Harvard Psychological Studies Part 8

Column _A_ contains the filled s.p.a.ces, columns _B_, _C_, _D_, _E_ the open s.p.a.ces that were judged equal. In _B_ the block was moved with the finger, and in _C_ against the finger as it traversed the filled s.p.a.ce, and in _D_ and _E_ the block was moved with and against the finger respectively as it pa.s.sed over the open s.p.a.ce. The block was always moved approximately one-half the distance of the filled s.p.a.ce.

I have given some of the results for one subject in Table XII. These results show at a glance how potent a factor the time element is. The quant.i.ty of tactual sensations received by the finger-tip enters into the judgment of s.p.a.ce to no appreciable extent. With one subject, after he had pa.s.sed his finger over a filled s.p.a.ce of 10 cm. the block was moved so as almost to keep pace with the finger as it pa.s.sed over the open s.p.a.ce. In this way the subject was forced to judge a filled s.p.a.ce of 10 cm. equal to only 2 cm. of the open s.p.a.ce. And when the block was moved in the opposite direction he was made to judge a distance of 10 cm. equal to an open distance of 16 cm.

The criticism may be made on these experiments that the subject has not in reality been obliged to rely entirely upon the time sense, but that he has equated the two s.p.a.ces as the basis of equivalent muscle or joint sensation, which might be considered independent of the sensations which yield the notion of time. I made some experiments, however, to prove that this criticism would not be well founded. By arranging the apparatus so that the finger-tip could be held stationary, and the block with the open and filled s.p.a.ces moved back and forth under it, the measurement by joint and muscle sensations was eliminated.

It will be observed that no uniform motion could be secured by simply manipulating the lever with the hand. But uniformity of motion was not necessary for the results at which I aimed here. Dresslar has laid great stress on the desirability of having uniform motion in his similar experiments. But this, it seems to me, is precisely what is not wanted. With my apparatus, I was able to give widely different rates of speed to the block as it pa.s.sed under the finger-tip. By giving a slow rate for the filled s.p.a.ce and a much more rapid rate for the open s.p.a.ce, I found again that the subject relied hardly at all on the touch sensations that came from the finger-tip, but almost entirely on the consciousness of the amount of time consumed in pa.s.sing over the s.p.a.ces. The judgments were made as in the previous experiments with this apparatus. When the subject reached the point in the open s.p.a.ce which he judged equal to the filled s.p.a.ce, he slightly depressed his finger and stopped the moving block. In this way, the subject was deprived of any a.s.sistance from arm-movements in his judgments, and was obliged to rely on the tactual impressions received at the finger-tip, or on his time sense. That these tactual sensations played here also a very minor part in the judgment of the distance was shown by the fact that these sensations could be doubled or trebled by doubling or trebling the amount of s.p.a.ce traversed, without perceptibly changing the judgment, provided the rate of speed was increased proportionately. s.p.a.ces that required the same amount of time in traversing were judged equal.

In all these experiments the filled s.p.a.ce was presented first. When the open s.p.a.ce was presented first, the results for four out of five subjects were just reversed. For short distances the filled s.p.a.ce was underestimated, for long distances the filled s.p.a.ce was overestimated.

A very plausible explanation for these anomalous results is again to be found in the influence of the time factor. The open s.p.a.ce seemed longer while it was being traversed, but rapidly foreshortened after it was left for the filled s.p.a.ce. While on the other hand, if the judgment was p.r.o.nounced while the subject was still in the midst of the filled s.p.a.ce, it seemed shorter than it really was. The combination of these two illusions is plainly again responsible for the underestimation of the short filled s.p.a.ces. The same double illusion may be taken to explain the opposite tendency for the longer distances.

IX.

The one generalization that I have thus far drawn from the investigation--namely, that the optical illusions are not reversed in pa.s.sing from the field of touch, and that we therefore have a safe warrant for the conclusion that sight and touch do function alike--has contained no implicit or expressed a.s.sertion as to the origin of our notion of s.p.a.ce. I have now reached the point where I must venture an explanation of the illusion itself.

