Sixteen Experimental Investigations from the Harvard Psychological Laboratory by Hugo Münsterberg (100 books to read .txt) 📕

second, that there is, in fact, no such thing as fusion, is a

supposition which is in such striking contrast to all previous

explanations of optical phenomena, that it should be accepted only if

no other theory can do justice to them. It is hoped that the following

pages will show that the facts do not demand such a theory.

Another simple observation is interesting in this connection. If at

any time, except when the eyes are quite fresh, one closes one’s eyes

and attends to the after-images, some will be found which are so faint

as to be just barely distinguishable from the idioretinal light. If

the attention is then fixed on one such after-image, and the eyes are

moved, the image will suddenly disappear and slowly emerge again after

the eyes have come to rest. This disappearance during eye-movements

can be observed also on after-images of considerable intensity; these,

however, flash back instantly into view, so that the observation is

somewhat more difficult. Exner,[2] in speaking of this phenomenon,

adds that in general “subjective visual phenomena whose origin lies in

the retina, as for instance after-images, Purkinje’s vessel-figure,

or the phenomena of circulation under discussion, are almost

exclusively to be seen when the eye is rigidly fixed on a certain

spot: as soon as a movement of the eye is made, the subjective

phenomena disappear.”

[2] Exner, Sigmund, _Zeitschrift f. Psychologie u. Physiologie

der Sinnesorgane_, 1890, I., S. 46.

The facts here mentioned in no wise contradict a phenomenon recently

discussed by McDougall,[3] wherein eye-movements revive sensations

which had already faded. Thus an eye-movement will bring back an

after-image which was no longer visible. This return to vividness

takes place after the movement has been completed, and there is no

contention that the image is seen just during the movement.

[3] McDougall, W., Mind, N.S., X., 1901, p. 52.

The disappearance of after-images during eye-movements is mentioned by

Fick and Gürber,[4] who seek to explain the phenomenon by ascribing it

to a momentary period of recovery which the retina perhaps undergoes,

and which would for the moment prevent further stimulations from being

transmitted to the optic nerve. Exner observes that this explanation

would not, however, apply to the disappearance of the vessel-figure,

the circulation phenomenon, the foveal figure, the polarization-sheaf

of Haidinger, Maxwell’s spot, or the ring of Löwe; for these phenomena

disappear in a similar manner during movement. Exner offers another

and a highly suggestive explanation. He says of the phenomenon (_op.

citat._, S. 47), “This is obviously related to the following fact,

that objective and subjective impressions are not to be distinguished

as such, so long as the eye is at rest, but that they are immediately

distinguished if an eye-movement is executed; for then the subjective

phenomena move with the eye, whereas the objective phenomena are not

displaced…. This neglect of the subjective phenomena is effected,

however, not by means of an act of will, but rather by some central

mechanism which, perhaps in the manner of a reflex inhibition,

withholds the stimulation in question from consciousness, without our

assistance and indeed without our knowledge.” The suggestion of a

central mechanism which brings about a reflex inhibition is the

significant point.

[4] Fick, Eug., and Gürber, A., _Berichte d. ophthalmologischen

Gesellschaft in Heidelberg_, 1889.

It is furthermore worth noting that movements of the eyelid and

changes in the accommodation also cause the after-images to disappear

(Fick and Gürber), whereas artificial displacement of the eye, as by

means of pressure from the finger, does not interfere with the images

(Exner).

Another motive for suspecting anæsthesia during eye-movement is found

by Dodge,[5] in the fact that, “One may watch one’s eyes as closely as

possible, even with the aid of a concave reflector, whether one looks

from one eye to the other, or from some more distant object to one’s

own eyes, the eyes may be seen now in one position and now in another,

but never in motion.” This phenomenon was described by Graefe,[6] who

believed it was to be explained in the same way as the illusion which

one experiences in a railway coach when another train is moving

parallel with the coach in which one sits, in the same direction and

at the same speed. The second train, of course, appears motionless.

[5] Dodge, Raymond, PSYCHOLOGICAL REVIEW, 1900, VII., p. 456.

[6] Graefe, A., Archiv f. Ophthalmologie, 1895, XLI., 3, S.

This explanation of Graefe is not to be admitted, however, since in

the case of eye-movement there are muscular sensations of one’s own

activity, which are not present when one merely sits in a coach. These

sensations of eye-movement are in all cases so intimately connected

with our perception of the movement of objects, that they may not be

in this case simply neglected. The case of the eye trying to watch its

own movement in a mirror is more nearly comparable with the case in

which the eye follows the movement of some independent object, as a

race-horse or a shooting-star. In both cases the image remains on

virtually the same point of the retina, and in both cases muscular

sensations afford the knowledge that the eye is moving. The

shooting-star, however, is perceived to move, and the question

remains, why is not the eye in the mirror also seen to move?

