autoscopic experience An autoscopic experience is the experience of seeing one’s body from an external perspective. It is related to, but distinct from, out-of-body experiences, which involve a change in the perceived location of the self. Heautoscopy, an intermediate form, involves autoscopy with some uncertainty or alternation regarding perceived self-location.
binocular rivalry An experimental method in which each eye is presented with a different (incompatible) image, so that conscious perception alternates repeatedly between each. Because sensory input remains constant, examining neural activity during binocular rivalry can help reveal the brain basis of consciousness.
coma A major disorder of consciousness and the nearest thing to brain death before actually dying. Patients in a comatose state show no signs of wakefulness or awareness. Brain activity during coma is much reduced and in some ways is similar to that observed during general anaesthesia.
global workspace theory This theory proposes that mental contents (such as perceptions, thoughts, actions) become conscious when they gain access to a brain-wide ‘global workspace’, which allows them to be used flexibly in the control of behaviour.
integrated information theory (IIT) The IIT, a mathematical theory, proposes that conscious experiences arise from the integration of large quantities of information by the brain.
neural correlate(s) of consciousness (NCCs) Defined by Francis Crick and Christof Koch as ‘the minimal neuronal mechanisms jointly sufficient for one specific percept’. The search for the NCC – or NCCs – is still the dominant approach in the neuroscience of consciousness.
neurotransmitters The chemical machinery of the brain. Neurotransmitters enable communication between neurons and synapses by crossing the divide (‘synaptic cleft’) between the axon of one neuron and the dendrite of another when a nerve impulse arrives. Neurotransmitters come in many varieties but can be broadly divided into excitatory and inhibitory subtypes.
prefrontal parietal network The network of regions comprising prefrontal and parietal cortical areas, which are involved in sensory integration, attention and higher-order cognitive functions. The prefrontal parietal network is frequently associated with consciousness, especially in so-called global workspace theories.
qualia A philosophical term broadly referring to the intrinsic properties of conscious experience – the redness of an evening sky, the sound of a bell, the warmth of a log fire and so on. Informally, it is simply ‘the way things seem’.
rapid eye movement (REM) sleep A stage of sleep in which the eyes (though closed) move rapidly (hence the name). REM sleep occupies about one-quarter of a night’s sleep and occurs mainly later on, towards the morning. Dreams are most commonly associated with REM sleep but they can occur during other sleep stages as well.
readiness potential A slowly rising electrical brain signal – detectable using EEG – that seems to precede voluntary decisions to perform actions. Controversy has raged over whether the existence of these potentials calls into doubt notions of ‘free will’.
supplementary motor area Part of the frontal lobes and a possible neural source of the readiness potential. Direct electrical stimulation of this region produces the experiences of wanting to make a movement.
vegetative state A major disorder of consciousness that can follow severe brain injury. Patients in this condition seem awake but not aware. The vegetative state is called ‘persistent’ when it lasts for more than a year. The state is distinct from coma, in which patients show no signs of wakefulness or awareness, and from the so-called ‘minimally conscious state’, in which patients show brief and transient signs of awareness.
Why are any physical processes, such as those happening in brains, ever accompanied by conscious experience? This is the ‘hard problem’, to be distinguished from the so-called ‘easy problem’ of explaining how the brain works. The hard problem has been with us for centuries – early ‘dualist’ thinkers, such as Descartes, divided the universe into ‘mind stuff’ and ‘material stuff’ – but it was only in the 1990s that the philosopher David Chalmers coined the term. Zombies – of the philosophical kind – also depend on the hard problem. A philosophical zombie (unlike the Hollywood type) is indistinguishable from a real person in all its behaviour, yet there is no conscious experience going on within. Not all philosophers believe in the hard problem. Daniel Dennett argues that consciousness is best defined in terms of the function that it supports and not by the raw essence of experience (so-called ‘qualia’). A challenge for this view is that no one is sure what consciousness is really for. Some people think the hard problem will prevent us from ever scientifically understanding consciousness, but this might be unfairly pessimistic. It could be that by solving the ‘easy’ problems of the brain, the hard problem will dissolve away.
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How does the water of the brain generate the wine of consciousness? Nobody knows … yet.
