Hallucinations are classified in a multitude of ways. As in all classifications, the resulting arrangements are subordinate to the purpose of the classification at hand, and to the guiding principles involved. An implicit purpose of psychiatric classification tends to be the delineation of groups of signs and symptoms endowed with a similar pathophysiology, endowed with a similar etiology, and/or requiring a similar therapeutic approach. The 19th-century British physician and Professor of medical jurisprudence Henry Maudsley (1835-1918) advocates the view that all classifications in psychiatry should be based on etiology. However, the history of psychiatry demonstrates that that aim has seldom been realized. A less ambitious goal of classification is to organize the area of interest, for example, as a means of enhancing communication. Starting from the general class of * perceptual disturbances, which comprises the groups of * sensory deceptions and * sensory distortions, hallucinations are classified as a variant of the group of sensory deceptions. Arrangements of types of hallucinations are legion. Using the supposed location of the initial impulse of hallucinatory activity as a guiding principle, they are traditionally subdivided into those of a *centrifugal nature (i.e. hallucinations mediated primarily by the higher sensory areas of the brain, or, in a dualist reading, mediated by the mind), and those of a * centripetal nature (i.e. hallucinations mediated primarily by the sense organs or the peripheral nervous system). Using the involvement ofsensory percepts as a guiding principle, a somewhat crude dichotomy has been constructed of * illusions and hallucinations (both belonging to the group of sensory deceptions). Arguably the most common way to classify hallucinations is an arrangement in accordance with the sensory modality involved. Traditionally the human *perceptual system is deemed to have five sensory modalities: olfactory, gustatory, visual, auditory, and tactile. However, this classification is not exhaustive.Hallucinations can also manifest in the form of somatic sensations (i.e. bodily sensations that would seem to come from within the body), vibrations, sensations of heat or cold, kinaesthetic sensations, proprioceptic sensations, feelings of sexual arousal (as in the *persistent sexual arousal syndrome), and the experience of time (as in * time distortions). It is debatable whether feelings of pain should be included as well (as in * hallucinated pain and * hallucinated headache, for example). Using context as a guiding principle, a historical tripartite division of hallucinations has been made, which consists of * dreams (i.e. 'hallucinations' occurring during sleep), * delirium, and hallucinations proper. The idea of a continuum between these states is sometimes attributed to ancient schools of thought, but the question of whether the ancients understood this kinship in a literal or metaphorical sense has yet to be settled by historians of psychiatry. For all we know, the German philosopher Immanuel Kant (1724-1804) may have taken the relation quite literally, as witness his famous dictum that "the madman is a waking dreamer". It would seem that 19th-century medicine took up this notion and expanded it in the direction of a worked-out theory of non-sensory perception. Thus the French classical scholar and dream researcher Louis-Ferdinand-Alfred Maury (1817-1892) suggests that the false perceptions of dreams, delirium, and hallucinations proper may well have a common origin. Using the vigilance state as a guiding principle yields a somewhat different tripartition, consisting of *hypnagogic hallucinations (occurring at the moment of falling asleep), * hypnopompic hallucinations (at the moment of waking up), and hallucinations proper (occurring during the waking state). Applying the guiding principle of complexity yields a subdivision into * elementary, * organized, * geometric, * complex, and * compound hallucinations. Elementary hallucinations are simple phenomena that confine themselves to a single sensory modality. They typically lack persistence and complexity. Some examples of elementary hallucinations are * photopsias, transient * paraesthesias, odours, and tastes, and sounds like humming, ticking, and coughing. It may be tempting to attribute the origin of these phenomena to peripheral neuronal discharges, but empirical studies indicate that they can also be mediated by central structures. Organized hallucinations are more complex in nature, ranging from simple geometrical patterns (or tunes, in the auditory modality) to full-colour, three-dimensional images (or symphonies). But they still confine themselves to a single sensory modality. The term complex hallucination is used to denote hallucinated symphonies, three-dimensional images, etc. Hallucinations occurring in more than one sensory modality at a time are referred to as compound or multimodal hallucinations. These latter phenomena can range from the combined sight and smell of a rose, to a full-blown imitation of everyday experience in all of the sensory modalities at once. On occasion, it is even possible for hallucinations to replace the entire sensory input, thus constituting a totally different reality for the individual affected. Such instances are referred to as * panoramic, * scenic, or * dissociative hallucinations. Obviously, classifications of hallucinatory phenomena are not mutually exclusive. A hallucinated command, for example, can at once be identified as a * verbal auditory hallucination, a * command hallucination, a complex hallucination, a compound hallucination (when co-occurring with a visual hallucination of the person giving the command, for example), a *brainstem hallucination (when attributed to aberrant neurophysiological activity in the brainstem), a * hypnagogic hallucination (when occurring during the intermediate state between wakefulness and sleep), a * synaesthesia (when occurring in reaction to a sense perception experienced in any of the other sensory modalities), and an illusion (when an actual voice is heard but misunderstood). Some examples ofhal-lucinations defined with regard to their etiology or pathophysiological substrate are * peduncular hallucinations, * brainstem auditory hallucinosis, * alcoholic hallucinosis, and cannabis-induced hallucinations.
References
Berrios, G.E., Porter, R., eds. (1999). A history ofclinical psychiatry. The origin and history of psychiatric disorders. London: Athlone Press. Maury, L.F.A. (1865). Le sommeil et les rêves. Études psychologiques sur ces phénomènes et les divers états qui s'y rattachent. Troisième édition. Paris: Librairie Académique Didier et Cie., Libraires-Éditeurs. Rabkin, R. (1970). Do you see things that aren't there? Construct validity of the concept "hallucination"'.In: Origin and mechanisms of hallucinations. Proceedings of the 14th annual meeting of the Eastern Psychiatric Research Association held in New York, November 1415, 1969. Edited by Keup, W. New York, NY: Plenum Press.