What is consciousness? No small question. To this day it hasn’t been entirely satisfied. 

 

Consider a conversation:

 

There are voices from the outside, stimuli that talk to all the sensory receptors that we have. They pass on messages to our fingertips that we are touching something cold; they tell our eyes that we are seeing certain wavelengths of light; and they tell the cochlea of our ears what sounds we are hearing. 

 

The sensory circuits of our bodies bring these words from outside and turn them inward, presenting them to the centre of our consciousness: Here - this is what we are experiencing. This is what we are taking from the world outside. 

 

But already, at the base of this consciousness, an idea of the world has been established. The central experience of our mind is built upon prediction: we are constantly conjuring up an estimate of how the outside world will be. The ‘Predictive Processing’ model of consciousness states that it is the conversation between this predictive perception of the world and the feedback from our sensory experience that defines what it is to feel consciousness (1). 

 

In 1971, Nature published the conclusions of a study titled, ‘Preliminary Observations on Tickling Oneself’ (2). In this experiment, a device was used to compare the experience of being tickled by an experimenter to the experience of tickling oneself, and both were compared to the intermediate of passively following the experimenter’s arm as they tickled the participant. The study concluded that the action of tickling oneself produced no effect as the planned action of tickling cancelled out the sensation of being tickled; the lack of an action in the case of the experimenter tickling the subject’s hand, allowed for a full ‘tickle’ sensation. Interestingly, the third process of passively following the tickling action was rated at a level in between these two responses. This showed that it was not the action of tickling alone that cancelled out the sensation of the stimulus as tickling, but that a knowledge of the tickle, a prediction of it, were enough to reduce the effect. 

 

This experiment reflects the idea that it is not just our planned actions and our sensory perception that drive consciousness, but that it is prediction that has a core place in driving experience. 

 

For centuries, hallucinations have been recognised as distortions of our sense of being conscious in the world.  In 1838, Esquirol wrote in his ‘Mental Maladies: A Treatise On Insanity’ that the experience of a hallucination is “a thorough conviction of the perception of a sensation, when no external object, suited to excite this sensation, has impressed the senses.” (3)

 

Anything that distorts our ‘perception’ or ‘sensation’ can therefore give rise to a hallucination. This can occur in the context of infection, psychosis, delirium, use of certain drugs - and the aptly named ‘exploding head syndrome’. Contrary to popular opinion, hallucinations are not a feature of psychotic disorders alone. In fact, analysis has shown that no single aspect of schizophrenia-related hallucinations is specific to this disease (4). 

 

In 2000, the idea of the ‘Tickling’ study was elaborated with respect to hallucinations in an investigation comparing the experience of self-produced and externally implemented stimuli for those who both did and did not suffer from hallucinations. It was shown in this study that for participants with hallucinatory disorders, there was a breakdown in the ability to differentiate between stimuli produced externally and internally (5). This study is in line with a certain theory of hallucination purported by Frith, who suggests in his discussion of positive symptoms of schizophrenia that the foundation of hallucination is a “fault in the system which internally monitors and compares intentions and actions” (6).

 

There is another interesting theory that describes hallucinations as memories released from suppression. The authors suggest that the hallucination itself is a cacophony of memory signals set loose, where normally they are shut out of our conscious mind. One study described auditory hallucinations in those with hearing loss as an “uninhibition syndrome”. They argued that in the cases studied, a lack of sensory auditory input seemed to “uninhibit neuronal groups storing auditory memory” (7). 

 

The brain is an incredibly complex organ and theories regarding consciousness and hallucinations abound. The question of greatest practical importance is what part of the process of hallucinations can we understand and therefore, what can be targeted when we are called to treat this system in a medical setting. 

 

Recent investigations have linked various molecules, receptors and genes to hallucinatory disorders or states, whilst imaging studies demonstrate networks and regions of the brain activated during hallucinations. Investigation of certain receptor-modulating drugs has revealed the place of certain molecules in delusion and sensation; and the association of certain genes to hallucination-prone phenotypes has established a genetic cause for susceptibilities to hallucination. This research yields molecular and genetic targets for therapies that can help reduce the burden of hallucinations on an individual.

 

It is a remarkable faculty of our minds, the ability to create a world - or aspects of the world - for ourselves and convince our own consciousness that it is real. Hallucinations reveal the capacity of the human brain for imagination; they show that all we experience is indeed creative, merely restricted by what we see as truth. But the grounding fact of knowing what is real is essential to functioning in society. Losing the ability to check our own creative experience of consciousness is exceedingly frightening and disempowering. Anything that helps us to maintain the right balance of conversation between the experiences we create and those we feel allow us to maintain a sense of self in the world. Elucidating the biology behind these conversations and the effects of hallucination itself can bring us closer to a definition of consciousness.