Various - Harvard Psychological Studies, Volume 1

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The fact that the anaesthesia commences appreciably later than the
movement so far favors this interpretation. For if the anaesthesia is
conditioned by high tension in the localization-centers, due to
incoming sensations from the eye-muscles, it could not possibly
commence synchronously with the movement. For, first the sensory
end-organs in the eye-muscles (or perhaps in the ligaments, surfaces
of the eye-sockets, etc.) have their latent period; then the
stimulation has to travel to the brain; and lastly it probably has to
initiate there a summation-process equivalent to another latent
period. These three processes would account very readily for what we
may call the latent period of the anaesthesia, as observed in the
experiments. It is true that this latent period was observed only in
long eye-and head-movements, but the experiments were not delicate
enough in this particular to bring out the finer points.

Finally, the conditioning of anaesthesia by movements of the head, if
really proved, would rather corroborate this interpretation. For of
course the position of the head on the shoulders is as important for
localization of the retinal picture as the position of the eyes in the
head, so that sensations of head-movements must be equally represented
in the localization centers; and head movements would equally raise
the tension on those centers against discharge-currents from the
color-centers.

The conclusion from the foregoing experiments is that voluntary
movements of the eyes condition a momentary, visual, central
anaesthesia.

* * * * *

TACTUAL ILLUSIONS.

BY CHARLES H. RIEBER.

I.

Many profound researches have been published upon the subject of
optical illusions, but in the field of tactual illusions no equally
extensive and serious work has been accomplished. The reason for this
apparent neglect of the illusions of touch is obviously the fact that
the studies in the optical illusions are generally thought to yield
more important results for psychology than corresponding studies in
the field of touch. Then, too, the optical studies are more attractive
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 near-lying 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 cooeperated 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 Thiery,[3] and Dresslar.[4] Thiery 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.
514.

[3] Thiery, A.: _Philos. Studien_, 1896, Bd. XII., S. 121.

[4] Dresslar, F.B.: _Amer. Journ. of Psy._, 1894, Vol. VI., p.
332.

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. Muensterberg[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] Muensterberg, H.: 'Beitraege 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 aesthesiometers, A and B, were arranged in a framework, so that
uniform stimulations could be given on both arms. The aesthesiometers
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 aesthesiometer could be conveniently adjusted
by the pins F and H. The shape of the cams was such that the descent
of the aesthesiometer 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 several forms and lengths, could be
easily detached from the upright rods at _K_ and _L_. Each of the
points by which the contacts were made moved easily along the sliding
rule, and could be also raised or lowered for accommodation to the
unevenness of the surface of the skin. These latter were the most
valuable two features of the apparatus. There were two sets of points,
one of hard rubber, the other of metal. This enabled me to take into
account, to a certain extent, the factor of temperature. A wide range
of apparent differences in temperature was secured by employing these
two stimuli of such widely different conductivity. Then, as each point
was independent of the rest in its movements, its weight could also be
changed without affecting the rest.

In the first series of experiments I endeavored to reproduce for touch
the optical illusion in its exact form. There the open and the filled
spaces are adjacent to each other, and are presented simultaneously
for passive functioning of the eye, which is what concerns us here in
our search for the analogue of passive touch. This was by no means an
easy task, for obviously the open and the filled spaces in this
position on the skin could not be compared directly, owing to the lack
of uniformity in the sensibility of different portions of the skin. At
first, equivalents had to be established between two collinear open
spaces for the particular region of the skin tested. Three points were
taken in a line, and one of the end points was moved until the two
adjacent open spaces were pronounced equal. Then one of the spaces was
filled, and the process of finding another open space equivalent to
this filled space was repeated as before. This finding of two
equivalent open spaces was repeated at frequent intervals. It was
found unsafe to determine an equivalent at the beginning of each
sitting to be used throughout the hour.

Two sets of experiments were made with the illusion in this form. In
one the contacts were made simultaneously; the results of this series
are given in Table I. In the second set of experiments the central
point which divided the open from the filled space touched the skin
first, and then the others in various orders. The object of this was
to prevent fusion of the points, and, therefore, to enable the subject
to pronounce his judgments more rapidly and confidently. A record of
these judgments is given in Table II. In both of these series the
filled space was always taken near the wrist and the open space in a
straight line toward the elbow, on the volar side of the arm. At
present, I shall not undertake to give a complete interpretation of
the results of these two tables, but simply call attention to two
manifest tendencies in the figures. First, it will be seen that the
short filled distance of four centimeters is underestimated, but that
the long filled distance is overestimated. Second, in Table II., which
represents the judgments when the contacts were made successively, the
tendency to underestimate the short distance is less, and at the same
time we notice a more pronounced overestimation of the longer filled
distances. I shall give a further explanation of these results in
connection with later tables.

TABLE I.

4 cm. 6 cm. 8 cm.
Filled. Open. Filled. Open. Filled. Open.

F. 5.3 4.7 7.8 7.6 9.3 10.5
F. 5.7 4.4 6.5 7.3 9.2 11.7
F. 6.0 5.6 8.2 7.3 8.7 10.8
--- --- --- --- --- ----
Av. 5.7 4.9 7.5 7.4 9.1 11.0

R. 5.7 5.1 6.7 6.8 9.3 10.2
R. 5.4 5.4 7.2 7.1 8.5 10.7
R. 4.6 4.2 8.1 8.1 9.1 11.4
--- --- --- --- --- ----
Av. 5.2 4.9 7.3 7.3 9.0 10.8

K. 5.6 5.1 6.8 6.7 8.1 9.6
K. 5.0 5.1 7.3 7.5 8.2 11.2
K. 4.9 4.9 8.2 8.1 10.1 10.1
--- --- --- --- ---- ----
Av. 5.2 5.0 7.4 7.4 8.8 10.3

TABLE II.

