While sleeping, we react to two different types of stimuli. Quantitative information, such as loud noises, require no special cognitive processing to wake us up. However, qualitative or salient information, like subtle smells or auditory cues, requires additional subconscious processing.
By studying sleep patterns in fruit flies, researchers from the Imperial College London (ICL) have managed for the first time to determine which parts of the brain are involved in salient subconscious processing. While tracking such processing in humans is more difficult due to the complexity of our brains, for fruit flies their entire “connectome” (their neuronal maps) has been defined, making it easier to study their neuronal patterns during sleep.
“Sleep puts you in a vulnerable position, such as being at risk of predation. Therefore, animals, including humans, need to be able to respond to potential threats so that they wake up and act. If it’s a sound, like a loud bang, the processing our brains need to do is relatively simple,” explained lead study author Dr. Alice French, a researcher in the Department of Life Sciences at ICL.
“However, to actively decode the sounds and smells around us that may or may not be relevant to us, different parts of the brain must remain alert. Now, in fruit flies, who share more in common with us than you might think, we have discovered one region of the sleeping brain stays awake to process smells.”
The research team tested the response to dozens of odors in well-fed and rested fruit flies, by placing them in tubes and monitoring them with an automatic system able to release odors when detecting that the flies were asleep. They discovered that aversive odors woke the flies up more often than attractive ones. By tracking the neurons involved with fluorescent markers, the scientists managed to map the brain regions responsible for salient processing during sleep.
Moreover, they also tested whether the flies’ response to salient information was fixed or “plastic,” that is, whether it would be different under different circumstances. They found that sleep-deprived or drunk flies were less responsive to odors, while starved flies where more responsive to odors specifically related to foods.
According to lead researcher Dr. Giorgio Gilestro, a senior lecturer at ICL, although fruit flies have relatively simple brains, even in their case different species have different patterns of sleep and wakefulness. “Investigating how different animals balance their needs can help us determine the ultimate function of sleep,” he concluded.
The study is published in the journal Nature.