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

has come to rest on a new fixation point (by supposition to the right

of the old), we should expect the streak to be localized to the left

of this, that is, to the right of the former fixation-point. In order

to be projected, a retinal image has to be localized with reference to

some point, generally the fixation-point of the eyes; and it is

therefore clear that when two such fixation-points are involved, the

localization will be ambiguous if for any reason the central apparatus

does not clearly determine which shall be the point of reference. With

regard to the oppositely moving streak Mach says:[9] “The streak is,

of course, an after-image, which comes to consciousness only on, or

shortly before, the completion of the eye-movement, nevertheless with

positional values which correspond, remarkably enough, not to the

later but to the earlier position and innervation of the eyes.” Mach

does not further attempt to explain the phenomenon.

[8] Mach, Ernst, ‘Beiträge zur Analyze der Empfindungen,’ Jena,

[9] Mach, op. citat., 2te Aufl., Jena, 1900, S. 96.

It is brought up again by Lipps,[10] who assumes that the streak ought

to dart with the eyes and calls therefore the oppositely moving streak

the ‘falsely localized image.’ For sake of brevity we may call this

the ‘false image.’ The explanation of Lipps can be pieced together as

follows (ibid., S. 64): “The explanation presupposes that sensations

of eye-movements have nothing to do with the projection of retinal

impressions into the visual field, that is, with the perception of the

mutual relations as to direction and distance, of objects which are

viewed simultaneously…. Undoubtedly, however, sensations of

eye-movements, and of head-and body-movements as well, afford us a

scale for measuring the displacements which our entire visual field

and every point in it undergo within the surrounding _totality of

space_, which we conceive of as fixed. We estimate according to the

length of such movements, or at least we deduce therefrom, the

distance through fixed space which our view by virtue of these

movements has traversed…. They themselves are nothing for our

consciousness but a series of purely intensive states. But in

experience they can come to indicate distance traversed.” Now in

turning the eye from a luminous object, O, to some other

fixation-point, P, the distance as simply contemplated is more or

less subdivided or filled in by the objects which are seen to lie

between O and P, or if no such objects are visible the distance is

still felt to consist of an infinity of points; whereas the muscular

innervation which is to carry the eye over this very distance is an

undivided unit. But it is this which gives us our estimate of the arc

we move through, and being thus uninterrupted it will appear shorter

than the contemplated, much subdivided distance OP, just as a

continuous line appears shorter than a broken line. “After such

analogies, now, the movement of the eye from O to P, that is, the

arc which I traverse, must be underestimated” (ibid., S. 67). There

is thus a discrepancy between our two estimates of the distance OP.

This discrepancy is felt during the movement, and can be harmonized

only if we seem to see the two fixation-points move apart, until the

arc between them, in terms of innervation-feeling, feels equal to the

distance OP in terms of its visual subdivisions. Now either O and

P can both seem to move apart from each other, or else one can seem

fixed while the other moves. But the eye has for its goal P, which

ought therefore to have a definite position. “P appears fixed

because, as goal, I hold it fast in my thought” (_loc. citat._). It

must be O, therefore, which appears to move; that is, O must dart

backward as the eye moves forward toward P. Thus Lipps explains the

illusion.

[10] Lipps, Th., _Zeitschrift f. Psychologie u. Physiologie der

Sinnesorgane_, 1890, I., S. 60-74.

Such an explanation involves many doubtful presuppositions, but if we

were to grant to Lipps those, the following consideration would

invalidate his account. Whether the feeling of innervation which he

speaks of as being the underestimated factor is supposed to be a true

innervation-feeling in the narrower sense, or a muscular sensation

remembered from past movements, it would in the course of experience

certainly come to be so closely associated with the corresponding

objective distance as not to feel less than this. So far as an

innervation-feeling might allow us to estimate distance, it could have

no other meaning than to represent just that distance through which

the innervation will move the organ in question. If OP is a distance

and i is the feeling of such an innervation as will move the eye

through that distance, it is inconceivable that i, if it represent

any distance at all, should represent any other distance than just

OP.

Cornelius[11] brought up the matter a year later than Lipps. Cornelius

criticises the unwarranted presuppositions of Lipps, and himself

suggests that the falsely localized streak is due to a slight rebound

which the eye, having overshot its intended goal, may make in the

opposite direction to regain the mark. This would undoubtedly explain

the phenomenon if such movements of rebound actually took place.

Cornelius himself does not adduce any experiments to corroborate this

account.

[11] Cornelius, C.S., _Zeitschrift f. Psychologie u.

Physiologie der Sinnesorgane_, 1891, II., S. 164-179.

The writer, therefore, undertook to find out if such movements

actually are made. The observations were made by watching the eyes of

several subjects, who looked repeatedly from one fixation-point to

another. Although sometimes such backward movements seemed indeed to

be made, they were very rare and always very slight. Inasmuch as the

‘false’ streak is often one third as long as the distance moved

through, a movement of rebound, such as Cornelius means, would have to

be one third of the arc intended, and could therefore easily have been

noticed. Furthermore, the researches of Lamansky,[12] Guillery,[13]

Huey,[14] Dodge and Cline,[15] which are particularly concerned with

the movements of the eyes, make no mention of such rebounds.

