HALLUCINATION, a psychological term which has been the subject of much controversy, and to which, although there is now fair agreement as to its denotation, it is still impossible to give a precise and entirely satisfactory definition (from Lat. alucinari or allucinari, to wander in mind, Gr. awoaav or qXoav, from axe, wandering). Hallucinations constitute one of the two great classes of all false sense-perceptions, the other class consisting of the "illusions," and the difficulty of definition is clearly to mark the boundary between the two classes. Illusion may be de fined as the misinterpretation of sense-impression, while hallucina tion, in its typical instances, is the experiencing of a sensory presentation, i.e., a presentation having the sensory vividness that distinguishes perceptions from representative imagery, at a time when no stimulus is acting on the corresponding sense-organ. There is, however, good reason to think that in many cases, pos sibly in all cases, some stimulation of the sense-organ, coming either from without or from within the body, plays a part in the genesis of the hallucination. This being so, we must be content to leave the boundary between illusions and hallucinations ill defined, and to regard as illusions those false perceptions in which impressions made on the sense-organ play a leading part in deter mining the character of the percept, and as hallucinations those in which any such impression is lacking, or plays but a subsidiary part and bears no obvious relation to the character of the false percept.
As in the case of illusion, hallucination may or may not involve delusion, or belief in the reality of the object falsely perceived. Among the sane the hallucinatory object is frequently recognized at once as unreal or at least as but quasi-real ; and it is only the insane, or persons in abnormal states, such as hypnosis, who, when an hallucination persists or recurs, fail to recognize that it corresponds to no physical impression from, or object in, the outer world. Hallucinations of all the senses occur, but the most commonly reported are the auditory and the visual, while those of the other senses seem to be comparatively rare. This apparent difference of frequency is no doubt largely due to the more striking character of visual and auditory hallucinations, and to the relative difficulty of ascertaining, in the case of percep tions of the lower senses, e.g., of taste and smell, that no impres sion adequate to the genesis of the percept has been made upon the sense-organ ; but, in so far as it is real, it is probably due in part to the more constant use of the higher senses and the greater strain consequently thrown upon them, in part also to their more intimate connection with the life of ideas.
.The hallucinatory perception may involve two or more senses, e.g., the subject may seem to see a human being, to hear his voice and to feel the touch of his hand. This is rarely the case in spon taneous hallucination, but in hypnotic hallucination the subject is apt to develop the object suggested to him, as present to one of his senses, and to perceive it also through other senses.
Among visual hallucinations the human figure, and among auditory hallucinations human voices, are the objects most com monly perceived. The figure seen always appears localized more or less definitely in the outer world. In many cases it appears related to the objects truly seen in just the same way as a real object ; e.g., it is no longer seen if the eyes are closed or turned away, it does not move with the movements of the eyes, and it may hide objects lying behind it, or be hidden by objects coming between the place that it appears to occupy and the eye of the percipient. Visual hallucinations are most often experienced when the eyes are open and the surrounding space is well or even brightly illuminated. Less frequently the visual hallucination takes the form of a self-luminous figure in a dark place or appears in a luminous globe or mist which shuts out from view the real objects of the part of the field of view in which it appears.
Auditory hallucinations, especially voices, seem to fall into two distinct classes: (I) those which are heard as coming from without, and are more or less definitely localized in outer space, (2) those which seem to be within the head or, in some cases, within the chest, and to have less definite auditory quality. It seems probable that the latter are hallucinations involving prin cipally kinaesthetic sensations, sensations of movement of the organs of speech.
Hallucinations occur under a great variety of bodily and mental conditions, which may conveniently be classified as follows: