarity of the voice made a big difference.

The researchers monitored brain activity using an electroencephalogram (EEG) and saw that one particular pattern of activity, known as the K-complex, was different depending on the familiarity of the voice. K-complexes are two-part spikes in brain activity during sleep in response to an external stimulus, such as noise, light or touch. The first part supresses neural activity to keep you asleep, while the second processes the information to decide if it is important enough to wake you up.

When sleeping subjects heard unfamiliar voices their brains registered more, and bigger, K-complexes than when the voice was familiar. Unfamiliar voices also produced more micro-arousals, a pattern of activity that is believed to indicate information processing during sleep. “This means the brain is processing, in its unconscious state, whether a voice is familiar or unfamiliar,” said Schabus. The work was published yesterday in the Journal of Neuroscience.

This ability is likely an evolutionary defense mechanism, as familiar voices can be safely tuned out to ensure a good night’s sleep, but unfamiliar voices could signal a threat to the defenseless sleeper.

“Unfamiliar voices should not be talking to you at night,” said Schabus. “If our recordings had continued beyond a single word, we might expect people to wake up.”

Thomas Andrillon, a neuroscientist at the Paris Brain Institute in France, says this study builds on the recent realization among neuroscientists that the sleeping brain is not completely disconnected from its surroundings, even while the individual is nonresponsive. Schabus’ work on the role of K-complexes in this and previous studies helps to answer the mystery of how the brain allows some information to get through and be processed while unconscious.

“It’s quite a smart mechanism that allows you to filter what’s relevant or not, and when it is relevant it will trigger a chain of processes facilitating the processing of that information without needing you to wake up and disrupt sleep,” he said. “K-complexes may be the key mechanism shaping how we sleep, helping the brain decide if we should stay asleep or wake up.”

In addition to scanning for potential threats, the subjects also seemed to be learning over the course of the night. The K-complex responses were less pronounced in the second half of the night, which the researchers attributed to the unfamiliar voices becoming more familiar through repetition. Schabus said this indicates that, in some situations, it could be possible to learn new information in your sleep.

That’s been the dream of lazy students for years, but it has never really worked. Schabus said that, as long as the information is relatively simple, is not presented too loudly, and does not go on for too long, you should be able to learn without being consciously aware of it. For now though, it would only be possible in a lab where technicians can constantly monitor the state of the subject’s sleep and switch off the stimulus if they start to wake up.

“If you tried it at home you would probably wake up all the time, and it would do more harm than good,” he said.

Schabus said that this research also shows just how important it is to get a solid night’s sleep.

“Sleep is not just an unconscious state with everything blocked out,” he said. “Your brain is always monitoring and processing information so you need a good night’s sleep to recover from all this activity.”