For example, drinking too much coffee increases the likelihood of hearing voices and seeing things that are not there, like sensing the presence of dead people. Research from Durham University indicates that this is due to the stress response and increased cortisol. And of course, a variety of drugs can induce wacky hallucinatory experiences.
Other common types of hallucinations which occur are hypnagogic and hypnopompic hallucinations. These occur when you are falling asleep or waking up. This hallucinatory experience varies from a simple impression of a line or dots to a convincing impression of an animal or person in the room.
Sensory Deprivation
Sensory deprivation can induce hallucinations in pretty fast time. In a study by the University of Cambridge, 19 ordinary volunteers began to experience hallucinations after only 15 minutes of being completely deprived of light or sound. If the brain has no input, it soon begins to create its own input. Interestingly, many religious experiences seem to take place where there is sensory deprivation, in such places as deserted areas or caves.
People who are losing their sight or hearing often experience visual or musical hallucinations, which can be quite disturbing and seemingly uncontrollable. And people who lose a limb may develop phantom limb syndrome, where they may experience a pain or itch in a limb, that is no longer there. Some people who experience migraine headaches have a type of "aura", while others can "see" little people running about, or other types of visions.
In reality, we all experience hallucinations because our eyes have a blind spot where the optic nerve enters the retina and our brain simply fills in the blank spot.
The Believing Brain: From Ghosts and Gods to Politics and Conspiracies—How We Construct Beliefs and Reinforce Them as Truth, by Michael Shermer.
