Various - Harvard Psychological Studies, Volume 1

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We must then consider Fig. 3, case 4. Here the eye moves from _B_ to
_B'_, through the same arc of 40 deg.. The wall _W_ is placed so that _L_
cannot be seen until the axis of vision has moved from _EB_ to _EL_,
but _then L_ is seen in direct vision. Its image falls full on the
fovea. But one streak, and that the correctly localized one, is seen.
This is like case 2, except that here the streak extending from _L_ to
the right quite reaches the final fixation-point _B'_. It is therefore
not the fact of a stimulation being foveal which conditions its being
seen in two places.

It should be added that this experiment involves no particular
difficulties of observation, except that in case 4 the eye tends to
stop midway in its movement when the spot of light _L_ comes in view.
Otherwise no particular training of the subject is necessary beyond
that needed for the observing of any after-image. Ten persons made the
foregoing observations and were unanimous in their reports.

This experiment leaves it impossible to doubt that the conjecture of
Schwarz, that the correct image is only the false one seen over again,
is perfectly true. It would be interesting to enquire what it is that
conditions the length of the false streak. It is never more than one
third that of the correct streak (Fig. 3:1; except of course under the
artificial conditions of Fig. 3:3) and may be less. The false streak
seems originally to _dart out_ from the light, as described by Lipps,
visibly growing in length for a certain distance, and then to be
suddenly eclipsed or blotted out _simultaneously_ in all its parts.
Whereas the fainter, correct streak flashes into consciousness _all
parts at once_, but disappears by fading gradually from one end, the
end which lies farther from the light.

Certain it is that when the false streak stops growing and is
eclipsed, some new central process has intervened. One has next to
ask, Is the image continuously conscious, suffering only an
instantaneous relocalization, or is there a moment of central
anaesthesia between the disappearance of the false streak and the
appearance of the other? The relative dimness of the second streak in
the _first moment_ of its appearance speaks for such a brief period of
anaesthesia, during which the retinal process may have partly subsided.

We have now to seek some experimental test which shall demonstrate
definitely either the presence or the absence of a central anaesthesia
during eye-movements. The question of head-movements will be deferred,
although, as we have seen above, these afford equally the phenomenon
of twice-localized after-images.

IV. THE PENDULUM-TEST FOR ANAESTHESIA.

A. Apparatus must be devised to fulfil the following conditions. A
retinal stimulation must be given during an eye-movement. The moment
of excitation must be so brief and its intensity so low that the
process shall be finished before the eye comes to rest, that is, so
that no after-image shall be left to come into consciousness _after_
the movement is over. Yet, on the other hand, it must be positively
demonstrated that a stimulation of this _very same_ brief duration and
low intensity is amply strong enough to force its way into
consciousness if no eye-movement is taking place. If such a
stimulation, distinctly perceived when the eye is at rest, should not
be perceptible if given while the eye is moving, we should have a
valid proof that some central process has intervened during the
movement, to shut out the stimulation-image during that brief moment
when it might otherwise have been perceived.

Obviously enough, with the perimeter arrangement devised by Dodge,
where the eye moves past a narrow, illuminated slit, the light within
the slit can be reduced to any degree of faintness. But on the other
hand, it is clearly impossible to find out how long the moment of
excitation lasts, and therefore impossible to find out whether an
excitation of the same duration and intensity is yet sufficient to
affect consciousness if given when the eye is not moving. Unless the
stimulation is proved to be thus sufficient, a failure to see it when
given during an eye-movement would of course prove nothing at all.

Perhaps the most exact way to measure the duration of a light-stimulus
is to let it be controlled by the passing of a shutter which is
affixed to a pendulum. Furthermore, by means of a pendulum a
stimulation of exactly the same duration and intensity can be given to
the moving, as to the resting eye. Let us consider Fig. 4:1. If _P_ is
a pendulum bearing an opaque shield _SS_ pierced by the hole _tt_, and
_BB_ an opaque background pierced by the hole _i_ behind which is a
lamp, it is clear that if the eye is fixed on _i_, a swing of the
pendulum will allow _i_ to stimulate the retina during such a time as
it takes the opening _tt_ to move past _i_. The shape of _i_ will
determine the shape of the image on the retina, and the intensity of
the stimulation can be regulated by ground-or milk-glass interposed
between the hole _i_ and the lamp behind it. The duration of the
exposure can be regulated by the width of _tt_, by the length of the
pendulum, and by the arc through which it swings.

