and the following statements are largely taken from his work. His method was peculiarly well-adapted to bring out the differences of figure and ground, in employing geometrical patterns which are phenomenally equivocal as to their figure-ground structure. A simple example of such a pattern has already been discussed by Schumann (55). If we make the distances in our fence aa, bb, . . equal to ab, bc, . . we have a striking instance. For now bb may be a stripe, bc a piece of the ground, or inversely, bc may be a stripe, and bb a piece of the ground. In either case we find our old difference, that the stripes are always bounded, whether they are formed by bb or bc, while the intervals are not. Another example is offered by the so-called subjective rhythm, whether auditory or visual, which corresponds to an objectively equal series of beats or flashes.
[p. 557] In such a phenomenal series we again meet with the difference of inter- and intrametric intervals, and again their coordination with the objective intervals is ambiguous. The cross in Figure 6, reproduced from Rubin, may be experienced either as a white cross on a black ground, or as a black cross on a white ground (neglecting other less important effects). Compare either cross with its ground and you can clearly recognize that the latter is always less definitely structured than the former; either the ground has no distinct shape at all, or else it approaches the comparatively simple form of a square.
Hand in hand with higher degrees of structure there goes a greater "liveliness" or vividness of the figure. As Schumann observed, the white space inside a figure is "whiter" than that outside, which can also be easily seen in the equidistant fence-design. A striking example of this is afforded by a certain kind of drawings, used frequently for advertising posters, where the contour is not fully drawn, but where, nevertheless, no gap appears in the figure. I may refer the reader to Jastrow's Editor, reproduced in Pillsbury's textbook (51, p. 158).
These last examples show what has already been pointed out, namely, that phenomenal figures have boundary lines even when the corresponding objective figures have none. A good figure is always a "closed" figure, which the boundary line has the function of closing. So this line, separating the fields of figure and ground, has a very different relation to each of these, for though it bounds the figure, it does not bound the ground. The ground is unaffected by the contour and is partly hidden by the figure, yet it lies without interruption behind the figure. The cross of the accompanying figure (Figure 7) will make this description clear. Look at the fields with the arcs for filling. When forming a cross, these become true arcs, i.e., cut-off pieces of circles, but when forming the ground they look quite differently, for they are no longer cut off, becoming now the visible parts of a phenomenal series of complete circles.
This property of the ground, that the figure's contour does not affect it, is closely related to the first characteristic we mentioned, namely, its lesser degree of structure. In our last instance this fact is revealed by the observation that the whole circles when they constitute a ground are simpler structures than the arcs which are necessary to the formation of the cross; for in place of each single circle there appear four arcs. The lesser degree of structure leads also to another indication noted by Rubin of the difference between [p. 558] ground and figure: the ground has more of a substance- and the figure more of a thing-character.
Let us return to the boundary line. From its variable relation to figure and ground there follows the inference that it must have two different sides, an inside and an outside; the one includes, the other excludes, or to use terms in this more general sense which have been suggested by v. Hornbostel (19), the one is concave, and the other is convex. Though these words are not psychological terms they are meant to indicate true psychological descriptions. Look at the left line-b in our fence-figure and you will understand what is meant by this description, for its left side is hard and
repelling, whereas its right side is soft and yielding. Very full descriptions of these properties are given by v. Hornbostel who reduces the illusions of reversible perspective to a change in these properties: to reverse a figure is to make concave what was convex, and convex what was concave.
One remark here to the reader who may raise the objection that our terms do not designate the existential properties of visual phenomena, but only their intentional meanings. I have said that I wished to point out true properties. Now consider that these properties need not be like those of traditional psychology, "dead" attributes, possessing a "so-being" only, but that many of them are alive and active, possessing a "so-functioning." A beam of wood, lying unused on the floor, may look like a beam carrying weight, yet [p. 559] an accurate description would have to note this fact by giving heed to the state of tension which must then exist. More generally speaking, a state of rest with an absence of force is different from a state of rest with an equilibrium of force, and the same thing holds true, in the writer's opinion, for phenomena. The border line of a figure performs a function. and this performance is one of its visual properties. Traditional psychology has defined the term "visual property" so as to include "dead" properties only. Consequently in looking for visual properties it has found only these. But this definition was arbitrary, and it proves to be inadequate, since it makes the investigator blind to facts of the highest significance.
Lest the reader should be inclined to consider the distinction hitherto offered as trivial, artificial, and secondary, we may turn to experiments with ambiguous patterns, where the different structures
correspond to two totally different forms, whereas in the previous examples the same form, a fence or a cross, appeared in both cases. Well-known puzzle pictures fall under this head, one of which is produced by Titchener (61, p. 278) -- a brain with fissures which assimilate as babies, while another example is given in Pillsbury's book (51, p. 162) as a duck's or a rabbit's head. The best example of this which I know was used by Rubin. It is a goblet, whose contours also form the profiles of two faces. Many similar patterns were employed by Martin (42). We need not, however, search for examples, since everyday life supplies us with any number of them. The simplest, perhaps, is an ordinary chessboard pattern, where at least six different phenomena may be aroused, and many others are frequently found in lace or wallpaper designs. Figure 8 is reproduced from the edging of a table cloth. You can see either the black T-shaped forms or the white leaves. On the actual frieze it is hardly possible to see both at the same time, though in our sample [p. 560] this is easier. Whenever you see one of the figures only, the remainder becomes a ground of the simplest possible description.