The favorite hypothesis for the explanation of the geometrical optical illusions is the movement theory. The most generally accepted explanation of the illusion with whose tactual counterpart this paper is concerned, is that given by Wundt.[15] Wundt's explanation rests on variation in eye movements. When the eye pa.s.ses over broken distances, the movement is made more difficult by reason of the frequent stoppages. The fact that the s.p.a.ce which is filled with only one point in the middle is underestimated, is explained by Wundt on the theory that the eye has here the tendency to fix on the middle point and to estimate the distance by taking in the whole s.p.a.ce at once without moving from this middle point. A different explanation for this illusion is offered by Helmholtz.[16] He makes use of the aesthetic factor of contrasts. Wundt insists that the fact that this illusion is still present when there are no actual eye movements does not demonstrate that the illusion is not to be referred to a motor origin. He says, "If a phenomenon is perceived with the moving eye only, the influence of movement on it is undoubtedly true. But an inference cannot be drawn in the opposite direction, that movement is without influence on the phenomenon that persists when there is no movement."[17]

[15] Wundt., W., 'Physiolog. Psych.,' 4te Aufl., Leipzig, 1893, Bd. II., S. 144.

[16] v. Helmholtz, H., 'Handbuch d. Physiol. Optik,' 2te Aufl., Hamburg u. Leipzig, 1896, S. 705.

[17] Wundt, W., _op. citat._, S. 139.

Satisfactorily as the movement hypothesis explains this and other optical illusions, it yet falls short of furnishing an entirely adequate explanation. It seems to me certain that several causes exist to produce this illusion, and also the illusion that is often a.s.sociated with it, the well-known Muller-Lyer illusion. But in what degree each is present has not yet been determined by any of the quant.i.tative studies in this particular illusion. I made a number of tests of the optical illusion, with these results: that the illusion is strongest when the attention is fixed at about the middle of the open s.p.a.ce, that there is scarcely any illusion left when the attention is fixed on the middle of the filled s.p.a.ce. It is stronger when the outer end-point of the open s.p.a.ce is fixated than when the outer end of the filled s.p.a.ce is fixated. For the moving eye, I find the illusion to be much stronger when the eye pa.s.ses over the filled s.p.a.ce first, and then over the open s.p.a.ce, than when the process is reversed.

Now, the movement hypothesis does not, it seems to me, sufficiently explain all the fluctuations in the illusion. My experiments with the tactual illusion justify the belief that the movement theory is even less adequate to explain all of the variations there, unless the movement hypothesis is given a wider and richer interpretation than is ordinarily given to it. In the explanation of the tactual illusion which I have here been studying two other important factors must be taken into consideration. These I shall call, for the sake of convenience, the aesthetic factor and the time factor. These factors should not, however, be regarded as independent of the factor of movement. That term should be made wide enough to include these within its meaning. The importance of the time factor in the illusion for pa.s.sive touch I have already briefly mentioned. I have also, in several places in the course of my experiments, called attention to the importance of the aesthetic element in our s.p.a.ce judgments. I wish now to consider these two factors more in detail.

The foregoing discussion has pointed to the view that the s.p.a.ce-perceiving and the localizing functions of the skin have a deep-lying common origin in the motor sensations. My experiments show that, even in the highly differentiated form in which we find them in their ordinary functioning, they plainly reveal their common origin. A formula, then, for expressing the judgments of distance by means of the resting skin might be put in this way. Let _P_ and _P'_ represent any two points on the skin, and let _L_ and _L'_ represent the local signs of these points, and _M_ and _M'_ the muscle sensations which give rise to these local signs. Then _M-M'_ will represent the distance between _P_ and _P'_, whether that distance be judged directly in terms of the localizing function of the skin or in terms of its s.p.a.ce-perceiving function. This would be the formula for a normal judgment. In an illusory judgment, the temporal and aesthetic factors enter as disturbing elements. Now, the point which I insist on here is that the judgments of the extent of the voluntary movements, represented in the formula by _M_ and _M'_, do not depend alone on the sensations from the moving parts or other sensations of objective origin, as Dresslar would say, nor alone on the intention or impulse or innervation as Loeb and others claim, but on the sum of all the sensory elements that enter, both those of external and those of internal origin. And, furthermore, these sensations of external origin are important in judgments of s.p.a.ce, only in so far as they are referred to sensations of internal origin. Delabarre says, "Movements are judged equal when their sensory elements are judged equal. These sensory elements need not all have their source in the moving parts.

