hallucination
- Key People:
- Jean-Étienne-Dominique Esquirol
hallucination, the experience of perceiving objects or events that do not have an external source, such as hearing one’s name called by a voice that no one else seems to hear. A hallucination is distinguished from an illusion, which is a misinterpretation of an actual stimulus.
A historical survey of the study of hallucinations reflects the development of scientific thought in psychiatry, psychology, and neurobiology. By 1838 the significant relationship between the content of dreams and of hallucinations had been pointed out. In the 1840s the occurrence of hallucinations under a wide variety of conditions (including psychological and physical stress) as well as their genesis through the effects of such drugs as stramonium and hashish had been described.
French physician Alexandre-Jacques-François Brierre de Boismont in 1845 described many instances of hallucinations associated with intense concentration, or with musing, or simply occurring in the course of psychiatric disorder. In the last half of the 19th century, studies of hallucinations continued. Investigators in France were particularly oriented toward abnormal psychological function, and from this came descriptions of hallucinosis during sleepwalking and related reactions. In the 1880s English neurologist John Hughlings Jackson described hallucination as being released or triggered by the nervous system.
Other definitions of the term emerged later. Swiss psychiatrist Eugen Bleuler (1857–1939) defined hallucinations as “perceptions without corresponding stimuli from without,” while the Psychiatric Dictionary in 1940 referred to hallucination as the “apparent perception of an external object when no such object is present.” A spirited interest in hallucinations continued well into the 20th century. Sigmund Freud’s concepts of conscious and unconscious activities added new significance to the content of dreams and hallucinations. It was theorized that infants normally hallucinate the objects and processes that give them gratification. Although the notion has since been disputed, this “regression” hypothesis (i.e., that hallucinating is a regression, or return, to infantile ways) is still employed, especially by those who find it clinically useful. During the same period, others put forth theories that were more broadly biological than Freud’s but that had more points in common with Freud than with each other.
The nature of hallucinations
The general theory of hallucinations here delineated rests upon two fundamental assumptions. One assumption states that life experiences influence the brain in such a way as to leave, in the brain, enduring physical changes that have variously been called neural traces, templates, or engrams. Ideas and images are held to derive from the incorporation and activation of these engrams in complex circuits involving nerve cells. Such circuits in the cortex (outer layers) of the brain appear to subserve the neurophysiology of memory, thought, imagination, and fantasy. The emotions associated with these intellectual and perceptual functions seem to be mediated through cortex connections with the deeper parts of the brain (the limbic system or “visceral brain,” for example), thus permitting a dynamic interplay between perception and emotion through transactions that appear to take place largely at unconscious levels.
Conscious awareness is found to be mediated by the ascending midbrain reticular activating system (a network of nerve cells in the brainstem). Analyses of hallucinations reported by sufferers of neurological disorders and by neurosurgical patients in whom the brain is stimulated electrically have shown the importance of the temporal lobes (at the sides of the brain) to auditory hallucinations, for example, and of other functionally relevant parts of the brain in this process.
A second assumption states that the total human personality is best understood in terms of the constant interplay of forces that continually emanate from inside (as internal physiological activity) and from outside the individual (as sensory stimuli). Such transactions between the environment and the individual may be said to exert an integrating and organizing influence upon memory traces stored in the nervous system and to affect the patterns in which sensory engrams are activated to produce experiences called images, fantasies, dreams, or hallucinations, as well as the emotions associated with these patterns. If such a constantly shifting balance exists between internal and external environmental forces, physiological considerations (e.g., brain function) as well as cultural and experiential factors emerge as major determinants of the content and meaning of hallucinations.
The brain is bombarded constantly by sensory impulses, but most of these are excluded from consciousness in a dynamically shifting, selective fashion. The exclusion seems to be accomplished through the exercise of integrative inner mechanisms that focus one’s awareness on selected parts of potential experience. (The sound of a ticking clock, for example, fades in and out of awareness.) Functioning simultaneously, these mechanisms survey information that is stored within the brain, select tiny samples needed to give adaptive significance to the incoming flow of information, and bring forth only a few items for actual recall from the brain’s extensive “memory banks.”
