Various - Harvard Psychological Studies, Volume 1

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[8] Mach, Ernst, 'Beitraege zur Analyze der Empfindungen,' Jena,
1886.

[9] Mach, _op. citat._, 2te Aufl., Jena, 1900, S. 96.

It is brought up again by Lipps,[10] who assumes that the streak ought
to dart with the eyes and calls therefore the oppositely moving streak
the 'falsely localized image.' For sake of brevity we may call this
the 'false image.' The explanation of Lipps can be pieced together as
follows (_ibid._, S. 64): "The explanation presupposes that sensations
of eye-movements have nothing to do with the projection of retinal
impressions into the visual field, that is, with the perception of the
mutual relations as to direction and distance, of objects which are
viewed simultaneously.... Undoubtedly, however, sensations of
eye-movements, and of head-and body-movements as well, afford us a
scale for measuring the displacements which our entire visual field
and every point in it undergo within the surrounding _totality of
space_, which we conceive of as fixed. We estimate according to the
length of such movements, or at least we deduce therefrom, the
distance through fixed space which our view by virtue of these
movements has traversed.... They themselves are nothing for our
consciousness but a series of purely intensive states. But in
experience they can come to _indicate_ distance traversed." Now in
turning the eye from a luminous object, _O_, to some other
fixation-point, _P_, the distance as simply contemplated is more or
less subdivided or filled in by the objects which are seen to lie
between _O_ and _P_, or if no such objects are visible the distance is
still felt to consist of an infinity of points; whereas the muscular
innervation which is to carry the eye over this very distance is an
undivided unit. But it is this which gives us our estimate of the arc
we move through, and being thus uninterrupted it will appear shorter
than the contemplated, much subdivided distance _OP_, just as a
continuous line appears shorter than a broken line. "After such
analogies, now, the movement of the eye from _O_ to _P_, that is, the
arc which I traverse, must be underestimated" (_ibid._, S. 67). There
is thus a discrepancy between our two estimates of the distance _OP_.
This discrepancy is felt during the movement, and can be harmonized
only if we seem to see the two fixation-points move apart, until the
arc between them, in terms of innervation-feeling, feels equal to the
distance _OP_ in terms of its visual subdivisions. Now either _O_ and
_P_ can both seem to move apart from each other, or else one can seem
fixed while the other moves. But the eye has for its goal _P_, which
ought therefore to have a definite position. "_P_ appears fixed
because, as goal, I hold it fast in my thought" (_loc. citat._). It
must be _O_, therefore, which appears to move; that is, _O_ must dart
backward as the eye moves forward toward _P_. Thus Lipps explains the
illusion.

[10] Lipps, Th., _Zeitschrift f. Psychologie u. Physiologie der
Sinnesorgane_, 1890, I., S. 60-74.

Such an explanation involves many doubtful presuppositions, but if we
were to grant to Lipps those, the following consideration would
invalidate his account. Whether the feeling of innervation which he
speaks of as being the underestimated factor is supposed to be a true
innervation-feeling in the narrower sense, or a muscular sensation
remembered from past movements, it would in the course of experience
certainly come to be so closely associated with the corresponding
objective distance as not to feel less than this. So far as an
innervation-feeling might allow us to estimate distance, it could have
no other meaning than to represent just that distance through which
the innervation will move the organ in question. If _OP_ is a distance
and _i_ is the feeling of such an innervation as will move the eye
through that distance, it is inconceivable that _i_, if it represent
any distance at all, should represent any other distance than just
_OP_.

Cornelius[11] brought up the matter a year later than Lipps. Cornelius
criticises the unwarranted presuppositions of Lipps, and himself
suggests that the falsely localized streak is due to a slight rebound
which the eye, having overshot its intended goal, may make in the
opposite direction to regain the mark. This would undoubtedly explain
the phenomenon if such movements of rebound actually took place.
Cornelius himself does not adduce any experiments to corroborate this
account.

[11] Cornelius, C.S., _Zeitschrift f. Psychologie u.
Physiologie der Sinnesorgane_, 1891, II., S. 164-179.