This difference has not escaped the psychologist, but has been discussed at length. The clearest statement is given by Titchener (61), whose report I shall closely follow in my interpretation. He would say that in the beginning the black T's are at the upper level of consciousness, while the rest is at a lower level. Suddenly a change takes place, the T's drop clear away from the upper level, and the white leaves stand out with all imaginable clearness, while the form of the T's is no clearer than the feel of the book in your hand. Had he written the last sentence only there would be no disagreement between us, for the "feel of the book" belongs truly to the "ground" of the whole situation. But what he does say leaves the existence of the T-phenomenon untouched by the change in its phenomenal aspect. It has merely shifted its level, having dropped from the crest of attention to its base, from whence the leaves have now risen.
In objecting to this interpretation (which has also been vigorously attacked by Rahn), and at the same time arguing against Wundt, Rubin states most emphatically that when the T's have disappeared and we see in their place a mere ground, the T's have indeed no clearness at all, for they have become nonexistent.
In Titchener's report we recognize the typical attempt of traditional psychology to elucidate phenomena by means of the cardinal concepts stated at the beginning of this paper. Something which ought to be there phenomenally, since a corresponding stimulus does exist, is not observable, and this contradiction is overcome with the aid of attention. Yet this is no longer a description of fact, but a hypothetical interpretation.[14] For I can describe only what I can observe, what is there before me, and to say that a figure is at so low a level of consciousness that it is not observable is not a description of what is present, even though in the next moment I can reëxperience what at the time was nonexistent. If I wish to describe truly I must report positively what that part of the total phenomenon looks like which lies at the so-called basis of attention; for it is not a description of it to tell how it does not look.
To infer how something looks when it is not observable from the data of its appearance when at the crest of the attention-wave, means the acceptance of the constancy-hypothesis and a final abandonment [p. 561] of every effort to obtain a factual verification. As Köhler has pointed out (34), if we stand by description proper, i.e., by verifiable description, we must recognize that the T's have ceased to exist the moment we see the leaves, and that the T-phenomenon has been replaced by a totally different ground-phenomenon, which corresponds to the same part of the stimulus-complex. We see now what an enormous change has been effected when a figure "emerges" from its ground. Rubin gives a striking description of the shock of surprise felt again and again in such a transition, even when he tried to imagine in advance what the new phenomenon would be like.
We have seen how the concept of attention has prevented the recognition and vitiated the pure description of a very marked phenomenal difference. Yet a connection exists between the figure-ground consciousness and the attention, so-called. But by observing the facts, what we find is a functional dependency, instead of a descriptive identity. As a rule the figure is the outstanding kernel of the whole experience. Whenever I give attention to a particular part of a field, this part appears in the figure-character. have frequently performed the following classroom experiment: using a photographic shutter I project Figure 8 for a short time upon the wall, and instruct beforehand one-half the audience to watch the white, and one-half the black parts of the picture. I then ask the whole audience to make sketches of what they have seen. Invariably the "black" half of the audience draws the T's, and the "white" half the leaves.
Is it possible to describe the attitude of the observer which is produced by the instruction to "watch"? Again we may refer to v. Hornbostel's inversion experiments. He finds that it is more difficult to invert the convex into the concave than the concave into the convex, because whatever I am looking at, watching, acting upon, stands forth grows fixed, becomes an object, while the rest recedes, grows empty, and becomes the ground. He also adds that since the objects obtrude themselves upon me, and come toward me, it is they I notice and watch rather than the holes between them. (19, p. 154.) We need only to apply this general description to our special case, and we shall see that attention has now a very definite meaning; for, in attending to the black parts, we adopt a "figure attitude" toward them by making them the center of our interest. At the same time, the part that has become the figure itself strives to become the center of our experience. This notion of the "center" will play an [p. 562] important part in the later expositions of our theory; here we have simply replaced the vague concept of attention with one which is well-defined.
The functional connection of figure- and center-consciousness is not absolute. Though it is natural to "attend" to the figure we can, for a time, at least, attend to the ground, and let the figure recede. If we continue this attitude too long, however, we run the risk of a change in the phenomenon; but that such an attitude is possible --and many observations reported in the foregoing prove that it is -- demonstrates that the figure-ground distinction cannot be identified with a mere difference of the attention-level.
All good psychological descriptions must find their justification in functional facts. Phenomena that are different in description must also prove to be different in function, if the description is tenable.
So we turn to the functional facts which underlie the figure-ground distinction.
Two sets of experiments have been performed by Rubin, both employing patterns of the type of Figure 9. These patterns are ambiguous, either the enclosed white space or the enclosing black space may appear as the figure. Let us call the first the positive, the second the negative reaction. According to the instructions given, it is possible for the O.'s to assume either a positive or a negative attitude before the exposure of the pattern. After some practice the attitude assumed will in most cases be effective, i.e., a positive reaction will ensue from a positive attitude, and vice versa. In his first series of experiments, Rubin presented a number of such patterns with either positive or negative instructions. After a certain interval the experiment was repeated with instructions prescribing an indifferent attitude, neither positive nor negative. The result was that in the majority of cases a pattern once reacted to in a [p. 563] certain manner was reacted to the next time in the same manner. Rubin calls this a "figural after-effect" (figurale Nachwikung). It proves that the structure by which we react to a given stimulus-complex remains in the memory of the individual, a fact of paramount importance for the theory of learning, as I have elsewhere (33) shown. The problem of the second series was to find out if a pattern seen the first time under one attitude, positive or negative, will be recognized when it is seen the second time under the reversed attitude. The procedure was similar to that of the previous experiment, except that the instruction of the test-series was either positive or negative. The result was in full accordance with the descriptive distinction, for when the reverse instruction was effective no recognition took place. By overlooking this fact many troublesome mistakes are committed even in everyday life.
