Harvard Psychological Studies, Volume 1 | Page 5

inch distant, three clear-cut round holes were seen
much brighter than the band of light out of which they seemed to
emerge. This was only possible when the velocity of the holes was
sufficient to keep their images at exactly the same spot on the retina
during the movement of the eye. The significant thing is that the
individual round spots of light thus seen were much more intense than
the fused line of light seen while the eyes were at rest. Neither my
assistant nor I was able to detect any difference in brightness between
them and the background when altogether unobstructed." Dodge finds
that this experiment 'disproves' the hypothesis of anæsthesia.
If by 'anæsthesia' is meant a condition of the retinal end-organs in
which they should be momentarily indifferent to excitation by
light-waves, the hypothesis is indeed disproved, for obviously the 'three
clear-cut round holes' which appeared as bright as the unobstructed
background were due to a summation of the light which reached the
retina during the movement, through three holes of the disc, and which
fell on the same three spots of the retina as long as the disc and the
eyeball were moving at the same angular rate. But such a momentary

anæsthesia of the retina itself would in any case, from our knowledge
of its physiological and chemical structure, be utterly inconceivable.
On the other hand, there seems to be nothing in the experiment which
shows that the images of the three holes were present to consciousness
just during the movement, rather than immediately thereafter. A central
mechanism of inhibition, such as Exner mentions, might condition a
central anæsthesia during movement, although the functioning of the
retina should remain unaltered. Such a central anæsthesia would just as
well account for the phenomena which have been enumerated. The
three luminous images could be supposed to remain unmodified for a
finite interval as positive after-images, and as such first to appear in
consciousness. Inasmuch as 'the arc of eye movements was 4.7°' only,
the time would be too brief to make possible any reliable judgment as
to whether the three holes were seen during or just after the
eye-movement. With this point in view, the writer repeated the
experiment of Dodge, and found indeed nothing which gave a hint as to
the exact time when the images emerged in consciousness. The results
of Dodge were otherwise entirely confirmed.
II. THE PHENOMENON OF 'FALSELY LOCALIZED
AFTER-IMAGES.'
A further fact suggestive of anæsthesia during movement comes from
an unexpected source. While walking in the street of an evening, if one
fixates for a moment some bright light and then quickly turns the eye
away, one will observe that a luminous streak seems to dart out from
the light and to shoot away in either of two directions, either in the
same direction as that in which the eye moved, or in just the opposite.
If the eye makes only a slight movement, say of 5°, the streak jumps
with the eye; but if the eye sweeps through a rather large arc, say of 40°,
the luminous streak darts away in the opposite direction. In the latter
case, moreover, a faint streak of light appears later, lying in the
direction of the eye-movement.
This phenomenon was probably first described by Mach, in 1886.[8]
His view is essentially as follows: It is clear that in whatever direction
the eye moves, away from its luminous fixation point, the streak

described on the retina by the luminous image will lie on the same part
of the retina as it would have lain on had the eye remained at rest but
the object moved in the opposite direction. Thus, if the eye moves to
the right, we should expect the streak to appear to dart to the left. If,
however, the streak has not faded by the time the eye 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