What happens to the brain when consciousness goes offline? Scientists uncover how disrupted brain communication can alter awareness, memory and our sense of self

For almost a hundred years now, general anaesthesia has made it possible for surgeons to perform operations where patients do not have any sense of what is going on in their body. But the agents which are responsible for this fascinating state are posing a fundamental mystery to neuroscientists: how does the process of loss of consciousness affect the brain?

The explanation is not as straightforward as just turning the brain off.

As stated by the BBC, research on general anaesthesia is showing that the drugs affect the communication process in the brain, causing some parts of the brain to shut down before others and drastically reducing the number of long-range connections typical of wakefulness.


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It is important to notice that the phenomenon of consciousness is not associated with a certain brain part. Rather, it comes out of the information exchange between several regions of the brain.

Thus, using general anaesthetics provides scientists with the unique opportunity to observe the process of change in the brain caused by altered consciousness.

And, indeed, the drugs used in operating theatre might be revealing to scientists the nature of human consciousness.

When the brain stops talking across long distances

There is no general anaesthetic. Whereas drugs like propofol have a different molecular mechanism from gases like xenon, various chemicals can induce the same outcome: a state of profound unawareness.

Brain scanning technology has provided researchers with one key clue. Under general anaesthesia, brain activity drops off generally, but the BBC says scientists are becoming increasingly interested in how the activity is organised as well as its quantity.

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Using fMRI scans, neuroscientist Andrea Luppi at the University of Cambridge has investigated this issue. He is researching functional connectivity, basically, the relationship between various parts of the brain.

That wider connectivity is severely curtailed under general anaesthesia.

What the effect does not create is a quiet brain, but apparently a brain where information flow is significantly constrained to certain paths.

Assuming that consciousness relies on information exchange between different parts of the brain, the interference with these links may be one way anaesthesia induces an absence of awareness.

According to the BBC, Luppi's study also demonstrated that the unique qualities of individual brains are reduced during anaesthesia, thus suggesting another hint that consciousness may rely on the complex structure of the brain, not the activity of a particular region.

The thalamus may be one of the first places to go quiet

Scientists have also started studying the areas of the brain which react first under the influence of anesthetics.

One such area gaining much attention lately is the thalamus, a small central area responsible for communications within the brain and arousal regulation.

According to Nick Franks, a professor of biophysics and anaesthetics at Imperial College London, brain imaging studies show that some areas of the brain are more susceptible to the influence of anaesthetics than others, and they switch off before other brain areas.

As reported by the BBC, scientists have managed to activate certain areas of the thalamus and trigger waking responses in anaesthetized animals. There has been similar research on whether stimulating the thalamus could have any impact on the level of consciousness in patients with serious problems of consciousness.

Another key brain area of particular interest is the cerebral cortex, covering the outer layers of the brain. The frontal and parietal lobes have also been found to be involved in the matter.

It appears therefore that anaesthesia is more of a complex process of disconnecting brain connections rather than a simple switch.

Are there really more than two states of consciousness?

Anaesthesia has also complicated the traditional idea that a person is either conscious or unconscious.

Most patients undergoing general anaesthesia have no awareness of the operation and remember nothing afterwards. But accidental awareness can occur. The BBC has reported estimates suggesting that roughly one in 15,000 patients undergoing surgery may recall some aspect of the procedure.

That phenomenon has prompted researchers to investigate whether consciousness can exist in degrees that do not fit neatly into the categories of awake and unconscious.

Jaideep Pandit, a consultant anaesthetist at Oxford University Hospitals, has proposed what he describes as a possible "third state".

In experiments, patients received an anaesthetic and a muscle-relaxing drug. A tourniquet was used to prevent the muscle relaxant from reaching one forearm, leaving the hand capable of movement.

Some patients who otherwise appeared unconscious were able to respond by squeezing an experimenter's fingers when instructed, according to the BBC.

The findings do not prove that there is a distinct third form of consciousness. But they raise an intriguing possibility: outward behaviour may not always reveal everything happening inside an anaesthetised brain.

That distinction is particularly important because modern anaesthesia is designed to prevent awareness, pain and memory during surgery. Understanding unusual responses could therefore have both scientific and medical implications.

Anaesthesia may reveal what makes each brain unique

One of the most striking clues comes from what happens to the brain's individuality during anaesthesia.

When people are awake, patterns of functional connectivity vary from person to person. Researchers can detect these differences almost like a neurological fingerprint.

Luppi and his colleagues found that these individual patterns become less pronounced under general anaesthesia, the BBC reported. Anaesthetised human brains also became less distinguishable from those of other primates.

The finding has an evolutionary dimension. Some of the brain networks most affected by anaesthesia appear to involve functions that became especially developed in humans.

The broader implication is that consciousness may not be generated by a single "consciousness centre". Instead, it may emerge from the coordinated activity of multiple regions that communicate across the brain.

This also helps explain why anaesthesia is scientifically useful. Sleep naturally changes consciousness, but general anaesthetics allow researchers to induce a comparable loss of awareness quickly and under controlled conditions.

The contrast with psychedelic drugs is equally interesting. While anaesthetics generally reduce communication between distant brain networks, research into substances such as LSD and psilocybin suggests that some psychedelic states can increase or alter communication between networks.

Neither tells scientists exactly what consciousness is.

But together, they offer different ways of examining what happens when the brain's normal organisation changes.

As Stanford University neuroscientist Boris Heifets told the BBC, understanding consciousness remains a remarkably unfinished scientific problem.

For now, general anaesthesia has provided an unusual window into that mystery: not by showing scientists where consciousness lives, but by revealing what changes when the brain can no longer communicate in its usual way.

FAQ

How does general anaesthesia affect the brain?

General anaesthesia suppresses and reorganises brain functions, including interactions among regions normally interacting during waking. How precisely it happens differs depending on drugs used; the exact mechanisms are being actively researched.

2. Does the brain switch off entirely during general anaesthesia?

Not always. Studies show that some brain functions persist, but interaction between regions is seriously disrupted. According to the BBC, certain regions may be more susceptible to anaesthesia than others.

3. What is the thalamus and why is it important for consciousness?

The thalamus is an area deep inside the brain which participates in brain region interaction and in the regulation of arousal and sleep. It has been found that stimulation of certain thalamic areas affects wakefulness; however, it has not been determined to be a consciousness centre.

4. Can a person be conscious under general anaesthesia?

Yes, rare accidental instances of consciousness during anaesthesia are known. The BBC mentions experiments where some apparently unconscious people performed a hand movement in response to instructions.