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

pierced at H by a one-centimeter hole. The distance EH is 34 cm.

The disc DD is so pivoted that the highest point of the circle of

holes lies in a straight line between the eye E and the lamp L.

The hole H lies also in this straight line. A piece of milk-glass

M intervenes between L and H, to temper the illumination. The

disc DD is geared to a wheel W, which can be turned by the hand of

the observer at E, or by a second person. As the disc revolves, each

hole in turn crosses the line EL. Thus the luminous hole H is

successively covered and uncovered to the eye E; and if the eye

moves, a succession of points on the retina is stimulated by the

successive uncovering of the luminous spot. No fixation-points are

provided for the eye, since such points, if bright enough to be of use

in the otherwise dark room, might themselves produce confusing

streaks, and also since an exact determination of the arc of

eye-movement would be superfluous.

[Illustration: Fig. 1.]

The eye was first fixated on the light-spot, and then moved

horizontally away toward either the right or the left. In the first

few trials (with eye-sweeps of medium length), the observations did

not agree, for some subjects saw both the false and the correct

streaks, while others saw only the latter. It was found later that

all the subjects saw both streaks if the arc of movement was large,

say 40°, and all saw only the correctly localized streak if the arc

was small, say 5°. Arcs of medium length revealed individual

differences between the persons, and these differences, though

modified, persisted throughout the experiments. After the subjects had

become somewhat trained in observation, the falsely localized streak

never appeared without the correctly localized one as well. For the

sake of brevity the word ‘streak’ is retained, although the appearance

now referred to is that of a series of separate spots of light

arranged in a nearly straight line.

The phenomena are as follows.—(1) If the arc of movement is small, a

short, correctly localized streak is seen extending from the final

fixation-point to the light-spot. It is brightest at the end nearer

the light. (2) If the eye-movement is 40° or more, a streak having a

length of about one third the distance moved through is seen on the

other side of the light from the final fixation-point; while another

streak is seen of the length of the distance moved through, and

extending from the final fixation-point to the light. The first is the

falsely, the second the correctly localized streak. The second, which

is paler than the first, feels as if it appeared a moment later than

this. The brighter end of each streak is the end which adjoins the

luminous spot. (3) Owing to this last fact, it sometimes happens, when

the eye-movement is 40° or a trifle less, that both streaks are seen,

but that the feeling of succession is absent, so that the two streaks

look like one streak which lies (unequally parted) on both sides of

the spot of light. It was observed, in agreement with Schwarz, that

the phenomenon was the same whether the head or the eyes moved. Only

one other point need be noted. It is that the false streak, which

appears in the beginning to dart from the luminous hole, does not

fade, but seems to suffer a sudden and total eclipse; whereas the

second streak flashes out suddenly in situ, but at a lesser

brilliancy than the other, and very slowly fades away.

These observations thoroughly confirmed those of Schwarz. And one

could not avoid the conviction that Schwarz’s suggestion of the two

streaks being separate localizations of the same retinal stimulation

was an extremely shrewd conjecture. The facts speak strongly in its

favor; first, that when the arc of movement is rather long, there is a

distinct feeling of succession between the appearances of the falsely

and the correctly localized images; second, that when both streaks are

seen, the correct streak is always noticeably dimmer than the false

streak.

It is of course perfectly conceivable that the feeling of succession

is an illusion (which will itself then need to be explained), and that

the streak is seen continuously, its spacial reference only undergoing

an instantaneous substitution. If this is the case, it is singular

that the correctly seen streak seems to enter consciousness so much

reduced as to intensity below that of the false streak when it was

eclipsed. Whereas, if a momentary anæsthesia could be demonstrated,

both the feeling of succession and the discontinuity of the

intensities would be explained (since during the anæsthesia the

after-image on the retina would have faded). This last interpretation

would be entirely in accordance with the observations of

McDougall,[17] who reports some cases in which after-images are

intermittently present to consciousness, and fade during their

eclipse, so that they reappear always noticeably dimmer than when they

disappeared.

[17] McDougall, Mind, N.S., X., 1901, p. 55, Observation II.

Now if the event of such an anæsthesia could be established, we should

know at once that it is not a retinal but a central phenomenon. We

should strongly suspect, moreover, that the anæsthesia is not present

during the very first part of the movement. This must be so if the

interpretation of Schwarz is correct, for certainly no part of the

streak could be made before the eye had begun to move; and yet

approximately the first third was seen at once in its original

intensity, before indeed the ‘innervation-feelings’ had reached

consciousness. Apparently the anæsthesia commences, it at all, after

the eye has accomplished about the first third of its sweep. And

finally, we shall expect to find that movements of the head no less

than movements of the eyes condition the anæsthesia, since neither by

Schwarz nor by the present writer was any difference observed in the

phenomena of falsely localized after-images, between the cases when

the head, and those when the eyes moved.

