For more than a century, anaesthesia has been a routine part of surgical procedures, and for most people, it does the job of suppressing consciousness and pain perception pretty seamlessly.
But despite how common it is, scientists have never actually been able to prove how it shuts down consciousness, and now new evidence suggests that the leading hypothesis is way off track. Turns out, the drug isn't simply blocking connections between brain areas - it's actively changing them.
"To this day, it is an open question in anaesthesia research how general anaesthesia leads to loss of consciousness," researchers from Goethe University in Germany explain.
For decades, the leading hypothesis for how anaesthesia blocks consciousness is that it disrupts connections between various parts of the brain, which reduces the amount of information that can be transferred between them.
This has been tested experimentally by measuring the amount of information that's transferred between certain areas in the brain, and making the assumption that reduced amounts of information transferred indicate a disruption of the usual connections.
But what if there was no information to begin with? What if the reduction in transferred information was the result of the information not being there in the first place?
"This alternative interpretation derives from the simple principle that information that is not available at the source of information transfer cannot be transferred," says one of the researchers, Patricia Wollstadt.
Wollstadt and her team tested their hypothesis by monitoring brain activity in ferrets that had been put under general anaesthetic using the drug isofluorane.
They followed the transfer of information across various parts of the brain, and found that the biggest decreases in information weren't at the 'target' area (referred to as V1) where information was being transferred to, but at the source (the prefrontal cortex).
If the problem was the connection between the prefrontal cortex, you'd expect to see the biggest reduction in information in V1, as the brain struggles to funnel it out of the prefrontal cortex.
But it now appears that the prefrontal cortex itself is struggling to generate the information it needs to transfer.
"[T]he decreases in information transfer could be related to less information being available in the source, rather than to a decoupling," the team reports.
"We tested this possibility measuring the information available in source brain areas, and found that it decreased under isoflurane anaesthesia."
The researchers suggest that rather than affecting the function of nerve cells that connect brain areas, anaesthesia leads to a loss of consciousness by messing with connections between nerve cells within brain areas.
More research is needed to confirm if this new hypothesis can explain the loss of consciousness that comes from anaesthesia. Perhaps those answers could help us figure out something even bigger: where consciousness actually comes from.
In previous studies, researchers have tried to link the formation of consciousness to a giant neuron wrapped around the entire circumference of a mouse brain, and a trio of brain regions on the brainstem and in the cortex (the outer layer of the brain).
And let's not forget 'Perceptronium' - the hypothetical state of matter that constitutes consciousness.
The origins of consciousness are still a massive question mark, but if we can figure out how to block and regenerate it, it might be just the edge we need.
The research has been published in PLOS Computational Biology.