We have assigned to the figure a "thing"-character, and to its ground a "substance"-character. This description has also been justified by experiments, for we learn from Gelb's investigation (11) that the color-constancy commonly called memory-color is dependent upon the color's "thing"-character and not upon its "surface"-character. This was clearly proved by two patients with brain lesions who saw no surface-colors (Oberflächenfarben) and yet they made the same brightness-equations between a lighted and an unlighted color as did normal O.'s. They reacted differently only in case of a shadow, and this was because their visual apprehension was not sufficiently restored to enable them to recognize a dark spot as a shadow cast upon an object.
Before Gelb's paper had been published the connection between color-constancy and "thing"-character was suggested to Rubin by the researches of Katz (24), and Rubin concluded that because of this connection the figure-ground difference ought also to appear when the color-constancy is altered. To test this conclusion he planned two ingenious experiments. In the shadow-experiment he used a cross of the type of Figure 6, and cast a light shadow upon one of the white sectors. His O.'s reported this shading to be stronger when the white sector was part of the ground than when it was part of the cross. In the color experiment the cross was colored and observed through differently colored glasses. The result was again that the figure offered a stronger resistance to change of color than did the ground.
Starting from the greater vividness of the figure as described, I [p. 564] devised the following experiment (32). I tested the power of figure and ground to resist so-called retinal rivalry. On the left side of a stereoscope I put a Rubin cross like that of Figure 6, composed of alternate blue and yellow sectors, while on the right side there was a regular blue octagon of homogeneous surface (comp. Figure 10). The left cross can appear either as a blue cross on a yellow ground, or as a yellow cross on a blue ground and in looking through the stereoscope it is easy to see either, since the left image, with its richer detail is superior in rivalry to the right image. Beginning with the yellow cross which is a very stable phenomenon, you can accentuate the right image by moving it, or by pointing at it with a pencil, without disturbing the yellow cross. But let the blue cross on the yellow field involuntarily appear, and then accentuate the right image but slightly and the cross will disappear as the blue octagon emerges. The
explanation is simple enough- There is a constant rivalry between the yellow sectors on the left and the corresponding blue space on the right, yet so long as the yellow forms the figure in the left image the structure is so strong and so fixed that it resists attack. When the yellow is ground, however, it is but loosely formed and can therefore b easily defeated by the right image. So the better formed the field is, the more vivid and more impressive (eindringlich) it will be, a fact which has been theoretically explained by Köhler (38, p. 206f). Discussing the electrical processes occurring in the optical system during stimulation, and making the well-founded assumption that the entire optical sector, periphery, optical tract, and cortical area together form one system, Köhler comes to the conclusion that the density of energy is always much greater in the figure-field than it is in the ground-field, and that the current (Strömung ) is much more concentrated in the former than in the latter. It is this condition [p. 565] of energy which helps figures to attain their phenomenal vividness, and also, as we say after our last experiment, their superiority in rivalry.
Phenomenally, the figure is always a stronger and more resistant structure than the ground, and in extreme cases the ground may be almost formless, a mere background. For this distinction we have also found a functional counterpart. Kenkel (25) has discovered that figures, when briefly exposed, appear with specific movements which expand with their appearance and contract with their disappearance. I have (27) advanced the hypothesis that this movement, called by Kenkel the gamma movement, is the expression of a structural process. This hypothesis has been tested and proved by an investigation of Lindemann (28) which will be more explicitly discussed in a later article. However, one experiment of this investigation belongs in the present context. Lindemann worked also with patterns that were ambiguous in their figure-ground structure. His figures were of the type of Figure 9 and of the goblet pattern described above. If Figure 9 is positively apprehended the O. sees violent outward movements of the white teeth, whereas, if observed negatively, the black indentures, particularly the lower claw-like one, move vigorously inwards. The goblet pattern behaves similarly. If the goblet is seen, it performs extensive expansions and contractions, whereas, if the profiles appear they tend toward one another, the direction of the movement being reversed, but, on account of the close proximity of these two structures movement is in this case notably checked. These experiments show that the gamma movement takes place in the figure and not in the ground, and since they reveal a constructing process, they prove that functionally the figure is better formed than the ground.
I shall repeat here another experiment performed in the Giessen Psychological Laboratory, which has not yet been published. Hartmann (29) has investigated the laws governing the fusion of two stimuli separated by a dark interval. The O. looked through a telescope, or in most of the experiments through a blackened tube, behind which the Schumann tachistoscope was rotating. In the rim of the wheel there were two slits, separated by a variable interval. Behind the wheel was the object which in this procedure was twice exposed during one revolution. The objects were transparent figures getting their light from the rear. In accordance with the facts known about the Talbot fusion (for instance, rotating discs), [p. 566] Hartmann found that the critical speed of the wheel was a direct function of the intensity of the stimulus which could easily be regulated by varying the amount of light passing through the exposed objects. By "critical" speed is meant that speed which is just capable of bringing about a complete fusion, after the last bit of flicker has disappeared. Hartmann then worked with Figure 6 as one of his objects, and he found a marked difference in the critical speed for the two phenomena, black cross on white ground and white cross on black ground. For instance, the time of revolution in the first case was 1.65 seconds while in the second case it was 1.3 seconds. Now the black sectors are no blacker than the dark interval, hence the flicker is produced by the white sectors alone; consequently the same field fuses under Hartmann's conditions more easily when it is a ground than when it is a figure. This proves again the close connection between construction or "formedness" and vividness or intensity. And this proof seems all the more convincing because it is based upon an effect which has hitherto been considered a purely physiological process of the retina. Besides, this experiment is not only qualitative, it is also quantitative, since the difference of critical speed for the two different phenomena corresponding to the same stimulus-intensity can be matched with another difference in critical speed between two corresponding phenomena (black vs. white cross) with different stimulus-intensities.
