Hallucination: A Normal Phenomenon?

A superb article Posted by Dartmouth University Journal of Science / In Fall 2009 / November 21, 2009

Original : http://dujs.dartmouth.edu/fall-2009/hallucination-a-normal-phenomenon#.VV8ttvlVhBc

The word “hallucination” conjures images of schizophrenics and drug abusers. Hallucinations seem to be the domain of psychosis and psychedelic drugs. But in reality, hallucinations can be common and ordinary, defined as a “sensory perception without external stimulation of the relevant sensory organ” (1).  The “creepy crawly” feeling one gets on the skin is an example of such a perception, or rather misperception, caused by a misfiring or other minor error in the nervous system.

Seeing Dead People

In general, hallucinations occur slightly more often in males than females. They are most common in males between ages 25-30, while females peak around age 40-50. Aging increases hallucinations in both sexes (2). The increase in hallucinations with age might have to do with the deaths of loved ones. It is not unusual to see dead friends and relatives; these hallucinations are considered normal, perhaps part of the grieving process. Almost half of widows and widowers have hallucinations of their dead spouse, most commonly in the first 10 years of widowhood (3). The occurrence of these hallucinations is unrelated to social isolation or depression— they actually increase with length of marriage, the happiness of the relationship, and parenthood. They are even considered helpful accompaniments and a coping mechanism of widowhood.

Hallucination can cause an altered sense of reality, which may manifest itself as visual or auditory distortions.


Hallucinations are most commonly associated with schizophrenia. Individuals may hear voices, or in rare cases see objects, that are not really there and believe that they are completely real, as opposed to a widow’s awareness that the vision of her dead husband is just in her head. Hallucinations experienced by schizophrenics also tend to be derogatory and hostile in tone.

Auditory hallucinations in schizophrenic individuals occur when normal cognitive processes are disturbed, such as monitoring of “self-generated verbal material.” These hallucinations are not a result of epileptic episodes in the auditory cortex, just small neural disturbances in regular brain activity. As a result, the auditory input seems just as vivid as a real perception.

Magnetic resonance imaging (MRI) brain scans during episodes of hallucinations show that the right hemisphere is highly active. The right hemisphere is associated with emotions and connections among the set of perceptions brought in through the sensory organs. It is thought to “make sense” of the surroundings by interpreting multiple sensory inputs. It is thought that during an episode, the schizophrenic individual may be trying to interpret and process what is being said by the voice in his head. Since these hallucinations are hostile in nature, the individual may be trying to form an emotional response to the input he is receiving (4). As a result, schizophrenic individuals become paranoid about the people or objects in their lives that the voices tell them are harmful.

Another illness associated with hallucination is Parkinson’s disease. Parkinson’s is a degenerative disease of the brain that leads to loss of motor skills, caused by the death of dopamine-secreting nerve cells in the brain. A first sign of Parkinson’s is an involuntary tic-like tremor or reduced mobility of certain body parts. Sensory processing then becomes disturbed, especially in vision. One study showed that almost 40% of people with Parkinson’s experience hallucinations, mostly visual. These individuals have “presence” hallucinations, where they perceive somebody else in the room. Some patients experience passage hallucinations, where they think they see something in their peripheral vision, probably due to a misinterpretation of a very flimsy perception in an early part of the visual cognitive process. Patients usually see strangers or domestic animals such as dogs, neutral objects that draw no emotional response (5).

Select individuals with Parkinson’s have more vivid hallucinations, perhaps due to other causes. One woman saw her dead son saying, “take care of yourself.” Another man, who was also diagnosed with dementia, saw “small incorporeal devils with a blurred face and a changing size.” He once thought that they were hurting him during an episode of lumbar pain, but for the most part the devils were not frightening to the man. In general, hallucinations are more prevalent in Parkinson’s patients with a longer duration of the disease. Also, the dopaminergic drug therapy administered to Parkinson’s patients is strongly associated with hallucinations (6). Hallucinations can also be induced with recreational drugs, such as LSD, by increasing activity in the same dopamine pathway (7).

Another neurological disorder that can cause hallucinations is called Charles Bonnet’s syndrome, or CBS. Frequent in elderly, visually handicapped individuals, CBS leads to “complex visual hallucinations without delusions or loss of insightful cognition.” The individuals are psychologically normal and aware that their hallucinations are not real. In a study, CBS was present in 11% of a group of 300 visually handicapped patients, 72 years old on average. The hallucinations ranged from mundane objects like a bottle or hat to funny situations, such as “two miniature policemen guiding a midget villain to a tiny prison van.” Others saw ghosts, dragons, or angels. Real images might combine with the imaginary, such as a hallucinated person in a real chair. Like the hallucinations experienced by Parkinson’s patients, the hallucinations generally have nothing to do with the person’s life (8).

Most of the individuals with CBS involved in the study did not tell their doctor about their hallucinations for fear of being ignored or considered insane. One individual who did tell the doctor received a response: “You’d better not talk about such silly things.” They also do not tell loved ones, due to the same fears. When the wife of a man with CBS expressed her amazement at her husband’s episodes and asked why he never told her, he simply replied, “I didn’t want to upset you” (9).

