Hallucinations.

I do not propose to go very fully into the description and explanation of hallucinations here, since they fall to a large extent under the category of distinctly pathological phenomena.  Yet our study of illusions would not be complete without a glance at this part of the subject.

Hallucination, by which I mean the projection of a mental image outwards when there is no external agency answering to it, assumes one of two fairly distinct forms:  it may present itself either as a semblance of an external impression with the minimum amount of interpretation, or as a counterfeit of a completely developed percept.  Thus, a visual hallucination may assume the aspect of a sensation of light or colour which we vaguely refer to a certain region of the external world, or of a vision of some recognizable object.  All of us frequently have incomplete visual and auditory hallucinations of the first order, whereas the complete hallucinations of the second order are comparatively rare.  The first I shall call rudimentary, the second developed, hallucinations.

Rudimentary hallucinations may have either a peripheral or a central origin.  They may first of all have their starting-point in those subjective sensations which, as we have seen, are connected with certain processes set up in the peripheral regions of the nervous system.  Or, secondly, they may originate in a certain preternatural activity of the sensory centres, or “sensorium,” in what has been called by German physiologists an automatic excitation of the central structures, which activity may probably diffuse itself downwards to the peripheral regions of the nerves.  Baillarger would call hallucinations of the former class “psycho-sensorial,” those of the latter class purely “psychical,” hallucinations.[57]

It is often a matter of great difficulty to determine which part of the nervous system is originally concerned in these rudimentary hallucinations.  It is probable that in normal life they are most frequently due to peripheral disturbance.  And it seems reasonable to suppose that where the hallucination remains in this initial stage of a very incompletely interpreted visual or auditory impression, whether in normal or abnormal life, its real physiological source is the periphery.  For the automatic excitation of the centres would pretty certainly issue in the semblance of some definite, familiar variety of sense-impression which, moreover, as a part of a complex state known as a percept, would instantly present itself as a completely formed quasi-percept.  In truth, we may pretty safely argue that if it is the centre which is directly thrown into a state of activity, it will be thrown into the usual complex, that is to say, perceptional, mode of activity.