The writer, therefore, undertook to find out if such movements
actually are made. The observations were made by watching the eyes of
several subjects, who looked repeatedly from one fixation-point to
another. Although sometimes such backward movements seemed indeed to
be made, they were very rare and always very slight. Inasmuch as the
'false' streak is often one third as long as the distance moved
through, a movement of rebound, such as Cornelius means, would have to
be one third of the arc intended, and could therefore easily have been
noticed. Furthermore, the researches of Lamansky,[12] Guillery,[13]
Huey,[14] Dodge and Cline,[15] which are particularly concerned with
the movements of the eyes, make no mention of such rebounds.
Schwarz[16] above all has made careful investigations on this very
point, in which a screen was so placed between the observer and the
luminous spot that it intervened between the pupil and the light, just
before the end of the movement. Thus the retina was not stimulated
during the latter part of its movement, just when Cornelius assumed
the rebound to take place. This arrangement, however, did not in the
least modify the appearance of the false streak.

[12] Lamansky, S., _Pflueger's Archiv f. d. gesammte
Physiologie_, 1869, II., S. 418.

[13] Guillery, _ibid._, 1898, LXXI., S. 607; and 1898, LXXIII.,
S. 87.

[14] Huey, Edmund B., _American Journal of Psychology_, 1900,
XI., p. 283.

[15] Dodge, Raymond, and Cline, T.S., PSYCHOLOGICAL REVIEW,
1901, VIII., PP. 145-157.

[16] Schwarz, Otto, _Zeitschrift J. Psychologie u. Physiologie
der Sinnesorgane_, 1892, III., S. 398-404.

This work of Schwarz certainly proves that the explanation of
Cornelius is not correct. Schwarz found that the phenomenon takes
place as well when the head moves and the eyes are fixed relatively to
the head, as when the eyes alone move. He furthermore made this
observation. Meaning by _a_ the point of departure and by _b_ the goal
of either the eye-or the head-movement, movement, he says (_ibid._,
S. 400-2): "While oftentimes the streak of the after-image extended
uninterruptedly to the point _b_, or better seemed to proceed from
this point,--as Lipps also reported--yet generally, under the
experimental conditions which I have indicated, _two streaks_ could be
seen, _separated by a dark space between_; firstly the anomalous one"
(the false streak) "rather brilliant, and secondly a fainter one of
about equal or perhaps greater length, which began at the new
fixation-point _b_ and was manifestly an after-image correctly
localized with regard to the situation of this point. This last
after-image streak did not always appear; but it appeared regularly if
the light at _a_ was bright enough and the background dark.... It was
impossible for this second after-image streak to originate in the
point _b_, because it appeared equally when _b_ was only an imaginary
fixation-point.... This consideration makes it already conceivable
that the two parts of the total after-image _are two manifestations of
the one identical retinal stimulation, which are differently
localized_.... Therefore we must probably picture to ourselves that
the sensation from the strip of the retina stimulated during the quick
eye-movement is, _during the interval of movement or at least during
the greater part of it, localized as if the axis of vision were still
directed toward the original fixation-point. And when the new position
of rest is reached and the disturbance on the retinal strip has not
wholly died away, then the strip comes once more into consciousness,
but this time correctly localized with reference to the new position
of the axis of vision_. By attending closely to the behavior as
regards time of both after-image streaks, I can generally see the
normal after-image develop a moment later than the anomalous one"
(that is, the false streak). Schwarz finally suggests (S. 404) that
probably between the first and second appearances of the streak an
'innervation-feeling' intervenes which affords the basis for
localizing the second streak ('correctly') with reference to the new
position of the eye.

After this digression we return to consider how this phenomenon is
related to the hypothesis of anaesthesia during eye-movements. If we
accept the interpretation of Schwarz, there is one retinal process
which is perceived as two luminous streaks in space, localized
differently and referred to different moments of time. It is
surprising, then, that a continuous retinal process is subjectively
interpreted as two quite different objects, that is, as something
discontinuous. Where does the factor of discontinuity come in? If we
suppose the retinal disturbance to produce a continuous sensation in
consciousness, we should expect, according to every analogy, that this
sensation would be referred to one continuously existing object. And
if this object is to be localized in two places successively, we
should expect it to appear to move continuously through all
intervening positions. Such an interpretation is all the more to be
expected, since, as the strobic phenomena show, even discontinuous
retinal processes tend to be interpreted as continuously existing
objects.

