Harvard Psychological Studies Part 4

I.

Many profound researches have been published upon the subject of optical illusions, but in the field of tactual illusions no equally extensive and serious work has been accomplished. The reason for this apparent neglect of the illusions of touch is obviously the fact that the studies in the optical illusions are generally thought to yield more important results for psychology than corresponding studies in the field of touch. Then, too, the optical studies are more attractive by reason of the comparative ease and certainty with which the statistics are gathered there. An optical illusion is discovered in a single instance of the phenomenon. We are aware of the illusion almost immediately. But in the case of most of the illusions of touch, a large number of experiments is often necessary in order to reveal any approximately constant error in the judgments. Nevertheless, it seems to me that the factors that influence our judgments of visual s.p.a.ce, though their effects are nearly always immediately apparent, are of no more vital significance for the final explanation of the origin of our notion of s.p.a.ce than the disturbing factors in our estimations of tactual s.p.a.ce whose effects are not so open to direct observation.

The present investigation has for its main object a critical examination of the tactual illusions that correspond to some of the well-known optical illusions, in the hope of segregating some of the various disturbing factors that enter into our very complex judgments of tactual s.p.a.ce. The investigation has unavoidably extended into a number of near-lying problems in the psychology of touch, but the final object of my paper will be to offer a more decisive answer than has. .h.i.therto been given to the question, _Are the optical illusions also tactual illusions, or are they reversed for touch?_

Those who have given their attention to illusions of sight and touch are rather unequally divided in their views as to whether the geometrical optical illusions undergo a reversal in the field of touch, the majority inclining to the belief that they are reversed.

And yet there are not wanting warm adherents of the opposite view. A comparison of the two cla.s.ses of illusions, with this question in view, appears therefore in the present state of divergent opinion to be a needed contribution to experimental psychology. Such an experimental study, if it succeeds in finding the solution to this debate, ought to throw some further light upon the question of the origin of our idea of s.p.a.ce, as well as upon the subject of illusions of sense in general. For, on the one hand, if touch and sight function alike in our judgment of s.p.a.ce, we should expect that like peripheral disturbances in the two senses would cause like central errors in judgment, and every tactual a.n.a.logue of an optical illusion should be found to correspond both in the direction of the error and, to a certain extent, quant.i.tatively with the optical illusion. But if, on the other hand, they are in their origin and in their developed state really disparate senses, each guided by a different psychological principle, the illusion in the one sense might well be the reverse of the corresponding illusion in the other sense. Therefore, if the results of an empirical study should furnish evidence that the illusions are reversed in pa.s.sing from one field to the other, we should be obliged to conclude that we are here in the presence of what psychologists have been content to call the 'una.n.a.lyzable fact' that the two senses function differently under the same objective conditions. But if, on the contrary, it should turn out that the illusions are not reversed for the two senses, then the theory of the ultimate uniformity of the psychical laws will have received an important defence.

These experiments were carried on in the Harvard Psychological Laboratory during the greater part of the years 1898-1901. In all, fifteen subjects cooperated in the work at different times.

The experimental work in the direction of a comparison of the optical illusions with the tactual illusions, to the time of the present investigation, has been carried on chiefly with the familiar optical illusion of the overestimation of filled s.p.a.ce. If the distance between two points be divided into two equal parts by a point midway between them, and the one of the halves be filled with intermediate points, the filled half will, to the eye, appear longer than the open half. James[1] says that one may easily prove that with the skin we underestimate a filled s.p.a.ce, 'by taking a visiting card, and cutting one edge of it into a saw-toothed pattern, and from the opposite edge cutting out all but two corners, and then comparing the feelings aroused by the two edges when held against the skin.' He then remarks, 'the skin seems to obey a different law here from the eye.' This experiment has often been repeated and verified. The most extensive work on the problem, however, is that by Parrish.[2] It is doubtless princ.i.p.ally on the results of Parrish's experiments that several authors of text-books in psychology have based their a.s.sertions that a filled s.p.a.ce is underestimated by the skin. The opposite conclusion, namely, that the illusion is not reversed for the skin, has been maintained by Thiery,[3] and Dresslar.[4] Thiery does not, so far as I know, state the statistics on which he bases his view. Dresslar's experiments, as Parrish has correctly observed, do not deal with the proper a.n.a.logue of the optical illusion for filled s.p.a.ce. The work of Dresslar will be criticised in detail when we come to the illusions for active touch.

