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What is consciousness?

In essence, consciousness is any kind of subjective experience. Being in pain; smelling onions frying; feeling humiliated; recognising a friend in the crowd; reflecting that you are wiser than you were last year 鈥� all of these are examples of conscious experiences. In a field fraught with disagreements, this is something that most, but not all, researchers agree on. Go any deeper, though, and the rifts open up.

The 17th-century French philosopher Ren茅 Descartes famously divided the universe into 鈥渕atter stuff鈥�, such as rocks and physical bodies, and 鈥渕ind stuff鈥�. In the 20th century, philosopher at New York University built on Descartes鈥檚 separation, known as 鈥渄ualism鈥�, and the work of later thinkers, to distinguish between 鈥渆asy problems of consciousness鈥� and 鈥渢he hard problem鈥�.

The easy stuff consists of explaining the brain processes associated with consciousness, such as the integration of sensory information, learning, thinking and being awake or asleep. Though we are making steady progress, these problems have yet to be cracked: they are easy only in the sense that the known strategies of cognitive and neuroscientific research should eventually provide full explanations.

, which Chalmers introduced at a scientific meeting in 1994, is to explain why and how we have subjective experiences at all. 鈥淐onsciousness poses the most baffling problem in the science of the mind,鈥� Chalmers said. When we think and perceive, there is a 鈥渨hir of information-processing鈥� in the brain, as he put it, but also very distinctive subjective states of mind. The puzzle is how a 1.3 kilogram can generate the feeling of being.

Some believe the hard problem can never be solved 鈥� it is beyond the capabilities of human cognition. Others think that, with a better understanding of the brain, it will simply go away. at the University of Sussex, UK, sees a third way. He thinks that tackling the easier stuff will allow researchers to address the hard problem indirectly. To this end, he has developed a framework that highlights different aspects of consciousness to be investigated. Other theorists divide it up differently, but for Seth the three main components are conscious content, conscious self and conscious level.

Conscious content is whatever you are conscious of, including awareness of sensory perceptions. Conscious self is a unique component of conscious content that refers to self-awareness 鈥� the subjective feeling of being you 鈥� and includes being aware of your own awareness and reflecting on your conscious thoughts, known as meta-consciousness. And conscious level refers to differences in so-called 鈥済lobal states鈥�, such as being asleep, under anaesthesia or being awake.

These aspects of consciousness aren鈥檛 independent, says Seth. 鈥淚f you explain all the different kinds of conscious content 鈥� everything you might be conscious of 鈥� then you鈥檝e probably explained everything,鈥� he says. But he thinks breaking consciousness down like this is the best approach to tackling what it is.

How many states of consciousness do humans have?

Consciousness was traditionally thought to be like a light switch: it is on when you are awake and off when you are sleeping, anaesthetised or in a coma. However, when we dream while asleep we often have subjective experiences that are similar to our waking ones. Taking this into account led to the conclusion that, instead of two states of consciousness, there are three.

This idea didn鈥檛 hold for long, though. One driver of the shift in thinking was research looking at people who are in a coma, where the brain is considered to be entirely unresponsive, and others who are in a persistent vegetative state, which is marked by a sleep-wake cycle, but with no response to stimuli. People in the latter state would appear to be more conscious than those in a coma, and both seem less conscious than someone under mild sedation. Neuroscientists and philosophers alike began arguing that there are many states of consciousness: it is more like a dimmer-switch or a ladder with zero consciousness at the bottom and maximum consciousness at the top.

This picture was further complicated by a startling finding: a few people diagnosed as being in a persistent vegetative state have brain activity indicative of some awareness. In 2010, for example, a team that included Adrian Owen, then at the University of Cambridge, reported several cases of people who could respond to questions using their brain activity, picked up by an fMRI brain scanner. These included a 29-year-old man who was able to give yes or no answers to queries by imagining doing a different activity for each word.

Next came claims of levels of consciousness beyond alert wakefulness. In 2017, a team led by Anil Seth at the University of Sussex, UK, reported that people who had taken LSD or psilocybin (the hallucinogenic compound in magic mushrooms) experienced a 鈥渉igher鈥� level of consciousness: at least, their brains showed more diverse electrical activity with the drugs than without them. The ladder of consciousness was expanded to accommodate this and other states, including hypnosis, sleepwalking, epilepsy-induced unawareness, being half asleep (sleep is no longer regarded as an all-or-nothing phenomenon) and daydreaming.

