The many faces of hallucination

Are you wondering about this strange picture I chose for this week’s blog? Well, you should because this picture isn’t less than a magical artifact, capable to let whole camels disappear. Just close your left eye, focus on the target with your right and move towards your screen. And Hocuspocus and Abrakadabra the camel is nada! Did you see it disappear? I hope so because this means your optic nerve leaving your eyeball creates an area where photoreceptors are absent. This results in a blind spot which is perfectly normal. But the interesting part is not that this exists, it’s the psychological reaction to this anatomical problem. It’s the filling in of this blind spot with visuals of the surrounding so we don’t even notice it in our daily life (Durgin, Tripathy & Levi, 1995). So, the reason we don’t see a void stain when we close an eye is that our brain creates an unreal image without the presence of an external stimulus. It’s hallucinating.

When talking about hallucinations most people think about psychedelics like LSD or Psilocybin. But these drug-induced states are just a small part of a much bigger picture. Hallucinations can occur without any external substances involved. In mental illnesses like schizophrenia, patients report hallucinations in various sensory systems. They can be olfactory, visual, tactile, auditory, or gustatory and the patients often experience them in more than just one sensory system (Goodwin & Rosenthal, 1971). On one hand, this makes sense because all our sensory areas in the cortex are neuronal strongly connected but on the other hand, it also seems that patients have individual connectivity patterns which would explain predominant hallucinations in one sense, most of the time visual or auditorial (Amad et al., 2014). But also, an imbalance in neurotransmitters, mostly dopamine and serotonin, seem to play a major role in schizophrenia. An imbalance that is known to cause hallucinations in other diseases like Parkinson’s syndrome (Stahl, 2016). All of these just mentioned hallucinations have a clinical background even though healthy people are just as capable of producing these illusions. All you have to do is to skip sleep for some nights (Waters, Chiu, Atkinson & Blom, 2018). Sleep and hallucinations seem to be related anyway in some kind of way and I’m not talking about your ordinary dreams. Those affected by sleep paralysis, which by the way would deserve an article on his own, report about vivid hallucinations of mostly frightening creatures like ghosts and shadow creatures. They are often seen by the sleeping person in the same room as they are, sometimes even touching them or sitting on top of their victim (Cheyne, Rueffer & Newby-Clark, 1999). Interestingly, a possible cause in the serotonin system is also discussed here, as is the case with hallucinations in schizophrenia (Jalal, 2018). But one does not need an unusual amount of a neurotransmitter to perceive things that aren’t really there. Sometimes the lack of a sensory information also causes hallucinations, as seen in the Charles Bonnet syndrome. Mostly elderly, visually impaired people see colorful pictures of people, faces and animals while being completely aware that these images are not real. And even if these scenes are mostly perceived as very entertaining and not threatening at all, there is often a fear of the associated stereotype. Those affected are afraid of being labeled as mentally unfit even though their cognitive function is faultless (Menon, Rahman, Menon & Dutton, 2003).

The list goes on and on. Sensual deprivation and sensual overload, prescription and unprescribed drugs, mentally ill and sane, young and old, hallucinations are omnipresent! And while there are still many unanswered questions about how they arise, we can already learn a lot from them about perception in general. Why we see our world the way we do. And most importantly, how individual this view is.

Bibliography :
  • Amad, A., Cachia, A., Gorwood, P., Pins, D., Delmaire, C., Rolland, B., & Jardri, R. (2014). The multimodal connectivity of the hippocampal complex in auditory and visual hallucinations. Molecular psychiatry, 19(2), 184-191.
  • Cheyne, J. A., Rueffer, S. D., & Newby-Clark, I. R. (1999). Hypnagogic and hypnopompic hallucinations during sleep paralysis: neurological and cultural construction of the night-mare. Consciousness and cognition, 8(3), 319-337.
  • Durgin, F. H., Tripathy, S. P., & Levi, D. M. (1995). On the Filling in of the Visual Blind Spot: Some Rules of Thumb. Perception, 24(7), 827–840.
  • Goodwin, D. W., & Rosenthal, R. (1971). Clinical significance of hallucinations in psychiatric disorders: a study of 116 hallucinatory patients. Archives of General Psychiatry, 24(1), 76-80.
  • Jalal, B. (2018). The neuropharmacology of sleep paralysis hallucinations: serotonin 2A activation and a novel therapeutic drug. Psychopharmacology, 235(11), 3083-3091.
  • Menon, G. J., Rahman, I., Menon, S. J., & Dutton, G. N. (2003). Complex visual hallucinations in the visually impaired: the Charles Bonnet Syndrome. Survey of ophthalmology, 48(1), 58-72.
  • Stahl, S. (2016). Parkinson’s disease psychosis as a serotonin-dopamine imbalance syndrome. CNS Spectrums, 21(5), 355-359. doi:10.1017/S1092852916000602
  • Waters, F., Chiu, V., Atkinson, A., & Blom, J. D. (2018). Severe sleep deprivation causes hallucinations and a gradual progression toward psychosis with increasing time awake. Frontiers in psychiatry, 9, 303.
Featured image :
  • Joachim Herz Stiftung. (2021). Experiment zum Blinden Fleck. Retrieved from:

Author : Max Frutiger