Hallucinations

Hallucinatory phenomena were recognized as long ago as the 4th century by Macarus and, until fairly recent times, were generally credited with occult significance which gave rise to a belief in the magical powers of the percipient. Such experiences have had a considerable effect on the lives of the hallucinators the hysterical crisis of Paul (who saw Jesus in a vision) was instrumental in his acceptance of the new faith; Socrates had his 'daemon' which warned and guided him from within; while Joan of Arc's visions and voices played a memorable role in altering the course of history. But it was the decline of the demoniacal model during the 18th and 19th centuries and its replacement by the medical model in the realm of insanity which led to a more objective evaluation of hallucinatory significance and an increasing concern with phenomenology and definition. The definition of a true hallucination has posed problems, both of inclusion and exclusion. On the one hand, observers have been keen to reach agreement on a set of positive defining criteria, while on the other to distinguish true hallucinations from similar kinds of non-veridical perceptual experience. The latter concern has given rise to the differentiation of true hallucinations from illusion (Esquirol, 1838), from pseudohallucinations (Kandinsky, 1885), from hypnagogic images (Maury, 1848) and from a large variety of other types of mental imagery. Horowitz (1970) has proposed that various kinds of hallucinatory and imagery experiences can be distinguished and classified on four dimensions: in terms of their vividness, the context in which they occur, their content, and the degree of interaction with veridical perception. From the viewpoint of positive defining criteria of a true hallucination, three such criteria would probably be considered essential by modern observers: namely (1) percept-like experience in the absence of an external stimulus, (2) percept-like experience which has the full force and impact of a real perception, and (3) percept-like experience which is unwilled, occurs spontaneously and cannot be readily controlled by the percipient. Each of these three criteria also serves to differentiate a true hallucination from other types of similar experience. The criterion of'absence of an external basis' is an obvious one and has been included in nearly every definition of the phenomenon since Esquirol. It serves to separate the different experiences of hallucination and illusion. The second criterion concerning the realistic nature of the experience has gained prominence in recent times. Jaspers (1911, 1963) distinguished true hallucinations from imagery and pseudo-hallucinations on the grounds that the latter occur in 'inner subjective space' while the former have an objective reality of their own. Sedman (1966) used similar criteria in distinguishing 'inner voices' which are a form of pseudohallucination from those that can be considered true hallucinations. In a series of recent phenomenological studies, Aggernaes (1972a, b; Aggernaes &Myeborg, 1972) has shown that hallucinatory experiences can be reliably classified on seven specific and relatively subtle dimensions of reality characteristics, including that of involuntarity. Moreover, he has demonstrated that the true hallucinations of chronic schizophrenic patients are akin to actual perceptions in having positive reality characteristics and in being clearly discriminable from imagined objects and people. The third criterion of'lack of control' on the part of the percipient is important for differentiating hallucinatory experiences from examples of memory and imagination imagery. The latter have been discussed at length by Richardson (1969).