III. THE PERIMETER-TEST OF DODGE, AND THE LAW OF THE LOCALIZATION OF

AFTER-IMAGES.

We have seen (above, p. 8) how the evidence which Dodge adduces to

disprove the hypothesis of anæsthesia is not conclusive, since,

although an image imprinted on the retina during its movement was

seen, yet nothing showed that it was seen before the eye had come to

rest.

Having convinced himself that there is after all no anæsthesia, Dodge

devised a very ingenious attachment for a perimeter ‘to determine just

what is seen during the eye-movement.’[18] The eye was made to move

through a known arc, and during its movement to pass by a very narrow

slit. Behind this slit was an illuminated field which stimulated the

retina. And since only during its movement was the pupil opposite the

slit, so only during the movement could the stimulation be given. In

the first experiments nothing at all of the illuminated field was

seen, and Dodge admits (ibid., p. 461) that this fact ‘is certainly

suggestive of a central explanation for the absence of bands of fusion

under ordinary conditions.’ But “these failures suggested an increase

of the illumination of the field of exposure…. Under these

conditions a long band of light was immediately evident at each

movement of the eye.” This and similar observations were believed ‘to

show experimentally that when a complex field of vision is perceived

during eye-movement it is seen fused’ (p. 462).

[18] Dodge, PSYCHOLOGICAL REVIEW, 1900, VII., p. 459.

Between the ‘failures’ and the cases when a band of light was seen, no

change in the conditions had been introduced except ‘an increase of

the illumination.’ Suppose now this change made just the difference

between a stimulation which left no appreciable after-image, and

one which left a distinct one. And is it even possible, in view of

the extreme rapidity of eye-movements, that a retinal stimulation of

any considerable intensity should not endure after the movement, to be

then perceived, whether or not it had been first ‘perceived during

the movement’?

Both of Dodge’s experiments are open to the same objection. They do

not admit of distinguishing between consciousness of a retinal process

during the moment of stimulation, and consciousness of the same

process just afterward. In both his cases the stimulation was given

during the eye-movement, but there was nothing to prove that it was

perceived at just the same moment. Whatever the difficulties of

demonstrating an anæsthesia during movement, an experiment which does

not observe the mentioned distinction can never disprove the

hypothesis.

[Illustration: Fig. 2.]

For the sake of a better understanding of these bands of light of

Dodge, a perimeter was equipped in as nearly the manner described by

him (ibid., p. 460) as possible. Experiments with the eye moving

past a very narrow illuminated slit confirmed his observations. If the

light behind the slit was feeble, no band was seen; if moderately

bright, a band was always seen. The most striking fact, however, was

that the band was not localized behind the slit, but was projected on

to that point where the eye came to rest. The band seemed to appear

at this point and there to hover until it faded away. This apparent

anomaly of localization, which Dodge does not mention, suggests the

localization which Schwarz describes of his streaks. Hereupon the

apparatus was further modified so that, whereas Dodge had let the

stimulation take place only during the movement of the eye across a

narrow slit between two walls, now either one of these walls could be

taken away, allowing the stimulation to last for one half of the time

of movement, and this could be either the first or the second half at

pleasure. A plan of the perimeter so arranged is given in Fig. 2.

PBCDB’P is the horizontal section of a semicircular perimeter of 30

cm. radius. E is an eye-rest fixed at the centre of the semicircle;

CD is a square hole which is closed by the screen S fitted into

the front pair of the grooves GG. In the center of S and on a

level with the eye E is a hole A, 2 cm. in diameter, which

contains a ‘jewel’ of red glass. The other two pairs of grooves are

made to hold pieces of milk-or ground-glass, as M, which may be

needed to temper the illumination down to the proper intensity. L is

an electric lamp. B and B’ are two white beads fixed to the

perimeter at the same level as E and A, and used as

fixation-points. Although the room is darkened, these beads catch

enough light to be just visible against the black perimeter, and the

eye is able to move from one to the other, or from A to either one,

with considerable accuracy. They leave a slight after-image streak,

which is, however, incomparably fainter than that left by A (the

streak to be studied), and which is furthermore white while that of

A is bright red. B and B’ are adjustable along a scale of

degrees, which is not shown in the figure, so that the arc of

eye-movement is variable at will. W is a thin, opaque, perpendicular

wall extending from E to C, that is, standing on a radius of the

perimeter. At E this wall comes to within about 4 mm. of the cornea,

and when the eye is directed toward B the wall conceals the red spot

A from the pupil. W