Harvard Psychological Studies Part 7

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 s.p.a.ces. 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 s.p.a.ces. 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 s.p.a.ces are judged longer and the longer s.p.a.ces shorter_ than they really were. Second, that an increase in the number of points in the filled s.p.a.ce 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 s.p.a.ce is increased. It will be noticed, also, that this is just the reverse of the result that was obtained for pa.s.sive 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 s.p.a.ces are always overestimated, but at the same time his figures show an increasing tendency towards an underestimation of the filled s.p.a.ces 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 pa.s.sive 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 s.p.a.ce, the limits between which the stopping point varies is a much wider range than when the finger-tip is drawn over two open s.p.a.ces. 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 s.p.a.ces. This shows plainly a greater accuracy in the judgments than when the judgments were on a filled and an open s.p.a.ce, 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 pa.s.sing to another. A glance at the diagram will show how p.r.o.nounced 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 s.p.a.ce 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 s.p.a.ces were at first underestimated, and then this underestimation slowly decreased.

[Ill.u.s.tration: FIG. 8.]

[Ill.u.s.tration: FIG. 9.]

None of the qualitative studies that have hitherto been made on this illusion have brought to light this significant wearing away of the illusion.

VII.

I have already spoken of the defects of the apparatus with which the experiments of the previous chapter were made. I shall now give an account of some experiments that were made with an apparatus designed to overcome these difficulties. This is shown in Fig. 10. The block _C_ was clamped to a table, while the block _A_ could be moved back and forth by the lever _B_, in order to bring up different lengths of filled s.p.a.ce for judgment. For each judgment the subject brought his finger back to the strip _D_, and by moving his finger up along the edge of this strip he always came into contact with the first point of the new distance. The lever was not used in the present experiment; but in later experiments, where the points were moved under the finger tip, which was held stationary, this lever was very useful in producing different rates of speed. In one series of experiments with this apparatus the filled s.p.a.ces were presented first, and in another series the open s.p.a.ces were presented first. In the previous experiments, so far as I have reported them, the filled s.p.a.ces were always presented first.

[Ill.u.s.tration: FIG. 10.]

In order to enable the subject to make proper connections with the first point in the filled s.p.a.ce, when the open s.p.a.ce was presented first, a slight depression was put in the smooth surface. This depression amounted merely to the suggestion of a groove, but it sufficed to guide the finger.

The general results of the first series of experiments with this apparatus were similar to those already given, but were based on a very much larger number of judgments. They show at once that the short filled s.p.a.ces are overestimated, while the longer s.p.a.ces are underestimated. The uniformity of this law has seemed to me one of the most significant results of this entire investigation. In the results already reported from the experiments with the former apparatus, I have mentioned the fact that the judgments upon the distances fluctuate more widely when one is filled and the other open, than when both are open. This fluctuation appeared again in a p.r.o.nounced way in the present experiments. I now set about to discover the cause of this variation, which was so evidently outside of the limits of Weber's law.

TABLE XI.

I. II.

Subjects. R. B. A. R. B. A.

2= 3.1 3.2 3.7 2.7 2.5 3.1 3= 4.5 4.4 4.1 4.1 4.0 3.6 4= 5.3 5.0 4.3 4.2 4.6 4.6 5= 6.0 5.1 5.8 5.9 5.2 4.3 6= 6.8 5.6 6.2 6.9 5.3 6.0 7= 7.4 7.2 6.9 7.6 7.3 6.8 8= 8.1 8.4 7.3 8.3 9.7 7.8 9= 9.3 9.0 8.5 9.5 8.9 8.7 Filled 10= 10.1 10.0 8.1 10.3 10.0 9.2 s.p.a.ces. 11= 10.5 9.3 9.7 10.6 8.7 9.6 12= 11.7 10.6 10.6 11.8 9.7 10.2 13= 12.3 10.9 10.9 11.1 10.2 9.6 14= 12.2 11.5 12.2 10.4 9.6 11.3 15= 13.6 12.3 11.9 13.1 10.1 9.6 16= 14.1 13.5 14.1 12.3 13.2 13.3 17= 14.9 12.9 14.6 14.1 12.6 13.7 18= 15.0 15.3 14.9 15.0 15.3 13.8 19= 15.2 14.6 15.2 14.1 13.9 14.2 20= 17.1 16.5 15.7 16.1 16.4 14.7

The first line of group I. reads: 'When the finger-tip was pa.s.sed over a filled s.p.a.ce of 2 cm., the subject _R_ measured off 3.1 cm. on the open s.p.a.ce, the subject _B_ 3.2 cm., and the subject _A_ 3.7.' In group II., the numbers represent the distance measured off when both s.p.a.ces were unfilled.

