Harvard Psychological Studies Part 6

Turning to Fig. 6, we notice that the tendency is now to locate the end points in the filled distance outside of the localization of these same points when given without the intermediate points. It will also be seen from the irregularities in these two longer curves that there is now a clear-cut tendency to single out the individual points. The fact that the curves here are again higher over point 4 simply signifies that at this, the wrist end, the failure to discover the presence of the points was less frequent than towards the elbow. But this does not disturb the relation of the two series of judgments. As I have before said, the first two sets of experiments described in Section II. showed that the shorter filled distances are underestimated, while the longer distances are overestimated, and that between the two there is somewhat of an 'indifferent zone.' In those experiments the judgments were made directly on the cutaneous distances themselves. In the experiments the results of which are plotted in these curves, the judgment of distances is indirectly reached through the function of localization. But it will be observed that the results are substantially the same. The longer distances are overestimated and the shorter distances underestimated. The curves in Figs. 4, 5 and 6 were plotted on the combined results for two subjects. But before the combination was made the two main tendencies which I have just mentioned were observed to be the same for both subjects.

It will be remembered also that in these experiments, where the judgment of distance was based directly on the cutaneous impression, the underestimation of the short, filled distance was lessened and even turned into an overestimation, by giving greater distinctness to the end points, in allowing them to come in contact with the skin just before or just after the filling. The results here are again the same as before. The tendency to underestimate is lessened by this device.

Whenever, then, a filled s.p.a.ce is made up of points which are distinctly perceived as discrete--and this is shown in the longer curves by the comparative accuracy with which the points are located--these s.p.a.ces are overestimated.

In all of these experiments on localization, the judgments were given with open eyes, by naming the visual points under which the tactual points seemed to lie. I have already spoken of the other method which I also employed. This consisted in marking points on paper which seemed to correspond in number and position to the points on the skin.

During this process the eyes were kept closed. This may appear to be a very crude way of getting at the illusion, but from a large number of judgments which show a surprising consistency I received the emphatic confirmation of my previous conclusion, that filled s.p.a.ces were overestimated. These experiments were valuable also from the fact that here the cutaneous s.p.a.ce was estimated by the muscle sense, or active touch, as it is called.

In the experiments so far described the filling in of the closed s.p.a.ce was always made by means of stationary points. I shall now give a brief account of some experiments which I regard as very important for the theory that I shall advance later. Here the filling was made by means of a point drawn over the skin from one end of a two-point distance to the other.

These experiments were made on four different parts of the skin--the forehead, the back of the hand, the abdomen, and the leg between the knee and the thigh. I here forsook the plan which I had followed almost exclusively hitherto, that of comparing the cutaneous distances with each other directly. The judgments now were secured indirectly through the medium of visual distances. There was placed before the subject a gray card, upon which were put a series of two-point distances ranging from 2 to 20 cm. The two-point distances were given on the skin, and the subject then selected from the optical distances the one that appeared equal to the cutaneous distance. This process furnished the judgments on open s.p.a.ces. For the filled s.p.a.ces, immediately after the two-point distance was given a blunt stylus was drawn from one point to the other, and the subject then again selected the optical distance which seemed equal to this distance filled by the moving point.

The results from these experiments point very plainly in one direction. I have therefore thought it unnecessary to go into any further detail with them than to state that for all subjects and for all regions of the skin the filled s.p.a.ces were overestimated. This overestimation varied also with the rate of speed at which the stylus was moved. The overestimation is greatest where the motion is slowest.

Vierordt[7] found the same result in his studies on the time sense, that is, that the more rapid the movement, the shorter the distance seems. But lines drawn on the skin are, according to him, underestimated in comparison with open two-point distances. Fechner[8]

also reported that a line drawn on the skin is judged shorter than the distance between two points which are merely touched. It will be noticed, however, that my experiments differed from those of Vierordt 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 p.r.o.nounced extremes. The slow rate was approximately 3, and the fast approximately 15 cm. per second.

[7] 'Zeitsinn,' Tubingen, 1858.

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

Theil, S. 328.

I have already said that these filled s.p.a.ces 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 s.p.a.ce 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 s.p.a.ce 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 s.p.a.ces, 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 s.p.a.ce, 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 s.p.a.ces 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 quant.i.tative studies in the optical illusion, especially in this particular illusion for open and filled s.p.a.ces, have observed and commented on the instability of the illusion.