If now the conditions are altered, as in Fig. 4:2, so that the opening
_tt_ (indicated by the dotted line) lies not in _SS_, but in the fixed
background _BB_, while the small hole _i_ now moves with the shield
_SS_, it necessarily follows that if the eye can move at just the rate
of the pendulum, it will receive a stimulation of exactly the same
size, shape, duration, and intensity as in the previous case where the
eye was at rest. Furthermore, it will always be possible to tell
whether the eye does move at the same rate as the pendulum, since if
it moves either more rapidly or more slowly, the image of _i_ on the
retina will be horizontally elongated, and this fact will be given by
a judgment as to the proportions of the image seen.

It may be said that since the eye does not rotate like the pendulum,
from a fulcrum above, the image of _i_ in the case of the moving eye
will be distorted as is indicated in Fig. 4, _a_. This is true, but
the distortion will be so minute as to be negligible if the pendulum
is rather long (say a meter and a half) and the opening _tt_ rather
narrow (say not more than ten degrees wide). A merely horizontal
movement of the eye will then give a practically exact superposition
of the image of _i_ at all moments of the exposure.

[Illustration: PSYCHOLOGICAL REVIEW. MONOGRAPH SUPPLEMENT, 17. PLATE PLATE II.
Fig. 4. Fig. 6.
HOLT ON EYE-MOVEMENT.]

Thus much of preliminary discussion to show how, by means of a
pendulum, identical stimulations can be given to the moving and to the
resting eye. We return to the problem. It is to find out whether a
stimulation given during an eye-movement can be perceived if its
after-image is so brief as wholly to elapse before the end of the
movement. If a period of anaesthesia is to be demonstrated, two
observations must be made. First, that the stimulation is bright
enough to be _unmistakably visible_ when given to the eye at rest;
second, that it is not visible when given to the moving eye. Hence, we
shall have three cases.

Case 1. A control, in which the stimulation is proved intense
enough to be seen by the eye at rest.

Case 2. In which the same stimulation is given to the eye
during movement.

Case 3. Another control, to make sure that no change in the
adaptation or fatigue of the eye has intervened during the
experiments to render the eye insensible to the stimulation.

Fig. 5 shows the exact arrangement of the experiment. The figure
represents a horizontal section at the eye-level of the pendulum of
Fig. 4, with accessories. _E_ is the eye which moves between the two
fixation-points _P_ and _P_'. _WONW_ is a wall which conceals the
mechanism of the pendulum from the subject. _ON_ is a rectangular hole
9 cm. wide and 7 cm. high, in this wall. _SS_ is the shield which
swings with the pendulum, and _BB_ is the background (cf. Fig. 4).
When the pendulum is not swinging, a hole in the shield lies behind
_ON_ and exactly corresponds with it. Another in the background does
the same. The eye can thus see straight through to the light _L_.

Each of these three holes has grooves to take an opaque card, _x_,
_y_, or _z_; there are two cards for the three grooves, and they are
pierced with holes to correspond to _i_ and _tt_ of Fig. 4. The
background _BB_ has a second groove to take a piece of milk-glass _M_.
These cards are shown in Fig. 6 (Plate II.) Card _I_ bears a hole 5
cm. high and shaped like a dumb-bell. The diameter of the end-circles
(_e_, _e_) is 1.3 cm., and the width of the handle _h_ is 0.2 cm. Card
_T_ is pierced by two slits _EE_, _EE_, each 9 cm. long and 1.3 cm.
high, which correspond to the two ends of the dumb-bell. These slits
are connected by a perforation _H_, 1.5 cm. wide, which corresponds to
the handle of the dumb-bell. This opening _EEHEE_ is covered by a
piece of ground-glass which serves as a radiating surface for the
light.

[Illustration: Fig. 5.]

The distance _EA_ (Fig. 5) is 56 cm., and _PP_' is 40 cm.; so that the
arc of eye-movement, that is, the angle _PEP_', is very nearly 40 deg.,
of which the 9-cm. opening _ON_ 9 deg. 11'. _SS_ is 2 cm. behind _ON_, and
_BB_ 2 cm. behind _SS_; these distances being left to allow the
pendulum to swing freely.