Schwarz[16] above all has made careful investigations on this very

point, in which a screen was so placed between the observer and the

luminous spot that it intervened between the pupil and the light, just

before the end of the movement. Thus the retina was not stimulated

during the latter part of its movement, just when Cornelius assumed

the rebound to take place. This arrangement, however, did not in the

least modify the appearance of the false streak.

[12] Lamansky, S., _Pflüger’s Archiv f. d. gesammte

Physiologie_, 1869, II., S. 418.

[13] Guillery, ibid., 1898, LXXI., S. 607; and 1898, LXXIII.,

S. 87.

[14] Huey, Edmund B., American Journal of Psychology, 1900,

XI., p. 283.

[15] Dodge, Raymond, and Cline, T.S., PSYCHOLOGICAL REVIEW,

1901, VIII., PP. 145-157.

[16] Schwarz, Otto, _Zeitschrift J. Psychologie u. Physiologie

der Sinnesorgane_, 1892, III., S. 398-404.

This work of Schwarz certainly proves that the explanation of

Cornelius is not correct. Schwarz found that the phenomenon takes

place as well when the head moves and the eyes are fixed relatively to

the head, as when the eyes alone move. He furthermore made this

observation. Meaning by a the point of departure and by b the goal

of either the eye-or the head-movement, movement, he says (ibid.,

S. 400-2): “While oftentimes the streak of the after-image extended

uninterruptedly to the point b, or better seemed to proceed from

this point,—as Lipps also reported—yet generally, under the

experimental conditions which I have indicated, two streaks could be

seen, separated by a dark space between; firstly the anomalous one”

(the false streak) “rather brilliant, and secondly a fainter one of

about equal or perhaps greater length, which began at the new

fixation-point b and was manifestly an after-image correctly

localized with regard to the situation of this point. This last

after-image streak did not always appear; but it appeared regularly if

the light at a was bright enough and the background dark…. It was

impossible for this second after-image streak to originate in the

point b, because it appeared equally when b was only an imaginary

fixation-point…. This consideration makes it already conceivable

that the two parts of the total after-image _are two manifestations of

the one identical retinal stimulation, which are differently

localized_…. Therefore we must probably picture to ourselves that

the sensation from the strip of the retina stimulated during the quick

eye-movement is, _during the interval of movement or at least during

the greater part of it, localized as if the axis of vision were still

directed toward the original fixation-point. And when the new position

of rest is reached and the disturbance on the retinal strip has not

wholly died away, then the strip comes once more into consciousness,

but this time correctly localized with reference to the new position

of the axis of vision_. By attending closely to the behavior as

regards time of both after-image streaks, I can generally see the

normal after-image develop a moment later than the anomalous one”

(that is, the false streak). Schwarz finally suggests (S. 404) that

probably between the first and second appearances of the streak an

‘innervation-feeling’ intervenes which affords the basis for

localizing the second streak (‘correctly’) with reference to the new

position of the eye.

After this digression we return to consider how this phenomenon is

related to the hypothesis of anæsthesia during eye-movements. If we

accept the interpretation of Schwarz, there is one retinal process

which is perceived as two luminous streaks in space, localized

differently and referred to different moments of time. It is

surprising, then, that a continuous retinal process is subjectively

interpreted as two quite different objects, that is, as something

discontinuous. Where does the factor of discontinuity come in? If we

suppose the retinal disturbance to produce a continuous sensation in

consciousness, we should expect, according to every analogy, that this

sensation would be referred to one continuously existing object. And

if this object is to be localized in two places successively, we

should expect it to appear to move continuously through all

intervening positions. Such an interpretation is all the more to be

expected, since, as the strobic phenomena show, even discontinuous

retinal processes tend to be interpreted as continuously existing

objects.

On the other hand, if there were a central anæsthesia during

eye-movement, the continuous process in the retina could not produce a

continuous sensation, and if the interval were long enough the image

might well be referred to two objects; since also, in the strobic

appearances, the stimulations must succeed at a certain minimal rate

in order to produce the illusion of continuous existence and movement.

This consideration seemed to make it worth while to perform some

experiments with the falsely localized after-images. The phenomenon

had also by chance been noted in the case of the eye moving past a

luminous dot which was being regularly covered and uncovered. The

appearance is of a row of luminous spots side by side in space, which

under conditions may be either falsely or correctly localized. Since

these dots seemed likely to afford every phenomenon exhibited by the

streaks, with the bare chance of bringing out new facts, apparatus was

arranged as in Fig. 1, which is a horizontal section.

DD is a disc which revolves in a vertical plane, 56 cm. in diameter

and bearing near its periphery one-centimeter holes punched 3 cm.

apart. E is an eye-rest, and L an electric lamp.