In my search for the cause of the variations reported previously I first tried the plan of obliging the subject to attend more closely to the filled s.p.a.ce as his finger was drawn over it. In order to do this, I held a piece of fine wire across the line of the filled s.p.a.ce, and after the subject had measured off the equal open s.p.a.ce he was asked to tell whether or not he had crossed the wire. The wire was so fine that considerable attention was necessary to detect it. In some of the experiments the wire was inserted early in the filled s.p.a.ce, and in some near the end. When it was put in near the beginning, it was interesting to notice, as ill.u.s.trating the amount of attention that was being given to the effort of finding the wire, that the subject, as soon as he had discovered it, would increase his speed, relax the attention, and continue the rest of the journey more easily.

The general effect of this forcing of the attention was to increase the apparent length of the filled s.p.a.ce. This conclusion was reached by comparing these results with those in which there was no compelled attention. When the obstacle was inserted early, the s.p.a.ce was judged shorter than when it came at the end of the filled s.p.a.ce. This shows very plainly the effect of continued concentration of attention, when that attention is directed intensely to the spot immediately under the finger-tip. When the attention was focalized in this way, the subject lost sight of the s.p.a.ce as a whole. It rapidly faded out of memory behind the moving finger-tip. But when this concentration of attention was not required, the subject was able to hold together in consciousness the entire collection of discrete points, and he overestimated the s.p.a.ce occupied by them. It must be remembered here that I mean that the filled s.p.a.ce with the focalized attention was judged shorter than the filled s.p.a.ce without such concentration of attention, but both of these s.p.a.ces were judged shorter than the adjacent open s.p.a.ce. This latter fact I shall attempt to explain later. Many other simple devices were employed to oblige the subject to fix his attention on the s.p.a.ce as it was traversed by the finger.

The results were always the same: the greater the amount of attention, the longer the distance seemed.

In another experiment, I tried the plan of tapping a bell as the subject was pa.s.sing over the filled s.p.a.ce and asking him, after he had measured off the equivalent open s.p.a.ce, whether the sound had occurred in the first half or in the second half of the filled s.p.a.ce.

When the finger-tip was drawn over two adjacent open s.p.a.ces, and during the first a bell was tapped continuously, this kind of filled s.p.a.ce was underestimated if the distance was long and overestimated if the distance was short. So, too, if a disagreeable odor was held to the nostrils while the finger-tip was being drawn over one of the two adjacent open s.p.a.ces, the s.p.a.ce thus filled by the sensations of smell followed the law already stated. But if an agreeable perfume was used, the distance always seemed shorter than when an unpleasant odor was given.

In all of these experiments with s.p.a.ces filled by means of other than tactual sensations, I always compared the judgment on the filled and open s.p.a.ces with judgments on two open s.p.a.ces, in order to guard against any error due to unsymmetrical, subjective conditions for the two s.p.a.ces. It is difficult to have the subject so seat himself before the apparatus as to avoid the errors arising from tension and flexion.

In one experiment, a piece of plush was used for the filled s.p.a.ce and the finger drawn over it against the nap. This filled s.p.a.ce was judged longer than a piece of silk of equal length. The sensations from the plush were very unpleasant. One subject said, even, that they made him shudder. This was of course precisely what was wanted for the experiment. It showed that the affective tone of the sensation within the filled s.p.a.ce was a most important factor in producing an illusory judgment of distance.

The overestimation of these filled s.p.a.ces is evidently due in a large measure to aesthetic motives. The s.p.a.ce that is filled with agreeable sensations is judged shorter than one which is filled with disagreeable sensations. In other words, the illusions in judgments on cutaneous s.p.a.ce are not so much dependent on the quality of sensations that we get from the outer world through these channels, as from the amount of inner activity that we set over against these bare sense-perceptions.