I believe that the functional facts I have adduced are sufficient to prove the essential difference between the figure and ground phenomena. This difference is fundamental and the figure-ground structure must therefore be considered one of the most primitive of all structures. I have (33) defended the view that this structure is also the first phenomenon experienced by the human infant; for instance a light patch on a dark ground instead of the various sensations with which, according to the traditional view, the baby's consciousness is supposed to be filled. This genetic consideration raises still another question. We have said that a figure cannot exist without a ground. Can a ground exist without a figure? In another connection (33, p. 97) I have tried to prove that it cannot, and that mere ground would be equivalent to no consciousness at all.
So far our observations have shown a superiority of the figure-phenomenon over the ground-phenomenon. This, however, must not lead us to disregard the latter, for the ground has a very important function of its own; it serves as a general level (niveau) upon [p. 567] which the figure appears. Now figure and ground together form a structure, consequently the former cannot be independent of the latter. On the contrary the quality of the figure must be very largely determined by the general level upon which it appears. This is a universal fact, observed in such products of culture as fashion and style. The same dress which is not only smart, but nice to look at, almost a thing of beauty, may become intolerable after the mode has passed. Again, put a heavy modern leather club-chair into a rococo salon and the effect will be hideous. Music offers any number of examples as to the influence exerted by the general level. Each tone, each harmony, has a specific meaning, inherent in its "sound" for a given key only; but this meaning changes with the key, so that G is the tonic of G major, but the dominant of C major.
The influence of the ground appears in many psychological experiments. As Hering (15) has shown, the question of the functional dependence of the brightness of a gray upon the amount of light reflected into our eyes is unanswerable because it is incompletely stated. To solve this problem we must determine the general level. If we allow the level to vary the same amount of light in the figure may arouse a black, a gray or a white, as can easily be proved by Hering's "hole method," and the same is true if we take color into consideration. Witness the following experiment with Hering's hole method which I have often used as demonstration in a classroom. Put a white screen (of about 50 x 50 cm.) with a hole in it of about 5 cm. diameter before a white wall. Put one or two ordinary electric lamps between screen and wall so that they throw their reddish-yellowish light on the wall, and close the shutters of the room. The wall will look fairly white, so will the hole in the screen so long as you perceive it as part of the wall seen through the hole in the screen. But you can also see the hole as part of the screen, which then becomes its ground; in which case the hole will seem to protrude somewhat from the screen and will have a distinct yellow tinge. Now throw on the screen white light from the arc-lamp of a projection lantern and arrange the intensities so that the amount of light upon the wall and upon the screen is approximately equal. The filling of the hole is then forced into the plane of the screen and has a fairly saturated color of a warm reddish-yellow. It is much more colored than it was before, while the screen now looks to be a light gray with a slight blue-greenish tint. Cover and uncover alternately the objective lens of the lantern and you can easily observe a great [p. 568] change in the hole's color. Now fixate the wall, looking right under the lower edge of the screen. Again open and close your lantern. In this case you will see the fairly white wall suffering but very little change, whereas when the screen is lit up with a clear blue-green color, the hole becomes invisible.
You may object that this experiment involves a combination of memory-color and contrast. But in the first place, Jaensch has proved that these are not two different effects, but special cases of one and the same law; in the second place these terms are not an explanation of the phenomena and the facts mentioned do not readily submit to the current theories of contrast.[15]
Let us describe the facts by means of our level-concept. Consider that objectively the filling of the hole is but slightly altered by the turning on of the lantern light, which only causes it to grow a little whiter. Since this effect is opposed to the phenomenon we have described, we may for simplicity neglect it altogether. But why does the "white" wall, when illumined by the yellowish lamp, still look fairly white? There is but one sort of light in the room, excepting the traces of daylight that are not excluded by the shutters.
The light-level of the room is therefore solely determined by the lamps, and the lighted part of the room is homogeneously colored. Let us now make the assumption that every general color-level tends to look white, that, in other words, white (including gray and black) is the, characteristic level-color. This will explain our fact. Now, as to the hole in the dark screen: it remains white when it appears as part of the wall, for it then belongs to this general level. But, if it appears on the screen it lies at the screen's level, and since the screen reflects no lamplight but only certain traces of daylight, the screen will therefore look almost black- (white-level). As a consequence, the hole as a figure upon this ground, reflecting a light which is different from its ground, can no longer retain the same color; accordingly it appears yellowish. The color-effect is not very marked because of the great difference in brightness between the ground and the figure. The explanation for this, which also involves a law of structure, will be given in the following article.
When the screen is illuminated by the white lantern light, it forms a pronounced level, and since by the conditions of the experiment the brightness-difference between ground and figure has been [p. 569] decreased, the figure now appears to have the color of the lamps behind the screen. The screen, reflecting white light only, does not look like a pure gray, but being much smaller than the wall, it is therefore influenced by the wall's general level, as the wall is also influenced by the "level" of the screen. Therefore the screen looks slightly blue-greenish, while the wall, in turn, is tinged with yellow. If the screen were larger, so as to cover the entire wall, it would look pure gray and the hole still more yellowish. The difference in illumination between screen and wall determines, primarily, a color-distance or a system of color-steps, the actual position of the steps being dependent upon other factors.
Turn to the second experiment. Here one remains at the unchanged level of the wall. The screen becomes. now the figure upon this constant level, and since the objectively yellowish level of the wall looks almost white, the screen must appear of a pronounced bluish color, though it, too, is objectively white.