4 cm. 6 cm. 8 cm.
Filled. Open. Filled. Open. Filled. Open.

F. 5.1 5.0 8.0 8.3 9.2 10.3
F. 5.8 4.7 7.2 7.9 8.7 10.9
F. 5.6 5.5 6.9 9.1 9.1 11.1
--- --- --- --- --- ----
Av. 5.5 5.1 7.4 8.4 9.0 10.8

R. 6.0 4.8 8.2 7.5 9.4 10.6
R. 5.7 5.4 6.5 7.4 10.1 9.4
R. 5.0 5.2 7.7 7.8 8.6 11.2
--- --- --- --- ---- ----
Av. 5.6 5.1 7.5 7.6 9.4 10.4

K. 4.8 4.8 8.2 8.3 8.1 9.8
K. 5.1 5.3 7.1 7.7 10.0 10.8
K. 4.7 5.0 8.1 8.6 8.6 9.4
--- --- --- --- ---- ----
Av. 4.9 5.0 7.8 8.2 8.9 10.0

The first two numbers in the first line signify that when an
open distance of 4 cm. was taken, an adjacent open distance of
4.7 cm. was judged equal; but when the adjacent space was
filled, 5.3 cm. was judged equal. Each number in the column of
filled distances represents an average of five judgments. All
of the contacts in Table I. were made simultaneously; in Table
II. they were made successively.

In the next series of experiments the illusion was approached from an
entirely different point of view. The two points representing the open
space were given on one arm, and the filled space on a symmetrical
part of the other arm. I was now able to use a much wider range of
distances, and made many variations in the weights of the points and
the number that were taken for the filled distance.

However, before I began this second series, in which one of the chief
variations was to be in the weights of the different points, I made a
brief preliminary series of experiments to determine in a general way
the influence of pressure on judgments of point distances. Only three
distances were employed, four, six and twelve centimeters, and three
weights, twelve, twenty and forty grams. Table III. shows that, for
three men who were to serve as subjects in the main experiments that
are to follow, an increase in the weight of the points was almost
always accompanied by an increase in the apparent distance.

TABLE III.

Distances. 4 cm. 6 cm. 12 cm.

Weights
(Grams). 12 20 40 12 20 40 12 20 40

R. 3.9 3.2 3.0 6.2 5.6 5.3 11.4 10.4 9.3
F. 4.3 4.0 3.6 6.1 5.3 5.5 12.3 11.6 10.8
B. 4.1 3.6 3.1 6.0 5.7 5.8 12.0 10.2 9.4
P. 4.3 4.1 3.7 5.9 5.6 5.6 13.1 11.9 10.7

In the standard distances the points were each weighted to 6
grams. The first three figures signify that a two-point
distance of 4 cm., each point weighing 6 grams, was judged
equal to 3.9 cm. when each point weighed 12 grams. 3.2 cm.
when each point weighed 20 grams, etc. Each figure is the
average of five judgments.

Now the application of this principle in my criticism of Parrish's
experiments, and as anticipating the direction which the following
experiments will take, is this: if we take a block such as Parrish
used, with only two points in it, and weight it with forty grams in
applying it to the skin, it is plain that each point will receive one
half of the whole pressure, or twenty grams. But if we put a pressure
of forty grams upon a block of eight points, each point will receive
only one eighth of the forty, or five grams. Thus, in the case of the
filled space, the end points, which play the most important part in
the judgment of the distance, have each only five grams' pressure,
while the points in the open space have each twenty grams. We should,
therefore, naturally expect that the open space would be
overestimated, because of the decided increase of pressure at these
significant points. Parrish should have subjected the blocks, not to
the same pressure, but to a pressure proportional to the number of
points in each block. With my apparatus, I was easily able to prove
the correctness of my position here. It will be seen in Tables IV. to
VIII. that, when the sum of the weights of the two end points in the
open space was only just equal to the sum of the weights of all the
points in the filled space, the filled space was underestimated just
as Parrish has reported. But when the points were all of the same
weight, both in the filled and the open space, the filled space was
judged longer in all but the very short distances. For this latter
exception I shall offer an explanation presently.

Having now given an account of the results of this digression into
experiments to determine the influence of pressure upon point
distances, I shall pass to the second series of experiments on the
illusion in question. In this series, as has been already stated, the
filled space was taken on one arm and the open on the other, and then
the process was reversed in order to eliminate any error arising from
a lack of symmetry between the two regions. Without, for the present,
going into a detailed explanation of the statistics of this second
series of experiments, which are recorded in Tables IV., V., VI.,
VII. and VIII., I may summarize the salient results into these general
conclusions: First, the short filled distance is underestimated;
second, this underestimation of the filled space gradually decreases
until in the case of the filled distance of 18 cm. the judgments pass
over into pronounced overestimations; third, an increase in the number
of points of contact in the shorter distances increases the
underestimation, while an increase in the number of points in the
longer distance increases the overestimation; fourth, an increase of
pressure causes an invariable increase in the apparent length of
space. If a general average were made of the results given in Tables
IV., V., VI., VII. and VIII., there would be a preponderance of
evidence for the conclusion that the filled spaces are overestimated.
But we cannot ignore the marked tendencies in the opposite direction
for the long and the short distances. These anomalous results, which,
it will be remembered, were also found in our first series, call for
explanation. Several hypotheses were framed to explain these
fluctuations in the illusion, and then some shorter series of
experiments were made in different directions with as large a number
of variations in the conditions as possible, in the hope of
discovering the disturbing factors.