Harvard Psychological Studies, Volume 1 | Page 9

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