Harvard Psychological Studies Part 2

We have now to seek some experimental test which shall demonstrate definitely either the presence or the absence of a central anaesthesia during eye-movements. The question of head-movements will be deferred, although, as we have seen above, these afford equally the phenomenon of twice-localized after-images.

IV. THE PENDULUM-TEST FOR ANaeSTHESIA.

A. Apparatus must be devised to fulfil the following conditions. A retinal stimulation must be given during an eye-movement. The moment of excitation must be so brief and its intensity so low that the process shall be finished before the eye comes to rest, that is, so that no after-image shall be left to come into consciousness _after_ the movement is over. Yet, on the other hand, it must be positively demonstrated that a stimulation of this _very same_ brief duration and low intensity is amply strong enough to force its way into consciousness if no eye-movement is taking place. If such a stimulation, distinctly perceived when the eye is at rest, should not be perceptible if given while the eye is moving, we should have a valid proof that some central process has intervened during the movement, to shut out the stimulation-image during that brief moment when it might otherwise have been perceived.

Obviously enough, with the perimeter arrangement devised by Dodge, where the eye moves past a narrow, illuminated slit, the light within the slit can be reduced to any degree of faintness. But on the other hand, it is clearly impossible to find out how long the moment of excitation lasts, and therefore impossible to find out whether an excitation of the same duration and intensity is yet sufficient to affect consciousness if given when the eye is not moving. Unless the stimulation is proved to be thus sufficient, a failure to see it when given during an eye-movement would of course prove nothing at all.

Perhaps the most exact way to measure the duration of a light-stimulus is to let it be controlled by the pa.s.sing of a shutter which is affixed to a pendulum. Furthermore, by means of a pendulum a stimulation of exactly the same duration and intensity can be given to the moving, as to the resting eye. Let us consider Fig. 4:1. If _P_ is a pendulum bearing an opaque shield _SS_ pierced by the hole _tt_, and _BB_ an opaque background pierced by the hole _i_ behind which is a lamp, it is clear that if the eye is fixed on _i_, a swing of the pendulum will allow _i_ to stimulate the retina during such a time as it takes the opening _tt_ to move past _i_. The shape of _i_ will determine the shape of the image on the retina, and the intensity of the stimulation can be regulated by ground-or milk-gla.s.s interposed between the hole _i_ and the lamp behind it. The duration of the exposure can be regulated by the width of _tt_, by the length of the pendulum, and by the arc through which it swings.

If now the conditions are altered, as in Fig. 4:2, so that the opening _tt_ (indicated by the dotted line) lies not in _SS_, but in the fixed background _BB_, while the small hole _i_ now moves with the shield _SS_, it necessarily follows that if the eye can move at just the rate of the pendulum, it will receive a stimulation of exactly the same size, shape, duration, and intensity as in the previous case where the eye was at rest. Furthermore, it will always be possible to tell whether the eye does move at the same rate as the pendulum, since if it moves either more rapidly or more slowly, the image of _i_ on the retina will be horizontally elongated, and this fact will be given by a judgment as to the proportions of the image seen.

It may be said that since the eye does not rotate like the pendulum, from a fulcrum above, the image of _i_ in the case of the moving eye will be distorted as is indicated in Fig. 4, _a_. This is true, but the distortion will be so minute as to be negligible if the pendulum is rather long (say a meter and a half) and the opening _tt_ rather narrow (say not more than ten degrees wide). A merely horizontal movement of the eye will then give a practically exact superposition of the image of _i_ at all moments of the exposure.

[Ill.u.s.tration: PSYCHOLOGICAL REVIEW. MONOGRAPH SUPPLEMENT, 17. PLATE PLATE II.

Fig. 4. Fig. 6.

HOLT ON EYE-MOVEMENT.]

Thus much of preliminary discussion to show how, by means of a pendulum, identical stimulations can be given to the moving and to the resting eye. We return to the problem. It is to find out whether a stimulation given during an eye-movement can be perceived if its after-image is so brief as wholly to elapse before the end of the movement. If a period of anaesthesia is to be demonstrated, two observations must be made. First, that the stimulation is bright enough to be _unmistakably visible_ when given to the eye at rest; second, that it is not visible when given to the moving eye. Hence, we shall have three cases.

