If you have ever been under general anesthetic, it is very unlikely you will recall anything that went on around you throughout the procedure. But according to researchers, the human brain can continue to show signs of perceptual awareness up to a certain point.

Researchers from the Centre for Functional Resonance Imaging of the Brain (FMRIB) at the University of Oxford in the UK, say through monitoring patients’ brain waves while under anesthetic, they were able to pinpoint degrees of consciousness dependent on the doses of anesthetic administered.

The researchers say their findings could lead to personalized methods for administering accurate doses of anesthetic to patients undergoing operations, potentially reducing associated health risks.

The study, published in the journal Science Translational Medicine, involved monitoring 16 patients who received propofol – a standard anesthetic – over an extended period of time.

While the patients were anesthetized, researchers exposed them to various stimuli, some of which required motor responses. During the process, electrical activity in the patients’ brains was monitored using electroencephalography (EEG).

From this, the researchers discovered similar patterns of behavior in all patients as they gradually lost consciousness under the anesthetic. However, these behaviors happened at different points for each individual.

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Researchers say that even when under general anesthetic, the human brain can still show signs of perceptual awareness up to a specific point.

Once the patients stopped responding to stimuli, it was found that slow-wave activity in the brain remained the same for each individual, even when higher does of anesthetic were administered.

Furthermore, the researchers uncovered a link between slow-wave saturation amplitude and grey matter volume in the frontal regions of their brains. Grey matter plays a part in routing sensory or motor stimuli to the central nervous system.

The researchers then used a combination of EEG and functional magnetic resonance imaging (fMRI) to measure the slow-wave activity in 12 of the patients’ brains.

This revealed that when the patients were experiencing slow-wave activity, it caused the brain to become “isolated to the external world.” Explaining this, the researchers say that the brain regions usually expected to respond to stimuli were no longer activated.

According to the researchers, this suggests there is an “optimum depth” of anesthesia that causes loss of perception, indicating that further anesthetic after this point is not needed.

At present, when a person receives anesthetic for an operation, their responses to the anesthetic are monitored through heart rate and rate of respiration.

However, the researchers say their findings suggest a more accurate method for monitoring anesthetic responses, by pinpointing a level of consciousness in an anesthetized brain.

Prof. Irene Tracey, director of the FMRIB at the University of Oxford and senior author of the study, says:

Despite the hundreds of thousands of anesthetics administered daily to patients, remarkably there is no robust, individualized indicator of perceptual awareness available.

While we can indirectly gauge whether a patient physically responds to their environment, this imaging method offers a much more nuanced approach.”

The research team say that the risk of side-effects as a result of anesthetic is low overall, and the chance of waking up through an operation is even lower. According to the Mayo Clinic (resource no longer available at www.mayoclinic.com), around one or two people in every 1,000 wake up briefly while under general anesthetic.

But the researchers note that the research may help older people, or those who have neurological or cardiac problems, who are more vulnerable to complications following high doses of anesthetic.

“With the growing use of anesthetics in the elderly and other at-risk groups, understanding the minimal dose required to induce the necessary level of anesthesia is hugely important,” says Prof. Hugh Perry, chair of the Neurosciences and Mental Health Board at the Medical Research Council who part-funded the study.

“This work is an excellent example of implementing cutting-edge imaging techniques in a way that provides an excellent scientific foundation for new ways of treating vulnerable patients.”

Earlier this year, Medical News Today reported on a study suggesting that general anesthesia increases the risk of dementia among elderly patients.