I have already spoken of the defects of this method of measuring off equivalent distances as a means of getting at the quant.i.tative amount of the illusion. The results that have come to light thus far have, however, amply justified the method. I had no difficulty, however, in adapting my apparatus to the other way of getting the judgments. I had a short curved piece of wire inserted in the handle, which could be held across the line traversed, and thus the end of the open s.p.a.ce could be marked out. Different lengths were presented to the subject as before, but now the subject pa.s.sed his finger in a uniform motion over the s.p.a.ces, after which he p.r.o.nounced the judgment 'greater,'

'equal,' or 'less.' The general result of these experiments was not different from those already given. The short, filled s.p.a.ces were overestimated, while the longer ones were underestimated. The only difference was found to be that now the transition from one direction to the other was at a more distant point. It was, of course, more difficult to convert these qualitative results into a quant.i.tative determination of the illusion.

Before pa.s.sing to the experiments in which the open s.p.a.ces were presented first, I wish to offer an explanation for the divergent tendencies that were exhibited through all the experiments of the last two sections, namely, that the short filled s.p.a.ces are overestimated and the long s.p.a.ces underestimated. Let us take two typical judgments, one in which a filled s.p.a.ce of 3 cm. is judged equal to an open s.p.a.ce of 4.2 cm., and then one in which the filled s.p.a.ce is 9 cm., and is judged equal to an open s.p.a.ce of 7.4 cm. In the case of the shorter distance, because of its shortness, after the finger leaves it, it is held in a present state of consciousness for some moments, and does not suffer the foreshortening that comes from pastness. This is, however, only a part of the reason for its overestimation. After the finger-tip has left the filled s.p.a.ce, and while it is traversing the first part of the open s.p.a.ce, there is a dearth of sensations. The tactual sensations are meager and faint, and muscular tensions have not yet had time to arise. It is not until the finger has pa.s.sed over several centimeters of the distance, that the surprise of its barrenness sets up the organic sensations of muscular strain. One subject remarked navely at the end of some experiments of this kind, that the process of judging was an easy and comfortable affair so long as he was pa.s.sing over the filled s.p.a.ce, but when he set out upon the open s.p.a.ce he had to pay far more strict attention to the experiment.

By a careful introspection of the processes in my own case, I came to the conclusion that it is certainly a combination of these two illusions that causes the overestimation of the short filled distances. In the case of the long distances, the underestimation of the filled s.p.a.ce is, I think, again due to a combination of two illusions. When the finger-tip leaves the filled s.p.a.ce, part of it, because of its length, has already, as it were, left the specious present, and has suffered the foreshortening effect of being relegated to the past. And, on the other hand, after the short distance of the open s.p.a.ce has been traversed the sensations of muscular strain become very p.r.o.nounced, and cause a premature judgment of equality.

One subject, who was very accurate in his judgments, and for whom the illusion hardly existed, said, when asked to explain his method of judging, that after leaving the filled s.p.a.ce he exerted a little more pressure with his finger as he pa.s.sed over the open s.p.a.ce, so as to get the same quant.i.ty of tactual sensations in both instances. The muscular tension that was set up when the subject had pa.s.sed out over the open s.p.a.ce a short way was very plainly noticeable in some subjects, who were seen at this time to hold their breath.

I have thus far continually spoken of the s.p.a.ce containing the tacks as being the filled s.p.a.ce, and the smooth surface as the open s.p.a.ce.

But now we see that in reality the name should be reversed, especially for the longer distances. The smooth surface is, after the first few centimeters, very emphatically filled with sensations arising from the organism which, as I have already intimated, are of the most vital importance in our spatial judgments. Now, according to the most generally accepted psychological theories, it is these organic sensations which are the means whereby we measure time, and our spatial judgments are, in the last a.n.a.lysis, I will not for the present say dependent on, but at any rate fundamentally related to our time judgments.

VIII.

In the last section I attempted to explain the overestimation of short filled s.p.a.ces, and the underestimation of long filled s.p.a.ces by active touch, as the result of a double illusion arising from the differences in the manner and amount of attention given to the two kinds of s.p.a.ces when they are held in immediate contrast. This explanation was of course purely theoretical. I have thus far offered no experiments to show that this double illusion of lengthening, on the one hand, and shortening, on the other, does actually exist. I next made some simple experiments which seemed to prove conclusively that the phenomenon does not exist, or at least not in so important a way, when the time factor is not permitted to enter.