Case 1. A control, in which the stimulation is proved intense enough to be seen by the eye at rest.

Case 2. In which the same stimulation is given to the eye during movement.

Case 3. Another control, to make sure that no change in the adaptation or fatigue of the eye has intervened during the experiments to render the eye insensible to the stimulation.

Fig. 5 shows the exact arrangement of the experiment. The figure represents a horizontal section at the eye-level of the pendulum of Fig. 4, with accessories. _E_ is the eye which moves between the two fixation-points _P_ and _P_'. _WONW_ is a wall which conceals the mechanism of the pendulum from the subject. _ON_ is a rectangular hole 9 cm. wide and 7 cm. high, in this wall. _SS_ is the shield which swings with the pendulum, and _BB_ is the background (cf. Fig. 4).

When the pendulum is not swinging, a hole in the shield lies behind _ON_ and exactly corresponds with it. Another in the background does the same. The eye can thus see straight through to the light _L_.

Each of these three holes has grooves to take an opaque card, _x_, _y_, or _z_; there are two cards for the three grooves, and they are pierced with holes to correspond to _i_ and _tt_ of Fig. 4. The background _BB_ has a second groove to take a piece of milk-gla.s.s _M_.

These cards are shown in Fig. 6 (Plate II.) Card _I_ bears a hole 5 cm. high and shaped like a dumb-bell. The diameter of the end-circles (_e_, _e_) is 1.3 cm., and the width of the handle _h_ is 0.2 cm. Card _T_ is pierced by two slits _EE_, _EE_, each 9 cm. long and 1.3 cm.

high, which correspond to the two ends of the dumb-bell. These slits are connected by a perforation _H_, 1.5 cm. wide, which corresponds to the handle of the dumb-bell. This opening _EEHEE_ is covered by a piece of ground-gla.s.s which serves as a radiating surface for the light.

[Ill.u.s.tration: Fig. 5.]

The distance _EA_ (Fig. 5) is 56 cm., and _PP_' is 40 cm.; so that the arc of eye-movement, that is, the angle _PEP_', is very nearly 40, of which the 9-cm. opening _ON_ 9 11'. _SS_ is 2 cm. behind _ON_, and _BB_ 2 cm. behind _SS_; these distances being left to allow the pendulum to swing freely.

It is found under these conditions that the natural speed made by the eye in pa.s.sing the 9-cm. opening _ON_ is very well approximated by the pendulum if the latter is allowed to fall through 23.5 of its arc, the complete swing being therefore 47. The middle point of the pendulum is then found to move from _O_ to _N_ in 110[sigma][19]. If the eye sweeps from _O_ to _N_ in the same time, it will be moving at an angular velocity of 1 in 11.98[sigma] (since the 9 cm. are 9 11'

of eye-movement). This rate is much less than that found by Dodge and Cline (_op. cit._, p. 155), who give the time for an eye-movement of 40 as 99.9[sigma], which is an average of only 2.49[sigma] to the degree. Voluntary eye-movements, like other voluntary movements, can of course be slow or fast according to conditions. After the pendulum has been swinging for some time, so that its amplitude of movement has fallen below the initial 47 and therewith its speed past the middle point has been diminished, the eye in its movements back and forth between the fixation-points can still catch the after-image of _i_ perfectly distinct and not at all horizontally elongated, as it would have to be if eye and pendulum had not moved just together. It appears from this that certain motives are able to r.e.t.a.r.d the rate of voluntary movements of the eye, even when the distance traversed is constant.

[19] The speed of the pendulum is measured by attaching a tuning-fork of known vibration-rate to the pendulum, and letting it write on smoked paper as the pendulum swings past the 9-cm. opening.

The experiment is now as follows. The room is darkened. Card _T_ is dropped into groove _z_, while _I_ is put in groove _y_ and swings with the pendulum. One eye alone is used.