All sensations which are added from other parts of the body and which are not recognized as coming from these distant sources, are mingled with the elements from the moving member, and influence the judgment."[18] The importance of these sensations of inner origin was shown in many of the experiments in sections VI. to VIII. In the instance where the finger-tip was drawn over an open and a filled s.p.a.ce, in the filled half the sensations were largely of external origin, while in the open half they were of internal origin. The result was that the s.p.a.ces filled with sensations of internal origin were always overestimated.

The failure to recognize the importance of these inwardly initiated sensations is the chief defect in Dresslar's reasoning. He has endeavored to make our judgments in the illusion in question depend entirely on the sensations of external origin. He insists also that the illusion varies according to the variations in quant.i.ty of these external sensations. Now my experiments have shown, I think, very clearly that it is not the numerical or quant.i.tative extent of the objective sensations which disturbs the judgment of distance, but the sensation of inner origin which we set over against these outer sensations. The piece of plush, because of the disagreeable sensations which it gives, is judged shorter than the s.p.a.ce filled with closely crowded tacks. Dresslar seems to have overlooked entirely the fact that the feelings and emotions can be sources of illusions in the amount of movement, and hence in our judgments of s.p.a.ce. The importance of this element has been pointed out by Munsterberg[19] in his studies of movement.

[18] Delabarre, E.B., 'Ueber Bewegungsempfindungen,' Inaug.

Dissert., Freiburg, 1891.

[19] Munsterberg, H., 'Beitrage zur Experimentellen Psychol.,'

Freiburg i. B., 1892, Heft 4.

Dresslar says again, "The explanations heretofore given, wholly based on the differences in the time the eye uses in pa.s.sing over the two s.p.a.ces, must stop short of the real truth." My experiments, however, as I have already indicated, go to prove quite the contrary. In short, I do not think we have any means of distinguishing our tactual judgments of time from our similar judgments of s.p.a.ce. When the subject is asked to measure off equal s.p.a.ces, he certainly uses time as means, because when he is asked to measure off equal times he registers precisely the same illusion that he makes in his judgments of spatial distances. The fact that objectively equal times were used by Dresslar in his experiments is no reason for supposing that the subject also regarded these times as equal. What I have here a.s.serted of active touch is true also of the resting skin. When a stylus is drawn over the skin, the subject's answer to the question, How long is the distance? is subject to precisely the same illusion as his answer to the question, How long is the time?

I can by a simple ill.u.s.tration show more plainly what I mean by the statement that the blending of the inner and outer sensations is necessary for the perception of s.p.a.ce. I shall use the sense of sight for the ill.u.s.tration, although precisely the same reasoning would apply to the sense of touch. Suppose that I sat in an entirely pa.s.sive position and gazed at a spot on an otherwise blank piece of paper before me. I am perfectly pa.s.sive so far as motion on my part is concerned. I may be engaged in any manner of speculation or be in the midst of the so-called active attention to the spot; but I must be and for the present remain motionless. Now, while I am in this condition of pa.s.sivity, suppose the spot be made to move slowly to one side by some force external to myself. I am immovable all the while, and yet am conscious of this movement of the spot from the first position, which I call _A_, to the new position, _A'_, where it stops. The sensation which I now have is qualitatively different from the sensation which I had from the spot in its original position. My world of experience thus far has been a purely qualitative one. I might go on to eternity having experiences of the same kind, and never dream of s.p.a.ce, or geometry, nor should I have the unique experience of a geometrical illusion, either optical or tactual. Now suppose I set up the bodily movements of the eyes or the head, or of the whole body, which are necessary to follow the path of that point, until I overtake it and once more restore the quality of the original sensation. This circle, completed by the two processes of external activity and restoration by internal activity, forms a group of sensations which const.i.tutes the ultimate atom in our spatial experience. I have my first spatial experience when I have the thrill of satisfaction that comes from overtaking again, by means of my own inner activity, a sensation that has escaped me through an activity not my own. A being incapable of motion, in a world of flux, would not have the spatial experience that we have. A being incapable of motion could not make the distinction between an outer change that can be corrected by an internal change, and an outer change that cannot so be restored. Such an external change incapable of restoration by internal activity we should have if the spot on the paper changed by a chemical process from black to red.

