Various - Harvard Psychological Studies, Volume 1

V >> Various >> Harvard Psychological Studies, Volume 1

Pages:
1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 | 20 | 21 | 22 | 23 | 24 | 25 | 26 | 27 | 28 | 29 | 30 | 31 | 32 | 33 | 34 | 35 | 36 | 37 | 38 | 39 | 40 | 41 | 42 | 43 | 44 | 45 | 46 | 47 | 48 | 49 | 50 | 51 | 52 | 53 | 54 | 55

[7] 'Zeitsinn,' Tuebingen, 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 Zoellner 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 naively 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
naive 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 Muensterberg 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 Mueller-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 better not to make this distinction so pronounced.
Here again I have concerned myself primarily with only one illusion,
the illusion which deals with open and filled spaces. This is the
illusion to which Dresslar[12] devoted a considerable portion of his
essay on the 'Psychology of Touch,' and which he erroneously thought
to be the counterpart of the optical illusion for open and filled
spaces. One of the earliest notices of this illusion is that given by
James,[13] who says, "Divide a line on paper into two equal halves,
puncture the extremities, and make punctures all along one of the
halves; then, with the finger-tip on the opposite side of the paper,
follow the line of punctures; the empty half will seem much longer
than the punctured half."

[12] Dresslar, F.B., _Am. Journ. of Psy._, 1894, VI., p. 313.

[13] James, W., 'Principles of Psychology,' New York, 1893,
II., p. 250.

James has given no detailed account of his experiments. He does not
tell us how many tests were made, nor how long the lines were, nor
whether the illusion was the same when the open half was presented
first. Dresslar took these important questions into consideration, and
arrived at a conclusion directly opposite to that of James, namely,
that the filled half of the line appears larger than the open half.
Dresslar's conclusion is, therefore, that sight and touch function
alike. I have already said that I think that Parrish was entirely
right in saying that this is not the analogue of the familiar optical
illusion. Nevertheless, I felt sure that it would be quite worth the
while to make a more extensive study than that which Dresslar has
reported. Others besides James and Dresslar have experimented with
this illusion. As in the case of the illusion for passive touch, there
are not wanting champions of both opinions as to the direction in
which this illusion lies.

I may say in advance of the account of my experiments, that I have
here also found a ground of reconciliation for these two divergent
opinions. Just as in the case of the illusion for passive touch, there
are here also certain conditions under which the filled space seems
longer, and other conditions under which it appears shorter than the
open space. I feel warranted, therefore, in giving in some detail my
research on this illusion, which again has been an extended one. I
think that the results of this study are equally important with those
for passive touch, because of the further light which they throw on
the way in which our touch sense functions in the perception of the
geometrical illusions. Dresslar's experiments, like those of James,
were made with cards in which one half was filled with punctures. The
number of punctures in each centimeter varied with the different
cards. Dresslar's conclusion was not only that the filled space is
overestimated, but also that the overestimation varies, in a general
way, with the number of punctures in the filling. Up to a certain
point, the more holes there are in the card, the longer the space
appears.

I had at the onset of the present experiment the same feeling about
Dresslar's work that I had about Parrish's work, which I have already
criticised, namely, that a large number of experiments, in which many
variations were introduced, would bring to light facts that would
explain the variety of opinion that had hitherto been expressed. I was
confident, however, that what was most needed was a quantitative
determination of the illusion. Then, too, inasmuch as the illusion,
whatever direction it takes, is certainly due to some sort of
qualitative differences in the two kinds of touch sensations, those
from the punctured, and those from the smooth half, it seemed
especially desirable to introduce as many changes into the nature of
the filling as possible. The punctured cards I found very
unsatisfactory, because they rapidly wear off, and thus change the
quality of the sensations, even from judgment to judgment.

[Illustration: FIG. 7.]

