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

TABLE XII.

A B C D E

4 7.1 2.6 2.4 6.5

5 8.3 3.1 3.3 8.7

6 8.2 3.3 4.1 9.2

7 9.7 3.6 3.7 10.1

8 10.5 3.7 4.5 10.6

9 12.4 4.8 5.1 11.5

10 13.1 4.7 5.3 13.2

11 13.3 5.3 6.1 14.6

12 13.7 6.9 7.2 12.7

13 14.6 7.5 8.1 13.2

14 15.3 8.2 9.4 15.6

15 15.7 8.7 10.3 14.9

Column A contains the filled spaces, columns B, C, D,

E the open spaces that were judged equal. In B the block

was moved with the finger, and in C against the finger as it

traversed the filled space, and in D and E the block was

moved with and against the finger respectively as it passed

over the open space. The block was always moved approximately

one-half the distance of the filled space.

I have given some of the results for one subject in Table XII. These

results show at a glance how potent a factor the time element is. The

quantity of tactual sensations received by the finger-tip enters into

the judgment of space to no appreciable extent. With one subject,

after he had passed his finger over a filled space of 10 cm. the block

was moved so as almost to keep pace with the finger as it passed over

the open space. In this way the subject was forced to judge a filled

space of 10 cm. equal to only 2 cm. of the open space. And when the

block was moved in the opposite direction he was made to judge a

distance of 10 cm. equal to an open distance of 16 cm.

The criticism may be made on these experiments that the subject has

not in reality been obliged to rely entirely upon the time sense, but

that he has equated the two spaces as the basis of equivalent muscle

or joint sensation, which might be considered independent of the

sensations which yield the notion of time. I made some experiments,

however, to prove that this criticism would not be well founded. By

arranging the apparatus so that the finger-tip could be held

stationary, and the block with the open and filled spaces moved back

and forth under it, the measurement by joint and muscle sensations was

eliminated.

It will be observed that no uniform motion could be secured by simply

manipulating the lever with the hand. But uniformity of motion was not

necessary for the results at which I aimed here. Dresslar has laid

great stress on the desirability of having uniform motion in his

similar experiments. But this, it seems to me, is precisely what is

not wanted. With my apparatus, I was able to give widely different

rates of speed to the block as it passed under the finger-tip. By

giving a slow rate for the filled space and a much more rapid rate for

the open space, I found again that the subject relied hardly at all on

the touch sensations that came from the finger-tip, but almost

entirely on the consciousness of the amount of time consumed in

passing over the spaces. The judgments were made as in the previous

experiments with this apparatus. When the subject reached the point in

the open space which he judged equal to the filled space, he slightly

depressed his finger and stopped the moving block. In this way, the

subject was deprived of any assistance from arm-movements in his

judgments, and was obliged to rely on the tactual impressions received

at the finger-tip, or on his time sense. That these tactual sensations

played here also a very minor part in the judgment of the distance was

shown by the fact that these sensations could be doubled or trebled by

doubling or trebling the amount of space traversed, without

perceptibly changing the judgment, provided the rate of speed was

increased proportionately. Spaces that required the same amount of

time in traversing were judged equal.

In all these experiments the filled space was presented first. When

the open space was presented first, the results for four out of five

subjects were just reversed. For short distances the filled space was

underestimated, for long distances the filled space was overestimated.

A very plausible explanation for these anomalous results is again to

be found in the influence of the time factor. The open space seemed

longer while it was being traversed, but rapidly foreshortened after

it was left for the filled space. While on the other hand, if the

judgment was pronounced while the subject was still in the midst of

the filled space, it seemed shorter than it really was. The

combination of these two illusions is plainly again responsible for

the underestimation of the short filled spaces. The same double

illusion may be taken to explain the opposite tendency for the longer

distances.

IX.

The one generalization that I have thus far drawn from the

investigation—namely, that the optical illusions are not reversed in

passing from the field of touch, and that we therefore have a safe

warrant for the conclusion that sight and touch do function alike—has

contained no implicit or expressed assertion as to the origin of our

notion of space. I have now reached the point where I must venture an

explanation of the illusion itself.

