If you’ve ever been put under anaesthesia, you might recall the disorienting feeling of blinking your eyes one moment and the next, waking up hours later.
It’s the first paper to track travelling brain waves in subjects all the way through the process of losing to regaining consciousness.
An interdisciplinary team has found that the commonly used anaesthetic, propofol, substantially alters how different frequencies of brain waves travel along the cortex – the surface of the brain – and the research has been published in the Journal of Cognitive Neuroscience.
Unconsciousness induced by propofol may be in part due to an increase in the strength and direction of slow delta traveling brain waves that disrupt higher-frequency waves associated with cognition.
“The rhythms that we associate with higher cognition are drastically altered by propofol,” explains senior author Earl Miller, professor of neuroscience with the Department of Brain and Cognitive Sciences at the Massachusetts Institute of Technology (MIT) in the US.
“The beta traveling waves seen during wakefulness are pushed aside, redirected by delta traveling waves that have been altered and made more powerful by the anaesthetic,” he says. “The deltas come through like a bull in a china shop.”
Conscious brains show a mixture of brain waves of different frequencies, which rotate or travel straight in various directions: you could think of them like the numerous waves on a choppy ocean.
But under propofol anaesthesia, unconscious brains become dominated by powerful, very low-frequency delta waves (∼1 Hz).
These delta waves roll straight outward in opposite directions, instead of slowly rotating around central points as they do during consciousness. Meanwhile, higher-frequency beta waves (8–30 Hz), became fewer and more erratically structured, traveling only in directions not dominated by the surging delta waves.
The authors used the analogy of these delta waves as being like an ocean liner, lumbering through the sea and flattening everything, leaving a powerful, parting wake.
Anaesthesia studied in non-human primates
In the study, researchers re-analysed travelling brain wave data recorded from two non-human primates as they underwent propofol anaesthesia, were kept in that state for a while and then were brought back to consciousness.
Travelling brain waves are thought to perform many important functions, as they coordinate the activity of brain cells over the large areas of the brain they cover.
After regaining consciousness, the primates’ brain wave patterns all returned to where they were before propofol was administered.
The authors say that these findings strongly suggest a connection between the two states of travelling brain waves that occur with and without anaesthesia and the state of consciousness.
“We hypothesise that the drastic breakdown of beta traveling waves and their redirection could contribute to loss of consciousness under propofol anaesthesia,” says lead author Sayak Bhattacharya, a postdoctoral researcher at MIT.
These findings also point to an important way in which anaesthesia differs from sleep.
In normal sleep delta waves travel in a rotating pattern that may provide the timing that induces a phenomenon called spike timing dependent plasticity, that is needed for consolidating memories of experiences from during the day.
Under propofol, however, delta waves roll straight outwards rather than rotate, disrupting this memory-aiding mechanism and depriving the brain of a key function of sleep.