New research published in the Proceedings of the National Academy of Sciences examines what happens to the brain when it recovers from the effects of anesthetic.
"I always found it remarkable that someone can recover from anesthesia, not only that you blink your eyes and can walk around, but you return to being yourself," says Alexander Proekt, a visiting fellow in Don Pfaff's Laboratory of Neurobiology and Behavior at Rockefeller University and an anesthesiologist at Weill Cornell Medical College, both in New York.
"So if you learned how to do something on Sunday and on Monday, you have surgery, and you wake up and you still know how to do it."
Fascinated by how the brain manages to recover normal function quickly after "significant perturbations" such as anesthesia, Proekt and colleagues turned to statistical analysis. They wanted to investigate a hypothesis that, as anesthetic washes out of the body, electrical activity gradually returns the brain to its usual conscious patterns.
However, the team's findings demonstrated that the reality may not be so simple. "Our research shows that the recovery from deep anesthesia is not a smooth, linear process," says Proekt.
"There are dynamic 'way stations' or states of activity the brain must temporarily occupy on the way to full recovery," say the researchers.
"Instead, there are dynamic 'way stations' or states of activity the brain must temporarily occupy on the way to full recovery," he explains, suggesting that these results may have implications for understanding how brain injury can disrupt someone's ability to recover consciousness.
Next, Proekt and team administered common veterinary anesthetic to laboratory rats and monitored the electrical potential outside neurons - also known as local field potentials (LFPs) - in the rats' brains as they recovered.
The LFPs monitored by the researchers were observed in regions of the brain associated with wakefulness and anesthesia in previous electrophysiological and pharmacological studies.
The brain - in humans and rats - generates an electrical voltage that oscillates slowly when the subject is asleep and more quickly when they are awake.
By examining patterns of these oscillations in the rats' brains, the researchers attempted to understand how the electrical activity changed in different brain regions as the rats recovered from the effects of anesthetic.
Co-author Andrew Hudson, now an assistant professor in anesthesiology at the University of California, Los Angeles, describes what the team found:
"Recordings from each animal wound up having particular features that spontaneously appeared, suggesting their brain activity was abruptly transitioning through particular states. We analyzed the probability of a brain jumping from one state to another, and we found that certain states act as hubs through which the brain must pass to continue on its way to consciousness."
Hudson explains that it seems there is an intrinsic way in which the unconscious brain recovers consciousness. "The anesthetic is just a tool for severely reducing brain activity in a way in which we can control," he says.
The team is also interested in understanding what happens to brain activity in comas, brain injuries and neurological diseases, but as the disruption to consciousness cannot be controlled in these scenarios, they are much harder to study.
Co-author and research associate Diany Paola Calderon speculates that, in these scenarios, "maybe a pathway has shut down, or a brain structure that was key for full consciousness is no longer working."
Calderon thinks that this study's findings may provide clues to understanding how the brain might recover from these disruptions, however.
"We don't know yet, but our results suggest the possibility that under certain circumstances, someone may be theoretically capable of returning to consciousness but, due to the inability to transition through the hubs we have identified, his or her brain is unable to navigate the way back."