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Page 12
This seems very simple. Yet it was slowly discovered that the
explanation is far too simple and that it does not in the least do
justice to the true experiences. With the advance of modern laboratory
psychology the experimental investigations frequently turned to the
analysis of our perception of movement. In the last thirty years many
researches, notably those of Stricker, Exner, Hall, James, Fischer,
Stern, Marbe, Lincke, Wertheimer, and Korte have thrown new light on the
problem by carefully devised experiments. One result of them came
quickly into the foreground of the newer view: the perception of
movement is an independent experience which cannot be reduced to a
simple seeing of a series of different positions. A characteristic
content of consciousness must be added to such a series of visual
impressions. The mere idea of succeeding phases of movement is not at
all the original movement idea. This is suggested first by the various
illusions of movement. We may believe that we perceive a movement where
no actual changes of visual impressions occur. This, to be sure, may
result from a mere misinterpretation of the impression: for instance
when in the railway train at the station we look out of the window and
believe suddenly that our train is moving, while in reality the train on
the neighboring track has started. It is the same when we see the moon
floating quickly through the motionless clouds. We are inclined to
consider as being at rest that which we fixate and to interpret the
relative changes in the field of vision as movements of those parts
which we do not fixate.
But it is different when we come, for instance, to those illusions in
which movement is forced on our perception by contrast and aftereffect.
We look from a bridge into the flowing water and if we turn our eyes
toward the land the motionless shore seems to swim in the opposite
direction. It is not sufficient in such cases to refer to contrasting
eye movements. It can easily be shown by experiments that these
movements and counter-movements in the field of vision can proceed in
opposite directions at the same time and no eye, of course, is able to
move upward and downward, or right and left, in the same moment. A very
characteristic experiment can be performed with a black spiral line on a
white disk. If we revolve such a disk slowly around its center, the
spiral line produces the impression of a continuous enlargement of
concentric curves. The lines start at the center and expand until they
disappear in the periphery. If we look for a minute or two into this
play of the expanding curves and then turn our eyes to the face of a
neighbor, we see at once how the features of the face begin to shrink.
It looks as if the whole face were elastically drawn toward its center.
If we revolve the disk in the opposite direction, the curves seem to
move from the edge of the disk toward the center, becoming smaller and
smaller, and if then we look toward a face, the person seems to swell up
and every point in the face seems to move from the nose toward the chin
or forehead or ears. Our eye which watches such an aftereffect cannot
really move at the same time from the center of the face toward both
ears and the hair and the chin. The impression of movement must
therefore have other conditions than the actual performance of the
movements, and above all it is clear from such tests that the seeing of
the movements is a unique experience which can be entirely independent
from the actual seeing of successive positions. The eye itself gets the
impression of a face at rest, and yet we see the face in the one case
shrinking, in the other case swelling; in the one case every point
apparently moving toward the center, in the other case apparently moving
away from the center. The experience of movement is here evidently
produced by the spectator's mind and not excited from without.
We may approach the same result also from experiments of very different
kind. If a flash of light at one point is followed by a flash at another
point after a very short time, about a twentieth of a second, the two
lights appear to us simultaneous. The first light is still fully visible
when the second flashes, and it cannot be noticed that the second comes
later than the first. If now in the same short time interval the first
light moves toward the second point, we should expect that we would see
the whole process as a lighted line at rest, inasmuch as the beginning
and the end point appear simultaneous, if the end is reached less than a
twentieth of a second after the starting point. But the experiment shows
the opposite result. Instead of the expected lighted line, we see in
this case an actual movement from one point to the other. Again we must
conclude that the movement is more than the mere seeing of successive
positions, as in this case we see the movement, while the isolated
positions do not appear as successive but as simultaneous.
Another group of interesting phenomena of movement may be formed from
those cases in which the moving object is more easily noticed than the
impressions of the whole field through which the movement is carried
out. We may overlook an area in our visual field, especially when it
lies far to one side from our fixation point, but as soon as anything
moves in that area our attention is drawn. We notice the movement more
quickly than the whole background in which the movement is executed. The
fluttering of kerchiefs at a far distance or the waving of flags for
signaling is characteristic. All indicate that the movement is to us
something different from merely seeing an object first at one and
afterward at another place. We can easily find the analogy in other
senses. If we touch our forehead or the back of our hand with two blunt
compass points so that the two points are about a third of an inch
distant from each other, we do not discriminate the two points as two,
but we perceive the impression as that of one point. We cannot
discriminate the one pressure point from the other. But if we move the
point of a pencil to and fro from one point to the other we perceive
distinctly the movement in spite of the fact that it is a movement
between two end points which could not be discriminated. It is wholly
characteristic that the experimenter in every field of sensations,
visual or acoustical or tactual, often finds himself before the
experience of having noticed a movement while he is unable to say in
which direction the movement occurred.
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