In other words, objective white looks white when it is the "ground" of the observation, and objective yellow looks yellow when it is a "figure" upon this "ground." Similarly, objective white looks bluish when it appears upon an objectively yellow ground, and objective yellow looks white when it forms a ground -- all of which may take place under the same objective conditions. From this we can draw the conclusion that a field, reflecting a certain amount and quality of light, depends for its phenomenal color-quality upon the ground on which it appears.
Thus our experiments are arguments in favor of our initial assumption, and this assumption furnishes a true psychological interpretation of the observation of Helmholtz, who maintained that we are unable to recognize a true white without comparison. Since, according to his theory, a sensation of white is composed of the sensations of the three cardinal colors mixed in certain proportions of intensity, and since the comparison of the intensities of colors is difficult and uncertain, therefore, in the absence of a true standard, we are very often mistaken and judge a sensation to be white, when in reality it is not white, but colored.
Our conception (Begriff) of white, is thus subject to change, while the sensation remains constant (13, II, p. 223f, 1st ed., p. 396f ). This theory involves the constancy-hypothesis, deducing the actual though misjudged sensation from the nature of its stimulus. [p. 570] Furthermore, it draws a distinction between the true sensation and our judgment of it. Having abandoned this position, we can resolve the statement of Helmholtz into our own terms by saying that if the general level is produced by a colored light, then we see it as white. Helmholtz characteristically bases his theory upon a number of experiments similar to those from which we started. Let us return to our experiments and leave everything unaltered except that instead of a white light, we throw saturated yellow light upon the screen. If the intensities upon the screen and the wall are fairly equal the objectively yellow hole will appear to be distinctly bluish. The explanation follows from what has already been said. In a third experiment we use the same arrangement as in the first-white light on the screen, both wall and screen receiving approximately the same intensity-and we see a hole slightly lighter or darker than the screen and of a different color. Now slowly change the illumination of the screen, for instance by moving the objective lens of the projection lantern, and a distinct change will take place in the objectively unaltered hole, whereas a change upon the screen is hardly noticeable.
This experiment shows that the general level offers a greater resistance to changes in the objective conditions than does a single figure. The physiological explanation follows from the general physiological theory of the figure-ground structure. Since the density of energy is greater in the figure-field than in the ground-field, any change of the whole system will appear with greater strength in the figure than in the ground. This relative stability of a general level is probably the fundamental fact in all our so-called "color-transformations." Nor does this fact contradict the results of the experiments previously described, in which the figure, by virtue of its "thing-character" proved to be the more constant; for in these other experiments the general level of the whole experience was never involved.
In the realm of space the general level plays a role no less important. Witasek (66) has described the following method of testing the single "space-values" of the retina. One single point of fight in a totally dark room is presented in different positions with head and eyes fixated. Under these so far unrealized conditions Witasek expected to secure an exact determination of pure space (local) sensations. Try this experiment yourself and you will find it altogether impossible; for after you have stayed some time in total darkness, a single point of light has no definite position at all; if [p. 571] continually exposed, it wanders about, even when fixated, making so-called autokinetic movements.[16]
If the exposure is only momentary the point of light is neither clearly nor fixedly localized, and the crudest mistakes in localization occur. After watching these autokinetic movements for some time, the floor under your feet, the very chair you sit on, begin to lose their hold.
All this means that a definite single phenomenal position exists only within a fixed spatial level. If the conditions for the formation and conservation of such a level are absent, localization is no longer possible; for just as the level grows unstable so does the single point within it.
The spatial level has, however, a marked tendency to remain constant, together with the common directions of "above" and "below" "right" and "left." We shall see in the next article what a strong influence these common directions exercise upon the formation of structure. For our present purpose we need only point out that "above" is not necessarily something depicted upon the vertical meridian of the eye below the fovea, since this is true only when the eyes are in a special position with head erect and eyes looking straight forward. When writing at my desk, for instance, this same part of my retina gives the impression of that which is farther away. It comes, to be sure, from the upper part of my manuscript but this is not "above" me. As a rule, the general level remains unaltered, despite changes in phenomena produced by movements of the eye, the head, the whole body, or indeed movements of the surrounding objects. But let yourself be rapidly turned around several times, or let the surroundings be revolved about you, and everything is changed; all orientation is lost and giddiness results. The effect when your surroundings revolve is produced by visual influences alone, but when you are yourself moved, the vestibuler organs play a part. This, however, does not impair our theoretical position for it only goes to show that spatial level is dependent upon these sense-organs.
A third system upon which our spatial level depends is formed by the sense-organs of skin and muscles. In a very ingenious investigation, Garten (9) has tested our capacity to recognize the position of the body relative to the vertical. He constructed a special tilt-table which could be immersed in water so that the effect of gravity [p. 572] could be almost totally neutralized. Under these conditions, orientation was considerably disturbed, which again indicates the importance of the sensory systems named.
