Sixteen Experimental Investigations from the Harvard Psychological Laboratory, Hugo Münsterberg [top fiction books of all time TXT] 📗
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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
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