Case 1. The eye is fixed in the direction _EA_. The pendulum is allowed to swing through its 47. The resulting visual image is shown in Fig. 7:1. Its shape is of course like _T_, Fig. 6, but the part _H_ is less bright than the rest because it is exposed a shorter time, owing to the narrowness of the handle of the dumb-bell, which swings by and mediates the exposure. Sheets of milk-gla.s.s are now dropped into the back groove of _BB_, until the light is so tempered that part _H_ (Fig. 7:1) is _barely but unmistakably_ visible as luminous.

The intensity actually used by the writer, relative to that of _EE_, is fairly shown in the figure. (See Plate III.)

It is clear, if the eye were now to move with the pendulum, that the same amount of light would reach the retina, but that it would be concentrated on a horizontally narrower area. And if the eye moves exactly with the pendulum, the visual image will be no longer like 1 but like 2 (Fig. 7). We do not as yet know how the intensities of _e_, _e_ and _h_ will relatively appear. To ascertain this we must put card _I_ into groove _x_, and let card _T_ swing with the pendulum in groove _y_. If the eye is again fixed in the direction _EA_ (Fig. 5), the retina receives exactly the same stimulation that it would have received before the cards were shifted if it had moved exactly at the rate of the pendulum. In the experiments described, the handle _h_ of this image (Fig. 7:2) curiously enough appears of the same brightness as the two ends _e_, _e_, although, as we know, it is stimulated for a briefer interval. Nor can any difference between _e_, _e_ and _h_ be detected in the time of disappearance of their after-images. These conditions are therefore generous. The danger is that _h_ of the figure, the only part of the stimulation which could possibly quite elapse during the movement, is still too bright to do so.

Case 2. The cards are replaced in their first positions, _T_ in groove _z_, _I_ in groove _y_ which swings. The subject is now asked to make voluntary eye-sweeps from _P_ to _P'_ and back, timing his moment of starting so as to bring his axis of vision on to the near side of opening _ON_ at approximately the same time as the pendulum brings _I_ on the same point. This is a delicate matter and requires practice.

Even then it would be impossible, if the subject were not allowed to get the rhythm of the pendulum before pa.s.sing judgment on the after-images. The pendulum used gives a slight click at each end of its swing, and from the rhythm of this the subject is soon able to time the innervation of his eye so that the exposure coincides with the middle of the eye-movement.

[Ill.u.s.tration: PSYCHOLOGICAL REVIEW. MONOGRAPH SUPPLEMENT, 17. PLATE III.

Fig. 7.

HOLT ON EYE-MOVEMENT.]

It is true that with every swing the pendulum moves more slowly past _ON_, and the period of exposure is lengthened. This, however, only tends to make the retinal image brighter, so that its disappearance during an anaesthesia would be so much the less likely. The pendulum may therefore be allowed to 'run down' until its swing is too slow for the eye to move with it, that is, too slow for a distinct, non-elongated image of _i_ to be caught in transit on the retina.

With these eye-movements, the possible appearances are of two cla.s.ses, according to the localization of the after-image. The image is localized either at _A_ (Fig. 5), or at the final fixation-point (_P_ or _P'_, according to the direction of the movement). Localized at _A_, the image may be seen in either of two shapes. First, it may be identical with 1, Fig. 7. It is seen somewhat peripherally, judgment of indirect vision, and is correctly localized at _A_. When the subject's eye is watched, it is found that in this case it moved either too soon or too late, so that when the exposure was made, the eye was resting quietly on one of the fixation-points and so naturally received the same image as in case 1, except that now it lies in indirect vision, the eye being directed not toward _A_ (as in case 1) but towards either _P_ or _P'_.