The first piece of apparatus that I used in the investigation of the
illusion for open and filled space with active touch is shown in Fig.
7. A thimble _A_, in which the finger was carried, moved freely along
the rod _B_. The filled spaces were produced by rows of tacks on the
roller _C_. By turning the roller, different kinds of fillings were
brought into contact with the finger-tip. The paper _D_, on which the
judgments were recorded by the subject, could be slowly advanced under
the roller _E_. Underneath the thimble carrier there was a pin so
arranged that, by a slight depression of the finger, a mark was made
on the record paper beneath. A typical judgment was made as follows;
the subject inserted his finger in the thimble, slightly depressed the
carrier to record the starting points, then brought his finger-tip
into contact with the first point in the filled space. The subject
was, of course, all the while ignorant of the length or character of
the filling over which he was about to pass. The finger-tip was then
drawn along the points, and out over the smooth surface of the roller,
until the open space passed over was judged equal to the filled space.
Another slight depression of the finger registered the judgment on the
paper below. The paper was then moved forward by turning the roller
_E_, and, if desired, a different row of pins was put in place for
judgment by revolving the roller _C_. The dividing line between the
open and filled spaces was continuously recorded on the paper from
below by a pin not shown in the illustration.

The rollers, of which I had three, were easily removed or turned
about, so that the open space was presented first. In one of the
distances on each roller both spaces were unfilled. This was used at
frequent intervals in each series and served somewhat the same purpose
as reversing the order in which the open and filled spaces were
presented. With some subjects this was the only safe way of securing
accurate results. The absolute distances measured off were not always
a sure criterion as to whether the filled space was under-or
overestimated. For example, one rather erratic subject, who was,
however, very constant in his erratic judgments, as an average of
fifty judgments declared a filled space of 4 cm. to be equal to an
open space of 3.7 cm. This would seem, on the surface, to mean that
the filled space had been underestimated. But with these fifty
judgments there were alternated judgments on two open spaces, in which
the first open space was judged equal to the second open space of 3.2
cm. From this it is obvious that the effect of the filling was to
cause an overestimation--not underestimation as seemed at first sight
to be the case.

In another instance, this same subject judged a filled space of 12.0
cm. to be equal to an open space of 12.9 cm., which would seem to
indicate an overestimation of the filled space. But an average of the
judgments on two open spaces that were given in alternation shows that
an equivalence was set up between the two at 13.7 cm. for the second
open space. This would show that the filling of a space really
produced an underestimation.

The same results were obtained from other subjects. In my experiments
on the illusion for passive touch, I pointed out that it is unsafe to
draw any conclusion from a judgment of comparison between open and
filled cutaneous spaces, unless we had previously determined what
might be called a standard judgment of comparison between two open
spaces. The parts of our muscular space are quite as unsymmetrical as
the parts of our skin space. The difficulties arising from this lack
of symmetry can best be eliminated by introducing at frequent
intervals judgments on two open spaces. As I shall try to show later,
the psychological character of the judgment is entirely changed by
reversing the order in which the spaces are presented, and we cannot
in this way eliminate the errors due to fluctuations of the attention.

The apparatus which I used in these first experiments possesses
several manifest advantages. Chief among these was the rapidity with
which large numbers of judgments could be gathered and automatically
recorded. Then, in long distances, when the open space was presented
first, the subject found no difficulty in striking the first point of
the filled space. Dresslar mentioned this as one reason why in his
experiments he could not safely use long distances. His subjects
complained of an anxious straining of the attention in their efforts
to meet the first point of the filled space.

There are two defects manifest in this apparatus. In the first place,
the other tactual sensations that arise from contact with the thimble
and from the friction with the carrier moving along the sliding rod
cannot be disregarded as unimportant factors in the judgments.
Secondly, there is obviously a difference between a judgment that is
made by the subject's stopping when he reaches a point which seems to
him to measure off equal spaces, and a judgment that is made by
sweeping the finger over a card, as in Dresslar's experiments, with a
uniform motion, and then, after the movement has ceased, pronouncing
judgment upon the relative lengths of the two spaces. In the former
case the subject moves his finger uniformly until he approaches the
region of equality, and then slackens his speed and slowly comes to a
standstill. This of course changes the character of the judgments.
Both of these defects I remedied in another apparatus which will be
described later. For my present purpose I may disregard these
objections, as they affect alike all the judgments.

In making the tests for the first series, the subject removed his
finger after each judgment, so that the position of the apparatus
could be changed and the subject made to enter upon the new judgment
without knowing either the approximate length or the nature of the
filling of this new test. With this apparatus no attempt was made to
discover the effects of introducing changes in the rate of speed. The
only requirement was that the motion should be uniform. This does not
mean that I disregarded the factor of speed. On the contrary, this
_time_ element I consider as of the highest consequence in the whole
of the present investigation. But I soon discovered, in these
experiments, that the subjects themselves varied the rate of speed
from judgment to judgment over a wide range of rates. There was no
difficulty in keeping track of these variations, by recording the
judgments under three groups, fast, slow and medium. But I found that
I could do this more conveniently with another apparatus, and will
tell at a later place of the results of introducing a time element. In
these first experiments the subject was allowed to use any rate of
speed which was convenient to him.