It is found under these conditions that the natural speed made by the
eye in passing the 9-cm. opening _ON_ is very well approximated by the
pendulum if the latter is allowed to fall through 23.5 deg. of its arc,
the complete swing being therefore 47 deg.. The middle point of the
pendulum is then found to move from _O_ to _N_ in 110[sigma][19]. If
the eye sweeps from _O_ to _N_ in the same time, it will be moving at
an angular velocity of 1 deg. in 11.98[sigma] (since the 9 cm. are 9 deg. 11'
of eye-movement). This rate is much less than that found by Dodge and
Cline (_op. cit._, p. 155), who give the time for an eye-movement of
40 deg. as 99.9[sigma], which is an average of only 2.49[sigma] to the
degree. Voluntary eye-movements, like other voluntary movements, can
of course be slow or fast according to conditions. After the pendulum
has been swinging for some time, so that its amplitude of movement has
fallen below the initial 47 deg. and therewith its speed past the middle
point has been diminished, the eye in its movements back and forth
between the fixation-points can still catch the after-image of _i_
perfectly distinct and not at all horizontally elongated, as it would
have to be if eye and pendulum had not moved just together. It appears
from this that certain motives are able to retard the rate of
voluntary movements of the eye, even when the distance traversed is
constant.

[19] The speed of the pendulum is measured by attaching a
tuning-fork of known vibration-rate to the pendulum, and
letting it write on smoked paper as the pendulum swings past
the 9-cm. opening.

The experiment is now as follows. The room is darkened. Card _T_ is
dropped into groove _z_, while _I_ is put in groove _y_ and swings
with the pendulum. One eye alone is used.

Case 1. The eye is fixed in the direction _EA_. The pendulum is
allowed to swing through its 47 deg.. The resulting visual image is shown
in Fig. 7:1. Its shape is of course like _T_, Fig. 6, but the part _H_
is less bright than the rest because it is exposed a shorter time,
owing to the narrowness of the handle of the dumb-bell, which swings
by and mediates the exposure. Sheets of milk-glass are now dropped
into the back groove of _BB_, until the light is so tempered that
part _H_ (Fig. 7:1) is _barely but unmistakably_ visible as luminous.
The intensity actually used by the writer, relative to that of _EE_,
is fairly shown in the figure. (See Plate III.)

It is clear, if the eye were now to move with the pendulum, that the
same amount of light would reach the retina, but that it would be
concentrated on a horizontally narrower area. And if the eye moves
exactly with the pendulum, the visual image will be no longer like 1
but like 2 (Fig. 7). We do not as yet know how the intensities of _e_,
_e_ and _h_ will relatively appear. To ascertain this we must put card
_I_ into groove _x_, and let card _T_ swing with the pendulum in
groove _y_. If the eye is again fixed in the direction _EA_ (Fig. 5),
the retina receives exactly the same stimulation that it would have
received before the cards were shifted if it had moved exactly at the
rate of the pendulum. In the experiments described, the handle _h_ of
this image (Fig. 7:2) curiously enough appears of the same brightness
as the two ends _e_, _e_, although, as we know, it is stimulated for a
briefer interval. Nor can any difference between _e_, _e_ and _h_ be
detected in the time of disappearance of their after-images. These
conditions are therefore generous. The danger is that _h_ of the
figure, the only part of the stimulation which could possibly quite
elapse during the movement, is still too bright to do so.

Case 2. The cards are replaced in their first positions, _T_ in groove
_z_, _I_ in groove _y_ which swings. The subject is now asked to make
voluntary eye-sweeps from _P_ to _P'_ and back, timing his moment of
starting so as to bring his axis of vision on to the near side of
opening _ON_ at approximately the same time as the pendulum brings _I_
on the same point. This is a delicate matter and requires practice.
Even then it would be impossible, if the subject were not allowed to
get the rhythm of the pendulum before passing judgment on the
after-images. The pendulum used gives a slight click at each end of
its swing, and from the rhythm of this the subject is soon able to
time the innervation of his eye so that the exposure coincides with
the middle of the eye-movement.

[Illustration: PSYCHOLOGICAL REVIEW. MONOGRAPH SUPPLEMENT, 17. PLATE III.
Fig. 7.
HOLT ON EYE-MOVEMENT.]

It is true that with every swing the pendulum moves more slowly past
_ON_, and the period of exposure is lengthened. This, however, only
tends to make the retinal image brighter, so that its disappearance
during an anaesthesia would be so much the less likely. The pendulum
may therefore be allowed to 'run down' until its swing is too slow for
the eye to move with it, that is, too slow for a distinct,
non-elongated image of _i_ to be caught in transit on the retina.

With these eye-movements, the possible appearances are of two classes,
according to the localization of the after-image. The image is
localized either at _A_ (Fig. 5), or at the final fixation-point (_P_
or _P'_, according to the direction of the movement). Localized at
_A_, the image may be seen in either of two shapes. First, it may be
identical with 1, Fig. 7. It is seen somewhat peripherally, judgment
of indirect vision, and is correctly localized at _A_. When the
subject's eye is watched, it is found that in this case it moved
either too soon or too late, so that when the exposure was made, the
eye was resting quietly on one of the fixation-points and so naturally
received the same image as in case 1, except that now it lies in
indirect vision, the eye being directed not toward _A_ (as in case 1)
but towards either _P_ or _P'_.