Now such a s.p.a.ce theory is plainly not to be confused with the theory that makes the reversibility of the spatial series its primary property. It is evident that we can have a series of sensations which may be reversed and yet not give the notion of s.p.a.ce. But we should always have s.p.a.ce-perception if one half of the circular process above described comes from an outer activity, and the other half from an inner activity. This way of describing the reversibility of the spatial series makes it less possible to urge against it the objections that Stumpf[20] has formulated against Bain's genetic s.p.a.ce-theory. Stumpf's famous criticism applies not only to Bain, but also to the other English empiricists and to Wundt. Bain says: "When with the hand we grasp something moving and move with it, we have a sensation of one unchanged contact and pressure, and the sensation is imbedded in a movement. This is one experience. When we move the hand over a fixed surface, we have with the feelings of movement a succession of feelings of touch; if the surface is a variable one, the sensations are constantly changing, so that we can be under no mistake as to our pa.s.sing through a series of tactual impressions.

This is another experience, and differs from the first not in the sense of power, but in the tactile accompaniment. The difference, however, is of vital importance. In the one case, we have an object moving and measuring time and continuous, in the other case we have coexistence in s.p.a.ce. The coexistence is still further made apparent by our reversing the movement, and thereby meeting the tactile series in the inverse order. Moreover, the serial order is unchanged by the rapidity of our movements."[21]

[20] Stumpf, K., 'Ueber d. psycholog. Ursprung d.

Raumvorstellung,' Leipzig, 1873, S. 54.

[21] Bain, A., 'The Senses and the Intellect,' 3d ed., New York, 1886, p. 183.

Stumpf maintained in his exhaustive criticism of this theory, first, that there are cases where all of the elements which Bain requires for the perception of s.p.a.ce are present, and yet we have no presentation of s.p.a.ce. Secondly, there are cases where not all of these elements are present, and where we have nevertheless s.p.a.ce presentation. It is the first objection that concerns me here. Stumpf gives as an example, under his first objection, the singing of a series of tones, C, G, E, F. We have here the muscle sensations from the larynx, and the series of the tone-sensations which are, Stumpf claims, reversed when the muscle-sensations are reversed, etc. According to Stumpf, these are all the elements that are required by Bain, and yet we have no perception of s.p.a.ce thereby. Henri[22] has pointed out two objections to Stumpf's criticism of Bain's theory. He says that Bain a.s.sumes, what Stumpf does not recognize, that the muscle sensations must contain three elements--resistance, time, and velocity--before they can lead to s.p.a.ce perceptions. These three elements are not to be found in the muscle sensations of the larynx as we find them in the sensations that come from the eye or arm muscles. In addition to this, Henri claims that Bain's theory demands a still further condition. If we wish to touch two objects, _A_ and _B_, with the same member, we can get a spatial experience from the process only if we insert between the touching of _A_ and the touching of _B_ a continual series of tactual sensations. In Stumpf's instance of the singing of tones, this has been overlooked. We can go from the tone C to the tone F without inserting between the two a continuous series of musical sensations.

[22] Henri, V., 'Ueber d. Raumwahrnehmungen d. Tastsinnes,'

Berlin, 1898, S. 190.

I think that all such objections to the genetic s.p.a.ce theories are avoided by formulating a theory in the manner in which I have just stated. When one says that there must be an outer activity producing a displacement of sensation, and then an inner activity retaining that sensation, it is plain that the singing of a series of tones ascending and then descending would not be a case in point.

TACTUAL TIME ESTIMATION.

BY KNIGHT DUNLAP.

I. GENERAL NATURE OF THE WORK.

The experiments comprised in this investigation were made during the year 1900-1901 and the early part of the year 1901-1902. They were planned as the beginning of an attempt at the a.n.a.lysis of the estimation of time intervals defined by tactual stimulations. The only published work in this quarter of the field so far is that of Vierordt,[1] who investigated only the constant error of time judgment, using both auditory and tactual stimulations, and that of Meumann,[2] who in his last published contribution to the literature of the time sense gives the results of his experiments with 'filled'

and 'empty' tactual intervals. The stimuli employed by Meumann were, however, not purely tactual, but electrical.

[1] Vierordt: 'Der Zeitsinn,' Tubingen, 1868.

[2] Meumann, E.: 'Beitrage zur Psychologie des Zeitbewusstseins,' III., _Phil. Studien,_ XII., S. 195-204.

