Psychology: an elementary text-book

by Hermann Ebbinghaus

Let us illustrate this by two figures. [Figure 16] may be perceived as a rabbit’s or as a duck’s head. When we perceive the figure as a rabbit’s head, the white streaks to the right of the eye are two separate sensation groups, each of them unified with respect to the effect produced by them in our nervous system. They are then the animal’s lips. At the same time the protrusions to the left make us conscious of softness, warmth, flexibility. Now perceive the figure as a duck’s head. Immediately those white streaks cease to be two separate units for our mind. Together with the darker parts surrounding them, they affect our mind as a single unit, the variegated back part of the duck’s head. And at the same time the protrusions to the left make us conscious of hardness, cold, rigidity. The sensory stimulations are exactly the same, but they are differently grouped together, and they bring about further nervous activities which greatly differ in these two perceptions.

[Figure 17], when shown to a person, is perceived as the result of a child’s careless handling of his ink bottle, as an ink spot. But ask this person if he does not see a boy falling downstairs, and immediately certain elements are grouped together and affect us as being the legs, other elements of sensation are perceived as the arms, and so on. And now suggest to the same person to turn the page slightly to the right and see a man trying to put on his shirt. Quickly the perception changes again; but this time not so much by the breaking up of the former units into their sensory elements and the formation of new units, as by a change of the accompanying ideas. The previous suggestion tends to make us perceive these sensations in one or the other way because it guides our attention. But this guidance is possible only because certain groups of sensational elements (for example, the groups illustrated by our figures) have very often occurred in our mind in consequence of the fact that they originate from external objects which have often been presented to our sense organs among greatly varying surroundings. Thus we have learned to group these elements together and to neglect, more or less, all other elements which may be presented simultaneously.

The total process of selective grouping and of furnishing the groups formed with additional mental contents has often been called apperception. But this meaning of the term apperception is not universally adopted. Some mean by apperception mainly the selective grouping of the elements, others mean by it exclusively the furnishing with ideational contents. Because of its ambiguity the term apperception has been entirely omitted from the present book, and the term perception is used in its broadest sense, including both the processes just mentioned. Perception thus means the working over by the mind of any aggregate of sensational elements given at the time through the sense organs.

[2.] Illusions

While the laws of perception are, on the whole, of the greatest benefit to the organism surrounded by a confusing multitude of physical elements bound together into a large number of more or less stable compounds, of things, there are exceptional cases in which these same laws lead the mind into a reaction not suitable to the situation presented.

That which has often occurred is likely to recur. But it does not regularly recur in the same manner. There are exceptions. It happens that certain things occur in surroundings different from their usual surroundings. These things are then perceived, that is, grouped together and supplemented by images, in harmony with their usual surroundings. But the perception is then in discord with the actual surroundings. To the inhabitant of the plains the colors of things appear rather saturated, and the outlines sharp, when these things are at a small distance from the observer. Walking toward them, he is soon able to lay hands on them. But when the air happens to be unusually moist, and because of its diminished weight, free from the particles of dust which have settled because of their weight, things look unusually near, and on walking toward them he discovers that it takes more time to reach them than he expected. The same happens when he goes to the mountains for his vacation, because there the air is always comparatively free from dust. We have here a foreseeing of what ordinarily becomes the subsequent experience, but fails to become it in this instance.

There is another kind of illusion based on the fact that sensations which have been imagined just before the stimuli became effective, are thereby favored and become unusually vivid. This law of attention holds good also when the stimuli are not in exact correspondence with the preceding images. In such a case the perception is more or less assimilated to those images, so that the same stimuli result in somewhat different percepts according to circumstances. “How heavy it is!” said a friend of Davy’s, when the discoverer of potassium placed a little piece of this metal on his finger. Potassium is so light that it floats on water, but the metallic appearance produced the image of pressure and changed the sensation into a percept of something heavy. When two pieces of gray paper, equally bright but of slightly different coloring, are put before me side by side, and I ask myself: is not the yellowish paper lighter than the bluish paper, immediately it seems to be lighter. But I begin to doubt and ask myself: is not the yellowish paper darker than the other; and immediately it looks darker.

Let no one say that this is only “imaginary,” meaning by this word that there are in my mind both the objectively true impression and an incorrect image of something similar. Such is not the case. There is no duality of consciousness. There is one unitary experience. Only scientific reflection reveals the fact that this unitary experience has two sources, one in the external stimulation, the other in the central nervous excitation. The result of these sources, the percept, does not betray the doubleness of its origin any more than a stream at its mouth shows the doubleness of its sources. It is a universal property of perception to be determined not by sensory stimulation alone, although this is the primary factor, but also by images, by nervous dispositions. The more vivid such images, the greater is their influence—now and then their deceptive influence—on our consciousness of the objectively existing. Suggestion is a name which has recently been accepted for such an influence. Illusion is another name for it, in case it is rather pronounced and ill adapted to the object.