The term "spatial level" (Raumlage) in the specific meaning here employed was used for the first time by Wertheimer (64) who also maintained that the Aubert phenomenon (A-P) depends largely upon a shifting of this level. This phenomenon and a number of related facts have been extensively investigated of late by G. E. Müller (46), but before we turn to these facts we must introduce some of the concepts used by Müller to explain the A-P, which are also applicable in our determination of the spatial level. Müller, investigating the localization of visual images (45), found that an ego-centric localization can be referred to three different systems of coordinates: the visual (Blick) system (V. system), the head system (H. system), and the "standpoint" system (S. system). The V. system may be defined by the three main axes of Hering's imaginary "Cyclopean" eye; the H. system is represented by the head, one axis being the basal line, the other two lying in the median plane of the head at right angles to the first; while the S. system is determined by the normal position of the trunk. In normal positions of trunk, head and eye the three systems fall together, while in other positions they may differ so that each in turn may determine the localization. Müller inferred these systems primarily from results obtained with images, but he could show also that they play a part in perception and recognition, for instance in reading.[17]
In the following consideration of the A-P we shall refer merely to the V. and S. systems. An O., inclining his head, say 90º sidewards, in a totally dark room, is shown a single vertical line of light (Leuchtlinie). He sees this line not as vertical, but inclined in a manner contrary to the inclination of his head. The inclination, considerable though it may be, never, or at least very rarely, reaches the full degree of the head's inclination, even when we deduct the effect produced by the compensatory swivel-rotation of the eye. This is the gross phenomenon of the A-P; it can be described, using Müller's terminology, in the following manner: "The apparent position of the vertical line lies between the two positions which it would have if either the V. or the S. system were alone operative["]. The V. system would make the line vertical, since it is parallel to the basal line which now is vertical, while the S. system would make the line horizontal, [p. 573] since normally a line cast upon the horizontal meridian of the eyes, as this vertical line now is, would be a horizontal line. For simplicity's sake we shall neglect the swivel rotation. Müller explains the actual apparent position of the line as a compound effect resulting from the competition of the two systems, and speaks, therefore, of the V. and S. components. This explanation is corroborated by another form of the phenomenon appearing mainly with slighter inclinations of the head, which we shall here omit.
For MüIler these systems are a product of experience and they work according to the general law of association. They can also be expressed by ascribing to each retinal point, not one, but two, values, a V. and an S. value. We see that Müller does not use our concept of a spatial level but operates with single elementary effects, the V. and S. components (resulting from corresponding space-values) which enter into an additive combination. But, like all theories of this sort, it must be supplemented, as we shall presently see, by the employment of such concepts as "apprehension" and "judgment." We may proceed by reporting from Müller's monograph (46) which contains an excellent summary of the existing literature, as well as a number of further facts.
1. We saw above that in eye- or head-movements our general level is not changed. Consequently objects do not seem to move when we move our head or eyes. But this holds only at a fixed level where the visual field contains points of "anchorage" (Verankerungspunkte). In the dark, where such points are missing, a single vertical line of light may appear to be moving about a vertical axis in a direction contrary to that of the head's movement. This shows that the effect of head-movements on visual objects is a function of the fixity of the spatial level, since, as we have already seen, in total darkness this level loses its stability.
2. If we observe the line of light with head inclined in a lighted room it appears to be vertical when the light is turned out, and with many O.'s it maintains at first its initial vertical position, and then passes gradually into its final oblique position. Müller considers this to be the effect of a general spatial perseverative tendency. (Beharrungstendenz). But what is it that perseveres; is it the line itself, or is it primarily the initial space-level of the lighted room? All the facts here adduced speak for the latter interpretation and against the former which Müller accepts. (See particularly No. 5, below.) [p. 574]
3. Some authors maintain that the A-P also appears, though in a lesser degree, when the head remains erect, if the rest of the body is turned about its sagittal axis. This result, which has not been confirmed by all investigators, seems largely dependent upon individual differences and upon the method by which the position of the body is maintained. Yet, like Müller, we have no reason to doubt that it may occur. Müller explains it by the associative law of substitution. I cannot here set forth the reasons why I am unable to accept this explanation, but the reader will understand that the Gestalt theory is fundamentally incompatible with the associationist's principles. According to our conception the fact under discussion signifies that the spatial level may be altered by unnatural positions of the body, even if the head remains in a normal position, and that this change of level is similar to that of the A-P proper. The individual differences, and the differences between the results of different authors, can then be also understood; for the stability of the spatial level is very different with different individuals, as has been clearly shown by Wertheimer (64), and different experimental conditions will therefore not be of equal effectiveness in producing a uniform change of level.
4. Many O.'s report that they feel very uncertain in judging the position of the line, since they have lost their standard of the vertical, and the same O.'s show great variability in their final judgments. Apparently Müller considers this only as a matter of judgment, but again we cannot accept the distinction he draws between the phenomenon itself and the judgment of it, in which marked properties of the judgment are not considered to be founded in the phenomenon. We must ask, instead, what are the properties of the phenomenon, and what are the causes of these properties which lead to an uncertain and variable judgment? Our answer is that such judgments are based upon uncertain and variable phenomena. "Uncertainty" or "undeterminateness" may quite well be a property of visual phenomena, as Katz (24) maintained for the "distance" of his so-called " film" colors (Flächenfarben), and both the undeterminateness and the variability of this phenomenon are readily explained from our point of view. We have recognized the paramount importance of visual points of anchorage for the spatial level. When these are lacking, the level loses its hold, since the position of any single object depends upon the general level, and if the O. no longer has this, the position itself is no longer fixed or unambiguously determined. [p. 575] Instead of employing this descriptively and functionally well-defined concept of the spatial level, Müller and his followers distinguish between the phenomenon and the means whereby we orientate ourselves to it, the absence of points of anchorage being for P. Busse (3, p. 19), the absence of objects that can give us information about the inclination of the observed line.
5. During a longer observation, the line of light does not, as a rule, maintain its position. In the majority of cases its angle of inclination is increased toward the vertical. Müller suggests several explanations for this, maintaining that the V. component loses in weight, since with the passing of time the impression of the head's inclination loses in intensity; since, however, this explanation is insufficient, he also suggests a tendency to decrease the influence of the V. component which is purely visual in origin. Yet the observed fact fits very readily into our explanations; for the longer the points of anchorage are lacking, the more the spatial level will change, and in consequence of its great instability the more it will deviate from its normal standard. The rarer cases in which a change takes place in the opposite direction simply prove again the general condition of instability; for they can be fully explained only when we know in detail all the factors upon which the level depends. In this connection we are reminded of the vestibular and the skin-muscle systems, to both of which we would ascribe a direct influence upon the level, and not merely an indirect influence upon our judgment concerning the head's inclination, as Müller states the case with reference to the vestibuler organ.