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Some people draw a parallel between the hard problem and the debate over vitalism in the history of biology. Not long ago it seemed inconceivable that any collection of biological processes could generate ‘life’ without some additional life-force, or ‘essence vital’. Nowadays, we are much more comfortable about the idea of life emerging from matter, even though there is a lot still to understand. Perhaps the same will happen with consciousness.
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NEURAL CORRELATES OF CONSCIOUSNESS
3-SECOND BIOGRAPHIES
RENÉ DESCARTES
1596–1650
Founder of dualism; discovered – or invented – the hard problem
DANIEL DENNETT
1942–
Famous for writing Consciousness Explained
DAVID CHALMERS
1966–
Coined the term ‘hard problem’
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Anil Seth
We spend about one-third of our lives sleeping and, when not dreaming, we are completely without consciousness. Even the humble fruit fly sleeps while some creatures, such as dolphins, sleep with half their brain at a time. Sleep matters: go one night without it and we suffer the next day; go too long and we would die. As we fall asleep, the rapid electrical activity found in normal waking fades away and slow, deep waves start coursing throughout the cortex. Most sensory input is blocked and nerve signals to muscles are interrupted, preventing us from acting out our dreams. Despite this, the brain remains almost as active as when we are awake. Sleep is usually divided into three stages of increasing depth, plus the ‘rapid eye movement’ (REM) stage in which brain activity is similar to waking and most dreams occur. There are many theories about why we sleep. Some researchers think sleep helps consolidate memories from the previous day. Others believe it rebalances neurotransmitter levels. Dreams are even more mysterious. Freud believed that they represent wish fulfilment of the subconscious. A more recent theory argues that they are just the brain making sense of its own activity when cut off from the world.
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Sleep is the brain’s way of dealing with being awake.
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Dream consciousness is different from normal consciousness. When dreaming, we easily accept bizarre events, show reduced self-awareness and generally lack experiences of volition. This may have to do with lower activity in the prefrontal cortex during dreaming. And dreaming is not limited to REM. Sleep–dream reports are also common after waking from early ‘slow wave’ sleep, though these dreams are relatively static, snapshot-like images and usually lack a ‘self’ character.
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SIGMUND FREUD
1856–1939
Proposed that dreams represent unfulfilled unconscious wishes
ALLEN HOBSON
1933–
An American psychiatrist notable for his ‘activation-input-modulation’ (AIM) theory of dreaming
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Anil Seth
Sleep gives your brain the chance to get on top of its paperwork, chuckle over your day’s events and enjoy some quality ‘me’ time.
Perhaps best known as one half of the Nobel-winning Crick and Watson partnership, the Lennon and McCartney of molecular biology, Francis Crick crammed enough into his illustrious career as a scientific theorist for a dozen great minds. Indeed, the strands of his interests are packed tightly together like the strands in the DNA helix. At the same time, huge upheavals, both intellectual and geographical, characterize his life, and his work is an exhilarating combination of hard work and inspired imaginative leaps, with occasional forays into more controversial areas, such as the panspermia theory and eugenics. The polar opposite of an ivory-tower academic, he believed that scientific theory could not be developed in isolation from the human experience. He was an energetic popularizer, writing several books explaining difficult concepts to eager but untrained minds.
Crick began academic life at University College, London, as a physicist, but in 1947 he switched to biology, studying for his PhD at Cambridge, before moving into genetics. After a 30-year career at Cambridge, he changed direction again, moved to the Salk Institute for Biological Research in La Jolla, California, taught himself neuroanatomy and focused his attention on theoretical neuroscience. In 1982, working with Graeme Mitchison, he produced a paper on possible functions of REM sleep and then from the early 1990s focused his talents squarely on re-establishing the investigation of consciousness at the heart of neuroscience. Crick had been struck by the reluctance of neuroscientists to tackle this central problem, observing in a 1990 paper with Christof Koch that ‘it is remarkable that most of the work in both cognitive science and the neurosciences makes no reference to consciousness’. His collaboration with Koch lasted until his death in 2004 and resulted in a series of influential theoretical articles about the relationship between brain activity and visual awareness, developing the notion of ‘neural correlates of consciousness’, or NCCs as they have come to be known.
