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

PHYSIOLOGICAL INTERPRETATION OF THE CENTRAL ANÆSTHESIA.

We have now to sum up the facts given by the experiments. The fact of

central anæsthesia during voluntary movement is supported by two

experimental proofs, aside from a number of random observations which

seem to require this anæsthesia for their explanation. The first proof

is that if an image of the shape of a dumb-bell is given to the retina

during an eye-movement, and in such a way that the handle of the

image, while positively above the threshold of perception, is yet of

brief enough duration to fade completely before the end of the

movement, it then happens that both ends of the dumb-bell are seen but

the handle not at all. The fact of its having been properly given to

the retina is made certain by the presence of the now disconnected

ends.

The second proof is that, similarly, if during an eye-movement two

stimulations of different colors are given to the retina, superposed

and at such intensity and rate of succession as would show to the

resting eye two successive phases of color (in the case taken,

reddish-orange and straw-yellow), it then happens that the first

phase, which runs its course and is supplanted by the second before

the movement is over, is not perceived at all. The first phase was

certainly given, because the conditions of the experiment require the

orange to be given if the straw-yellow is, since the straw-yellow

which is seen can be produced only by the addition of green to the

orange which is not seen.

These two phenomena seem inevitably to demonstrate a moment during

which a process on the retina, of sufficient duration and intensity

ordinarily to determine a corresponding conscious state, is

nevertheless prevented from doing so. One inclines to imagine a

retraction of dendrites, which breaks the connection between the

central end of the optic nerve and the occipital centers of vision.

The fact of anæsthesia demonstrated, other phenomena are now available

with further information. From the phenomena of the ‘falsely

localized’ images it follows that at least in voluntary eye-movements

of considerable arc (30° or more), the anæsthesia commences

appreciably later than the movement. The falsely localized streak is

not generated before the eye moves, but is yet seen before the

correctly localized streak, as is shown by the relative intensities of

the two. The anæsthesia must intervene between the two appearances.

The conjecture of Schwarz, that the fainter streak is but a second

appearance of the stronger, is undoubtedly right.

We know too that the anæsthesia depends on a mechanism central of the

retina, for stimulations are received during movement but not

transmitted to consciousness till afterward. This would be further

shown if it should be found that movements of the head, no less than

those of the eyes, condition the anæsthesia. As before said, it is not

certain that the eyes do not move slightly in the head while the head

moves. The movement of the eyes must then be very slight, and the

anæsthesia correspondingly either brief or discontinuous. Whereas, the

phenomena are the same when the head moves 90° as when the eyes move

that amount. It seems probable, then, that voluntary movements of the

head do equally condition the anæsthesia.

We have seen, too, that in reflex eye-or head-movements no anæsthesia

is so far to be demonstrated. The closeness with which the eye follows

the unexpected gyrations of a slowly waving rush-light, proves that

the reflex movement is produced by a succession of brief impulses

(probably from the cerebellum), each one of which carries the eye

through only a very short distance. It is an interesting question,

whether there is an instant of anæsthesia for each one of these

involuntary innervations—an instant too brief to be revealed by the

experimental conditions employed above. The seeming continuity of the

sensation during reflex movement would of course not argue against

such successive instants of anæsthesia, since no discontinuity of

vision during voluntary movement is noticeable, although a relatively

long moment of anæsthesia actually intervenes.

But decidedly the most interesting detail about the anæsthesia is that

shown by the extreme liability of the eye to stop reflexly on the red

or the green light, in the second experiment with the pendulum.

Suppose the eye to be moving from P to P’ (Fig. 5); the

anæsthesia, although beginning later than the movement, is present

when the eye reaches O, while it is between O and N, that is,

during the anæsthetic moment, that the eye is reflexly caught and held

by the light. This proves again that the anæsthesia is not retinal,

but it proves very much more; namely, that _the retinal stimulation is

transmitted to those lower centers which mediate reflex movements, at

the very instant during which it is cut off from the higher, conscious

centers_. The great frequency with which the eye would stop midway in

its movements, both in the second pendulum-experiment and in the

repetition of Dodge’s perimeter-test, was very annoying at the time,

and the observation cannot be questioned. The fact of the habitual

reflex regulation of voluntary movements is otherwise undisputed.

Exner[24] mentions a variety of similar instances. Also, with the

moving dumb-bell, as has been mentioned, the eye having begun a

voluntary sweep would often be caught by the moving image and carried

on thereafter reflexly with the pendulum. These observations hang

together, and prove a connection between the retina and the reflex

centers even while that between the retina and the conscious centers

is cut off.

[24] Exner, Sigmund, ‘Entwurf zu einer physiologischen

Erklärung der psychischen Erscheinungen,’ Leipzig und Wien,

1894, S. 124-129.