TABLE IX.

Subjects P R F Rr
2= 3.8 3.6 2.9 2.8
3= 4.1 4.1 4.2 3.9
4= 4.7 5.1 4.3 4.3
Filled 5= 5.2 5.6 5.8 6.0
Spaces. 6= 6.0 6.3 6.4 5.2
7= 6.8 6.5 6.6 7.0
8= 7.5 7.6 7.2 7.4
9= 8.3 8.1 8.2 8.6
10= 8.9 9.1 8.7 8.5

TABLE X.

Subjects P R F Rr
2= 4.0 3.8 3.2 2.6
3= 4.3 4.2 4.4 3.6
4= 4.6 5.6 4.6 4.8
Filled 5= 5.4 6.1 5.6 5.7
Spaces. 6= 6.2 6.4 6.8 6.9
7= 7.3 6.8 7.9 7.2
8= 7.8 7.4 7.3 7.8
9= 8.6 8.0 7.9 8.9
10= 9.3 9.1 8.9 8.5

TABLES IX. AND X.

First line reads: 'When the finger-tip was drawn over a filled
distance of 2 cm., the subject _P_ measured off 3.8 on the
open surface, the subject _R_ 3.6, etc.' Each number is the
average of five judgments. In Table IX. the points were set at
regular intervals. In Table X. the filling was made irregular
by having some points rougher than the others and set at
different intervals.

I can give here only a very brief summary of the results with this
apparatus. In Tables IX. and X. I give a few of the figures which will
show the tendency of the experiments. In these tests a different
length and a different filling were given for each judgment. The
result of the experiments of this group is, first, that the _shorter
filled spaces are judged longer and the longer spaces shorter_ than
they really were. Second, that an increase in the number of points in
the filled space causes no perceptible change in the apparent length.
Third, that when the filling is so arranged as to produce a tactual
rhythm by changing the position or size of every third point, the
apparent length of the space is increased. It will be noticed, also,
that this is just the reverse of the result that was obtained for
passive touch. These facts, which were completely borne out by several
other experiments with different apparatus which I shall describe
later, furnish again a reason why different investigators have
hitherto reported the illusion to exist, now in one direction, now in
the other. Dresslar drew the conclusion from his experiments that the
filled spaces are always overestimated, but at the same time his
figures show an increasing tendency towards an underestimation of the
filled spaces as the distances increased in length. I shall later, in
connection with similar results from other experiments on this
illusion, endeavor to explain these anomalous facts.

In section IV. I mentioned the fact that I found the illusion for
passive touch to be subject to large fluctuations. This is true also
of the illusion for active touch. When the finger-tip is drawn over
the filled, and then out over the open space, the limits between which
the stopping point varies is a much wider range than when the
finger-tip is drawn over two open spaces. In the latter case I found
the variation to follow Weber's Law in a general way. At first I
thought these erratic judgments were mere guesses on the part of the
subject; but I soon discovered a certain consistency in the midst of
these extreme fluctuations. To show what I mean, I have plotted some
diagrams based on a few of the results for three subjects. These
diagrams are found in Fig. 8. It will be observed that the curve which
represents the collection of stopping points is shorter and higher
where the judgments were on two open spaces. This shows plainly a
greater accuracy in the judgments than when the judgments were on a
filled and an open space, where the curves are seen to be longer and
flatter. This fluctuation in the illusion becomes important in the
theoretical part of my discussion, and, at the risk of apparently
emphasizing unduly an insignificant matter, I have given in Fig. 9 an
exact copy of a sheet of judgments as it came from the apparatus. This
shows plainly how the illusion wears away with practice, when one
distance is given several times in succession. The subject was allowed
to give his judgment on the same distance ten times before passing to
another. A glance at the diagram will show how pronounced the illusion
is at first, and how it then disappears, and the judgment settles down
to a uniform degree of accuracy. It will be seen that the short filled
space is at first overestimated, and then, with the succeeding
judgments, this overestimation is gradually reduced. In the case of
the longer filled distances (which could not be conveniently
reproduced here) the spaces were at first underestimated, and then
this underestimation slowly decreased.