Always a theorist rather than an experimenter, Crick’s greatest gift was the ability to discern patterns and links, to take the wider view without losing the detail and to understand how different scientific disciplines need to work together to build a better idea. If scientific theory was a sport, Crick would have been a top coach, headhunting players from different disciplines to build a team that would achieve the desired result. His last book, The Astonishing Hypothesis: The Scientific Search for the Soul, vigorously promoted the idea that neurobiology had all the tools and skills it needed to work out the age-old question of why (and how) we are conscious.
Born Francis Harry Compton Crick in Weston Favell, Northampton
1937
BSc in Physics from University College London
1939–45
Worked for the Admiralty as one of a team of scientists
1947
Began to study biology; worked at the Strangeways Research Laboratory, Cambridge
1950
Research student at Gonville & Caius College, Cambridge
1952
Met and became friends with James D. Watson
1953
Proposed the double helical structure of DNA with Watson
1954
PhD from Cambridge on X-ray diffraction, polypeptides and proteins
1959
Fellow of the Royal Society
1962
Shared Nobel Prize for Physiology or Medicine with James D. Watson and Maurice Wilkins (but not Rosalind Franklin) for their work on the structure of DNA
1967
Published Of Molecules and Men
1977
Left UK to work full time at the Salk Institute; simultaneously held a professorship at the University of San Diego
1981
Published Life Itself: Its Origin and Nature
1982
Published paper with Graeme Mitchison on the function of REM sleep
1988
Published What Mad Pursuit: A Personal View of Scientific Discovery
1990
Began work with Christof Koch on vision, short-term memory and consciousness
1994
Published The Astonishing Hypothesis: The Scientific Search for the Soul
28 July 2004
Died in San Diego
A popular way to investigate consciousness is to compare brain activity in unconscious and conscious conditions, either for different conscious states, such as sleeping or dreaming, or for different conscious experiences, such as seeing a house or a face. In binocular rivalry, one image (let’s say a house) is shown to one eye and at the same time another (a face) is shown to the other eye. Because the brain cannot resolve this ambiguous sensory input into a single image, one’s conscious experience alternates between the house and the face. Comparing brain activity between these two conditions should reveal the neural correlates of consciousness (NCCs) of the house (or face). Francis Crick (co-discoverer of the structure of DNA – see here) and his colleague Christof Koch defined the NCC as the ‘minimal neuronal mechanisms jointly sufficient for one specific percept’. Current experiments do not yet allow us to achieve such a detailed view. Showing that certain brain regions – or types of activity – correlate with consciousness does not show that they are sufficient, because other brain processes might be involved. Still, much has been learned, for example that consciousness is generally associated with activation of a large swathe of cortex involving prefrontal and parietal regions, and that ‘top-down’ connections from higher brain regions to sensory regions are also vital.
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Brain activity can correlate with conscious experience, but does it explain it?
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A big problem with the NCC approach is that it is difficult to ensure that the only thing that changes is the conscious experience. Usually, when we are conscious of ‘X’, we also say so, either verbally or by (for example) pushing a button. Thus, it is hard to disentangle the NCC from related brain processes, such as attention, memory and behavioural report.
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VOLITION, INTENTION & ‘FREE WILL’
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CHRISTOF KOCH
1956–
A long-term collaborator of Crick and an early pioneer in the NCC approach with a focus on visual consciousness
GERAINT REES
1967–
A cognitive neuroscientist who has generated many insights into NCCs of visual consciousness; pioneered research looking into structural NCCs
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Anil Seth
Part of any conscious scene is the experience of owning and identifying with a particular body. This seems so obvious we may take it for granted, but plenty of evidence shows that our body experience is actively constructed by the brain, just like our experience of the external world. In the now classic ‘rubber hand illusion’, synchronous stroking of a fake hand and a person’s real hand, with visual attention focused on the fake hand, leads them to experience the fake hand as part of their body. This means that the brain infers which parts of the world belong to its body and which do not, on the basis of correlations between different senses. On the other hand (so to speak), if you are unlucky enough to lose a limb, you may continue to experience sensations emanating from that limb even though it no longer exists (so-called ‘phantom limb syndrome’). This again shows that the brain builds a ‘body model’ that doesn’t always match the physical body. Recent studies have taken this view even further. Using clever combinations of virtual reality, head-mounted cameras, multisensory stimulation and insights from ‘out-of-body experiences’, researchers have induced ‘autoscopic’ experiences that lead them to not only experience a fake hand, but an entire body – filmed or virtual – as their own.
