The human brain is the only reason our species has survived on this planet for so long, but even it has a blind spot. When it’s unconscious — whether because of sleep, anesthesia, or a coma — we’re sitting ducks. That doesn’t mean the brain is completely shut down, though. Studies of the unconscious brain have revealed that it’s still active, but what it’s actually doing is anyone’s guess.

In a trio of papers published on Wednesday, neuroscientists from the University of Michigan Medical School’s Center for Consciousness Science presented their latest stab at understanding the machinations of the unconscious mind. Rather than shutting down completely, one team explained in the Trends in Neurosciences paper, some of the brain’s communication pathways shut down during unconsciousness, which is why processes don’t play out normally even though the different regions of the brain are still active.

“We examined unconsciousness across three different conditions: physiological, pharmacological and pathological,” said University of Michigan Medical School anesthesiology professor George A. Mashour, Ph.D., lead author on the study, in a statement on Wednesday.

“We found that during unconsciousness, disrupted connectivity in the brain and greater modularity are creating an environment that is inhospitable to the kind of efficient information transfer that is required for consciousness.”

When the brain is unconscious, its different regions become more inwardly connected, creating brain "islands."

This study, together with the related papers in Frontiers in Human Neuroscience and the Journal of Neuroscience, was based on Mashour’s long-standing hypothesis that anesthesia doesn’t shut the brain down but rather cuts off communication between its different regions. Consciousness, as we understand it, is dependent on rapid-fire signals sent from one area to the next. Maintaining it is like ensuring food can be supplied to all the cities in a state. Even if farms are producing goods and cities are ready to receive them, the whole process breaks down if the roads are blocked.

“Instead of seeing a highly connected brain network, anesthesia results in an array of islands with isolated cognition and processing,” said Mashour.

Mashour and his colleagues supported this hypothesis by looking at the brains of people in unconscious states, whether medically induced with anesthesia, sedated into a sleep-like state, or vegetative. In the Journal of Neuroscience article, Mashour and Anthony G. Hudetz, Ph.D., the paper’s senior author and also a professor of anesthesiology, showed that brains in the early stages of sedation take much longer to process information, which is in line with the hypothesis that communication slows down during unconsciousness. They also showed that the individual regions of the brain started to focus their activity inward rather than externally, further supporting the idea that unconsciousness manifests as brain “islands.”

“That tightening might lead to the inability to connect with distant areas,” said Hudetz in a statement.

In the Frontiers in Human Neuroscience paper, the team worked with physicist UnCheol Lee, Ph.D., to quantify the amount of “information integration” in the brain during unconsciousness. The key idea in integrated information theory, a prominent (and very complex) explanation for consciousness, is that “a system is conscious if it possesses a property called Φ, or phi, which is a measure of the system’s “integrated information,” as Scientific American put it in 2015. Lee and colleagues figured that if the brain were isolating its regions into little islands during unconsciousness, then it would be less integrated. Measuring phi as people’s brains slipped into unconsciousness, they found that it did indeed decrease.

Understanding unconsciousness is especially important to anesthesiologists, whose jobs depend on putting people into that state and, crucially, getting them out unharmed. Knowing which highways of the brain shut down and how the reduced flow of signals affects its different regions will someday also help scientists better understand people in comas — and how, or whether, they can be returned to a conscious state.

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