Second, the image correctly localized may be like 2 (Fig. 7), and then
it is seen to move past the opening _ON_. The handle _h_ looks as
bright as _e_, _e_. This appearance once obtained generally recurs
with each successive swing of the pendulum, and scrutiny of the
subject's eye shows it to be moving, not by separate voluntary
innervations from _P_ to _P'_ and then from _P'_ to _P_, but
continuously back and forth with the swing of the pendulum, much as
the eye of a child passively follows a moving candle. This movement is
purely reflex,[20] governed probably by cerebellar centers. It seems
to consist in a rapid succession of small reflex innervations, and is
very different from the type of movement in which one definite
innervation carries the eye through its 42 deg., and which yielded the
phenomena with the perimeter. A subject under the spell of this reflex
must be exercised in innervating his eye to move from _P_ to _P'_ and
back in single, rapid leaps. For this, the pendulum is to be
motionless and the eye is not to be stimulated during its movement.

[20] Exner, Sigmund, _Zeitschrift f. Psychologie u. Physiologie
der Sinnesorgane_, 1896, XII., S. 318. 'Entwurf zu einer
physiologischen Erklaerung der psychischen Erscheinungen,'
Leipzig u. Wien, 1894, S. 128. Mach, Ernst, 'Beitraege zur
Analyse der Empfindungen,' Jena, 1900, S. 98.

These two cases in which the image is localized midway between _P_ and
_P'_ interest us no further. Localized on the final fixation-point,
the image is always felt to flash out suddenly _in situ_, just as in
the case of the 'correctly localized' after-image streaks in the
experiments with the perimeter. The image appears in one of four
shapes, Fig. 7: 2 or 3, 4 or 5.

First, the plain or elongated outline of the dumb-bell appears with
its handle on the final fixation-point (2 or 3). The image is plain
and undistorted if the eye moves at just the rate of the pendulum,
elongated if the eye moves more rapidly or more slowly. The point that
concerns us is that the image appears _with its handle_. Two
precautions must here be observed.

The eye does not perhaps move through its whole 42 deg., but stops instead
just when the exposure is complete, that is, stops on either _O_ or
_N_ and considerably short of _P_ or _P'_. It then follows that the
exposure is given at the _very last_ part of the movement, so that the
after-image of even the handle _h_ has not had time to subside. The
experiment is planned so that the after-image of _h_ shall totally
elapse during that part of the movement which occurs after the
exposure, that is, while the eye is completing its sweep of 42 deg., from
_O_ to _P_, or else from _N_ to _P'_. If the arc is curtailed at point
_O_ or _N_, the handle of the dumb-bell will of course appear. The
fact can always be ascertained by asking the subject to notice very
carefully where the image is localized. If the eye does in fact stop
short at _O_ or _N_, the image will be there localized, although the
subject may have thoughtlessly said before that it was at _P_ or _P'_,
the points he had nominally had in mind.

But the image 2 or 3 may indeed be localized quite over the final
fixation-point. In this case the light is to be looked to. It is too
bright, as it probably was in the case of Dodge's experiments. It must
be further reduced; and with the eye at rest, the control (case I)
must be repeated. In the experiments here described it was always
found possible so to reduce the light that the distinct, entire image
of the dumb-bell (2, Fig. 7) never appeared localized on the final
fixation-point, although in the control, _H_, of Fig. 7:1, was always
distinctly visible.

With these two precautions taken, the image on the final
fixation-point is like either 3, 4, or 5. Shape 5 very rarely appears,
while the trained subject sees 4 and 3 each about one half the times;
and either may be seen for as many as fifteen times in succession.

Shape 4 is of course exactly the appearance which this experiment
takes to be crucial evidence of a moment of central anaesthesia, before
the image is perceived and during which the stimulation of the handle
_h_ completely elapses. Eight subjects saw this phenomenon distinctly
and, after some training in timing their eye-movements, habitually.
The first appearance of the handleless image was always a decided
surprise to the subject (as also to the writer), and with some
eagerness each hastened to verify the phenomenon by new trials.