But shall we suppose that the ‘connection’ between the retina and the

conscious centers is cut off during the central anæsthesia? All that

the facts prove is that the centers are at that time not conscious. It

would be at present an unwarrantable assumption to make, that these

centers are therefore disconnected from the retina, at the optic

thalami, the superior quadrigeminal bodies, or wheresoever. On broad

psychological grounds the action-theory of Münsterberg[25] has

proposed the hypothesis that cerebral centers fail to mediate

consciousness not merely when no stimulations are transmitted to them,

but rather when the stimulations transmitted are not able to pass

through and out. The stimulation arouses consciousness when it finds a

ready discharge. And indeed, in this particular case, while we have no

other grounds for supposing stimulations to the visual centers to be

cut off, we do have other grounds for supposing that egress from

these cells would be impeded.

[25] Münsterberg, Hugo, ‘Grundzüge der Psychologie,’ Leipzig,

1900, S. 525-561.

The occipital centers which mediate sensations of color are of course

most closely associated with those other centers (probably the

parietal) which receive sensations from the eye-muscles and which,

therefore, mediate sensations which furnish space and position to the

sensations of mere color. Now it is these occipital centers, mediators

of light-sensations merely, which the experiments have shown most

specially to be anæsthetic. The discharge of such centers means

particularly the passage of excitations on to the parietal

localization-centers. There are doubtless other outlets, but these are

the chief group. The movements, for instance, which activity of these

cells produces, are first of all eye-movements, which have to be

directly produced (according to our present psychophysical

conceptions) by discharges from the centers of eye-muscle sensation.

The principal direction of discharge, then, from the color-centers is

toward the localization-centers.

Now the experiment with falsely and correctly localized after-images

proves that before the anæsthesia all localization is with reference

to the point of departure, while afterwards it is with reference to

the final fixation-point. The transition is abrupt. During the

anæsthesia, then, the mechanism of localization is suffering a

readjustment. It is proved that during this interval of readjustment

in the centers of eye-muscle sensation the way is closed to oncoming

discharges from the color-centers; but it is certain that any such

discharge, during this complicated process of readjustment, would take

the localization-centres by surprise, as it were, and might

conceivably result in untoward eye-movements highly prejudicial to the

safety of the individual as a whole. The much more probable event is

the following:

Although Schwarz suggests that the moment between seeing the false and

seeing the correct after-image is the moment that consciousness is

taken up with ‘innervation-feelings’ of the eye-movement, this is

impossible, since the innervation-feelings (using the word in the only

permissible sense of remembered muscle-sensations) must precede the

movement, whereas even the first-seen, falsely localized streak is not

generated till the movement commences. But we do have to suppose that

during the visual anæsthesia, muscle-sensations of present movement

are streaming to consciousness, to form the basis of the new

post-motum localization. And these would have to go to those very

centers mentioned above, the localization-centers or eye-muscle

sensation centers. One may well suppose that these incoming currents

so raise the tension of these centers that for the moment no discharge

can take place thither from other parts of the brain, among which are

the centers for color-sensations. The word ‘tension’ is of course a

figure, but it expresses the familiar idea that centers which are in

process of receiving peripheral stimulations, radiate that energy

to other parts of the brain (according to the neural dispositions),

and probably do not for the time being receive communications

therefrom, since those other parts are now less strongly excited. It

is, therefore, most probable that during the incoming of the

eye-muscle sensations the centers for color are in fact not able to

discharge through their usual channels toward the localization-centers,

since the tension in that direction is too high. If, now, their other

channels of discharge are too few or too little used to come into

question, the action-theory would find in this a simple explanation of

the visual anæsthesia.

The fact that the anæsthesia commences appreciably later than the

movement so far favors this interpretation. For if the anæsthesia is

conditioned by high tension in the localization-centers, due to

incoming sensations from the eye-muscles, it could not possibly

commence synchronously with the movement. For, first the sensory

end-organs in the eye-muscles (or perhaps in the ligaments, surfaces

of the eye-sockets, etc.) have their latent period; then the

stimulation has to travel to the brain; and lastly it probably has to

initiate there a summation-process equivalent to another latent

period. These three processes would account very readily for what we

may call the latent period of the anæsthesia, as observed in the

experiments. It is true that this latent period was observed only in

long eye-and head-movements, but the experiments were not delicate

enough in this particular to bring out the finer points.

Finally, the conditioning of anæsthesia by movements of the head, if

really proved, would rather corroborate this interpretation. For of

course the position of the head on the shoulders is as important for

localization of the retinal picture as the position of the eyes in the

head, so that sensations of head-movements must be equally represented

in the localization centers; and head movements would equally raise

the tension on those centers against discharge-currents from the

color-centers.

The conclusion from the foregoing experiments is that voluntary

movements of the eyes condition a momentary, visual, central

anæsthesia.

*

TACTUAL ILLUSIONS.

BY CHARLES H. RIEBER.

I.

Many profound researches have been published upon the subject of

optical illusions, but in the field of tactual illusions no equally

extensive and serious work has been accomplished. The reason for this

apparent neglect of the illusions of touch is obviously the fact that

the studies in the optical illusions are generally thought to yield

more important results for psychology than corresponding studies in

the field of touch. Then,