In a nutshell: States of consciousness have many dimensions and shouldn’t be boiled down to a single scale from awake to comatose

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The big picture:

Who has the higher level of consciousness: a person sipping coffee, or someone in bed asleep?

Consciousness researchers and the public alike would usually give the latte-sipper a “higher” level of consciousness. Levels of consciousness is also enshrined in medicine through diagnostic scales such as the Glasgow Coma Scale.

But the concept of consciousness levels is mostly theoretical and has never been closely scrutinised. Until now.

Here, Monash philosopher Jakob Hohwy and colleagues argue that the idea of levels of consciousness is wrong. People can be ordered on the basis of their height or blood pressure, but not for how conscious they are.

Consciousness, the team argues, has many dimensions — the ability to put down new memories, for example, or recognize voices and feel pain — and they don’t necessarily change in lockstep from high to low.

If Hohwy and his colleagues are correct, current attempts to find a single measure that reflects different consciousness levels — which could be used to assist with anaesthesia delivery and end-of-life decisions for people in comas — are likely misguided.

While the idea of a sliding scale of consciousness levels has its origins in severe brain injury, the same concept is applied to theories that predict which non-human animals are conscious.

The Hohwy team argues that the notion of a single scale doesn’t fit with the little we know about consciousness.

Firstly, take subjective experience, a hallmark of consciousness. It feels like something to be who we are, a feeling that disappears when we fall into dreamless sleep or coma.

But like membership of the United Nations, subjective experience cannot be partial, says Hohwy. “It’s something you either have or you don’t.”

And if subjective experience can’t be put on a sliding scale, neither can consciousness, he says.

Another problem is that consciousness science needs to explain many different consciousness states – from those attained after brain injury to those held by bees or dogs – and ranking them on a single theoretical scale is neither possible nor helpful.

A better way of thinking about consciousness, they argue, is to view it as having many dimensions, which change in different ways in different types of consciousness.

For example, a person in a vegetative state can’t lift her arm if you ask her to, while a conscious person can. But ask either to imagine themselves playing tennis — as a now famous study by co-author Adrian Owen of the University of Western Ontario in Canada, did — and their brains may well light up in the same way, indicating both may share an awareness and an ability to respond to verbal commands.

Understanding the nuances of consciousness states could lead to more ethical end-of-life decisions for people with severe brain injuries, says Hohwy.  “You’re always either conscious or not conscious, but then it’s a matter of how much you’re conscious of, and what you’re doing with that consciousness.”

Rather than asking how conscious a person is, a better question might be to ask what the person is conscious of. Are they conscious of loved ones’ voices at their bedside, or only of unintelligible noises? This could mean additional tests — such as brain imaging — to assess a person’s cognitive abilities.

The team’s concept of consciousness, if widely accepted, will also change how theories of consciousness take shape and are tested, as they will now have to take into account that states of consciousness are multidimensional.

Next steps:
Philosophers, neuroscientists, and clinicians will nut out the concept of multidimensional states of consciousness, and may use it to guide new theories, experiments and, eventually, end-of-life decisions.

Bayne, T., Hohwy, J., and Owen, A. M. (2016). Are There Levels of Consciousness? Trends in cognitive sciences, 20(6), 405-413.

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