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

and Fechner in one essential respect. This difference, I think, is

sufficient to explain the different results. In my experiments the

two-point distance was held on the skin, while the stylus was moved

from one point to the other. In their experiments the line was drawn

without the points. This of course changes the objective conditions.

In simply drawing a line on the skin the subject rapidly loses sight

of the starting point of the movement. It follows, as it were, the

moving point, and hence the entire distance is underestimated. I made

a small number of tests of this kind, and found that the line seemed

shorter than the point distance as Fechner and Vierordt declared. But

when the point distance is kept on the skin while the stylus is being

drawn, the filling is allowed its full effect in the judgment,

inasmuch as the end points are perceived as stationary landmarks. The

subjects at first found some difficulty in withholding their judgments

until the movement was completed. Some subjects declared that they

frequently made a preliminary judgment before the filling was

inserted, but that when the moving point approached the end point,

they had distinctly the experience that the distance was widening. In

these experiments I used five sorts of motion, quick and heavy, quick

and light, slow and heavy, slow and light, and interrupted. I made no

attempt to determine either the exact amount of pressure or the exact

rate. I aimed simply at securing pronounced extremes. The slow rate

was approximately 3, and the fast approximately 15 cm. per second.

[7] ‘Zeitsinn,’ Tübingen, 1858.

[8] Fechner, G. Th., ‘Elem. d. Psychophysik,’ Leipzig, 1889; 2.

Theil, S. 328.

I have already said that these filled spaces were invariably

overestimated and that the slower the movement, the greater, in

general, is the overestimation. In addition to the facts just stated I

found also, what Hall and Donaldson[9] discovered, that an increase in

the pressure of a moving point diminishes the apparent distance.

[9] Hall, G. St., and Donaldson, H.H., ‘Motor Sensations on the

Skin,’ Mind, 1885, X., p. 557.

Nichols,[10] however, says that heavy movements seem longer and light

ones shorter.

[10] Op. citat., p. 98.

V.

There are several important matters which might properly have been

mentioned in an earlier part of this paper, in connection with the

experiments to which they relate, but which I have designedly omitted,

in order not to disturb the continuity in the development of the

central object of the research. The first of these is the question of

the influence of visualization on the judgments of cutaneous

distances. This is in many ways a most important question, and

confronts one who is making studies in tactual space everywhere. The

reader may have already noticed that I have said but little about the

factor of visualization in any of my experiments, and may have

regarded it as a serious omission. It might be offered as a criticism

of my work that the fact that I found the tactual illusions to exist

in the same sense as the optical illusions was perhaps due to the

failure to exclude visualization. All of the subjects declare that

they were unable to shut out the influence of visualizing entirely.

Some of the subjects who were very good visualizers found the habit

especially insistent. I think, however, that not even in these latter

cases does this factor at all vitiate my conclusions.

It will be remembered that the experiments up to this time fall into

two groups, first, those in which the judgments on the cutaneous

distances were reached by direct comparisons of the sensations

themselves; and secondly, those in which the sensations were first

localized and then the judgment of the distance read from these

localizations. Visualizing, therefore, entered very differently into

the two groups. In the first instance all of the judgments were made

with the eyes closed, while all of the localizations were made with

the eyes open. I was uncertain through the whole of the first group of

experiments as to just how much disturbance was being caused in the

estimation of the distance by visualizing. I therefore made a series

of experiments to determine what effect was produced upon the illusion

if in the one set of judgments one purposely visualized and in the

other excluded visualizing as far as possible. In my own case I found

that after some practice I could give very consistent judgments, in

which I felt that I had abstracted from the visualized image of the

arm almost entirely. I did not examine these results until the close

of the series, and then found that the illusion was greater for those

judgments in which visualization was excluded; that is, the filled

space seemed much larger when the judgment was made without the help

of visualization. It is evident, therefore, that the tactual illusion

is influenced rather in a negative direction by visualization.

In the second group of experiments, where the judgments were obtained

through the localization of the points, it would seem, at first sight,

that the judgments must have been very largely influenced by the

direct vision used in localizing the points. The subject, as will be

remembered, looked down at a card of numbered points and named those

which were directly over the contacts beneath. Here it should seem

that the optical illusion of the overestimation of filled spaces,

filled with points on the card, would be directly transmitted to the

sensation on the skin underneath. Such criticism on this method of

getting at the illusion has already been made orally to me. But this

is obviously a mistaken objection. The points on the card make a

filled space, which of course appears larger, but as the points

expand, the numbers which are attached to them expand likewise, and

the optical illusion has plainly no influence whatever upon the

tactual illusion.