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The experience of our bodily self and its location in space is actively constructed by the brain and is surprisingly open to change.
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The brain’s body model depends on multiple brain regions, including the parietal and somatosensory cortices, temporo-parietal junction and angular gyrus. Damage to these areas can lead to a variety of bizarre syndromes, including somatoparaphrenia (in which one denies ownership of a body part, sometimes attributing it to someone else) and xenomelia (the desire to amputate a completely healthy limb). Electrical stimulation and damage to the angular gyrus in patients can lead to out-of-body experiences.
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V. S. RAMACHANDRAN
1951–
Pioneer in investigation of phantom limb syndrome and developer of a novel low-tech ‘mirror box’ therapy
OLAF BLANKE
1969–
Known for his work on the neural basis of self-consciousness
THOMAS METZINGER
1958–
Philosopher and author of the ‘self model theory of subjectivity’, a theory of consciousness
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Anil Seth
Is this a hand I see before me? Is it mine? The rubber hand illusion tricks the brain into thinking the fake hand is real.
Many neuroscientists believe that the critical processes for consciousness involve the integration of neural activity among different brain regions. According to ‘global workspace theory’, specific mental contents (such as perceptions, thoughts, or intentions to act) remain unconscious unless or until they gain access to a ‘global workspace’, which broadcasts their contents across the brain, making them available for the flexible control of behaviour. This theory invites us to picture a theatre in which mental contents become conscious only when illuminated on a main stage where they can be seen by – and interact with – an audience. Integrated information theory (IIT) is also about networks, but its starting point is that every conscious experience is unique – one among a repertoire of possible experiences – resulting in the generation of an enormous amount of information. Consciousness is also integrated, in the sense that all the sounds, sights, thoughts and emotions we experience at any moment are bound together into a single conscious scene. IIT suggests this combination of information and integration can be quantified mathematically and this should correspond to the level of consciousness experienced. ‘Integrated information’ should be high during normal wakefulness and low during unconscious states, such as dreamless sleep.
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There is no consciousness ‘hot spot’ in the brain. Consciousness depends on the integration of neural activity among different brain regions.
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Brain-imaging studies suggest that the prefrontal and parietal cortices are particularly important for consciousness and may form part of a global workspace. However, it is hard to know whether these regions generate consciousness itself, or whether they implement associated processes, such as attention, memory and verbal report of conscious experiences. The IIT is less specific about the underlying neuroanatomy, though it does stress the importance of interactions between thalamus and cortex.
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BERNARD BAARS
1946–
Psychologist, originator of global workspace theory and author of A Cognitive Theory of Consciousness (1988)
GIULIO TONONI
1960–
Neuroscientist and psychiatrist, originator of the integrated information theory of consciousness; also known for his work on the neurophysiology of sleep
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Anil Seth
In the 1980s, Benjamin Libet performed one of the most notorious experiments in modern neuroscience. He asked participants to raise one finger, at a time of their own choosing, and to notice, by looking at a rotating clock hand, when they consciously felt the urge to do so. While they did this, he recorded electrical activity in their brains, finding reliable patterns of activity – ’readiness potentials’ – that preceded the time of the urge by about half a second. Some cite this as evidence that the brain commits to an action (raising a finger) before we ourselves are aware of the intention to do so, apparently challenging commonsense notions of ‘free will’. However, many find Libet’s results unsurprising – all events that depend on the brain, whether they are behaviours (raising a finger) or experiences (the conscious intention to do so), should have prior causes in the brain. The readiness potentials recorded by Libet are associated with a brain region called the ‘presupplementary motor area’. Indeed, the neurosurgeon Itzhak Fried found that mild electrical stimulation of this area brings about an experience of intending to move, while stronger stimulation leads to actual movement. But the controversy rolls on even now. It seems that the idea of conscious free will is one we are determined to hang on to.
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While there may be no such thing as ‘free will’, the experience of willing or intending behaviour certainly exists and can be localized within the brain.
