When people go under general anesthesia, they appear to lose consciousness or at least they stop reacting to any outside stimuli.

In a new study, Finnish researchers wanted to know whether total consciousness is truly lost during anesthesia or if it persists in the brain in an altered state.

Their findings show that general anesthesia may resemble normal sleep more than previously thought.

For the study, researchers at the University of Turku and the Hospital District of Southwest Finland looked at the changes caused by anesthetics while patients were being monitored with electroencephalogram (EEG) and positron emission tomography (PET).

First, healthy volunteers were anesthetized either with dexmedetomidine or propofol. The drugs were administered with computer-driven target-controlled infusions until the subject just barely lost responsiveness.

From this state, the participants could be woken up with light shaking or a loud voice without changing the drug infusion. As soon as the volunteers regained responsiveness, they were asked whether they experienced anything during the anesthesia period.

Nearly all participants reported dream-like experiences that sometimes mixed with reality, said professor of psychology Dr. Antti Revonsuo.

The participants were exposed to Finnish sentences during the anesthesia, half of which ended as expected and half in an unexpected word, such as “The night sky was filled with shimmering tomatoes.”

Normally, when a person is awake, the unexpected word causes a response in the EEG, which reflects how the brain processes the meaning of the sentence and word. In this case, the research team tested whether the participants could detect and understand words or entire sentences during anesthesia.

The EEG findings showed that the brain could not tell the difference between normal and bizarre sentences when under anesthesia. Both the unexpected and the expected words yielded a significant response, meaning that the brain was trying to interpret the meaning of the words.

However, once the volunteers woke up, they did not remember the sentences they had heard, said adjunct professor Dr. Katja Valli, a senior researcher.

The participants were also exposed to unpleasant sounds during the anesthesia. After the they woke up, the sounds were played again and, surprisingly, they reacted faster to these sounds than to new sounds they had not heard before. Participants who were given dexmedetomidine also recognized the played sounds better than by chance, even though they could not recall them spontaneously.

This suggests that the brain can process sounds and words even though the subject cannot recall it afterwards. Against common belief, anesthesia does not require full loss of consciousness, as it is sufficient to just disconnect the patient from the environment, says Scheinin.

The EEG results were mostly similar to those of previous studies. However, the new study used constant infusion both when the participants were asleep and awake, which allowed the researchers to differentiate the effects of the drugs on consciousness from other possible direct or indirect effects.

The study also looked at the effects of four different anaesthetics on regional cerebral glucose metabolism with PET imaging. The findings alleviated the concern for potential harmful effects of dexmedetomidine on the ratio of cerebral blood flow and metabolism. In the future, the research will further investigate the link between cerebral blood flow or metabolism and the state of consciousness.

Overall, the findings show that consciousness is not necessarily fully lost during anesthesia, even though the person is no longer reacting to their environment. Dream-like experiences and thoughts might still float in consciousness, and the brain might still register speech and try to decipher words.

Even so, the person will not understand or remember them consciously, and the brain cannot construe full sentences from them.

Source: University of Turku