A really serious objection to this indirect method of approaching the

illusion is, that the character of the cutaneous sensation is never so

distinctly perceived when the eyes are open as when they are closed.

Several subjects often found it necessary to close their eyes first,

in order to get a clear perception of the locality of the points;

they then opened their eyes, to name the visual points directly above.

Some subjects even complained that when they opened their eyes they

lost track of the exact location of the touch points, which they

seemed to have when their eyes were closed. The tactual impression

seems to be lost in the presence of active vision.

On the whole, then, I feel quite sure in concluding that the

overestimation of the filled cutaneous spaces is not traceable to the

influence of visualization. Parrish has explained all sporadic cases

of overestimation as due to the optical illusion carried over in

visualization. I have already shown that in my experiments

visualization has really the opposite effect. In Parrish’s experiments

the overestimation occurred in the case of those collections of points

which were so arranged as to allow the greatest differentiation among

the points, and especially where the end-points were more or less

distinct from the rest. This, according to my theory, is precisely

what one would expect.

Those who have made quantitative studies in the optical illusion,

especially in this particular illusion for open and filled spaces,

have observed and commented on the instability of the illusion.

Auerbach[11] says, in his investigation of the quantitative variations

of the illusion, that concentration of attention diminishes the

illusion. In the Zöllner figure, for instance, I have been able to

notice the illusion fluctuate through a wide range, without

eye-movements and without definitely attending to any point, during

the fluctuation of the attention. My experiments with the tactual

illusion have led me to the conclusion that it fluctuates even more

than the optical illusion. Any deliberation in the judgment causes the

apparent size of the filled space to shrink. The judgments that are

given most rapidly and naïvely exhibit the strongest tendency to

overestimation; and yet these judgments are so consistent as to

exclude them from the category of guesses.

[11] Auerbach, F., _Zeitsch. f. Psych. u. Phys. d.

Sinnesorgane_, 1874, Bd. VII., S. 152.

In most of my experiments, however, I did not insist on rapid and

naïve judgments; but by a close observation of the subject as he was

about to make a judgment I could tell quite plainly which judgments

were spontaneous and which were deliberate. By keeping track of these

with a system of marks, I was able to collect them in the end into

groups representing fairly well the different degrees of attention.

The illusion is always greatest for the group of spontaneous

judgments, which points to the conclusion that all illusions, tactual

as well as visual, are very largely a function of attention.

In Section II. I told of my attempt to reproduce the optical illusion

upon the skin in the same form in which we find it for sight, namely,

by presenting the open and filled spaces simultaneously, so that they

might be held in a unitary grasp of consciousness and the judgment

pronounced on the relative length of these parts of a whole. However,

as I have already said, the filled space appears longer, not only when

given simultaneously, but also when given successively with the open

space. In the case of the optical illusion I am not so sure that the

illusion does not exist if the two spaces are not presented

simultaneously and adjacent, as Münsterberg asserts. Although, to be

sure, for me the illusion is not so strong when an interval is allowed

between the two spaces, I was interested to know whether this was true

also in the case of a touch illusion. My previous tables did not

enable me to compare the quantitative extent of the illusion for

successive and simultaneous presentation. But I found in two series

which had this point directly in view, one with the subject F and

one in which G served as subject, that the illusion was emphatically

stronger when the open and filled spaces were presented simultaneously

and adjacent. In this instance, the illusion was doubtless a

combination of two illusions—a shrinking of the open space, on the

one hand, and a lengthening of the filled space on the other hand.

Binet says, in his studies on the well-known Müller-Lyer illusion,

that he believes the illusion, in its highest effects at any rate, to

be due to a double contrast illusion.

This distortion of contrasted distances I have found in more than one

case in this investigation—not only in the case of distances in which

there is a qualitative difference, but also in the case of two open

distances. In one experiment, in which open distances on the skin were

compared with optical point distances, a distance of 10 cm. was given

fifty times in connection with a distance of 15 cm., and fifty times

in connection with a distance of 5 cm. In the former instance the

distance of 10 cm. was underestimated, and in the other it was

overestimated.

The general conclusion of the entire investigation thus far may be

summed up in the statement: _Wherever the objective conditions are the

same in the two senses, the illusion exists in the same direction for

both sight and touch._

VI.

Thus far all of my experiments were made with passive touch. I

intend now to pursue this problem of the relation between the

illusions of sight and touch into the region of active touch. I have

yielded somewhat to the current fashion in thus separating the passive

from the active touch in this discussion. I have already said that I

believe it would be