Philosophers are fascinated by the idea of zombies. This thought experiment of theirs supposes hypothetical beings that behave indistinguishably from humans but lack consciousness, so “zombies”. For some reason, if they existed, they think it would prove that there is something besides brain activity that produce subjective experience. I don’t get it, since I know that people can walk around, have conversations and deny recollection of conscious experience when told what the did after the fact.
Understanding the brain by what’s missing
One of my main tools as a neurologist is to examine patients with a deficit and figure out what part of the brain has been injured. It’s our tradition to understand how different parts of the brain participate in behavior by looking at patients where some function is missing or impaired and correlate function to the damaged area of the brain. For example, a small stroke in Broca’s area, located in the frontal lobe of the cerebral cortex (specifically the posterior part of the inferior frontal gyrus in Brodmann areas 44 and 45) causes what we call an expressive aphasia, a loss in fluency when trying to speak. If the injury is limited to this area, say by a small stroke, the patient understands speech perfectly well and can read text with no problems. So we conclude by seeing this correlation over and over again that the critical brain area for speech production resides in Broca’s area.
Of course that’s not to say that the ability to produce speech is only represented there like some computer subroutine. The content of speech is fed through Broca’s area from a wide range of other areas that know about the world. The decision to speak is being triggered from the prefrontal cortex, particularly the dorsolateral prefrontal cortex (DLPFC) which is associated with higher-order executive functions, such as decision-making, planning, and goal-setting. A lesion in the prefrontal cortex causes apathy and lack of initiative as was seen in patients who had the psychosurgery of prefrontal lobotomy in the 1940s. The surgery was replaced by a pharmacological dampening of the broader dopamine control system with antipsychotics.
The localization of consciousness
We now know that maintenance of consciousness is located in one particular brain network consisting of the cerebral cortex and the more deeply located structure called the thalamus. Now we need to be careful in separating this network which controls level of consciousness from the mechanisms that provide for the content of consciousness, the particular sensory channel being activated for conscious awareness. The level of consciousness is how conscious the person is, ranging from being asleep, being in a coma or being wide awake and attending to some aspect of the sensory environment.
While there are brain lesions with profound effects on the level of consciousness, we also have an array of drugs that we use to alter level of consciousness for medical procedures. These drugs are quite capable of creating the zombie the philosophers are always hypothesizing, that its to say, a person who looks awake and is behaving as if they’re conscious but lacking awareness of their actions.
There’s actually a choice of drugs to create zombies, all of which activate the GABA inhibitory neurotransmitter system in one way or another. Among them are alcohol, gamma-hydroxybutyrate (GHB), benzodiazepines (like valium, midazolam and many others used for anxiety or insomnia) and general anesthetics both inhaled (like halothane) and injectable (like propofol).
Selective effects of propofol on consciousness
In the neuroscience literature on level of consciousness, you’ll see the intravenous anesthetic propofol studied most commonly. That’s a matter of convenience and suitability. It’s easy to use infusions in animal and human studies, the dose is easily controlled by rate of infusion, and the effects are very rapid, both coming on and wearing off.
The effects of propofol on the cerebral cortex are most easily seen by EEG, a recording of voltage differences at the scalp which reflect the electrical activity of the neurons under the electrodes as conducted though the skull and scalp. In an awake person, the electrical waves are chaotic and fast, reflecting all of the fluctuating activity across the cortex as sensory information comes in, is reflected to association areas and motor activity initiated. Even though our awareness is limited to one channel at a time through attentional systems, there’s activity across all of the systems and they are talking to each other.
Start a propofol infusion and the activity starts to slow. The EEG analysis shows a drop in EEG frequency across the spectrum. With enough propofol, we can induce a profound coma to where the signal becomes very nearly flat. We do this clinically at times to treat brain trauma and uncontrollable seizures.
Zombies are an in-between state where awareness is lost
An awake person is interesting to interact with while someone in profound coma isn’t so engaging. But it’s the in between zone where we create a zombie. If you’ve ever had general anesthesia, whether with propofol or inhalation anesthesia, you’ve had the unique experience of having your mind suddenly switched off and then back on again in what seems subjectively like no time passing. Even though hours may have elapsed on the clock in the operating room, one second they’re wheeling you in, the next second you awake in the recovery room. Its a disturbing interruption of self that doesn’t happen when you’re drowsy or asleep.
So yes, many of us can subjectively confirm that these drugs turn consciousness off. You have no experience of anything during that time. The EEG is slowed, but the cortex is continuing its business without awareness. In fact, most electical recordings from neurons in the lab are done on anesthetized animals. I did that during my PhD studies. It turns out that light anesthesia has very little effect on information flow though the visual system or autonomic control system. Hubel and Weisels pioneering recording from the visual system where the found edge detection neurons, cortical columns and surround inhibition were all done in anesthetized animals. True, spontaneous behavior disappears so it can’t be studied, but most brain circuits function pretty normally, well enough that their basic characteristics can be studied.
Behavior during sedation without subjective awareness = Zombie!
But you’ll object that the anesthetized person, even though their cortex continues processing sensory symptoms is not a zombie since there’s no behavior. Well, at just the right level of infusion, a level often called “Twilight Sleep” by the medical profession, but more appropriately just “sedation”, you can ask the patient to perform simple tasks like squeezing your hand or giving short answers to questions. That much of the cortical processing for input and output is working. If sedation gets too light, you get the problem that spontaneous behavior returns but the patient is still not conscious. They’ll try to get off the procedure table or at least move around to get comfortable. Not good during a colonoscopy. It’s just that the frontal lobe system to trigger behavior is active enough to try to get out of bed, but the thalamo-cortical network for awareness and attention is selectively turned off by the propofol infusion.
Unfortunately, this state of being unconscious but behaving is not uncommon in the real world when alcohol, benzos or GHB is circulating in the blood and activating the brain’s GABA system. It’s not uncommon for people to drink to excess, take pills or even be slipped a dose of powerful sedative like GHB. They’ll continue to act like they are awake but, just like the state of anesthesia or sedation, have a gap in the continuity of their awareness suggesting that they were behaving, but not aware. Clearly some supervisory, attentional mechanisms are active when the drinker gets a ride home from the bar and awakens with a gap. You tell the drinker how much fun they had last night and they recall none of it.
Memory is consciousness is self identity
You may realize that we’ve ended up conflating continuous awareness with memory of awareness. Since the subjective report relies on recall, they can’t two can’t be untangled. And of course, knowing who you are, that you’re the same person this morning that went to sleep last night is dependent on memory.
Actually, turning of memory storage is another way to create a zombie pharmacologically. But as I’ll argue in the next posts, much of our day passes in the zombie state. Most of the time our brains attend to controlling behavior, processing sensory input and responding to the environment but without awareness of self. Most of the time, we don’t need be anything other than a zombie. It feels strange when self awareness is gone because of external causes like sedation, not when we disengage the mechanism ourselves.