MENTAL FUNCTION 425 



It is unfortunate that so relatively little yet is known about its 

 psychical status as well as about its physical basis. Especially does 

 science need to know the nature of the break in memory between the 

 sleep-consciousness and that of waking hours. 



Hallucinations, Illusions, and Delusions. These three sorts of mental 

 experiences are, strictly speaking, more or less abnormal, but they are of 

 such fundamental importance in the theory of insanity and so instructive 

 physiologically that we may describe them briefly here. It is, however, 

 only the exceptional "normal" man (and normality is a very indefinite 

 sort of thing) who does not often experience in some degree, usually as a 

 victim, various illusions and delusions. Furthermore, about one-tenth 

 of the public have some time or other, if only for a few seconds, been 

 under the strange spell of a true hallucination. (See the "Census of 

 Hallucinations" made by H. Sidgwick.) 



An hallucination is a percept without objective occasion or representa- 

 tion, while an illusion is a wrongly interpreted percept. A delusion, on 

 the other hand, is of a conceptual rather than perceptual nature, and is 

 a system of notions and beliefs contrary to the facts. 



It is plain from these definitions that hallucinations and illusions are 

 of the same general nature and they probably have the same neural basis. 

 In normal perception the stimulus comes in through a sense-organ and 

 goes on into the perceptual brain-centers, each percept employing doubt- 

 less very many neural paths. In normal imagination these same brain- 

 centers are doubtless employed, but whether the sense-organs and the 

 afferent paths also ar,e we do not know. When an hallucination occurs 

 these perceptual centers are stimulated in the same way or to the same 

 degree as when a percept is formed, and not in the manner of the imagina- 

 tion, and no one ever mistakes the product of imagination for that of 

 perception. How it is that these centers are stimulated so as to give 

 the same sort of experience as in perception, no one as yet knows. It is a 

 process of imagination changed to the vividness, objectivity, and reality 

 of actual perception. Sometimes the individual is deceived into sup- 

 posing the hallucination a true perception, but more often, owing to 

 contradiction between the hallucination and perceptions, the false 

 "object" is realized to be fallacious. 



In illusion there is some sort of objective occasion or stimulus, and the 

 afferent neural apparatus (end-organ and nerve) employed is the same 

 as in perception. The result given to the brain is wrongly interpreted 

 by that organ, so that the person perceives differently from the objective 

 reality. For example, if in looking at the short curved lines of dried 

 bacilli on a microscope-slide one sees them bend and infers therefore 

 that they are alive, he is the victim of illusion, and illusions are easy and 

 very common in microscopy. A white stump in a swamp at midnight is 

 much more apt to seem a ghost than as a portion of a dead tree. These 

 are illusions and obviously differ from hallucinations in being wrongly 

 interpreted perceptions; in hallucinations the latter are wholly absent. 

 The physical basis of both of these experiences is evidently much like 