Second, the image correctly localized may be like 2 (Fig. 7), and then it is seen to move past the opening _ON_. The handle _h_ looks as bright as _e_, _e_. This appearance once obtained generally recurs with each successive swing of the pendulum, and scrutiny of the subject's eye shows it to be moving, not by separate voluntary innervations from _P_ to _P'_ and then from _P'_ to _P_, but continuously back and forth with the swing of the pendulum, much as the eye of a child pa.s.sively follows a moving candle. This movement is purely reflex,[20] governed probably by cerebellar centers. It seems to consist in a rapid succession of small reflex innervations, and is very different from the type of movement in which one definite innervation carries the eye through its 42, and which yielded the phenomena with the perimeter. A subject under the spell of this reflex must be exercised in innervating his eye to move from _P_ to _P'_ and back in single, rapid leaps. For this, the pendulum is to be motionless and the eye is not to be stimulated during its movement.

[20] Exner, Sigmund, _Zeitschrift f. Psychologie u. Physiologie der Sinnesorgane_, 1896, XII., S. 318. 'Entwurf zu einer physiologischen Erklarung der psychischen Erscheinungen,'

Leipzig u. Wien, 1894, S. 128. Mach, Ernst, 'Beitrage zur a.n.a.lyse der Empfindungen,' Jena, 1900, S. 98.

These two cases in which the image is localized midway between _P_ and _P'_ interest us no further. Localized on the final fixation-point, the image is always felt to flash out suddenly _in situ_, just as in the case of the 'correctly localized' after-image streaks in the experiments with the perimeter. The image appears in one of four shapes, Fig. 7: 2 or 3, 4 or 5.

First, the plain or elongated outline of the dumb-bell appears with its handle on the final fixation-point (2 or 3). The image is plain and undistorted if the eye moves at just the rate of the pendulum, elongated if the eye moves more rapidly or more slowly. The point that concerns us is that the image appears _with its handle_. Two precautions must here be observed.

The eye does not perhaps move through its whole 42, but stops instead just when the exposure is complete, that is, stops on either _O_ or _N_ and considerably short of _P_ or _P'_. It then follows that the exposure is given at the _very last_ part of the movement, so that the after-image of even the handle _h_ has not had time to subside. The experiment is planned so that the after-image of _h_ shall totally elapse during that part of the movement which occurs after the exposure, that is, while the eye is completing its sweep of 42, from _O_ to _P_, or else from _N_ to _P'_. If the arc is curtailed at point _O_ or _N_, the handle of the dumb-bell will of course appear. The fact can always be ascertained by asking the subject to notice very carefully where the image is localized. If the eye does in fact stop short at _O_ or _N_, the image will be there localized, although the subject may have thoughtlessly said before that it was at _P_ or _P'_, the points he had nominally had in mind.

But the image 2 or 3 may indeed be localized quite over the final fixation-point. In this case the light is to be looked to. It is too bright, as it probably was in the case of Dodge's experiments. It must be further reduced; and with the eye at rest, the control (case I) must be repeated. In the experiments here described it was always found possible so to reduce the light that the distinct, entire image of the dumb-bell (2, Fig. 7) never appeared localized on the final fixation-point, although in the control, _H_, of Fig. 7:1, was always distinctly visible.

With these two precautions taken, the image on the final fixation-point is like either 3, 4, or 5. Shape 5 very rarely appears, while the trained subject sees 4 and 3 each about one half the times; and either may be seen for as many as fifteen times in succession.

Shape 4 is of course exactly the appearance which this experiment takes to be crucial evidence of a moment of central anaesthesia, before the image is perceived and during which the stimulation of the handle _h_ completely elapses. Eight subjects saw this phenomenon distinctly and, after some training in timing their eye-movements, habitually.

The first appearance of the handleless image was always a decided surprise to the subject (as also to the writer), and with some eagerness each hastened to verify the phenomenon by new trials.

The two ends (_e_, _e_) of the dumb-bell seem to be of the same intensity as in shape 2 when seen in reflex movement. But there is no vestige whatsoever of a handle. Two of the subjects stated that for them the place where the handle should have been, appeared of a velvety blackness more intense than the rest of the background. The writer was not able to make this observation. It coincides interestingly with that of von Kries,[21] who reports as to the phases of fading after-images, that between the disappearance of the primary image and the appearance of the 'ghost,' a moment of the most intense blackness intervenes. The experiments with the pendulum, however, brought out no ghost.

