38 | NewScientist | 18 May 2013

IF YOU met a zombie in the street, would you notice? Spotting a horror-film zombie should be easy enough, but

the zombies of philosophers’ thought experiments are a different matter. They behave almost exactly like everybody else except for one crucial difference: they are not conscious.

Stick this zombie with a pin and it will say “ouch” and recoil. But that’s just a reflex – it feels no pain. In fact, this zombie has no subjective sensory experiences, or “qualia”, at all. So how do you know for sure that the people around you aren’t zombies?

The message you are supposed to take home from this thought experiment is that you have no way of knowing that other people are conscious. Or, in a slightly less radical form, that you can’t know if other people experience consciousness in the same way you do. It is a way of exploring one of the most awkward properties of consciousness: its intractable subjectivity.

But there is another – arguably more fruitful – reason for thinking about zombies. What (if any) survival advantage would a conscious human have over a zombie? In other words, what is the function of consciousness? Why did it evolve?

We are used to thinking of physical and mental traits as having adaptive functions. Language, colour vision and upright walking, for example, all have obvious survival advantages. For consciousness, however, this line of inquiry is beset by a problem, points out Geraint Rees at University College London. It is hard to work out what something is for when you are not sure what it is. Equally, it is hard to discover what something is when you don’t know what it is for. “That limits our speculation,” says Rees.

Despite this catch-22, we have made some inroads into discovering what consciousness is, at least in terms of what we can see going on in the brain using MRI scanners and electrodes on the scalp. One of the leading theories, the global neuronal workspace model, says that sensory stimuli, such as sights and sounds, are initially processed separately and locally at an unconscious level. Only the most salient information emerges into consciousness when it ignites activity in broader networks of neurons across the rest of the brain (see page 32).

According to this theory, the function of consciousness is to carry out difficult or complex mental tasks – ones that require information from multiple sources to be combined and integrated. Daniel Bor, a neuroscientist at the Sackler Centre for Consciousness Science in Brighton, UK, thinks this is plausible, but has a slightly different take on it. He believes that a key function of consciousness is to combine information in a way that leads to innovation and problem-solving, in particular through a process known as “chunking”.

We can usually hold only a few things in our working memory at once, but if we clump together related items it is easier to manipulate more concepts simultaneously. “Maybe consciousness is a way of binding components together in order to chunk them,” says Bor.

The idea is speculative but there is some supporting evidence. Two of the three areas of the brain seen as the home of consciousness – the prefrontal and parietal cortices – light up more strongly for mental tasks that require chunking than for anything else (Frontiers in Psychology, vol 3, p 63).

Another strand of support comes when we consider whether any other creatures are conscious: in theory all other animals

” what, if any, survival advantage would a conscious human have over a zombie”

So much time is spent working out what consciousness is, says Clare Wilson, that we sometimes neglect an equally important question…

Why be conscious?

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18 May 2013 | NewScientist | 39

could be zombies. Yet many people who study consciousness do not see it as an all-or-nothing quality: while other animals may not have the highly developed and special form of consciousness that we have, some species probably have a glimmer of it. And those animals we think of as most likely to be conscious – apes and dolphins, for instance – are also innovative problem-solvers and toolmakers. “I think that’s an important clue,” says Bor.

These are not the only theories about the function of consciousness. In the 1970s, the idea emerged that it was the need to understand other people’s minds that made us aware of our own. “It is more difficult to anticipate the perceptions of others if you cannot perceive your own,” says David Barash, a psychologist at the University of

Washington in Seattle. That might suggest human consciousness scaled greater heights as our ape-like ancestors started living in larger social groups, with the ensuing daily potential for aggression and competition.

It is not the only theory that relates the evolution of consciousness to group living. But for neuroscientist Chris Frith of University College London, the benefits concern cooperation rather than competition. “It’s so that we can talk to each other about experiences,” he says.

Combined sensesFrith’s group has shown that people make better decisions in laboratory tasks if they are allowed to mull over the pros and cons of the evidence with a partner (Science, vol 329, p 5995). That might sound obvious, but it is hard to imagine a zombie being able to do so as it requires reflection and introspection, key traits of consciousness. “We have to be able to reflect upon our experiences before we can talk about them,” says Frith.

Rees, who works with Frith, gives the example of two early humans regarding a distant dust cloud and trying to work out if it signals a herd of buffalo or a pack of lions. The better they are at reflecting on their feelings and judgements, the better their collective decision-making about whether to hunt or flee. “If you can combine the forces of your sensory systems that becomes a useful advantage,” says Rees.

Yet Frith thinks a better example of the benefits of consciousness would be the early humans discussing the characteristic flavour of buffalo meat, and thereby deducing where the herd had been grazing.

Of course, consciousness could have evolved for multiple reasons – or perhaps none. Some think that rather than having a survival advantage, it is an “epiphenomenon”, simply emerging as an automatic property of intelligence.

Yet that can feel like a cop-out. “My guess is that consciousness, because of its complexity and costliness, in fact conferred adaptive value on its possessors,” says Barash, “but I can’t think of any way to prove it.” n

Clare Wilson is New Scientist’s medical features editor

In the 1960s film Dr Strangelove, the lead character had a bizarre affliction. His right arm seemingly had a mind of its own. Such a condition really does exist, although it is vanishingly rare.

People with so-called anarchic hand syndrome find that their affected limb reaches out and grabs things they have no wish to pick up. They might try restraining it with their other hand, and if that doesn’t work, “they sometimes come to the surgery with their hand tied up”, says Sergio Della Sala, a neuroscientist at the University of Edinburgh, UK, who studies the condition.

The cause is injury to the brain, usually in a region known as the supplementary motor area (SMA). Work on monkeys has shown that another part of the brain, the premotor cortex, generates some of our actions unconsciously in response to things we see around us. The SMA then kicks in to allow the movement or stop it, but damage to the SMA can wreck this control – hence the anarchic hand, acting on every visual cue.

A few people are unfortunate enough to have damage to the SMA on both sides of the brain, and experience both hands acting outside their control. They are at the mercy of environmental triggers, says Della Sala.

The system sounds like the very opposite of free will – Della Sala calls it “free won’t”. The findings suggest that, while it feels like our actions are always under our conscious control, in fact there is a lot of unconscious decision-making going on too.

If that sounds implausible, have you ever been driving somewhere on a day off and found yourself heading towards the office the moment you hit part of your normal route to work? That’s your premotor cortex responding to an environmental cue right there. Clare Wilson

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