This article provides an overview on Hallucination.
Hallucination:
An Hallucination is a Subjective Perception:
It is a memory image invested with sensory vividness. It is protected to external space and mistaken for an object of perception. It is a memory image mistaken for a sensation. There is no external stimulus corresponding to it.
An illusion is produced by an external stimulus. But an hallucination is not produced by an external stimulus. It is an image invested with sensory vividness, which is not distinguished from an actual perception.
A murderer sees the ghost of the murdered person. There is no external stimulus here. There is a memory image invested with sensory vividness in the mind of the murderer. The image is excited by his brain in an abnormal condition.
He hears the voice of the ghost that he will avenge his death. The murderer haunted by a secret sense of guilt imagines the voice. There may be a faint sense-impression in an hallucination, but it plays an insignificant part. Imagination plays the major part in hallucination.
McDougall defines an hallucination as “seeing things which are not there” or thinking of remote objects with sensory vividness. The object of hallucination is not present. It is either recalled or imagined. It is an object of thinking, in a wide sense.
But an hallucination has sensory vividness. It is so vivid that it appears to the subject to be an object of actual perception existing in external space. It may be excited by a faint sense-impression which plays an insignificant part. Or, it may be entirely lacking in sense-impression. It is subjective perception.
Hallucinations are of various kinds. Visual and auditory hallucinations are common. There are tactile hallucinations also. For example, the patient sees an object which does not exist, or hears an imaginary voice. He hears a voice which generally refers to subjects closely related to his most intimate life, and often consists of a reproach.
He may hear a voice constantly, announcing that he will be shortly put to death on account of the sins he has committed. He often constructs some fantastic hypothesis that it is the message of the spirit.
Hallucinations may be deduced in some healthy persons who are highly suggestible. Verbal suggestion may induce hallucinations of various kinds in most hypnotic subjects during hypnosis and in some subjects in the post hypnotic state. Visual hallucinations may be induced in others by crystal gazing. Persons who are readily hypnotised can easily see crystal visions.
Illusion and Hallucination:
An illusion is excited by peripheral stimulation, while an hallucination is excited by central stimulation. In an illusion there is wrong interpretation of a sensation produced by an external stimulus. When a person mistakes a rope for a snake, the rope produced an impression in his mind, but he wrongly interprets it.
His idea of the snake does not tally with the rope. This is an illusion. But in an hallucination there is no external stimulus. It is produced by the pathological condition of the brain. It is centrally excited.
McDougall observes that it is difficult to draw a sharp line of demarcation between an illusion and an hallucination because even in an hallucination some sense impression, however faint it may be, plays a part in producing it. For example, obscure sounds due to internal stimulation of the auditory nerve may produce auditory hallucination (e.g., the voice of a ghost).
In illusion a sense- impression is always present, which is wrongly interpreted. In hallucination a sense-impression is not always present. But sometimes a sense-impression is present, but it does not play and important part; central interpretative factors play the major role in hallucination. They are of primary importance, while sensory stimulation is of secondary importance.