Inverse zombies studied using anaesthesia

Hospital medicine takes a pretty crude approach to consciousness. You’re considered mentally AWOL if you don’t respond to simple commands or physical prodding. But studies of post-operative patients have found that many of them recall having dreamt during anaesthesia. And in some disturbing cases they’ve even felt pain or heard the surgeons talking. This suggests that it’s possible to be outwardly dead to the world, but conscious inside (locked-in patients and imaging studies of brain-injured patients in a persistent vegetative state also imply the same thing). Researchers have nicknamed people in this state “inverse zombies” – a play on the standard philosophical zombie concept, in which a person may appear to be outwardly conscious, but is in fact, dead inside.

A problem with much of the research into “inverse zombies” is that it’s been conducted opportunistically in hospitals. The experimental set-up is messy, the patients have a variety of health complications, and they’ve often been given a cocktail of anaesthetic drugs. These studies have found rates of awareness during anaesthesia at around 0.023 to 1 per cent and rates of anaesthesia dreaming at rates of 6 to 53 per cent.

Now Valdas Noreika and his collaborators have performed a carefully controlled lab study of subjective (or “phenomenal”) consciousness during anaesthesia, with the help of 40 healthy male university students. These brave souls were given progressively higher doses of one of four different anaesthetic drugs: dexmedetomidine; propofol (the drug that tragically killed Michael Jackson, who was using it as a sleeping aid); sevoflurane; and xenon. Dexmedetomidine and propofol are given intravenously; the other two are inhaled.

After the doping had begun, the researchers gave the participants the verbal command “Open your eyes!” at five minute intervals. Once a participant stopped responding they were considered to be unconscious in the traditional medical sense and the dose was gradually lowered until they responded again. Throughout, the researchers recorded the surface electrical activity from the front of the participants’ brains using a “Bispectral Index Monitor (BIS)” – a form of electroencephalography (EEG), which provided an objective measure of the depth of sedation.

The induction phase – from the last response to “Open your eyes!” to the loss of responsiveness – lasted typically from around 5 to 10 minutes; the period of sedation or loss of responsiveness itself lasted around 10 minutes; this was followed by a 2 minute recovery phase and then 5 minutes of EEG scanning. At this point, the participants were interviewed about their subjective experiences during the time they were knocked out.

The key finding is that dreams or sensations were experienced during nearly 60 per cent of the anaesthesia sessions. These ranged from perceptual sensations (including “quick visual experiences”; out-of-body sensations; an altered sense of time); dream-like experiences (had a fragmentary dream about “a trip in Eastern Europe” said one participant); vision-based dreams related to the lab situation (“one of the nurses got suspended from her work”); and dreams with auditory content based on the lab situation (“a friend’s roommate … sitting next to me here in the lab, telling me we have to go to the city”). Sometimes these experiences were accompanied by negative emotions (“a bit anxious”); other times positive (“felt extraordinarily good”). The type of experiences didn’t vary with the particular anaesthetic given.

Noreika and his team say these findings are important because they highlight the inadequacy of the standard medical definition of loss of consciousness (i.e. a loss of responsiveness), which is used in many anaesthesia-based studies into the neural correlates of consciousness. This standard definition, they argue, fails to take into account the frequent persistence of phenomenal consciousness in the absence of responsiveness. “Arguably, if one aims to explore the neural correlates of phenomenal consciousness, it would be fruitful to contrast the neural activity during dreaming anaesthesia vs. the neural activity during dreamless anaesthesia,” they said.

The study is vulnerable to some obvious criticisms. The depth of sedation was shallower than is typically used in surgery, so the results may not generalise to higher doses of anaesthesia. Also, the participants were forewarned that they would be interviewed about any experiences they had whilst unconscious, which could have led them to come up with the kind of answers that they felt the researchers were after. Defending the validity of their results, Noreika’s team pointed out that subjective reports of experience were more frequent when the objective BIS measure indicated shallower sedation – just as you’d expect if the experiences were real. “The results confirm that subjective experience may occur during clinically defined unresponsiveness,” the researchers said.


Noreika, V., Jylhänkangas, L., Móró, L., Valli, K., Kaskinoro, K., Aantaa, R., Scheinin, H., and Revonsuo, A. (2011). Consciousness lost and found: Subjective experiences in an unresponsive state. Brain and Cognition, 77 (3), 327-334 DOI: 10.1016/j.bandc.2011.09.002

Further reading: Check out this recent New Scientist feature article on consciousness and anaesthesia.

Post written by Christian Jarrett for the BPS Research Digest.

One thought on “Inverse zombies studied using anaesthesia”

  1. This is an over-simplistic view demonstrating crass ignorance of 'awareness under anaesthesia' and of the many methodologies of induction and maintenance of anaesthesia and indeed the pharmaceutical agents used. The 'awareness' phenomenom has been known for decades. Traditionally the most at risk patients are those who, in adition to true unconciousness inducing drugs (induction and maintenance agents) may also received 'muscle relaxants' either to deliberately stop breathing such that it may be controlled by the anaesthetist, or to relax muscles to permit ease of access for the surgical team. These agents stop all neuromuscular activity such as breathing, limb movement and eye movement. If a patient should become 'light of anaesthesia' whilst still under the influence of the muscle relaxant, then they will be able to hear (and had their eyeslids not been taped down by the anaesthesia team, would be able to see) and may also be receptive to pain – especially if insufficient analgesics have been given – but, they will be totally unable to move, blink or speak or in any other way advise the 'outside' world of their state of awareness. The first noticeable sign may be a transient raise in blood pressure and/or increases in heart rate. It is for this reason that patients undergo extensive vital signs monitoring.

    Historically, patients undergoing Caesarian section (LSCS) were most likely to endure 'awareness' under anaesthesis; this is because it was necessary to give muscle relaxants at induction and to ensure that not too deep a plane of anaesthesia was aquired due to the transgress of drugs through the placental barrier and thus to the baby. The danger was a respiratory depressed baby who could not breath (blue baby). This became less of an issue with the introduction of a dedicated neonatal resuscitation team who ensured respiratry and circulatory function in the newborn. Nowadays, the vast majority of C-sections are carried out with the mother fully awake using 'regional analgesic' procedures such as epidural, spinal or epi-spinal techniques during the peri-operative phase, with the added benefit of being used for ppost-operative analgesia too.

    More recent experiments to assess levels of anaesthesia have included peri-operative electro-encephalogram (EEG) measurement. I was involved in conducting such studies in 1992-1993 at the Institute of Psychiatry in the Post Graduate Neurosurgical Science Unit based at The Maudsley hospital and also at the adjacent King's College hospital. This provided an intersting spectrum of patients for analysis. In the neurosurgical field there were patients (eg for stereotactic Parkinson's treatment) for whom it was necessary to wake up the patient mid operation to permit cognitive assessment whilst ablating/resecting critical portions of the brain. At king's College there was the oportunity to monitor patients undergoing coronary artery bypass surgery (CABG) wherein anaesthesia was essentially stopped upon the patient going 'on-bypass' and entering a state of induced hypothermia. Statistically, it is this group of patients who report the highest incidence of 'out of body' experiences during procedures and many have been able to recant, in great detail, conversations held by members of the theate team.

    I should be willing to illucidate further as required.

    Dr John Michael Richards D.Sc., MA, Dip.Ed.
    (Registered Operating Department Practitioner)

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