Induction of hallucinations
Direct brain stimulation
Hallucinatory experiences can be brought about through direct stimulation of the exposed brain, as in cases of local cortical arousal under the neurosurgeon’s stimulating electrode. Under circumstances other than surgery or such situations as, for example, some forms of epilepsy, however, there must be a decrease in the forces that ordinarily dominate consciousness before hallucinations occur. These inhibiting forces require for their maintenance a relatively high level of sensory input of appropriate quality and frequency.
Sleep
The ways in which the reticular network of cells in the brainstem acts as a regulatory and integrating system for these relationships remain under intensive study. Since levels of brain arousal during sleep and wakefulness also are mediated via reticular formation activity, sleeping and dreaming merit consideration as hallucinatory activities. In the process of falling asleep, a person passes through a period of “partial sleep” in which awareness of the environment drops rapidly but in which the level of cortical arousal (which falls less rapidly) remains sufficiently high to permit some appreciation of external stimulation. Thus, the so-called hypnagogic (induced by drowsiness) phenomena occur.
Hypnagogic hallucinations
Common hypnagogic hallucinations may be visual (e.g., scenes from the previous few hours appear) or auditory (e.g., one seems to hear one’s name called). A frequently occurring hypnagogic hallucination is the sensation of loss of support or balance, perhaps accompanied by a fragmentary “dream” of falling, followed immediately by a jerking reflex recovery movement (the myoclonic jerk) that may jolt the sleeper back into wakefulness.
Dreams
The first stages of sleep appear to be dream-free. During this phase, measurements taken by an electroencephalograph (EEG) will record the sleeper producing large, slow brain waves. Sensory stimuli from without (such as noise or cold) or stimuli from within the body (such as dyspepsia or anxiety), plus a somewhat regular spontaneous fluctuation in the depth of sleep, will periodically (perhaps every 90 minutes or so) bring the sleeper into a state that favours perceptual release, in which case dreaming tends to take place. This state (with specific EEG signs and rapid movements of the closed eyes) probably occurs several times every night, so that more than 20 percent of an average person’s sleep is taken up with several dreams, each of about 10 to 15 minutes’ duration. At the time of awakening, the typical sleeper again passes through a period of perceptual release, often experiencing dreams that increase in intensity, and perhaps may have the hallucinations of “partial sleep,” these now being called hypnopompic (associated with a semiconscious state before awakening) experiences.
A simplified but perhaps helpful way of characterizing these conditions might be to imagine someone standing at a window opposite a fireplace, looking out at a garden in the sunset. The person may be thoroughly absorbed by the view of the outside world and fail to visualize the interior of the room at all. As it becomes darker outside, images of the objects in the room behind the person are reflected dimly in the window glass. For a time, then, the viewer may see either the garden (if gazing into the distance) or the reflection of the room’s interior (when focusing on the glass). Night falls, but the fire still illuminates the room. The watcher now sees in the glass a reflection of the interior of the room, which appears to be outside the window. This illusion becomes dimmer as the fire dies down, and, finally, when it is dark both outside and within, nothing more is seen. If the fire flares up from time to time, the visions in the glass reappear.
Another analogy might be that dreams, like the stars, are shining all the time, even though the stars are not often seen in daytime. If, however, there is an eclipse of the sun, or if a viewer watches the sky after sunset or before sunrise, then the stars, like dreams, though often forgotten, may be seen.
A more brain-related concept is that of a continuous information-processing activity (a kind of “preconscious stream”) that is influenced by both conscious and unconscious forces and that constitutes the potential supply of dream content. Hallucinations in the waking state also would involve the same phenomenon, produced by a somewhat different set of psychological or physiological circumstances.
Excessive excitation
It is valuable to consider the probable relationship between the level of physiological arousal in the brain and information processing during the waking state. The functions of consciousness apparently reach an optimal point in relation to level of arousal, beyond which they disorganize progressively as arousal increases. The presence of marked arousal (produced, for example, by extreme anxiety or by chemical stimulation of the brain) is accompanied by marked disturbance of concentration. Again, contact with external stimuli is impaired, this time by excessive input that “jams the circuits,” in which case spontaneous dissociative experiences may occur. As arousal increases further, the hallucinations of full-blown delirium or psychotic excitement may appear with frightening vividness, intensity, and emotional accompaniment. Greater brain arousal might result in generalized seizure phenomena, as in epilepsy.
