Human Consciousness

What is consciousness? The notion of consciousness is one of science’s big mysteries, and in this article we will begin to explore what we do know about what consciousness is, how it evolves and how it affects our daily realities.

The mystery of human consciousness has plagued the minds of scientists and philosophers alike for centuries and remains one of the most incomplete areas of scientific research to date. Famously, the French philosopher Descartes once said, ‘I think, therefore I am’, capturing an idea that would drive research forward for many centuries to come, the idea that consciousness, broadly defined as the ‘mind’s subjective experience’, is a defining feature of our existence. Everything, from the taste of coffee on a cold winters morning, to the sense of pride you feel when you excel in an exam, to the knowledge you’ll gain from reading this article, is consciousness. How, then, does the brain, the most complex piece of active matter in the known universe, turn its electrical activity into the feeling of life itself?

Although the emergence of consciousness on the evolutionary timeline is unclear, it is widely accepted that the existence of a sufficiently complex communication system is necessary for its existence. In biology, this communication system often takes the form of a nervous system, and in our case, a brain. The idea that the brain acts as the seat of consciousness does not seem revolutionary. Yet, it is less obvious that it might first appear. After all, many internal organs, such as the liver, contain excitable tissues, much like the brain. Our digestive system is host to its very own enteric nervous system, consisting of ~150-200 million neurons, and yet we do not ‘experience’ digestion in the same way as a lengthy dog walk, for example. In fact, large amounts of excitable neural tissue, such as that of the spinal cord and cerebellum, are essentially redundant when it comes to the generation of human consciousness. Quadriplegic patients, suffering from full body paralysis induced by trauma of the spinal cord, often retain an entirely normal consciousness despite loss of conscious motor control below the injury.

So, which regions of the brain are necessary for consciousness? All available evidence points to key involvement of the cerebral cortex – the outer surface of the brain. It is a laminated sheet of intricately interconnected nervous tissue, the size and width of a 14-inch pizza. Two of these sheets, highly folded, along with their hundreds of millions of wires – the white matter – are crammed into the skull. Recently, scientists have investigated the contribution of different cortical regions to consciousness by electrically stimulating the surface of the brain in live, conscious patients. During the activation of different cortical regions, patients reported a diversity of distinct sensations and feelings, including flashes of light, geometric shapes, distortions of faces, auditory or visual hallucinations, a feeling of familiarity or unreality, the urge to move a specific limb, and so on. Although consciousness is dependent on the cumulative activity of the entire cortex, one region appears to stand out: the ‘posterior hot zone’. Activity in this area of cortex, located at the back of the brain, is normally associated with visual processing and sensory integration, but may also qualify as the minimal requirement for the generation of human consciousness. Further research will be necessary to tease apart the contribution of this area to different aspects of consciousness, such as how we see, think and feel.   

Although the cortex may have all the pieces of the ‘puzzle’ of human consciousness, it requires another region of the brain to put them together. This coordination is provided by the thalamus, which acts as a ‘relay centre’ for neural information, conveyed through innumerable synapses connecting neurons to each other. The importance of this connectivity could help explain how human consciousness develops during the transition to adulthood. Although human foetuses can react to sensory stimuli, and even exhibit the ability to alter their facial expression, these responses are likely pre-programmed, and of a subcortical origin. Even after birth, infants display only a minimal level of consciousness. However, during postnatal development, the brain undergoes enormous change, and as it matures, so does the consciousness it gives rise too. A burst of growth dramatically enhances connectivity between structures in the brain, giving rise to essential routes of communication between the thalamus and cortex known as ‘thalamocortical connections’ – an important step towards the generation of complex consciousness seen in adults. At the same time, these connections are refined during a process known as ‘synaptic pruning’, continuing well into adolescence, in which unnecessary connections between structures within the infant brain are eliminated, much like a potter who continually adds and strips away unnecessary clay from their sculpture to create the perfect shape.

A model showing the inside of the human brain

As well as the maturation of our consciousness during our transition into adulthood, our subjective experience of the world constantly fluctuates on a 24-hour cycle. Every night, all humans feel the need to crawl into bed, close their eyes, and fall unconscious – the need to sleep. Curiously, in certain phases of the sleep cycle, neural activity begins to resemble that of an awake human. Magnetic scanning of the human brain during sleep has showed that the posterior hot zone ‘lights up’ during dreaming, suggesting that a basic level of consciousness can be experienced during certain stages of sleep. The functional benefits of dreaming, in which we consciously perceive a world constructed from our own memories and imagination, often extending beyond the realms of physiological realism, are unknown. Some believe that dreaming provides a way of consolidating our subjective experience of the past period of wakefulness or even regulate our mood, whilst others believe it is simply a restorative process necessary for the maintenance of normal activity in the brain.

Whilst the loss of consciousness during sleep is almost certainly followed by the regaining of consciousness in healthy humans, the same cannot be said for some patients. Severe brain trauma can lead to patients existing in non-communicative states – ‘comas’ – in which their ability to engage in any physical movement is massively compromised. Yet, some of these patients retain their consciousness. Trapped in their own bodies, these patients become ‘locked-in’, capable of hearing, seeing and feeling, but without the capacity to respond. Discerning how conscious these patients are is essential when considering how to manage their long-term healthcare. One pioneering technique involves creating electrical ‘echoes’, which can be recorded as they pass through different regions of the brain. When patients are unconscious, during dreamless sleep, for example, the echoes that are produced are simple. But in the conscious brain, the echoes are complex and spread widely over the surface of the cerebral cortex. In the future, by comparing the complexity of these echoes, we may be able to determine whether patients are ‘locked-in’, or truly unconscious. Similarly, the need to reliably detect how consciousness is altered in patients under general anaesthesia is becoming increasingly urgent. Every year, thousands of patients become conscious while under general anaesthesia, known as ‘dysanaesthesia’, during live surgical operations. They cannot move or speak, but they might be able to hear voices or equipment noises, and to feel pain. The experience can be traumatic and is fraught with ethical and legal ramifications for the doctors who are caring for them. By monitoring changes in their consciousness, doctors may be able to develop methods of communicating with unresponsive patients, as well as ways of looking for signs of discomfort in such people.

Clearly, the mystery of human consciousness is still very much a mystery. But there is one thing we can be certain of: consciousness is not static – it evolves. Our consciousness is malleable. As well as its Darwinian evolution, consciousness evolves as we grow into adults, it wanes and ebbs as we sleep, and it can be snatched away in dramatic fashion as we fall to general anaesthesia or brain injury. As our technology advances and human consciousness becomes increasingly relevant to clinical practice, research will continue to unravel the neurobiology of consciousness, and how a 3-pound lump of grey matter, floating in a bony cage of fluid, can give rise to the essence of life itself.   

Tom Johnson, 4th year Medicine student at St John’s College

Further resources

Lovely, well-illustrated introduction to human consciousness, provided by TED-Ed.

An interesting overview of the problem of human consciousness by The Economist on YouTube. It effectively introduces some key scientists/ philosophers in the field and their take on the matter.

Advanced (lengthy) lecture into the role of the brain in the generation of consciousness, presented by Christof Koch (Chief Scientist and President of the Allen Institute for Brain Science), a prolific scientist and expert in the field of consciousness. This lecture provides in-depth insights into the ‘neural correlates of consciousness’ and the future of research into consciousness.

A ‘reflection on the world’ titled ‘How to make a consciousness meter’, written for Scientific American by Christof Koch. The article offers an insight into the latest research into the creation of a ‘consciousness meter’ for measuring consciousness in a medical setting.