When we dream, something unexpected happens within our brains, causing us to experience something similar to being awake, but also quite different from being awake, and scientists are still attempting to figure out exactly what's going on in this in-between state.
A new paper has discovered that while we're asleep, the ability to detect sounds or to identify them gets switched off, and it may help us understand how our brains dream.
It's difficult to do a good job of mapping living organisms while awake and asleep. Few of us would want electrodes implanted in our skulls during our day-to-day activities but here the team took advantage of medical research being conducted on epilepsy patients.
"We were able to utilize a special medical technique in which electrodes were placed in the brains of epilepsy patients, monitoring activity in various parts of their brain for diagnosis and treatment," according to Tel Aviv University in Israel.
"The patients volunteered to assist in studying the brain's response to auditory stimulation in wakefulness versus sleep."
When patients were awake right down to individual neurons, the electrodes enabled the researchers to see the differences in the cerebral cortex response.
The researchers played a variety of sounds through speakers on the bedsides of volunteers for the purposes of the study. Over the course of eight years, data on over 700 neurons (about 50 per patient) was collected.
While the brain's response to sound remained largely switched on during sleep, there was an increase in the frequency of alpha-beta waves associated with attention and expectation. It appears incoming sounds are being studied, but not passed to the conscious.
This goes against previous belief: sound-related signals quickly deteriorate in the brain. In fact, they remain stronger and richer than we expected, only that there is a subtle difference in how they are processed while we're snoozing.
"In all but one feature, the strength of brain response during sleep was comparable to that observed during wakefulness," says the first author, neuroscientist Hanna Hayat from Tel Aviv University.
These alpha-beta waves (10-30Hz) are controlled by higher up in the brain this feedback (including whether or not sounds are new) helps our minds identify which sounds are important and should be listened to.
Patients who have been anesthetic have previously seen an increase in alpha-beta wave patterns, but this hasn't been seen while sleeping. Researchers describe it as a way of understanding the "fascinating mystery" of how the conscious brain differs from the unconscious brain.
This gives scientists a quantitative and reliable way of evaluating whether someone is really unconscious or not: during hospital visits, in comatose individuals, when checking for signs of dementia, and so on.
Nir adds, "Our findings have wide implications beyond this particular experiment." "We intend to investigate further the mechanisms responsible for this variation in future research."
The research has been published in Nature Neuroscience.