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Libet himself was uncomfortable with the implications of his own experiments. He proposed that consciousness might impose a ‘veto’ in between the onset of the readiness potential and the execution of the corresponding movement. This implies ‘free won’t’ instead of ‘free will’. But, whether it is ‘will’ or ‘won’t’, the idea that consciousness can somehow intervene directly into brain processes is deeply problematic. Recent experiments have, therefore, looked for neural signatures of these conscious ‘vetoes’.
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BENJAMIN LIBET
1916–2007
Renowned for his original experiments on the timing of conscious intentions
PATRICK HAGGARD
1965–
Cognitive neuroscientist who has expanded on Libet’s work in a variety of interesting ways, including looking for brain signatures of conscious vetoes
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Anil Seth
Imagine a world without anaesthesia. Trips to the dentist would be one thing, but major surgery would be quite another. The development of general anaesthetics (GAs) – substances that reversibly induce full unconsciousness – has revolutionized medicine in the last century. It has also provided a powerful window into the brain basis of consciousness itself. We now know that a range of different substances can act as GAs, but we still don’t really know how they work. What we do know is that the brain’s electrical activity under deep anaesthesia is different to either waking or sleeping states, resembling more closely profound states of unconsciousness such as the vegetative state. Brain-imaging studies have shown that GAs affect many brain regions, including the parietal cortex and the thalamus, with weaker effects in sensory areas, such as the primary visual cortex. According to the ‘thalamic switch’ theory, GAs turn consciousness off by reducing activity in specific parts of the thalamus. However, it is not yet known whether this deactivation is a cause or a consequence of loss of consciousness. It could be that the thalamus is needed to allow other cortical areas to communicate and it is the loss of this communication that leads to unconsciousness.
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General anaesthesia taps into the on/off switch for consciousness – but is there just one switch, or many? We may not know how anaesthetics work, but it’s a good thing that they do.
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Unconsciousness should be distinguished from unresponsiveness. General anaesthetics can inhibit behavioural responses by acting on the brain stem and other paralyzing medicines are sometimes used in combination with them to prevent body reflexes during surgery. There are (rare) occasions when patients wake up during surgery and by relying on behavioural responses alone, we would not be able to tell. This is one reason why improved technologies for ‘measuring consciousness’ are worth developing.
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3-SECOND BIOGRAPHY
WILLIAM T. G. MORTON
1819–68
Pioneer of general anaesthesia; in 1846 he became the first to demonstrate the use of ether as an anaesthetic
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Anil Seth
If you are unlucky enough to experience major brain damage, you might end up in a coma in which consciousness is absent and the brain’s activity much diminished. If you survive, you might recover to a vegetative state in which you would seem to be awake but not aware. This condition can persist for years, even decades. These states are usually diagnosed by looking at patients’ behaviour, but modern brain imaging has challenged many assumptions. In 2006, Adrian Owen and his colleagues asked an apparently vegetative patient to imagine either playing tennis or walking around their house while inside an fMRI scanner. Despite showing no external signs of understanding, the patient showed brain activity related to each task at the appropriate times. While these cases remain rare, they show that at least some patients previously thought to be unconscious are in fact conscious. The method can also be used to communicate with patients by asking them to imagine different activities for ‘yes’ and ‘no’, with answers read out on the scanner. Unfortunately, we are a long way from developing cures for these conditions. New techniques such as deep-brain stimulation are showing some promise, but only in a small minority of patients.
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Being awake doesn’t always mean being aware, and appearing unconscious on the outside doesn’t always mean that consciousness is really gone.
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Now that it is possible to communicate with some behaviourally unresponsive patients, what should we ask them? Asking whether they are in pain seems reasonable, but should we ask them if they want to stay alive? The ethics for all this has yet to be properly worked out. Also, fMRI scanners are too expensive and bulky to be widely used for these purposes, so recent research has explored the use of relatively cheap and portable EEG equivalents.
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3-SECOND BIOGRAPHIES
FRED PLUM
1924–2010
First to characterize the vegetative state
ADRIAN OWEN
1966–
Pioneer in the neuroscience of consciousness disorders, and originator of the house/tennis experiment
STEVEN LAUREYS
1968–
Pioneer in the brain imaging of vegetative and coma patients
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