[1] James, William: 'Principles of Psychology,' New York, 1893, Vol. II., p. 141.

[2] Parrish, C.S.: _Amer. Journ. of Psy._, 1895, Vol. VI., p.

514.

[3] Thiery, A.: _Philos. Studien_, 1896, Bd. XII., S. 121.

[4] Dresslar, F.B.: _Amer. Journ. of Psy._, 1894, Vol. VI., p.

332.

At the beginning of the present investigation, the preponderance of testimony was found to be in favor of the view that filled s.p.a.ce is underestimated by the skin; and this view is invariably accompanied by the conclusion, which seems quite properly to follow from it, that the skin and the eye do not function alike in our perception of s.p.a.ce. I began my work, however, in the belief that there was lurking somewhere in the earlier experiments a radical error or oversight. I may say here, parenthetically, that I see no reason why experimental psychologists should so often be reluctant to admit that they begin certain investigations with preconceptions in favor of the theory which they ultimately defend by the results of their experiments. The conclusions of a critical research are in no wise vitiated because those conclusions were the working hypotheses with which the investigator entered upon his inquiry. I say frankly, therefore, that although my experiments developed many surprises as they advanced, I began them in the belief that the optical illusions are not reversed for touch. The uniformity of the law of sense perception is prejudiced if two senses, when affected by the same objective conditions, should report to consciousness diametrically opposite interpretations of these same objective facts. I may say at once, in advance of the evidence upon which I base the a.s.sertion, that the belief with which I began the experiments has been crystallized into a firm conviction, namely, that neither the illusion for open or filled s.p.a.ces, nor any other optical illusion, is genuinely reversed for touch.

II.

I began my work on the problem in question by attempting to verify with similar apparatus the results of some of the previous investigations, in the hope of discovering just where the suspected error lay. It is unnecessary for me to give in detail the results of these preliminary series, which were quite in agreement with the general results of Parrish's experiments. Distances of six centimeters filled with points varying in number and position were, on the whole, underestimated in comparison with equal distances without intermediate point stimulations. So, too, the card with saw-toothed notches was judged shorter than the card of equal length with all but the end points cut out.

After this preliminary verification of the previous results, I was convinced that to pa.s.s from these comparatively meager statistics, gathered under limited conditions in a very special case, to the general statement that the optical illusion is reversed in the field of touch, is an altogether unwarranted procedure. When one reads the summarized conclusions of these previous investigators, one finds it there a.s.sumed or even openly a.s.serted that the objective conditions of the tactual illusion are precisely the same as those of the optical illusion. But I contend that it is not the real a.n.a.logue of the optical illusion with which these experiments have been concerned.

The objective conditions are not the same in both. Although something that is very much like the optical illusion is reversed, yet I shall attempt to prove in this part of my paper, first, that the former experiments have not been made with the real counterpart of the optical illusion; second, that the optical illusion can be quite exactly reproduced on the skin; third, that where the objective conditions are the same, the filled cutaneous s.p.a.ce is overestimated, and the illusion thus exists in the same sense for both sight and touch.

Let me first call attention to some obvious criticisms on Parrish's experiments. They were all made with one distance, namely, 6.4 centimeters; and on only one region, the forearm. Furthermore, in these experiments no attempt was made to control the factor of pressure by any mechanical device. The experimenter relied entirely on the facility acquired by practice to give a uniform pressure to the stimuli. The number of judgments is also relatively small. Again, the open and filled s.p.a.ces were always given successively. This, of course, involves the comparison of a present impression with the memory of a somewhat remote past impression, which difficulty can not be completely obviated by simply reversing the order of presentation.

In the optical illusion, the two s.p.a.ces are presented simultaneously, and they lie adjacent to each other. It is still a debated question whether this illusion would exist at all if the two s.p.a.ces were not given simultaneously and adjacent. Munsterberg[5] says of the optical illusion for the open and filled s.p.a.ces, "I have the decided impression that the illusion does not arise from the fact of our comparing one half with the other, but from the fact that we grasp the line as a whole. As soon as an interval is inserted, so that the perception of the whole line as const.i.tuted of two halves vanishes, the illusion also disappears." This is an important consideration, to which I shall return again.

[5] Munsterberg, H.: 'Beitrage zur Exper. Psy.,' Freiburg i.B., 1889, Heft II., S. 171.

Now, in my experiments, I endeavored to guard against all of these objections. In the first place, I made a far greater number of tests.