On the other hand, if there were a central anaesthesia during
eye-movement, the continuous process in the retina could not produce a
continuous sensation, and if the interval were long enough the image
might well be referred to two objects; since also, in the strobic
appearances, the stimulations must succeed at a certain minimal rate
in order to produce the illusion of continuous existence and movement.

This consideration seemed to make it worth while to perform some
experiments with the falsely localized after-images. The phenomenon
had also by chance been noted in the case of the eye moving past a
luminous dot which was being regularly covered and uncovered. The
appearance is of a row of luminous spots side by side in space, which
under conditions may be either falsely or correctly localized. Since
these dots seemed likely to afford every phenomenon exhibited by the
streaks, with the bare chance of bringing out new facts, apparatus was
arranged as in Fig. 1, which is a horizontal section.

_DD_ is a disc which revolves in a vertical plane, 56 cm. in diameter
and bearing near its periphery one-centimeter holes punched 3 cm.
apart. _E_ is an eye-rest, and _L_ an electric lamp. _SS_ is a screen
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 deg., and all saw only the correctly localized streak if the arc
was small, say 5 deg.. 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 deg. 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 deg. 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 anaesthesia could be demonstrated,
both the feeling of succession and the discontinuity of the
intensities would be explained (since during the anaesthesia 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 anaesthesia 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 anaesthesia 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 anaesthesia 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 anaesthesia, 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 anaesthesia 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 anaesthesia, 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 anaesthesia 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_ can at will be transferred to the position
_ED_. _A_ is then hidden if the eye looks toward _B'_.

The four conditions of eye-movement to be studied are indicated in
Fig. 3 (Plate 1.). The location of the retinal stimulation is also
shown for each case, as well as the corresponding appearance of the
streaks, their approximate length, and above all their localization.
For the sake of simplicity the refractive effect of the lens and
humors of the eye is not shown, the path of the light-rays being in
each case drawn straight. In all four cases the eye moved without
stopping, through an arc of 40 deg..

[Illustration: PSYCHOLOGICAL REVIEW. MONOGRAPH SUPPLEMENT, 17. PLATE I.
Fig. 3.
HOLT ON EYE-MOVEMENT.]

To take the first case, Fig. 3:1. The eye fixates the light _L_, then
sweeps 40 deg. toward the right to the point _B'_. The retina is
stimulated throughout the movement, _l-l'_. These conditions yield the
phenomenon of both streaks, appearing as shown on the black rectangle.

In the second case (Fig. 3:2) the wall _W_ is in position and the eye
so adjusted in the eye-rest that the light _L_ is not seen until the
eye has moved about 10 deg. to the right, that is, until the axis of
vision is at _Ex_. Clearly, then, the image of _L_ falls at first a
little to the right of the fovea, and continues in indirect vision to
the end of the movement. The stimulated part of the retina is _l-l'_
(Fig. 3:2). Here, then, we have no stimulation of the eye during the
first part of its movement. The corresponding appearance of the streak
is also shown. Only the correctly localized streak is seen, extending
from the light _L_ toward the right but not quite reaching _B'_. Thus
by cutting out that portion of the stimulation which was given during
the first part of the movement, we have eliminated the whole of the
false image, and the right-hand (foveal) part of the correct image.

Fig. 3:3 shows the reverse case, in which the stimulation is given
only during the first part of the movement. The wall is fixed on the
right of _L_, and the eye so adjusted that _L_ remains in sight until
the axis of vision reaches position _Ex_, that is, until it has moved
about 10 deg.. A short strip of the retina next the fovea is here
stimulated, just the part which in case 2 was not stimulated; and the
part which in case 2 was, is here not stimulated. Now here the false
streak is seen, together with just that portion of the correct streak
which in the previous case was not seen. The latter is relatively dim.

Thus it looks indeed as if the streak given during the first part of
an eye-movement is seen twice and differently localized. But one may
say: The twice-seen portion was in both cases on the fovea; this may
have been the conditioning circumstance, and not the fact of being
given in the early part of the movement.