When we're awake, higher frequency waves (beta waves) dominate the brain; yet, under the effect of propofol-based general anesthesia, extremely slow-frequency traveling waves (delta waves) appear to be far more prevalent.
These waves alter from moving in all directions to pointing in the same direction when they pass through the cortex, the outermost layer of brain tissue. Beta waves can still be found, but only in small regions that aren't covered by delta waves.
"Propofol substantially alters the rhythms that we connect with higher cognition," says neuroscientist Earl Miller of the Massachusetts Institute of Technology (MIT).
"The beta traveling waves that are present during wakefulness are pushed aside, and delta traveling waves, which have been changed and made more forceful by the anesthetic, take their place. Like a bull in a china store, the deltas come through."
Another important result is that brain activity under anaesthesia differs dramatically from that witnessed while sleeping. The delta waves rotate in sleep, possibly to aid memory consolidation; here, they're smoothed down.
The findings are based on an examination of two macaque monkeys' brain scans, and the study is one of the first to follow patients from start to finish as they lose and regain consciousness.
It's crucial to be able to see how traveling waves change in the same patients as they go from awake to sedated and again. The research expands on a previous examination of the same data that was released last year.
"We continuously monitored how these waves behaved when the animals were awake, how they changed when loss of consciousness occurred, and then how they changed again when recovery of consciousness occurred – all in the same animals, in a continuous experimental session," says Sayak Bhattacharya, a computational biologist at MIT.
"We were able to see how the brain circuits that create the waves were changed in real time as a result of this."
The animals' brain wave patterns returned to normal once they recovered consciousness. This clearly shows a link between the activity changes and the use of the propofol anesthetic.
While scientists aren't quite clear what brain waves do as they travel around, they're thought to play a range of critical roles in regulating brain activity, such as storing information in memory, managing time, and seeing the world around us.
When the brain is anesthetized, all of this goes gone, and it appears that the breakdown of beta waves causes a loss of awareness. Scientists are continuously investigating how various anesthetic medications affect the brain in various ways.
"The traveling waves produced by propofol help us grasp that anesthetics trigger a variety of dynamical events that might lead to altered arousal states like unconsciousness," explains MIT neuroscientist Emery Brown.
"It is thus illogical to believe that all anesthetics have the same mechanism of action."
The study was published in the journal Journal of Cognitive Neuroscience.
