Sixteen Experimental Investigations from the Harvard Psychological Laboratory - Hugo Münsterberg (best life changing books txt) 📗
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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
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