The favorite hypothesis for the explanation of the geometrical optical

illusions is the movement theory. The most generally accepted

explanation of the illusion with whose tactual counterpart this paper

is concerned, is that given by Wundt.[15] Wundt’s explanation rests on

variation in eye movements. When the eye passes over broken

distances, the movement is made more difficult by reason of the

frequent stoppages. The fact that the space which is filled with only

one point in the middle is underestimated, is explained by Wundt on

the theory that the eye has here the tendency to fix on the middle

point and to estimate the distance by taking in the whole space at

once without moving from this middle point. A different explanation

for this illusion is offered by Helmholtz.[16] He makes use of the

æsthetic factor of contrasts. Wundt insists that the fact that this

illusion is still present when there are no actual eye movements does

not demonstrate that the illusion is not to be referred to a motor

origin. He says, “If a phenomenon is perceived with the moving eye

only, the influence of movement on it is undoubtedly true. But an

inference cannot be drawn in the opposite direction, that movement is

without influence on the phenomenon that persists when there is no

movement.”[17]

[15] Wundt., W., ‘Physiolog. Psych.,’ 4te Aufl., Leipzig, 1893,

Bd. II., S. 144.

[16] v. Helmholtz, H., ‘Handbuch d. Physiol. Optik,’ 2te Aufl.,

Hamburg u. Leipzig, 1896, S. 705.

[17] Wundt, W., op. citat., S. 139.

Satisfactorily as the movement hypothesis explains this and other

optical illusions, it yet falls short of furnishing an entirely

adequate explanation. It seems to me certain that several causes exist

to produce this illusion, and also the illusion that is often

associated with it, the well-known Müller-Lyer illusion. But in what

degree each is present has not yet been determined by any of the

quantitative studies in this particular illusion. I made a number of

tests of the optical illusion, with these results: that the illusion

is strongest when the attention is fixed at about the middle of the

open space, that there is scarcely any illusion left when the

attention is fixed on the middle of the filled space. It is stronger

when the outer end-point of the open space is fixated than when the

outer end of the filled space is fixated. For the moving eye, I find

the illusion to be much stronger when the eye passes over the filled

space first, and then over the open space, than when the process is

reversed.

Now, the movement hypothesis does not, it seems to me, sufficiently

explain all the fluctuations in the illusion. My experiments with the

tactual illusion justify the belief that the movement theory is even

less adequate to explain all of the variations there, unless the

movement hypothesis is given a wider and richer interpretation than is

ordinarily given to it. In the explanation of the tactual illusion

which I have here been studying two other important factors must be

taken into consideration. These I shall call, for the sake of

convenience, the æsthetic factor and the time factor. These factors

should not, however, be regarded as independent of the factor of

movement. That term should be made wide enough to include these within

its meaning. The importance of the time factor in the illusion for

passive touch I have already briefly mentioned. I have also, in

several places in the course of my experiments, called attention to

the importance of the æsthetic element in our space judgments. I wish

now to consider these two factors more in detail.

The foregoing discussion has pointed to the view that the

space-perceiving and the localizing functions of the skin have a

deep-lying common origin in the motor sensations. My experiments show

that, even in the highly differentiated form in which we find them in

their ordinary functioning, they plainly reveal their common origin. A

formula, then, for expressing the judgments of distance by means of

the resting skin might be put in this way. Let P and P’ represent

any two points on the skin, and let L and L’ represent the local

signs of these points, and M and M’ the muscle sensations which

give rise to these local signs. Then M-M’ will represent the

distance between P and P’, whether that distance be judged

directly in terms of the localizing function of the skin or in terms

of its space-perceiving function. This would be the formula for a

normal judgment. In an illusory judgment, the temporal and æsthetic

factors enter as disturbing elements. Now, the point which I insist on

here is that the judgments of the extent of the voluntary movements,

represented in the formula by M and M’, do not depend alone on the

sensations from the moving parts or other sensations of objective

origin, as Dresslar would say, nor alone on the intention or impulse

or innervation as Loeb and others claim, but on the sum of all the

sensory elements that enter, both those of external and those of

internal origin. And, furthermore, these sensations of external origin

are important in judgments of space, only in so far as they are

referred to sensations of internal origin. Delabarre says, “Movements

are judged equal when their sensory elements are judged equal. These

sensory elements need not all have their source in the moving parts.

All sensations which are added from other parts of the body and which

are not recognized as coming from these distant sources, are mingled

with the elements from the moving member, and influence the

judgment.”[18] The importance of these sensations of inner origin was

shown in many of the experiments in sections VI. to VIII. In the

instance where the finger-tip was drawn over an open and a filled

space, in the filled half the sensations were largely of external

origin, while in the open half they were of internal origin. The

result was that the spaces filled with sensations of internal origin

were always overestimated.

The failure to recognize the importance of these inwardly initiated

sensations is the chief defect in Dresslar’s reasoning. He has

endeavored to make our judgments in the illusion in question depend

entirely on the sensations of external origin. He insists also that

the illusion varies according to the variations in quantity of these

external sensations. Now my experiments have shown, I think, very

clearly