When you are sleeping, how much does your brain pay attention to the outside world?

Posted on August 22, 2016

When I was 13, I once dreamt that a beautiful woman was sensuously stroking the palm of my hand, as a family of fridges hummed in the background. In reality, a huge, buzzing wasp had landed on my right hand. It idly walked around for a bit, then stung me. After the shock had worn off, I was puzzled why my dreaming brain had stopped me from waking up to this potential danger.

In a recent paper in the Journal of Neuroscience, Thomas Andrillon and his colleagues have discovered intriguing clues that start to answer these questions. They used electroencephalography (EEG) to record their participants’ electrical brain activity while they completed a simple task: listening to a series of words and pressing a button with one hand if a given word was an animal, or the other hand if it was an object. Whenever the participants made an appropriate hand response, this was always preceded by a spike of brain activity – a well-known EEG marker, called the Lateral Readiness Potential (LRP), indicating extra activity over the motor cortex on the opposite side of the brain to the hand response.

Crucially, the researchers had the participants complete this test while awake and then on into sleep and they were interested in how the participants’ brains responded differently in these states. To make sure the participants didn’t “cheat” by memorising word and response mappings when awake (for instance, that the word “table” always means “press right”), there were separate lists for when they were awake and in different stages of sleep.

Intriguingly, as the participants drifted off to a light sleep, their LRP brain signal persisted, even after they’d stopped physically responding. This suggests that their brains were still working out the meaning of the words and how to respond. However, no such result was found for deep non-REM (rapid eye movement) sleep or REM sleep – the stage where most dreaming occurs.

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Source material from The British Psychological Society

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