Then my apparatus enabled me, firstly, to use a very wide range of distances. Where the points are set in a solid block, the experiments with long distances are practically impossible. Secondly, the apparatus enabled me to control accurately the pressure of each point.

Thirdly, the contacts could be made simultaneously or successively with much precision. This apparatus (Fig. 1) was planned and made in the Harvard Laboratory, and was employed not only in our study of this particular illusion, but also for the investigation of a number of allied problems.

[Ill.u.s.tration: FIG. 1.]

Two aesthesiometers, A and B, were arranged in a framework, so that uniform stimulations could be given on both arms. The aesthesiometers were raised or lowered by means of the crank, C, and the cams, D and E. The contacts were made either simultaneously or successively, with any interval between them according to the position of the cams on the crank. The height of the aesthesiometer could be conveniently adjusted by the pins F and H. The shape of the cams was such that the descent of the aesthesiometer was as uniform as the ascent, so that the contacts were not made by a drop motion unless that was desired. The sliding rules, of which there were several forms and lengths, could be easily detached from the upright rods at _K_ and _L_. Each of the points by which the contacts were made moved easily along the sliding rule, and could be also raised or lowered for accommodation to the unevenness of the surface of the skin. These latter were the most valuable two features of the apparatus. There were two sets of points, one of hard rubber, the other of metal. This enabled me to take into account, to a certain extent, the factor of temperature. A wide range of apparent differences in temperature was secured by employing these two stimuli of such widely different conductivity. Then, as each point was independent of the rest in its movements, its weight could also be changed without affecting the rest.

In the first series of experiments I endeavored to reproduce for touch the optical illusion in its exact form. There the open and the filled s.p.a.ces are adjacent to each other, and are presented simultaneously for pa.s.sive functioning of the eye, which is what concerns us here in our search for the a.n.a.logue of pa.s.sive touch. This was by no means an easy task, for obviously the open and the filled s.p.a.ces in this position on the skin could not be compared directly, owing to the lack of uniformity in the sensibility of different portions of the skin. At first, equivalents had to be established between two collinear open s.p.a.ces for the particular region of the skin tested. Three points were taken in a line, and one of the end points was moved until the two adjacent open s.p.a.ces were p.r.o.nounced equal. Then one of the s.p.a.ces was filled, and the process of finding another open s.p.a.ce equivalent to this filled s.p.a.ce was repeated as before. This finding of two equivalent open s.p.a.ces was repeated at frequent intervals. It was found unsafe to determine an equivalent at the beginning of each sitting to be used throughout the hour.

Two sets of experiments were made with the illusion in this form. In one the contacts were made simultaneously; the results of this series are given in Table I. In the second set of experiments the central point which divided the open from the filled s.p.a.ce touched the skin first, and then the others in various orders. The object of this was to prevent fusion of the points, and, therefore, to enable the subject to p.r.o.nounce his judgments more rapidly and confidently. A record of these judgments is given in Table II. In both of these series the filled s.p.a.ce was always taken near the wrist and the open s.p.a.ce in a straight line toward the elbow, on the volar side of the arm. At present, I shall not undertake to give a complete interpretation of the results of these two tables, but simply call attention to two manifest tendencies in the figures. First, it will be seen that the short filled distance of four centimeters is underestimated, but that the long filled distance is overestimated. Second, in Table II., which represents the judgments when the contacts were made successively, the tendency to underestimate the short distance is less, and at the same time we notice a more p.r.o.nounced overestimation of the longer filled distances. I shall give a further explanation of these results in connection with later tables.

TABLE I.

4 cm. 6 cm. 8 cm.

Filled. Open. Filled. Open. Filled. Open.

F. 5.3 4.7 7.8 7.6 9.3 10.5 F. 5.7 4.4 6.5 7.3 9.2 11.7 F. 6.0 5.6 8.2 7.3 8.7 10.8 --- --- --- --- --- ---- Av. 5.7 4.9 7.5 7.4 9.1 11.0

R. 5.7 5.1 6.7 6.8 9.3 10.2 R. 5.4 5.4 7.2 7.1 8.5 10.7 R. 4.6 4.2 8.1 8.1 9.1 11.4 --- --- --- --- --- ---- Av. 5.2 4.9 7.3 7.3 9.0 10.8

K. 5.6 5.1 6.8 6.7 8.1 9.6 K. 5.0 5.1 7.3 7.5 8.2 11.2 K. 4.9 4.9 8.2 8.1 10.1 10.1 --- --- --- --- ---- ---- Av. 5.2 5.0 7.4 7.4 8.8 10.3

TABLE II.

