Our nights are lulled by brain waves

During sleep our perception of our environment diminishes. Nevertheless the way in which the human brain reacts to surrounding noise over the course of sleep still remains poorly understood. In a study published this week in the Proceedings of the National Academy of Sciences (PNAS), researchers at the University of Liège have used cerebral imaging to analyse the brain's responses to sound during sleep. They have demonstrated that the activity of our brain is very tightly controlled by the specific brain waves which our sleep is composed of. In particular waves called 'sleep spindles' prevent sounds being transmitted to the regions of the brain responsible for hearing. Conversely, when sounds are linked to the cerebral waves called 'K-complex' the activation of hearing regions is enhanced.  Our perception of the environment is thus not reduced in a constant manner during sleep, but varies throughout it under the influence of particular waves produced by our brain.

In this study the science team led by Dr Thanh Dang-Vu and Professor Pierre Maquet (the University of Liège's Cyclotron Research Centre) shows that the cerebral activity induced by sounds during the course of sleep depends closely on the brain waves which are sleep is made up of.

In using functional magnetic resonance imaging (fMRI) techniques combined with electroencephalography (EEG), the researchers have demonstrated that the cerebral regions responsible for hearing remain active during in response to sounds during sleep (see the illustration, left hand panels), apart from when these sounds occur whilst the brain is producing waves called 'sleep spindles.' The study in effect shows that the spindles prevent the transmission of sounds to the auditory cortex (see the illustration, right hand panels)...

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