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

ED. A is then hidden if the eye looks toward B’.

The four conditions of eye-movement to be studied are indicated in

Fig. 3 (Plate 1.). The location of the retinal stimulation is also

shown for each case, as well as the corresponding appearance of the

streaks, their approximate length, and above all their localization.

For the sake of simplicity the refractive effect of the lens and

humors of the eye is not shown, the path of the light-rays being in

each case drawn straight. In all four cases the eye moved without

stopping, through an arc of 40°.

[Illustration: PSYCHOLOGICAL REVIEW. MONOGRAPH SUPPLEMENT, 17. PLATE I.

Fig. 3.

HOLT ON EYE-MOVEMENT.]

To take the first case, Fig. 3:1. The eye fixates the light L, then

sweeps 40° toward the right to the point B’. The retina is

stimulated throughout the movement, l-l’. These conditions yield the

phenomenon of both streaks, appearing as shown on the black rectangle.

In the second case (Fig. 3:2) the wall W is in position and the eye

so adjusted in the eye-rest that the light L is not seen until the

eye has moved about 10° to the right, that is, until the axis of

vision is at Ex. Clearly, then, the image of L falls at first a

little to the right of the fovea, and continues in indirect vision to

the end of the movement. The stimulated part of the retina is l-l’

(Fig. 3:2). Here, then, we have no stimulation of the eye during the

first part of its movement. The corresponding appearance of the streak

is also shown. Only the correctly localized streak is seen, extending

from the light L toward the right but not quite reaching B’. Thus

by cutting out that portion of the stimulation which was given during

the first part of the movement, we have eliminated the whole of the

false image, and the right-hand (foveal) part of the correct image.

Fig. 3:3 shows the reverse case, in which the stimulation is given

only during the first part of the movement. The wall is fixed on the

right of L, and the eye so adjusted that L remains in sight until

the axis of vision reaches position Ex, that is, until it has moved

about 10°. A short strip of the retina next the fovea is here

stimulated, just the part which in case 2 was not stimulated; and the

part which in case 2 was, is here not stimulated. Now here the false

streak is seen, together with just that portion of the correct streak

which in the previous case was not seen. The latter is relatively dim.

Thus it looks indeed as if the streak given during the first part of

an eye-movement is seen twice and differently localized. But one may

say: The twice-seen portion was in both cases on the fovea; this may

have been the conditioning circumstance, and not the fact of being

given in the early part of the movement.

We must then consider Fig. 3, case 4. Here the eye moves from B to

B’, through the same arc of 40°. The wall W is placed so that L

cannot be seen until the axis of vision has moved from EB to EL,

but then L is seen in direct vision. Its image falls full on the

fovea. But one streak, and that the correctly localized one, is seen.

This is like case 2, except that here the streak extending from L to

the right quite reaches the final fixation-point B’. It is therefore

not the fact of a stimulation being foveal which conditions its being

seen in two places.

It should be added that this experiment involves no particular

difficulties of observation, except that in case 4 the eye tends to

stop midway in its movement when the spot of light L comes in view.

Otherwise no particular training of the subject is necessary beyond

that needed for the observing of any after-image. Ten persons made the

foregoing observations and were unanimous in their reports.

This experiment leaves it impossible to doubt that the conjecture of

Schwarz, that the correct image is only the false one seen over again,

is perfectly true. It would be interesting to enquire what it is that

conditions the length of the false streak. It is never more than one

third that of the correct streak (Fig. 3:1; except of course under the

artificial conditions of Fig. 3:3) and may be less. The false streak

seems originally to dart out from the light, as described by Lipps,

visibly growing in length for a certain distance, and then to be

suddenly eclipsed or blotted out simultaneously in all its parts.

Whereas the fainter, correct streak flashes into consciousness _all

parts at once_, but disappears by fading gradually from one end, the

end which lies farther from the light.

Certain it is that when the false streak stops growing and is

eclipsed, some new central process has intervened. One has next to

ask, Is the image continuously conscious, suffering only an

instantaneous relocalization, or is there a moment of central

anæsthesia between the disappearance of the false streak and the

appearance of the other? The relative dimness of the second streak in

the first moment of its appearance speaks for such a brief period of

anæsthesia, during which the retinal process may have partly subsided.

We have now to seek some experimental test which shall demonstrate

definitely either the presence or the absence of a central anæsthesia

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 ANÆSTHESIA.

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 passing 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-glass 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.

[Illustration: 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 anæsthesia 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