A hallucination is the perception of stimuli in the absence of stimuli. Hallucinations are vivid, substantial, and usually perceived as external, but not always “real.” Hallucinations differ from illusions (a confused perception of a real sensation) and delusions (a perception of a real sensation is given an unreal, impossible, fantastic cause). Mild hallucinations are called “disturbances,” for example seeing movement in peripheral vision, or hearing faint noises and/or voices.
Hallucinations can occur from a variety of catalysts including: seizures, brain injuries, diseases, sensory deprivation, drug use, fever, stress, extreme grief, mental or neurological illnesses, depression, sleep deprivation, or upon loss of a sense (vision or hearing) or even phantom limb syndrome. The person experiencing the illusion needn’t be mentally injured or have a mental disease.
Hallucinations can occur as one or multiple senses including visual, auditory, olfactory, gustatory, tactile, proprioceptive, equilibrioceptive, nociceptive, thermoceptive and chronoceptive. Visual hallucinations can run a wide range from geometric, to very real, to surreal, to cartoons.
Hypnagogic and hypnopompic hallucinations happen in a high proportion of the population (as much as 37%) and are considers a normal phenomena that occur when falling asleep (Hypnagogic) or when waking (hypnopompic). These hallucinations can last from a few seconds to minutes and the person usually remains aware of their true nature. These types of hallucinations are sometimes associated with narcolepsy or brainstem abnormalities (rare).
Command hallucinations “hearing voices” are either auditory or inside of the person’s mind and are characterized as an outside voice instructing the individual. They are often associated with schizophrenia but can occur in healthy individuals under extreme stress.
Charles Bonnet syndrome: In 1760 the naturalist and philosophical writer Charles Bonnet described a condition where psychologically normal people experienced vivid, complex visual hallucinations. The syndrome typically occurs with people who have lost their eyesight or older people who have gone nearly blind from cataracts. Individuals with Charles Bonnet syndrome experience vivid hallucinations but are aware that they are not real. It is estimated that 10 percent of visually impaired people have Charles Bonnet syndrome, but only about one percent report it. Neurologist Oliver Sacks describes this syndrome in his TED Talk: What hallucination reveals about our minds. Interestingly Sacks suffers from the syndrome.
Augmented reality could be classified as an hallucination.