Consciousness feels like an on-off phenomenon: either you鈥檙e awake and experiencing the world or you鈥檙e not. But finding the button, or even the dimmer switch that allows our brains to move between these states, is tricky.
鈥淐onsciousness is not something we see, it鈥檚 something through which we see, which makes it challenging to study,鈥� says , director of the Center for Consciousness Science at the University of Michigan in Ann Arbor.
One common definition of human consciousness is 鈥渢he thing that abandons us when we fall into a dreamless sleep, and returns when we wake up鈥�. But it probably comes in different levels. Say I anaesthetise you: you may be able to hear my voice, but not respond to it; you may be dreaming and not hear my voice; or you may hear or experience nothing at all.
Advertisement
Read more: What is consciousness?
How your brain creates the feeling of being is the biggest problem in neuroscience. But we are coming closer to cracking it
Because we are reliant on people鈥檚 subjective descriptions of their experience, it is difficult to correlate these different levels of awareness with specific patterns of brain activity. 鈥淭he bottom line is, I really don鈥檛 know if you鈥檙e conscious of me, conscious of something else or not conscious at all,鈥� says Mashour.
We do know there are certain brain regions that, when damaged or electrically stimulated, will result in loss of consciousness. The claustrum 鈥� a thin, sheet-like structure buried deep inside the brain 鈥� is one of them. But there are others, and many of the leading theories that aim to describe consciousness veer away from a single anatomical site being its seat.
Take global workspace theory. It hinges on the idea that information coming in from the outside world competes for attention in the outer layer of the brain (the cortex) and a structure in the centre of the brain called the thalamus. We only become conscious of something 鈥� a ringing telephone, say 鈥� if the brain signals it generates outcompete other information that is being generated at the same time. If this happens, the signal will be broadcast to other areas of the brain, and we consciously register it.
Then there鈥檚 information integration theory, which ignores the brain鈥檚 anatomy, and simply suggests that consciousness is the result of data being combined so that it is more than the sum of its parts. 鈥淎 key commonality in many theories is that there has to be some degree of communication across different brain areas,鈥� says Mashour. 鈥淯nconsciousness doesn鈥檛 necessarily need to be mediated by brain regions being shut down or extinguished, but rather by a communication breakdown.鈥�
That鈥檚 backed up by a recent study that scanned people鈥檚 brains as they were slowly anaesthetised. It found that complete unconsciousness was marked by a failure of the cortex to propagate messages to the rest of the brain. This might also explain how a drug like ketamine manages to knock people out: while it ramps up activity in many of the brain areas that promote wakefulness, this potent tranquilliser depresses communication between different brain regions. So it seems unlikely consciousness has a single on-off switch 鈥� perhaps its seat is spread through the networked structure of the brain.
Read more: 11 scientific wonders we know exist 鈥� but we鈥檝e never seen
