Indirect Central Stimulation.

Besides these direct central stimulations, there are others which, in contradistinction, may be called indirect, depending on some previous excitation.  These are, no doubt, the conditions of a very large number of our dream-images.  There must, of course, be some primary cerebral excitation, whether that of a present peripheral stimulation, or that which has been termed central and spontaneous; but when once this first link of the imaginative chain is supplied, other links may be added in large numbers through the operation of the forces of association.  One may, indeed, safely say that the large proportion of the contents of every dream arise in this way.

The very simplest type of dream excited by a present sensation contains these elements.  To take an example, I once dreamt, as a consequence of the loud barking of a dog, that a dog approached me when lying down, and began to lick my face.  Here the play of the associative forces was apparent:  a mere sensation of sound called up the appropriate visual image, this again the representation of a characteristic action, and so on.  So it is with the dreams whose first impulse is some central or spontaneous excitation.  A momentary sight of a face or even the mention of a name during the preceding day may give the start to dream-activity; but all subsequent members of the series of images owe their revival to a tension, so to speak, in the fine threads which bind together, in so complicated a way, our impressions and ideas.

Among the psychic accompaniments of these central excitations visual images, as already hinted, fill the most conspicuous place.  Even auditory images, though by no means absent, are much less numerous than visual.  Indeed, when there are the conditions for the former, it sometimes happens that the auditory effect transforms itself into a visual effect.  An illustration of this occurred in my own experience.  Trying to fall asleep by means of the well-known device of counting, I suddenly found myself losing my hold on the faint auditory effects, my imagination transforming them into a visual spectacle, under the form of a path of light stretching away from me, in which the numbers appeared under the grotesque form of visible objects, tumbling along in glorious confusion.

Next to these visual phantasms, certain motor hallucinations seem to be most prominent in dreams.  By a motor hallucination, I mean the illusion that we are actually moving when there is no peripheral excitation of the motor organ.  Just as the centres concerned in passive sensation are susceptible of central stimulation, so are the centres concerned in muscular sensation.  A mere impulse in the centres of motor innervation (if we assume these to be the central seat of the muscular feelings) may suffice to give rise to a complete representation of a fully executed movement.  And thus in our sleep we seem to walk, ride, float, or fly.