Wakeful dreaming

There is a strong stigma against hallucinations, as they are considered severely abnormal and a marker of a major psychiatric disorder. Since our society also has a strong stigma against psychological illnesses, people may hide their hallucinations. However, hallucinations are not really that extraordinary. Mahowald, Woods, and Schenk propose that hallucinations are really just like dreams in a conscious, wakeful state. In a normal awake state, the brain ignores activity generated internally while attending to the external stimulation it perceives. In a hallucination, whether induced by drugs, sleeplessness, or sensory deprivation, the brain processes the internal activity that it normally ignores, creating a hallucinogenic episode. Mahowald, Woods, and Schenk label this “wakeful dreaming” (10).

A strong social stigma against hallucination causes many people to conceal them. But some scientists argue that hallucinations are a wakeful occurence of the brain’s dream imagery.

Our body is covered with sensory organs that transmit information to the thalamus, part of the cerebrum, which is the part of the brain that controls many functions including sensory processing. The thalamus has two modes to control whether information goes to the forebrain (relay) or stops in the thalamus (oscillatory), controlling the input of environmental information into the cortex. The thalamus is in relay mode in wakefulness and REM (rapid eye movement) sleep, the sleep stage when vivid dreams tend to occur. The rest of the time, in non-REM (NREM) sleep, the thalamus is in oscillatory mode and the cortex is deprived of all environmental sensory information (11).

According to J. Allen Hobson, a revolutionary in dream research, dreams have five common features: hallucinatory imagery, bizarre cognitive features like discontinuity and incongruity, strong emotion, uncritical acceptance, and memory deficits (most dreams are forgotten) (12). These features would be considered very abnormal in a wakeful state. One explanation for hallucinations is based on Louis West’s theory of perceptual release. According to this theory, our life history and experiences create permanent neural changes that reprise themselves as memories, thoughts, and fantasies. Using these changes and the constant sensory input received from both the environment and inside the body, the midbrain gives rise to consciousness (13). With normal environmental sensory input, the midbrain organizes and limits the brain from reintroducing memory traces within itself. When the brain is deprived of sensory input (such as by blindness), brain circuitry encoding the memory of previous perceptions is released, causing the memories to be re-experienced as hallucinations (10).

Don’t try this at home

Hallucinations do not occur naturally in healthy people without psychological disorders. Extreme physiological stress can induce hallucinations, such as heatstroke or fasting. Religious or spiritual people use this form of physical exertion to experience “awakenings” and supernatural visions that can allow them to leave the normal realm of human perception.

In addition to recreational drugs, some prescription medication for disorders like ADHD can cause hallucinations, though only in small numbers. In one study, psychosis-mania events occurred in 11 out of 748 individuals treated with drugs. Children commonly had visual or tactile sensations of insects, snakes, or worms, described as a “creepy crawly” feeling (14).

Another interesting way hallucinations can arise is through intentional sensory deprivation. One study showed that healthy, normal sighted people who were blindfolded for several days started having visual hallucinations. Thirteen subjects wore blindfolds for five consecutive days. Ten people had visual hallucinations ranging from bright spots of light to complex hallucinations like faces, landscapes, and ornate objects. They started occurring on the second day of blindfolding and the subjects were aware that their hallucinations were not real (15).

For the most part, hallucinations are harmless and no real cause for worry, unless the person starts to believe that they are real. In that case, the person may need to seek medical treatment, though ironically, the person would not recognize that he needs help. Even in psychologically normal people like those with Charles Bonnet’s syndrome, hallucinations can be unwanted annoyances that cause emotional distress. But they can be healing or reassuring in some way to the person experiencing a hallucination. Martin Luther King reportedly heard God reassuring him during hard times in the form of an inner voice, saying, “Lo, I will be with you” (16). Hallucinations occur in many situations in different ways. They range from adverse symptoms of severe mental illness, to occasional, harmless dream-like events. Perhaps hallucinations are just one indication of our physical or psychological state of being.


1. American Psychiatric Association, Diagnostic and statistical manual of mental disorders (American Psychiatric Association, Washington, D.C., ed. 3, 1987), pp. 398.
2. A. Y. Tien, Soc. Psych. Psych. Epid. 26, 287-292 (1991).
3. W. D. Rees, Br. Med. J. 4, 37-41 (1971).
4. S. S. Shergill, M. J. Brammer, S. C. R. Williams, R. M. Murray, P. K. McGuire, Arch. Gen. Psychiatry 57, 1033-1038 (2000).
5. G. Fenelon, F. Mahieux, R. Huon, M. Ziegler, Brain 123, 733-745 (2000).
6. C. G. Goetz, C. M. Tanner, H. L. Klawans, Am. J. Psychiatry 139, 494-49 (1982).
7. P. Seeman. Synapse 1, 133-152 (2009).
8. R. J. Teunisse, J. R. M. Cruysberg, W. H. L. Hoefnagels, A. L. M. Verbeek, F. G. Zitman, Lancelet 347, 794-797 (1996).
9. M. W. Mahowald, S. R. Woods, C. H. Schenk, Dreaming 8, 89-102 (1998).
10. J. A. Hobson, R. Stickgold, Conscious Cogn. 3, 1-15 (1994). [as quoted in Teunisse 1996]
11. J. Monahan, H. J. Steadman, Violence and Mental Disorder: Developments in Risk Assessment (University of Chicago Press, Chicago, 1996)
12. A. D. Mosholder, K. Gelperin, T. A. Hammad, K. Phelan, R. Johann-Liang, Pediatrics 123, 611-616 (2009).
13. L. B. Merabet, D. Maguire, A. Warde, K. Alterescu, R. Stickgold, A. Pascual-Leone, J. Neuro-Ophthal. 24, 109-113 (2004).
14. B. J. Scott, J. Relig. Health 36, 53-64 (1997).


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