Sixteen Experimental Investigations from the Harvard Psychological Laboratory | Page 3

continuously, the
objects outside swim past as confusedly as ever, and the image of the
fly remains always distinct. Here the eye is moving, and it may be
rapidly, yet both the fly and the blurred landscape testify to a thorough
awareness of the retinal stimulations. There seems to be no anæsthesia
here. It may be, however, that the eye-movement which follows a
moving object is different from that which strikes out independently
across the visual field; and while in the former case there is no
anæsthesia, perhaps in the latter case there is anæsthesia.
Cattell,[1] in considering a similar experience, gives his opinion that
not the absence of fusion for the moving eye, but its presence for the
resting eye, needs explanation. "More than a thousand interruptions per
second," he believes, "give a series of sharply defined retinal
processes." But as for the fusion of moving objects seen when the eyes
are at rest, Cattell says, "It is not necessary and would probably be
disadvantageous for us to see the separate phases." Even where distinct
vision would be 'disadvantageous' he half doubts if fusion comes to the
rescue, or if even the color-wheel ever produces complete fusion. "I
have never been able," he writes, "to make gray in a color-wheel from
red and green (with the necessary correction of blue), but when it is as
nearly gray as it can be got I see both red and green with an appearance
of translucence."
[1] Cattell, J. McK., PSYCHOLOGICAL REVIEW, 1900, VII., p. 325.
That the retina can hold apart more than one thousand stimulations per
second, that there is, in fact, no such thing as fusion, is a supposition
which is in such striking contrast to all previous explanations of optical
phenomena, that it should be accepted only if no other theory can do
justice to them. It is hoped that the following pages will show that the
facts do not demand such a theory.

Another simple observation is interesting in this connection. If at any
time, except when the eyes are quite fresh, one closes one's eyes and
attends to the after-images, some will be found which are so faint as to
be just barely distinguishable from the idioretinal light. If the attention
is then fixed on one such after-image, and the eyes are moved, the
image will suddenly disappear and slowly emerge again after the eyes
have come to rest. This disappearance during eye-movements can be
observed also on after-images of considerable intensity; these, however,
flash back instantly into view, so that the observation is somewhat
more difficult. Exner,[2] in speaking of this phenomenon, adds that in
general "subjective visual phenomena whose origin lies in the retina, as
for instance after-images, Purkinje's vessel-figure, or the phenomena of
circulation under discussion, are almost exclusively to be seen when
the eye is rigidly fixed on a certain spot: as soon as a movement of the
eye is made, the subjective phenomena disappear."
[2] Exner, Sigmund, Zeitschrift f. Psychologie u. Physiologie der
Sinnesorgane, 1890, I., S. 46.
The facts here mentioned in no wise contradict a phenomenon recently
discussed by McDougall,[3] wherein eye-movements revive sensations
which had already faded. Thus an eye-movement will bring back an
after-image which was no longer visible. This return to vividness takes
place after the movement has been completed, and there is no
contention that the image is seen just during the movement.
[3] McDougall, W., Mind, N.S., X., 1901, p. 52.
The disappearance of after-images during eye-movements is mentioned
by Fick and Gürber,[4] who seek to explain the phenomenon by
ascribing it to a momentary period of recovery which the retina perhaps
undergoes, and which would for the moment prevent further
stimulations from being transmitted to the optic nerve. Exner observes
that this explanation would not, however, apply to the disappearance of
the vessel-figure, the circulation phenomenon, the foveal figure, the
polarization-sheaf of Haidinger, Maxwell's spot, or the ring of Löwe;
for these phenomena disappear in a similar manner during movement.
Exner offers another and a highly suggestive explanation. He says of

the phenomenon (op. citat., S. 47), "This is obviously related to the
following fact, that objective and subjective impressions are not to be
distinguished as such, so long as the eye is at rest, but that they are
immediately distinguished if an eye-movement is executed; for then the
subjective phenomena move with the eye, whereas the objective
phenomena are not displaced.... This neglect of the subjective
phenomena is effected, however, not by means of an act of will, but
rather by some central mechanism which, perhaps in the manner of a
reflex inhibition, withholds the stimulation in question from
consciousness, without our assistance and indeed without our
knowledge." The suggestion of a central mechanism which brings
about a reflex inhibition is the significant point.