Auerbach[11] says, in his investigation of the quant.i.tative variations of the illusion, that concentration of attention diminishes the illusion. In the Zollner 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 s.p.a.ce to shrink. The judgments that are given most rapidly and navely 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 nave 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 s.p.a.ces simultaneously, so that they might be held in a unitary grasp of consciousness and the judgment p.r.o.nounced on the relative length of these parts of a whole. However, as I have already said, the filled s.p.a.ce appears longer, not only when given simultaneously, but also when given successively with the open s.p.a.ce. In the case of the optical illusion I am not so sure that the illusion does not exist if the two s.p.a.ces are not presented simultaneously and adjacent, as Munsterberg a.s.serts. Although, to be sure, for me the illusion is not so strong when an interval is allowed between the two s.p.a.ces, 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 quant.i.tative 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 s.p.a.ces were presented simultaneously and adjacent. In this instance, the illusion was doubtless a combination of two illusions--a shrinking of the open s.p.a.ce, on the one hand, and a lengthening of the filled s.p.a.ce on the other hand.

Binet says, in his studies on the well-known Muller-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 _pa.s.sive_ 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 pa.s.sive from the active touch in this discussion. I have already said that I believe it would be better not to make this distinction so p.r.o.nounced.

Here again I have concerned myself primarily with only one illusion, the illusion which deals with open and filled s.p.a.ces. 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 s.p.a.ces. 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 a.n.a.logue 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 pa.s.sive 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 pa.s.sive touch, there are here also certain conditions under which the filled s.p.a.ce seems longer, and other conditions under which it appears shorter than the open s.p.a.ce. 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 pa.s.sive 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 s.p.a.ce 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 s.p.a.ce 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 quant.i.tative 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.

[Ill.u.s.tration: FIG. 7.]

The first piece of apparatus that I used in the investigation of the illusion for open and filled s.p.a.ce 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 s.p.a.ces 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 s.p.a.ce. The subject was, of course, all the while ignorant of the length or character of the filling over which he was about to pa.s.s. The finger-tip was then drawn along the points, and out over the smooth surface of the roller, until the open s.p.a.ce pa.s.sed over was judged equal to the filled s.p.a.ce.

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 s.p.a.ces was continuously recorded on the paper from below by a pin not shown in the ill.u.s.tration.

The rollers, of which I had three, were easily removed or turned about, so that the open s.p.a.ce was presented first. In one of the distances on each roller both s.p.a.ces 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 s.p.a.ces 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 s.p.a.ce 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 s.p.a.ce of 4 cm. to be equal to an open s.p.a.ce of 3.7 cm. This would seem, on the surface, to mean that the filled s.p.a.ce had been underestimated. But with these fifty judgments there were alternated judgments on two open s.p.a.ces, in which the first open s.p.a.ce was judged equal to the second open s.p.a.ce 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 s.p.a.ce of 12.0 cm. to be equal to an open s.p.a.ce of 12.9 cm., which would seem to indicate an overestimation of the filled s.p.a.ce. But an average of the judgments on two open s.p.a.ces that were given in alternation shows that an equivalence was set up between the two at 13.7 cm. for the second open s.p.a.ce. This would show that the filling of a s.p.a.ce really produced an underestimation.

The same results were obtained from other subjects. In my experiments on the illusion for pa.s.sive touch, I pointed out that it is unsafe to draw any conclusion from a judgment of comparison between open and filled cutaneous s.p.a.ces, unless we had previously determined what might be called a standard judgment of comparison between two open s.p.a.ces. The parts of our muscular s.p.a.ce are quite as unsymmetrical as the parts of our skin s.p.a.ce. The difficulties arising from this lack of symmetry can best be eliminated by introducing at frequent intervals judgments on two open s.p.a.ces. As I shall try to show later, the psychological character of the judgment is entirely changed by reversing the order in which the s.p.a.ces 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 s.p.a.ce was presented first, the subject found no difficulty in striking the first point of the filled s.p.a.ce. 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 s.p.a.ce.

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 s.p.a.ces, 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, p.r.o.nouncing judgment upon the relative lengths of the two s.p.a.ces. 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.