[21] Von Kries, J., _Zeitschr. f. Psych, u. Physiol. d.

Sinnesorgane_, 1896, XII., S. 88.

We must now enquire why in about half the cases shape 3 is still seen, whereas shape 5 occurs very rarely. Some of the subjects, among whom is the writer, never saw 5 at all. We should expect that with the intensity of _H_ sufficiently reduced 4 and 5 would appear with equal frequency, whereas 3 would be seen no oftener than 2; shape 5 appearing when the eye did not, and 4 when it did, move at just the rate of the pendulum. It is certain that when 4 is seen, the eye has caught just the rate of the pendulum, and that for 3 or 5 it has moved at some other rate. We have seen above (p. 27) that to move with the pendulum the eye must already move decidedly more slowly than Dodge and Cline find the eye generally to move. Nothing so reliable in regard to the rate of voluntary eye-movements as these measurements of Dodge and Cline had been published at the time when the experiments on anaesthesia were carried on, and it is perhaps regrettable that in the 'empirical' approximation of the natural rate of the eye through 40 the pendulum was set to move so slowly.

In any case it is highly probable that whenever the eye did not move at just the rate of the pendulum, it moved _more rapidly_ rather than more slowly. The image is thus horizontally elongated, by an amount which varies from the least possible up to 9 cm. (the width of the opening in _T_), or _even more_. And while the last of the movement (_O_ to _P_, or _N_ to _P'_), in which the stimulation of _H'_ is supposed to subside, is indeed executed, it may yet be done so _rapidly_ that after all _H'_ cannot subside, not even although it is now less intense by being horizontally spread out (that is, less concentrated than the vanished _h_ of shape 4). This explanation is rendered more probable by the very rare appearance of shape 5, which must certainly emerge if ever the eye were to move more slowly than the pendulum.

The critical fact is, however, that shape 4 _does_ appear to a trained subject in about one half the trials--a very satisfactory ratio when one considers the difficulty of timing the beginning of the movement and its rate exactly to the pendulum.

Lastly, in some cases no image appears at all. This was at first a source of perplexity, until it was discovered that the image of the dumb-bell, made specially small so as to be contained within the area of distinct vision, could also be contained on the blind-spot. With the pendulum at rest the eye could be so fixed as to see not even the slight halo which diffuses in the eye and seems to lie about the dumb-bell. It may well occur, then, that in a movement the image happens to fall on the blind-spot and not on the fovea. That this accounts for the cases where no image appears, is proved by the fact that if both eyes are used, some image is always seen. A binocular image under normal convergence can of course not fall on both blind-spots. It may be further said that the shape 4 appears as well when both eyes are used as with only one. The experiment may indeed as well be carried on with both eyes.

Some objections must be answered. It may be said that the image of _h_ happens to fall on the blind-spot, _e_ and _e_ being above and below the same. This is impossible, since the entire image and its halo as well may lie within the blind-spot. If now _h_ is to be on the blind-spot, at least one of the end-circles _e_, _e_ will be there also, whereas shape 4 shows both end-circles of the dumb-bell with perfect distinctness.

Again, it cannot properly be urged that during the movement the attention was distracted so as not to 'notice' the handle. The shape of a dumb-bell was specially chosen for the image so that the weaker part of the stimulation should lie between two points which should be clearly noticed. Indeed, if anything, one might expect this central, connecting link in the image to be apperceptively filled in, even when it did not come to consciousness as immediate sensation. And it remains to ask what it is which should distract the attention.

In this connection the appearance under reflex eye-movement compares interestingly with that under voluntary. If the wall _WONW_ (Fig. 5) is taken from before the pendulum, and the eye allowed to move reflexly with the swinging dumb-bell, the entire image is seen at each exposure, the handle seeming no less bright than the end-circles.

Moreover, as the dumb-bell opening swings past the place of exposure and the image fades, although the handle must fade more quickly than the ends, yet this is not discernible, and the entire image disappears without having at any time presented the handleless appearance.