An Objective Measure of Consciousness...?

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By Neuroskeptic
Dec 22, 2011 9:20 PMNov 5, 2019 12:15 AM


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Could a puff of air in the eye offer a way to evaluate whether someone is conscious or not?

Yes it could, say Cambridge's Tristan Bekinschtein and colleagues in a new paper about Sea slugs, subliminal pictures, and vegetative state patients.

It's all about classical conditioning of the kind made famous by Pavlov. This is learning caused by the pairing of two stimuli, one of them somehow meaningful (usually unpleasant). So if I were to ring a little bell before, say, pepper spraying you, and I did that repeatedly, you would probably close your eyes whenever I rang that bell. Or just punch me, but you see the point.

Anyway, the key is that there are two kinds of classical conditioning. In the unhelpfully named "delay" conditioning, the warning stimulus overlaps with the painful one. Like if I started ringing my bell, then kept ringing it while I sprayed you with my other hand. In other words, there is no delay between the two stimuli... I said it was badly named.

By contrast in "trace", conditioning there is a delay - the warning stops shortly before the second stimulus. Bekinschtein et al argue that trace conditioning requires conciousness. While delay conditioning can occur without awareness of the link between the two stimuli, only conscious awareness can bridge the time gap in trace conditioning.

In trace experiments (in which rather than pepper spray, the unpleasant stimulus is just a puff of air in the eye), people who, when asked, can't explain the relationship ("sound means puff") don't learn to blink when they hear the sound. But with delay conditioning, this "unconscious" conditioning can occur. Likewise, under anaesthesia, trace conditioning is lost.

At first glance this looks like a piece of psychological trivia, but it could have literally life-or-death consequences. If trace conditioning is a measure of concious awareness then it could be used as a way of working out whether brain-injured people in a "coma" or "vegetative state" are aware or not.

This paper is in fact a follow-up to the author's own 2009 study showing that some people in a vegetative state do show trace conditioning - and the ones who did were more likely to subsequently wake up.

One snag is that the humble sea slug, Aplysia, can undergo trace conditioning, yet it is presumably not conscious, at least not in any recognizable sense.


Bekinschtein et al say that trace conditioning is a product of convergent evolution. Alplysia can do it and we can do it, but we use different means to the same end. Their argument is that while in Alpysia

trace conditioning is known to be dependent on just a handful of individual neurons in the creature's tiny "brain", in humans it requires an intact hippocampus (containing millions of cells). People with hippocampal damage, who suffer amnesia, also can't do trace conditioning.

That's a good point but does that mean such hippocampal patients aren't conscious? That would be weird because, apart from the amnesia, they seem perfectly normal. Presumably they're just not conscious of the relationship between things separated in time...

Also, primitive pathways for conditioning might still exist in humans, able to reactivate under special conditions. They do acknowledge this with a discussion of experiments showing that trace conditioning in the absence of conscious awareness of the relationship can occur but only when the warning stimuli are "scary", like pictures of snakes. They say that with generic, neutral stimuli there is no good evidence of unconscious trace conditioning, but this seems like a fairly fine distinction.

Ultimately, it's a very nice idea but only more studies on "unconscious" patients will tell us whether it's really able to measure consciousness in a useful way.

Bekinschtein TA, Peeters M, Shalom D, and Sigman M (2011). Sea slugs, subliminal pictures, and vegetative state patients: boundaries of consciousness in classical conditioning. Frontiers in psychology, 2 PMID: 22164148

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