In these new experiments the filled and the open s.p.a.ces were compared separately with optical distances. After the finger-tip was drawn over the filled path, judgment was given on it at once by comparing it directly with an optical distance. In this way the foreshortening effect of time was excluded. In all these experiments it was seen that the filled s.p.a.ce was judged longer when the judgment was p.r.o.nounced on it at once than when an interval of time was allowed, either by drawing the finger-tip out over the open s.p.a.ce, as in the previous experiment, or by requiring the subject to withhold his judgment until a certain signal was given. Any postponement of the judgment resulted in the disappearance of a certain amount of the illusion. The judgments that were made rapidly and without deliberation were subject to the strongest illusion. I have already spoken of the unanimous testimony which all who have made quant.i.tative studies in the corresponding optical illusions have given in this matter of the diminution of the illusion with the lapse of time. The judgments that were made without deliberation always exhibited the strongest tendency to illusion.

I have already said that the illusion for pa.s.sive touch was greatest when the two s.p.a.ces were presented simultaneously and adjacent.

Dresslar has mentioned in his studies on the 'Psychology of Touch,'

that the time factor cannot enter into an explanation of this illusion; but the experiments of which I have just spoken seem to point plainly to a very intimate relation between this illusion and the illusions in our judgments of time. We have here presented on a diminutive scale the illusions which we see in our daily experience in comparing past with present stretches of time. It is a well-known psychological experience that a filled time appears short in pa.s.sing, but long in retrospect, while an empty time appears long in pa.s.sing, but short in retrospect. Now this illusion of the open and filled s.p.a.ce, for the finger-tip, is at every point similar to the illusion to which our time judgment is subject. If we p.r.o.nounce judgment on a filled s.p.a.ce or filled time while we are still actually living in it, it seems shorter than it really is, because, while we pay attention to the discrete sensations of external origin, we lose sight of the sensations of internal origin, which are the sole means whereby we measure lapse of time, and we consequently underestimate such stretches of time or s.p.a.ce. But when the sensations from the outer world which enter into such filled s.p.a.ces or times exist only in memory, the time-measuring sensations of internal origin are allowed their full effect; and such s.p.a.ces and times seem much longer than when we are actually pa.s.sing through them.

I dwell on this illusion at a length which may seem out of proportion to its importance. My object has been to show how widely different are the objective conditions here from what they are in the optical illusion which has so often been called the a.n.a.logue of this.

James[14] has said of this tactual illusion: 'This seems to bring things back to the una.n.a.lyzable laws, by reason of which our feeling of size is determined differently in the skin and in the retina even when the objective conditions are the same.' I think that my experiments have shown that the objective conditions are not the same; that they differ in that most essential of all factors, namely, the time element. Something very nearly the a.n.a.logue of the optical illusion is secured when we take very short open and filled tactual s.p.a.ces, and move over them very rapidly. Here the illusion exists in the same direction as it does for sight, as has already been stated.

On the other hand, a phenomenon more nearly parallel to the tactual illusion, as reported in the experiments of James and Dresslar, is found if we take long optical distances, and traverse the open and filled s.p.a.ces continuously, without having both parts of the line entirely in the field of view at any one moment. I made a few experiments with the optical illusion in this form. The filled and open s.p.a.ces were viewed by the subject through a slot which was pa.s.sed over them. These experiments all pointed in the direction of an underestimation of a filled s.p.a.ce. Everywhere in this illusion, then, where the objective conditions were at all similar for sight and touch, the resulting illusion exists in the same direction for both senses.

[14] James, William, 'Principles of Psychology,' New York, II., p. 250.

Throughout the previous experiments with the illusion for active touch we saw the direct influence of the factor of time. I have yet one set of experiments to report, which seems to me to prove beyond the possibility of a doubt the correctness of my position. These experiments were made with the apparatus shown in Fig. 10. The subjects proceeded precisely as before. The finger-tip was pa.s.sed over the filled s.p.a.ce, and then out over the open s.p.a.ce, until an equivalent distance was measured off. But while the subject was drawing his fingers over the s.p.a.ces, the block _A_ was moved in either direction by means of the lever _B_. The subjects were all the while kept ignorant of the fact that the block was being moved. They all expressed great surprise on being told, after the experiments were over, that the block had been moved under the finger-tip through such long distances without their being able to detect it. The block always remained stationary as the finger pa.s.sed over one s.p.a.ce, but was moved either with or against the finger as it pa.s.sed over the other s.p.a.ce.

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