Sixteen Experimental Investigations from the Harvard Psychological Laboratory by Hugo Münsterberg (100 books to read .txt) 📕

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 space,

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 space than the disturbing factors in our estimations of

tactual space 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 space. The investigation has unavoidably extended into a

number of nearlying problems in the psychology of touch, but the

final object of my paper will be to offer a more decisive answer than

has hitherto 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 classes 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 space, 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 space, we should expect that like peripheral

disturbances in the two senses would cause like central errors in

judgment, and every tactual analogue of an optical illusion should be

found to correspond both in the direction of the error and, to a

certain extent, quantitatively 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 passing 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 ‘unanalyzable 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 coöperated 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 space. 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 space, ‘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

principally on the results of Parrish’s experiments that several

authors of text-books in psychology have based their assertions that a

filled space is underestimated by the skin. The opposite conclusion,

namely, that the illusion is not reversed for the skin, has been

maintained by Thiéry,[3] and Dresslar.[4] Thiéry 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 analogue of the optical illusion for filled space. 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.

[3] Thiéry, A.: Philos. Studien, 1896, Bd. XII., S. 121.

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

At the beginning of the present investigation, the preponderance of

testimony was found to be in favor of the view that filled space 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 space. 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 assertion, 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 spaces, 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 pass 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 assumed or even openly asserted 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 analogue 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 space 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 spaces 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 spaces 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 spaces were not

given simultaneously and adjacent. Münsterberg[5] says of the optical

illusion for the open and filled spaces, “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 constituted of two halves vanishes,

the illusion also disappears.” This is an important consideration, to

which I shall return again.

[5] Münsterberg, H.: ‘Beiträge 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.

[Illustration: FIG. 1.]

Two æsthesiometers, A and B, were arranged in a framework, so that

uniform stimulations could be given on both arms. The æsthesiometers

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 æsthesiometer could be conveniently adjusted

by the pins F and H. The shape of the cams was such that the descent

of the æsthesiometer 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