Dreaming is thought to play a vital role in numerous cognitive processes, such as consolidating memories and maintaining emotional balance. It’s therefore pretty important that nothing disturbs us while we slumber, ensuring we are able to gain maximum benefit from our nightly wanderings through the dreamscape. According to new research, the brain selectively filters out the sounds it deems most likely to wake us while we are dreaming, thereby eliminating any interruptions.
Published in the journal Current Biology, the new study describes how a team of researchers used electroencephalography (EEG) to measure activity in the auditory cortices of volunteers as they flowed through the various phases of sleep. Before beginning, participants were played two recordings at the same time, one of which featured normal speech while the other consisted of nonsensical noises, akin to the fictional Jabberwocky language from Lewis Carroll’s famous poem. The two streams of noises were processed separately yet simultaneously in the brains of participants while awake, which was to be expected. However, once volunteers fell asleep, things started to get interesting.
With both recordings still playing as the participants dozed, the study authors noticed that the brain began to alter the way it processed each stream. The first major shift came during the tonic rapid eye movement (REM) sleep phase, which is a form of deep sleep that isn’t usually associated with dreaming. At this point in the sleep cycle, processing of the normal speech was amplified in the auditory cortex, indicating that the brain was selecting these particular sounds as meaningful and therefore paying extra attention to them.
The sleepers then progressed to the phasic REM state, which is normally when dreams occur. At this point, the processing of the normal speech recording was suppressed, suggesting that the brain deliberately filters out sounds that it recognizes as meaningful and therefore likely to disturb us or wake us. The Jabberwocky sounds, meanwhile, were unaffected, indicating that they were deemed to be mere background noise and therefore not a threat to the integrity of a dream.
It has long been known that people are harder to wake up during phasic REM sleep than tonic REM sleep, and this new research may finally provide an explanation as to why this is: During tonic REM, the brain is actively listening for meaningful sounds in the external environment, while during phasic REM it tunes them out in order to prevent them from intruding on our dreams.