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Sleeping Brains Understand Words

Have you ever heard someone describe a task as being so easy that they ‘could do it in their sleep’? A fascinating new study from a team of French neuroscientists shows that this statement may be literally true, far more often than you’d think: Inducing Task-Relevant Responses to Speech in the Sleeping Brain

Sid Kouider and colleagues’ elegant experiment went as follows. Volunteers were asked to perform a word categorization task: spoken words were played to them and they had to press a button with their left hand (say) if the word was a kind of animal, or press a button with their right hand if it was an object.

So far, so simple – but the kicker was that participants were allowed to fall asleep during the task. The experiment took place in a quiet, dark room to help them nod off. Once a volunteer was soundly asleep, the task continued – more animal and object words were played to them while they slept.

The key question was: did the volunteers’ brains continue to perform the task while they were asleep? This might seem like a hard hypothesis to test – how can a brain ‘perform’ a button pressing task, without pressing any buttons, and how would we know even if it? Well, the participants were wired up to an EEG system to record brain electrical activity, before the experiment began. Based on the EEG data from the awake phase of the experiment, Kouider et al were able to record the different neural activations that accompanied pressing a button with either the left or the right hand. (These activations happen on opposite sides of the brain, fittingly.)


The authors then examined whether these same ‘button pressing’ patterns occurred in response to the stimuli presented during sleep – and amazingly, they did, in most cases. The truly remarkable result was that the sleeping brains ‘produced’ the correct responses to the stimuli. If an animal word was played, the brain’s activity was usually consistent with it making a (say) left hand button press.

So this is pretty amazing and suggests that the brain can perform a high-level language task, involving understanding the meaning of words, while asleep. There are some questions, of course. As Kouider et al say:

First, one might question whether participants in our study were truly asleep… in order to be fully confident that the trials that we included in our analysis genuinely reflect a state of sleep, microarousals and arousals (associated with button presses or not) were detected and trials in the direct vicinity of these events were discarded.

Finally, this paper made me think of the Chinese Room – a philosophical thought-experiment in which a man with an elaborate instruction book is able to respond, in Chinese, to questions posed in Chinese, even though he doesn’t know the language and has no (conscious) understanding of what he’s saying. Is a sleeping brain rather like that man? A sleeping brain has no conscious experience of the outside world, so far as we know. Yet somehow it knows how to respond to words…!

ResearchBlogging.orgKouider S, Andrillon T, Barbosa LS, Goupil L, & Bekinschtein TA (2014). Inducing task-relevant responses to speech in the sleeping brain. Current Biology, 24 (18), 2208-14 PMID: 25220055

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