F. Ostwald[7] refutes the explanation of Graefe from quite different

considerations, and gives one of his own, which depends on the

geometrical relations subsisting between the axes of vision of the

real eye and its reflected image. His explanation is too long to be

here considered, an undertaking which indeed the following

circumstance renders unnecessary. While it is true that the eye cannot

observe the full sweep of its own movement, yet nothing is easier than

to observe its movement through the very last part of the arc. If one

eye is closed, and the other is brought to within about six inches of

an ordinary mirror, and made to describe little movements from some

adjacent part of the mirror to its own reflected image, this image can

almost without exception be observed as just coming to rest. That is,

the very last part of the movement can be seen. The explanation of

Ostwald can therefore not be correct, for according to it not alone

some parts of the movement, but absolutely all parts alike must remain

invisible. It still remains, therefore, to ask why the greater part of

the movement eludes observation. The correct explanation will account

not only for the impossibility of seeing the first part of the

movement but also for the possibility of seeing the remainder.

[7] Ostwald, F., Revue Scientifique, 1896, 4e Série, V., p.

Apart from the experience of the eye watching itself in a glass, Dodge

(_loc. citat._) found another fact which strongly suggested

anæsthesia. In the course of some experiments on reading, conducted by

Erdmann and Dodge, the question came up, how “to explain the meaning

of those strangely rhythmic pauses of the eye in reading every page of

printed matter.” It was demonstrated (ibid., p. 457) “that the

rhythmic pauses in reading are the moments of significant

stimulation…. If a simple letter or figure is placed between two

fixation-points so as to be irrecognizable from both, no eye-movement

is found to make it clear, which does not show a full stop between

them.”

With these facts in view Dodge made an experiment to test the

hypothesis of anæsthesia. He proceeded as follows (ibid., p. 458):

“A disc of black cardboard thirteen inches in diameter, in which a

circle of one-eighth inch round holes, one half inch apart, had been

punched close to the periphery all around, was made to revolve at such

a velocity that, while the light from the holes fused to a bright

circle when the eye was at rest, when the eye moved in the direction

of the disc’s rotation from one fixation point, seen through the fused

circle of light, to another one inch distant, three clear-cut round

holes were seen much brighter than the band of light out of which they

seemed to emerge. This was only possible when the velocity of the

holes was sufficient to keep their images at exactly the same spot on

the retina during the movement of the eye. The significant thing is

that the individual round spots of light thus seen were much more

intense than the fused line of light seen while the eyes were at rest.

Neither my assistant nor I was able to detect any difference in

brightness between them and the background when altogether

unobstructed.” Dodge finds that this experiment ‘disproves’ the

hypothesis of anæsthesia.

If by ‘anæsthesia’ is meant a condition of the retinal end-organs in

which they should be momentarily indifferent to excitation by

light-waves, the hypothesis is indeed disproved, for obviously the

‘three clear-cut round holes’ which appeared as bright as the

unobstructed background were due to a summation of the light which

reached the retina during the movement, through three holes of the

disc, and which fell on the same three spots of the retina as long as

the disc and the eyeball were moving at the same angular rate. But

such a momentary anæsthesia of the retina itself would in any case,

from our knowledge of its physiological and chemical structure, be

utterly inconceivable.

On the other hand, there seems to be nothing in the experiment which

shows that the images of the three holes were present to consciousness

just during the movement, rather than immediately thereafter. A

central mechanism of inhibition, such as Exner mentions, might

condition a central anæsthesia during movement, although the

functioning of the retina should remain unaltered. Such a central

anæsthesia would just as well account for the phenomena which have

been enumerated. The three luminous images could be supposed to remain

unmodified for a finite interval as positive after-images, and as such

first to appear in consciousness. Inasmuch as ‘the arc of eye

movements was 4.7°’ only, the time would be too brief to make possible

any reliable judgment as to whether the three holes were seen during

or just after the eye-movement. With this point in view, the writer

repeated the experiment of Dodge, and found indeed nothing which gave

a hint as to the exact time when the images emerged in consciousness.

The results of Dodge were otherwise entirely confirmed.

II. THE PHENOMENON OF ‘FALSELY LOCALIZED AFTER-IMAGES.’

A further fact suggestive of anæsthesia during movement comes from an

unexpected source. While walking in the street of an evening, if one

fixates for a moment some bright light and then quickly turns the eye

away, one will observe that a luminous streak seems to dart out from

the light and to shoot away in either of two directions, either in the

same direction as that in which the eye moved, or in just the

opposite. If the eye makes only a slight movement, say of 5°, the

streak jumps with the eye; but if the eye sweeps through a rather

large arc, say of 40°, the luminous streak darts away in the opposite

direction. In the latter case, moreover, a faint streak of light

appears later, lying in the direction of the eye-movement.

This phenomenon was probably first described by Mach, in 1886.[8] His

view is essentially as follows: It is clear that in whatever direction

the eye moves, away from its luminous fixation point, the streak

described on the retina by the luminous image will lie on the same

part of the retina as it would have lain on had the eye remained at

rest but the object moved in the opposite direction. Thus, if the eye

moves to the right, we should expect the streak to appear to dart to

the left.