6. Some O.'s, particularly those who report uncertainty of judgment, show a tendency to persist in judging the line vertical. Again Müller explains this as a tendency to judge "without sufficient foundation," but he also admits the possibility of an illusory perception, caused by the O.'s imagination, which, in some persons, exercises a strong influence over the apparent position of the line. He calls this the "vertical tendency." From this vertical tendency he distinguishes such cases as those in which the O., with head but slightly inclined, judges the line of light, when momentarily exposed, as either uncertain or vertical. In cases of this sort he says that the line was not apprehended long enough. Apart from this method of interpretation the facts are as follows: When the level is unstable -- and the influence of the imagination is nothing but an expression of its instability -- the O. can see the line at will in different positions; [p. 576] the more prominent character of the vertical direction can then be influential. A momentary exposure is a favorable condition for instability because the peripheral (stimulus-determined) conditions of the whole (physiological) process, comprising the entire optical system, are weakened. Such a weakening always increases the effectiveness of purely structural factors. This has appeared clearly in Lindemann's investigations (28).
The conclusion we draw from these facts is not that Müller's theory is altogether wrong; for when we discard from it the concepts of apprehension, judgment, imagination and association [18], the competition among the two or three components remains. Only we would refer their effect to the general spatial level and not directly to the line. The components, therefore, find a place in our system as functional but not as descriptive facts.
Experimentally, we can destroy a fixed spatial level; we can also make one level give place to another as has been shown by an experiment of Wertheimer's (64). Put a mirror in an inclined position upon a table. That part of the room seen in the mirror will then look abnormal. Objectively vertical lines will be inclined, and if a person visible in the mirror drops an object, it does not appear to fall vertically. Now hold a tube to your eyes excluding the whole "real" room from your vision and continue looking into tile mirror. Let other persons walk about and do things in the visible section of the room. Very soon everything will be all right again; the floor will assume its horizontal position, the chairs will stand vertically upon it and objects will no longer be seen in an angle smaller than 90º. You can measure the change by executing an apparently vertical line at the beginning and at the end of the experiment, and then determine the angle between these two.
In the three systems, the V., the H., and the S. system, we have found factors which enter into the constitution of our spatial level, and this last experiment has shown that the visible world itself is a concurrent factor. This is a fact of very general significance. Standing in a room of average size the direction "straight ahead" is not under all conditions the sagittal axis of my Cyclopean eye; for the most part it is the direction toward the wall, with which [p. 577] the plane of my face forms the smallest angle. It is I who am turned out of the main direction when I gaze obliquely towards the wall. This influence of the objective room-structure upon the space level is very different with different individuals. Yet the normal effect for the majority of persons can be shown by the following experiment. Since the discovery of v. Hornbostel and Wertheimer (20), we know that the apparent direction of a noise depends upon the time-difference with which the sound-waves strike the two ears. By inserting pipes of variable length, like trombone pipes, between the source of the sound and each ear one can, by drawing out or pushing in these pipes, readily make the noise wander from one side of the head to the other. One can also try to bring it to the middle or straight ahead. After some practice, this can be done with great precision and subjective certainty if one sits in a "good" position, i.e., if one of the walls of the room serves as a frontal-parallel orientation. But if a wall is lacking, or if one sits Somewhat obliquely, the same task becomes very trying. When I had acquired an enormous practice after several thousand experiments, working with closed eyes, I was still unable to find a good middle position for the sound under these conditions. The auditory middle, the phenomenon provoked by the time-difference zero, coincides with the sagittal axis of the Cyclopean eye, but in an oblique position this was not "straight ahead" for me, since the walls of the room influenced my spatial level, and consequently the auditory cue failed. Referring back to the beginning of this paragraph, I may add that these experiments also indicate descriptively the existence of an auditory space-level; for when the noise of a metronome stroke occurs, it enters into a thus far empty, yet phenomenally existing, auditory space. We find, too, that the stability of objects within a given level depends upon the quality of the object. Thus Busse (3) found that fine black vertical threads were much less stable than thicker brown ones which carried red and black wooden beads at fixed distances.