The two ends (_e_, _e_) of the dumb-bell seem to be of the same
intensity as in shape 2 when seen in reflex movement. But there is no
vestige whatsoever of a handle. Two of the subjects stated that for
them the place where the handle should have been, appeared of a
velvety blackness more intense than the rest of the background. The
writer was not able to make this observation. It coincides
interestingly with that of von Kries,[21] who reports as to the phases
of fading after-images, that between the disappearance of the primary
image and the appearance of the 'ghost,' a moment of the most intense
blackness intervenes. The experiments with the pendulum, however,
brought out no ghost.

[21] Von Kries, J., _Zeitschr. f. Psych, u. Physiol. d.
Sinnesorgane_, 1896, XII., S. 88.

We must now enquire why in about half the cases shape 3 is still seen,
whereas shape 5 occurs very rarely. Some of the subjects, among whom
is the writer, never saw 5 at all. We should expect that with the
intensity of _H_ sufficiently reduced 4 and 5 would appear with equal
frequency, whereas 3 would be seen no oftener than 2; shape 5
appearing when the eye did not, and 4 when it did, move at just the
rate of the pendulum. It is certain that when 4 is seen, the eye has
caught just the rate of the pendulum, and that for 3 or 5 it has moved
at some other rate. We have seen above (p. 27) that to move with the
pendulum the eye must already move decidedly more slowly than Dodge
and Cline find the eye generally to move. Nothing so reliable in
regard to the rate of voluntary eye-movements as these measurements of
Dodge and Cline had been published at the time when the experiments on
anaesthesia were carried on, and it is perhaps regrettable that in the
'empirical' approximation of the natural rate of the eye through 40 deg.
the pendulum was set to move so slowly.

In any case it is highly probable that whenever the eye did not move
at just the rate of the pendulum, it moved _more rapidly_ rather than
more slowly. The image is thus horizontally elongated, by an amount
which varies from the least possible up to 9 cm. (the width of the
opening in _T_), or _even more_. And while the last of the movement
(_O_ to _P_, or _N_ to _P'_), in which the stimulation of _H'_ is
supposed to subside, is indeed executed, it may yet be done so
_rapidly_ that after all _H'_ cannot subside, not even although it is
now less intense by being horizontally spread out (that is, less
concentrated than the vanished _h_ of shape 4). This explanation is
rendered more probable by the very rare appearance of shape 5, which
must certainly emerge if ever the eye were to move more slowly than
the pendulum.

The critical fact is, however, that shape 4 _does_ appear to a trained
subject in about one half the trials--a very satisfactory ratio when
one considers the difficulty of timing the beginning of the movement
and its rate exactly to the pendulum.

Lastly, in some cases no image appears at all. This was at first a
source of perplexity, until it was discovered that the image of the
dumb-bell, made specially small so as to be contained within the area
of distinct vision, could also be contained on the blind-spot. With
the pendulum at rest the eye could be so fixed as to see not even the
slight halo which diffuses in the eye and seems to lie about the
dumb-bell. It may well occur, then, that in a movement the image
happens to fall on the blind-spot and not on the fovea. That this
accounts for the cases where no image appears, is proved by the fact
that if both eyes are used, some image is always seen. A binocular
image under normal convergence can of course not fall on both
blind-spots. It may be further said that the shape 4 appears as well
when both eyes are used as with only one. The experiment may indeed as
well be carried on with both eyes.

Some objections must be answered. It may be said that the image of _h_
happens to fall on the blind-spot, _e_ and _e_ being above and below
the same. This is impossible, since the entire image and its halo as
well may lie within the blind-spot. If now _h_ is to be on the
blind-spot, at least one of the end-circles _e_, _e_ will be there
also, whereas shape 4 shows both end-circles of the dumb-bell with
perfect distinctness.

Again, it cannot properly be urged that during the movement the
attention was distracted so as not to 'notice' the handle. The shape
of a dumb-bell was specially chosen for the image so that the weaker
part of the stimulation should lie between two points which should be
clearly noticed. Indeed, if anything, one might expect this central,
connecting link in the image to be apperceptively filled in, even when
it did not come to consciousness as immediate sensation. And it
remains to ask what it is which should distract the attention.

In this connection the appearance under reflex eye-movement compares
interestingly with that under voluntary. If the wall _WONW_ (Fig. 5)
is taken from before the pendulum, and the eye allowed to move
reflexly with the swinging dumb-bell, the entire image is seen at each
exposure, the handle seeming no less bright than the end-circles.
Moreover, as the dumb-bell opening swings past the place of exposure
and the image fades, although the handle must fade more quickly than
the ends, yet this is not discernible, and the entire image disappears
without having at any time presented the handleless appearance.