4 cm. 6 cm. 8 cm.

Filled. Open. Filled. Open. Filled. Open.

F. 5.1 5.0 8.0 8.3 9.2 10.3 F. 5.8 4.7 7.2 7.9 8.7 10.9 F. 5.6 5.5 6.9 9.1 9.1 11.1 --- --- --- --- --- ---- Av. 5.5 5.1 7.4 8.4 9.0 10.8

R. 6.0 4.8 8.2 7.5 9.4 10.6 R. 5.7 5.4 6.5 7.4 10.1 9.4 R. 5.0 5.2 7.7 7.8 8.6 11.2 --- --- --- --- ---- ---- Av. 5.6 5.1 7.5 7.6 9.4 10.4

K. 4.8 4.8 8.2 8.3 8.1 9.8 K. 5.1 5.3 7.1 7.7 10.0 10.8 K. 4.7 5.0 8.1 8.6 8.6 9.4 --- --- --- --- ---- ---- Av. 4.9 5.0 7.8 8.2 8.9 10.0

The first two numbers in the first line signify that when an open distance of 4 cm. was taken, an adjacent open distance of 4.7 cm. was judged equal; but when the adjacent s.p.a.ce was filled, 5.3 cm. was judged equal. Each number in the column of filled distances represents an average of five judgments. All of the contacts in Table I. were made simultaneously; in Table II. they were made successively.

In the next series of experiments the illusion was approached from an entirely different point of view. The two points representing the open s.p.a.ce were given on one arm, and the filled s.p.a.ce on a symmetrical part of the other arm. I was now able to use a much wider range of distances, and made many variations in the weights of the points and the number that were taken for the filled distance.

However, before I began this second series, in which one of the chief variations was to be in the weights of the different points, I made a brief preliminary series of experiments to determine in a general way the influence of pressure on judgments of point distances. Only three distances were employed, four, six and twelve centimeters, and three weights, twelve, twenty and forty grams. Table III. shows that, for three men who were to serve as subjects in the main experiments that are to follow, an increase in the weight of the points was almost always accompanied by an increase in the apparent distance.

TABLE III.

Distances. 4 cm. 6 cm. 12 cm.

Weights (Grams). 12 20 40 12 20 40 12 20 40

R. 3.9 3.2 3.0 6.2 5.6 5.3 11.4 10.4 9.3 F. 4.3 4.0 3.6 6.1 5.3 5.5 12.3 11.6 10.8 B. 4.1 3.6 3.1 6.0 5.7 5.8 12.0 10.2 9.4 P. 4.3 4.1 3.7 5.9 5.6 5.6 13.1 11.9 10.7

In the standard distances the points were each weighted to 6 grams. The first three figures signify that a two-point distance of 4 cm., each point weighing 6 grams, was judged equal to 3.9 cm. when each point weighed 12 grams. 3.2 cm.

when each point weighed 20 grams, etc. Each figure is the average of five judgments.

Now the application of this principle in my criticism of Parrish's experiments, and as antic.i.p.ating the direction which the following experiments will take, is this: if we take a block such as Parrish used, with only two points in it, and weight it with forty grams in applying it to the skin, it is plain that each point will receive one half of the whole pressure, or twenty grams. But if we put a pressure of forty grams upon a block of eight points, each point will receive only one eighth of the forty, or five grams. Thus, in the case of the filled s.p.a.ce, the end points, which play the most important part in the judgment of the distance, have each only five grams' pressure, while the points in the open s.p.a.ce have each twenty grams. We should, therefore, naturally expect that the open s.p.a.ce would be overestimated, because of the decided increase of pressure at these significant points. Parrish should have subjected the blocks, not to the same pressure, but to a pressure proportional to the number of points in each block. With my apparatus, I was easily able to prove the correctness of my position here. It will be seen in Tables IV. to VIII. that, when the sum of the weights of the two end points in the open s.p.a.ce was only just equal to the sum of the weights of all the points in the filled s.p.a.ce, the filled s.p.a.ce was underestimated just as Parrish has reported. But when the points were all of the same weight, both in the filled and the open s.p.a.ce, the filled s.p.a.ce was judged longer in all but the very short distances. For this latter exception I shall offer an explanation presently.