The conclusion is that normally we possess a general spatial level within which we are anchored. When we lose this anchorage, we are practically lost. Yet even this effect of optical vertigo has been explained by experience! When a room is rotated around us, "experience" should tell us that the room is fixed and that consequently we are ourselves rotating. Think of the man who daily, operates such a machine of deception and who knows by experience [p. 578] of long standing that the room does move; will this man entering the rotating chamber with his knowledge grow giddy, or will he not? Practice in the room may no doubt modify the effect, just as does practice on the merry-go-round. But it is only by practice and not by knowledge or experience that the individual can succeed in maintaining a fixed level of any sort under these trying circumstances. With the aid of our level-concept we can also understand the so-called (physiological) relativity of movement. A person looking from a bridge into a rapid stream soon has the impression of being himself moved. Seated in a train which is standing in a station, we are often unable to decide whether it is our train or the one on a neighboring track which is beginning to move. One explanation of the former effect actually maintains that the movement of a small piece of the bridge which belongs to one's field of vision is more probable than the movement of so large a surface as that of the stream! But we should say that normally there is no choice as to where we shall place our anchorage; for, in most cases, even with the strongest impulse of our will, we cannot alter this anchorage (see Wertheimer 64). Normally it is something quite independent of our will-a compulsory perception founded in properties of the objective field which determine for us what parts are to appear as figures and what parts as ground; as v. Hornbostel (19) puts it: things are not holes in the world of experience. On the other hand ambiguous situations occur in which two or more anchorages are equally possible, though here, too, law reigns and not chance. The chief rule for these ambiguous cases is this: that the objects which form the (dynamic) center of our visual world are at the same time our points of anchorage. When I am playing cards in my compartment I see the train move on the next track even if it is in reality my own train which is moving, but when I am looking at the other train, searching perhaps for an acquaintance in the coach, then it is my own train which seems to be moving. Psychologically, i.e., phenomenologically, there is no relativity of movement.19
But our level-concept has still a wider application. We have already referred to certain instances, such as fashion and style. Experimental psychology has also studied certain facts about the phenomenon of the level without recognizing them to be such. What I mean is best explained by referring to some of Hollingworth's experiments upon the indifference-point (I.P.) (17, 18). Many [p. 579] investigators, testing a scale of magnitudes, have found the existence of an I.P.; that is, while most members of the scale were estimated with a constant error, positive or negative, small magnitudes being overestimated and large ones underestimated, there comes a point where no constant error occurs. Though the fact has been confirmed over and over again, and in very different fields, yet, strangely enough, there has been a wide divergence of opinion as to the absolute position of the I.P. This startling fact suggested to Hollingworth the idea that there must be a mistake somewhere in the way the question is put. Is there, he asks, an absolute I.P. independent of the position and extent of the test-series, or is this I.P. a function of the total scale? He was able to demonstrate by a number of ingenious experiments that the latter is the correct assumption. Working with the reproduction of hand-and-arm movements, he arranged three series of experiments; A, including magnitudes of 10 to 70 mm. (with increments of 10 mm.); B, magnitudes of 30 to 150 mm. (with increments of 20 mm.); and C, magnitudes Of 70 to 250 mm. (with increments of 30 mm.); each scale consisting of seven different magnitudes. Upon a given day only one of these series was used. The I.P. of series A fell at about 40 mm., of B, at about 75 mm., and of C at about 125 mm., that is, it was always found to be approximately at the center of the scale. Smaller magnitudes were overestimated and larger ones underestimated. There was no absolute I.P. The magnitude of 70 mm., being the upper limit of A, and the lower limit of C, and near the middle of B, was underestimated in A (minus 10.2), overestimated in C (plus 16.5), and reproduced fairly accurately in B (plus 1.7; p.e. 10.3). To check this result, four months later, the three magnitudes of 10 mm. (always overestimated), 250 mm- (always underestimated) and 70 mm. (variable with the series) were tested singly on occasions several days apart, but for none of these three did a constant error occur.
In still another very clear experiment, the shifting of the I.P. itself was demonstrated. A set of standard magnitudes was prepared ranging from 10 mm. to 60 mm. (by increments of 10), from 60 mm. to 150 mm. (by increments of 15) and on to 250 mm. (by increments of 20). The standards of the 10 mm.-60 mm. were now given and reproduced in chance order, five trials being given for each magnitude. Then, without the knowledge of the O., the next magnitude was added and again five trials made of each standard. [p. 580] This was continued until the whole series of seventeen standards had been offered. The success of this experiment was remarkable. The I.P. rose with the introduction of each new standard magnitude; constant errors which were positive from the beginning, increased throughout the series, while constant errors which were negative in the beginning likewise underwent a continual change, decreasing to the zero point and emerging again as positive increments.
Hollingworth concludes, "that the phenomenon of the indifference point . . . is of purely central origin" (17, p. 21), and this theory is as close to the one we propose as the general theoretical position of psychology at the time of his investigation would admit. According to his results, the I.P.-phenomenon belongs not to memory but to perception, and as an analogy he refers to type-concepts, such as race and class (18, p. 468). He also speaks of a "mental set", meaning by this "that we are adjusted for or tend to expect the average magnitude, and to assimilate all other magnitudes toward it, to accept them in place of it" (17, p. 39). But he insists on employing the term "judgment"; the error to which this tendency leads, he says, "is distinctly an error of judgment, and is quite independent of sensory or physiological conditions" (18, p. 469).
Again the distinction drawn between sensory components and judgments of peripheral and central factors vitiates his theory. Leaving these out of account, and referring the reader to the next article for a discussion of the difference between peripheral and central factors, which takes on a very different aspect in our theory, we may here draw the following conclusions. In reacting to a definite scale of stimuli we establish a general level which, in the case described, as in many others, is both motor and sensory. The effect of each single stimulus is dependent upon this level, much as the figure is dependent upon its ground. And secondly, the general level holds together the whole group of phenomena corresponding to the scale of stimuli. Although they may rise or fall from this level, the phenomena never lose their existential connection with it, and being attracted by the level, the result is often a wrong judgment or a false reproduction. This attracting or assimilating effect of the level is a special case of our general law of leveling (discussed above). We see further that this level adapts itself automatically to the scale and this process of adaptation must therefore be explicable in terms of our general physiological theory. [p. 581]
Hollingworth rightly gives a wide application to his results, comparing the I.P.'s of different investigators with the range of their scales, and he has himself confirmed his "law of central tendency" in a purely sensory field by experiments upon the size of gray squares. I may also add in this connection that what G. E. Müller calls the "absolute impression" (Absoluter Eindruck) is just such a rise or fall from a general level. Whenever an O. makes a judgment that is not based upon a comparison between two stimuli, he is reacting not to a stepwise phenomenon, but to an emergence from the general level. With this I must bring my first article to a close.