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Why You Can't Hear While Asleep

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Have you ever fallen asleep at a party? Or slept right through a noisy event? What are our brains doing to keep us immersed in a dream and unaware of all the external stimuli?

               According to a new study, your brain is still recording everything that’s going on around you while you dream.  Specifically, it is filtering out external stimuli during different states of sleep—but it still knows that things are occurring in the outside world.

               To investigate this phenomenon, researchers from Australia and France teamed up with 18 volunteers.  The volunteers were connected to an EEG (electroencephalogram) while they listened during sleep to audio tapes.  The audio tapes contained actual, coherent stories in French that were combined with incoherent sounds resembling language.  The brain activity of the volunteers revealed that the brain recorded all of the sounds going on while they slept, and the brain recognized actual human speech only in non-REM states of dreaming (light sleep).  This was confirmed by neural responses in the auditory cortex that were consistent with the same responses seen in an awake brain after the exposure to sounds.

               During REM sleep, speech was filtered out.  REM sleep is associated with dreaming, and this led the researchers to the conclusion that the brain decides when to actively register speech, based on if it is engaged in dreaming.  The suppression of the brain’s response to external informative speech was especially noticeable during slow brain waves.  The researchers were able to differentiate between sounds that held information (such as human speech) from other noises that were heard during sleep  because there was no overall decline in how the brain encoded all the auditory signals; it only decreased selectively during the periods of informative speech.

               It appears to be a mechanism employed by the brain to keep  some portion of sleep reserved for undisturbed dreaming, which explains why dreams are usually so out of context from the immediate environment where we are sleeping.  It isn’t often that elements of what’s occurring around us as we sleep finds its way into our dreams.  If it does occur, it’s likely because you’re outside the actual phase of REM sleep.

References

Koroma, Matthieu, et al. “Sleepers Selectively Suppress Informative Inputs during Rapid Eye Movements.” Current Biology, vol. 30, 22 June 2020, pp. 1–7., https://www.cell.com/current-biology/pdf/S0960-9822(20)30561-3.pdf?_returnURL=https://linkinghub.elsevier.com/retrieve/pii/S0960982220305613?showall=true.

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