The limitation of time intervals by tactual stimulations offers, however, a rich field of variations, which promise a.s.sistance in the a.n.a.lytical problem of the psychology of time. The variations may be those of locality, area, intensity, rigidity, form, consecutiveness, and so on, in addition to the old comparisons of filled and empty intervals, intervals of varying length, and intervals separated by a pause and those not so separated.

To begin with, we have selected the conditions which are mechanically the simplest, namely, the comparison of two empty time intervals, both given objectively with no pause between them. We have employed the most easily accessible dermal areas, namely, that of the fingers of one or both hands, and introduced the mechanically simplest variations, namely, in locality stimulated and intensity of stimulation.

It was known from the results of nearly all who have studied the time sense experimentally, that there is in general a constant error of over- or underestimation of time intervals of moderate length, and from the results of Meumann,[3] that variations in intensity of limiting stimulation influenced the estimation decidedly, but apparently according to no exact law. The problem first at hand was then to see if variations introduced in tactual stimulations produce any regularity of effect, and if they throw any new light on the phenomena of the constant error.

[3] Meumaun, E.: 'Beitrage zur Psychologie des Zeitsinns,' II., _Phil. Studien_, IX., S. 264.

The stimulations employed were light blows from the cork tip of a hammer actuated by an electric current. These instruments, of which there were two, exactly alike in construction, were similar in principle to the acoustical hammers employed by Estel and Mehner. Each consisted essentially of a lever about ten inches in length, pivoted near one extremity, and having fastened to it near the pivot an armature so acted upon by an electromagnet as to depress the lever during the pa.s.sage of an electric current. The lever was returned to its original position by a spring as soon as the current through the electromagnet ceased. A clamp at the farther extremity held a small wooden rod with a cork tip, at right angles to the pivot, and the depression of the lever brought this tip into contact with the dermal surface in proximity with which it had been placed. The rod was easily removable, so that one bearing a different tip could be subst.i.tuted when desired. The whole instrument was mounted on a compact base attached to a short rod, by which it could be fastened in any desired position in an ordinary laboratory clamp.

During the course of most of the experiments the current was controlled by a pendulum beating half seconds and making a mercury contact at the lowest point of its arc. A condenser in parallel with the contact obviated the spark and consequent noise of the current interruption. A key, inserted in the circuit through the mercury cup and tapping instrument, allowed it to be opened or closed as desired, so that an interval of any number of half seconds could be interposed between successive stimulations.

In the first work, a modification of the method of right and wrong cases was followed, and found satisfactory. A series of intervals, ranging from one which was on the whole distinctly perceptible as longer than the standard to one on the whole distinctly shorter, was represented by a series of cards. Two such series were shuffled together, and the intervals given in the order so determined. Thus, when the pile of cards had been gone through, two complete series had been given, but in an order which the subject was confident was perfectly irregular. As he also knew that in a given series there were more than one occurrence of each compared interval (he was not informed that there were exactly two of each), every possible influence favored the formation each time of a perfectly fresh judgment without reference to preceding judgments. The only fear was lest certain sequences of compared intervals (_e.g._, a long compared interval in one test followed by a short one in the next), might produce unreliable results; but careful examination of the data, in which the order of the interval was always noted, fails to show any influence of such a factor.

To be more explicit with regard to the conditions of judgment; two intervals were presented to the subject in immediate succession. That is, the second stimulation marked the end of the first interval and the beginning of the second. The first interval was always the standard, while the second, or compared interval, varied in length, as determined by the series of cards, and the subject was requested to judge whether it was equal to, or longer or shorter than the standard interval.

In all of the work under Group 1, and the first work under Group 2, the standard interval employed was 5.0 seconds. This interval was selected because the minimum variation possible with the pendulum apparatus ( sec.) was too great for the satisfactory operation of a shorter standard, and it was not deemed advisable to keep the subject's attention on the strain for a longer interval, since 5.0 sec. satisfied all the requirements of the experiment.

In all work here reported, the cork tip on the tapping instrument was circular in form, and 1 mm. in diameter. In all, except one experiment of the second group, the areas stimulated were on the backs of the fingers, just above the nails. In the one exception a spot on the forearm was used in conjunction with the middle finger.