Researchers have gained new information on brain activity during general anesthesia by recording changes in the electrical activity of the brain. They discovered that changes in electroencephalogram correlated with the loss of consciousness itself and also by the non-specific effects of the drugs. Nearly all participants recalled dreaming during unresponsiveness and, somewhat surprisingly, words presented during anesthesia were still processed in the brain even though the participants were unable to recall them later.

When people are administered an anesthetic, they seem to lose consciousness, or at least they stop reacting to their environment. In the study, the changes caused by the anesthetics were monitored with electroencephalogram (EEG) and positron emission tomography (PET).

Brain Dreams and Processes Words during Anaesthesia

In the first part of the study, healthy voluntary participants were anesthetized either with dexmedetomidine or propofol. The drugs were administered with computer-driven target-controlled infusions until the subject just barely lost responsiveness. 

Anesthesia Period

Immediately after the subjects regained responsiveness, they were asked whether they experienced anything during the anesthesia period. Nearly all participants reported dream-like experiences that sometimes mixed with the reality. The researchers tested whether the subjects detected and understood words or entire sentences while under anesthesia.

Use Of Dexmedetomidine

The responses in the EEG showed that the brain could not differentiate between normal and bizarre sentences when under anesthesia. When we used dexmedetomidine, also the expected words created a significant response, meaning that the brain was trying to interpret the meaning of the words. 

In other words, the brain can process sounds and words even though the subject did not recall it afterward. Against common belief, anesthesia does not require the full loss of consciousness, as it is sufficient to just disconnect the patient from the environment.

The Applied Study Design Enabled Separation of Consciousness from Other Drug Effects

The perceived changes in the EEG were mostly similar to earlier studies. However, the current study used constant infusion both when the participants were asleep and awake, which enabled the researchers to differentiate the effects of the drugs on consciousness from other possible direct or indirect effects.

Partly because these effects get mixed, it is still a great challenge to estimate the depth of anesthesia during surgery. In the future, the project will further analyze the association between cerebral blood flow or metabolism and the state of consciousness.

Consciousness Is in a Dream-like State during Anaesthesia

All in all, the findings indicate that consciousness is not necessarily fully lost during anesthesia, even though the person is no longer reacting to their environment. However, dream-like experiences and thoughts might still float in consciousness. 

The brain might still register speech and try to decipher words, but the person will not understand or remember them consciously, and the brain cannot construe full sentences from them.

1. The state of consciousness induced by anesthetics can be similar to natural sleep. While sleeping, people dream and the brain observes the occurrences and stimuli in their environment subconsciously.

2. Anesthesia could resemble normal sleep more than we have previously thought