Many people think of sleep as the process of turning off your brain. That seems about right when you consider your mind, memory, and consciousness during these sleeping hours. A long gap in your thinking occurs each night when you seem to turn off your mind until the alarm rings. Yet, that alarm is a key to better understanding how the mind works during sleep. If it were as simple as turning off the brain, then the sound of an alarm would not be able to intervene; there must be something going on with the sense of hearing during sleep!
Setting aside that other vast puzzle of dreaming, we know that some sensory functions remain in operation during sleep, so it isn’t quite so simple as turning off our brains for those nightly hours. Along with automatic brain functions, such as regulating breathing and heartbeat, our senses remain on hold, ready to receive stimulus that warrants a response. Just how does the sense of hearing work during this “stand by” mode of operation? Considering how our minds work in a very noisy environment can actually give us a clue into how hearing works during sleep, as well.
The Cocktail Party Effect
When you are in a room full of people, the combination of voices can turn into one mess of indecipherable sound. In fact, the voice of the person standing directly behind you might be much louder than the sound of the person standing in front of you. When you are trying to make sense of this combination of sound, how do you tell one voice from the next? Our sense of hearing is remarkably astute at differentiating one sound from another, simply given the process of mental attention.
Experiments in perception have revealed that our minds prefer sound that makes sense to us. When presented with two sonic stimuli, one that is a collection of random phonemes, that the researchers called “Jabberwocky,” and another that is in our own language, it’s as if the mind turns up the volume on the sound of the language we know. A similar process of attention is linked to hearing during sleep, except that the mind turns down the volume on sounds that it deems unimportant. For this reason, the sound of an alarm—quite important—can wake us up, while the sound of a motorcycle passing by our window might not.
Brain Imaging Studies
In order to get a better sense of the way that we hear during sleep, researchers at Johns Hopkins University engaged in brain imaging studies to witness the process. With prior knowledge of the region of the brain that is engaged by hearing, undergraduates took an EEG reading during sleep. When we are awake, the primary auditory cortex is engaged in hearing. During sleep, however, not only the primary auditory cortex but also the frontal lobe are engaged. The frontal lobe plays a crucial role in vigilance operations, making it possible to ignite a chain reaction of nervous system functioning when the brain notices a threatening or otherwise dangerous sound. This portion of the brain is engaged both by a person who hears the sound of a bear and a baby crying. Interestingly enough, this part of the brain is engaged in hearing activity only during sleep. Perhaps this brain imaging helps us better understand the process that makes it possible to hear only what is deemed necessary during sleep, as well.
Have you ever had the experience of a sound integrating into the narrative of a dream? Perhaps your alarm sounded within that dream, or it might have been the voice of someone nearby. In these cases, you might be witness to the slippage between waking and sleeping hearing processes.
Normally the brain can keep sleeping during subtle or unproblematic sounds, but it’s as if we’ve trained our minds to respond to certain sounds both through instincts and through habits. These findings reveal even more about the process of perception that links hearing with thought, and they open space for further investigation, as well. As researchers continue to seek to understand how we hear during sleep, these puzzles will remain as fascinating as the process of sleep itself.