Consciousness is, for each of us, all there is: the world, the self, everything. But consciousness is also subjective and difficult to define. The closest we have to a consensus definition is that consciousness is something it is like to be. There is something it is like to be me or you but presumably there is nothing it is like to be a table or an iPhone.

How do our conscious experiences arise? Its a longstanding question, one that has perplexed scientists and philosophers for hundreds, if not thousands, of years. The orthodox scientific view today is that consciousness is a property of physical matter, an idea we might call physicalism or materialism. But this is by no means a universally held view, and even within physicalism there is little agreement about how consciousness emerges from, or otherwise relates to, physical stuff.

Neuroscientists have found important clues by looking at the activity of the 86 billion neurons and trillions of neural connections inside the human brain. One of the first questions they asked was which parts of the brain of any brain are associated with consciousness. For instance, you might instinctively assume that conscious experiences are more likely if a brain or brain region contains a large number of neurons.

Surprisingly, though, the human cerebellum a sort of mini brain hanging off the back of your cortex contains about three-quarters of the neurons in your brain but seems to have almost nothing to do with consciousness. One reason we know this is because some people are born without a functioning cerebellum, and while they experience some problems, a lack of consciousness is not one of them.

There are, however, some bundles of neurons that do appear to be vital for consciousness. If damage occurs to specific parts of the thalamus, or to a particular region of the brain stem, the result can be permanent unconsciousness. But are these brain regions actually central to generating conscious experiences, or are they more like a power socket that simply allows whatever is plugged into it to work?

Work involving brain imaging techniques such as electroencephalography (EEG) paints a more complex picture. Several decades ago, neuroscientists including Francis Crick and Christof Koch began to search for what they called the neural correlates of consciousness: particular patterns of brain activity that relate to given conscious states the experience of a painful toothache, for example.

As studies like this have progressed it has become clearer that consciousness depends on specific ways that different parts of the brain particularly the cortex communicate with one another. For example, by injecting a pulse of energy into the brain using transcranial magnetic stimulation (TMS), and using electroencephalography (EEG) to monitor the response, a team of neuroscientists led by Giulio Tononi and Marcello Massimini found that the electrical echo generated by the energy pulse will bounce all around a conscious brain, but stays very localised in an unconscious brain. In other words, the conscious brain is much more connected.

Do experiments like this bring us closer to understanding what consciousness is? Some might argue not. In the 1990s, the philosopher David Chalmers made an influential contribution to the consciousness debate by distinguishing between what he termed the easy problem, or problems, and the hard problem of consciousness.

The easy problems involve understanding how the brain and body gives rise to functions like perception, cognition, learning and behaviour. These problems are called easy not because they are trivial, but because there seems no reason why they cant be solved in terms of physical mechanisms albeit potentially very complex ones.

The hard problem is the enigma of why and how any of this should be accompanied by conscious experience at all: why do we each have an inner universe?

To address this hard problem, we need theories of consciousness that can bridge the gap from the world of physical processes to the world of conscious experiences: that can take us from correlation towards explanation.

There are now many theories of consciousness out there in the field of cognitive neuroscience: higher-order theories, global workspace theories, and integrated information theories, theories that in their strongest form imply that consciousness is spread widely throughout universe, and that even an electron may be conscious. There are even illusionist theories which attempt to persuade us that consciousness doesnt really exist at least not in the way we normally think about it.

The theory Ive been developing is a version of predictive processing theory. When I see a chair in front of me, its not that the eyes are transparent windows out onto the world and my brain just reads out chair. Instead there are noisy sensory signals impacting my retina and my brain has to use its prior expectations about what might be out there in order to interpret this ambiguous sensory data.

In a little more detail, the idea is that the brain is constantly calibrating its perceptual predictions using data from the senses. Predictive processing theory has it that perception involves two counterflowing streams of signals. There is an inside-out or top down stream that conveys predictions about the causes of sensory inputs.

Then there are outside-in or bottom up prediction errors the sensory signals which report the differences between what the brain expects and what it gets. By continually updating its predictions to minimise sensory prediction errors, the brain settles on an evolving best guess of its sensory causes, and this is what we consciously perceive. We dont passively perceive our worlds we actively generate them.

Predictive processing is well suited for explaining why a particular experience is the way it is and not some other way, because we can understand these differences in terms of the different kinds of perceptual predictions the brain is making. In my theory, these differences are particularly significant when it comes to the experience of being a self, which I argue is not an inner essence that does the perceiving, but rather a collection of perceptions itself. The self, in my view, is a special kind of controlled hallucination that has been shaped by evolution to regulate and control the living body.

Its not exactly a theory of consciousness, but you could call it a theory for consciousness. And its through ideas like this that I believe we will eventually come up with a satisfying scientific account of consciousness. Instead of solving the hard problem head on, we may end up dissolving it by developing and testing detailed explanations of how the properties of consciousness depend on their underlying mechanisms. In this way, we will have solved what I call the real problem of consciousness.

More:
What is consciousness? | New Scientist