Multi-faceted state

Now, the ladder model itself is being challenged. Owen is in the vanguard. He and two colleagues have 鈥�. They argue that, just as someone who is blind is no less conscious than a sighted person, so someone who has taken a mild sedative, say, is no less conscious than someone in a regular state of wakefulness.

Think about a toddler鈥檚 experience of the world, says Owen. An adult might look at the Eiffel Tower and be aware of its name and location and what was happening in their life when they last saw it, whereas a toddler might be conscious of just a big tower. In some ways, but not all, their conscious experience of the world isn鈥檛 the same as an adult鈥檚. If that adult developed Alzheimer鈥檚 disease, they may then come to have a similar awareness of the Eiffel Tower as a toddler, but their conscious experiences wouldn鈥檛 be the same. 鈥淐onsciousness is multi-faceted,鈥� says Owen, 鈥渁nd trying to measure it along any single dimension, including 鈥榙epth鈥�, will inevitably be futile.鈥�

Instead, he and his colleagues propose a more complex, multidimensional framework. Rather than being points on a line, states of consciousness would be represented by a spider鈥檚-web-type shape, with spikes indicating the extent to which someone possesses each dimension. Unfortunately, we don鈥檛 yet know enough about consciousness to say what many of these dimensions are, let alone measure them. That is a concern to researchers, including Seth. Nevertheless, the idea has been influential. 鈥淚t鈥檚 on the money,鈥� says Seth.

Is consciousness detectable in the brain?

One of the most famous outstanding wagers in science is that by 2023, a specific signature of consciousness in the brain will have been identified. Betting against this wager 鈥� made in 1998 and with a case of fine wine at stake 鈥� is at New York University, the originator of the 鈥渉ard problem鈥� of consciousness. Betting on it is , who heads the Allen Institute for Brain Science in Seattle, Washington. With two years to go, where do things stand?

We aren鈥檛 there yet. Progress has been made in understanding which brain regions and networks are involved, but there is still fierce debate.

Some researchers ascribe a vital role to the prefrontal cortex (PFC), which is responsible for sophisticated cognitive processes including attention, decision-making and planning. They argue that for sensory information to become a conscious perception 鈥� for you to actually see a red apple, say 鈥� it has to be processed not just by the sensory cortex, but also here. Neuroimaging studies of people and macaques support this idea. But sceptics say that the PFC activity they show could relate to thinking about a stimulus and planning a response, rather than being conscious of it. Koch also points to patients who have had large regions of their PFC surgically removed due to tumours or epileptic seizures. 鈥淭hey go on living, by and large, a normal life, never complaining that they have been turned into zombies,鈥� he says.

To probe further, research published this March that involved into various parts of human brains and asking volunteers about the effects. at New York University and his colleagues concluded that stimulation of only two regions of the PFC, the orbitofrontal cortex and the anterior cingulate cortex, sometimes alters reports of conscious experience. Based on what is known about the function of these subregions and what the participants reported, it seems likely that they support the emotional aspects of conscious experience, as well as self-consciousness and meta-consciousness (the awareness of being aware). But they may not be involved in more fundamental sensory perceptual awareness. It appears that an area towards the rear of the brain鈥檚 cortex 鈥� the 鈥減osterior hot zone鈥�, as Koch calls it 鈥� is crucial for this.

Joined-up thinking

One posterior region apparently important for consciousness is the parietal cortex, which processes sensory information from the body. Earlier this year, and at the University of Wisconsin-Madison and others reported research on macaques that were sleeping, anaesthetised or awake. They concluded that connectivity between the parietal cortex and two other brain regions, the striatum and the thalamus, is a . 鈥淥ur findings highlight the importance of integration between parietal and subcortical structures and challenge a key role of the frontal cortex in consciousness,鈥� says Afrasiabi.

Studies of people who have regained wakeful consciousness after being in a vegetative state also support the idea that integration of activity in different regions is key for various aspects of consciousness. This notion is central to the global neuronal workspace model too. 鈥淲hile there is much disagreement about which brain areas are most crucial for subjective experience, most theories do agree that consciousness is dependent on interactions and communication within and between different brain areas,鈥� says Redinbaugh. The exact nature of the interactions isn鈥檛 yet clear as current technology can鈥檛 measure everything that needs to be measured at once.

With the clock ticking on his wager, Koch anticipates defeat. 鈥淭he extent to which more frontal regions of the cortex, let alone other brain regions, contribute to consciousness will remain open for many years to come,鈥� he says. 鈥淎fter all, the brain is the most complex piece of active matter in the known universe.鈥�

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