Various - Harvard Psychological Studies, Volume 1

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B. Another test for this anaesthesia during movement is offered in the
following experiment. It is clear that, just as a light-stimulation is
not perceived if the whole retinal process begins and ends during a
movement, so also a particular phase of it should not be perceived if
that phase can be given complete within the time of the movement. The
same pendulum which was used in the previous experiment makes such a
thing possible. If in place of the perforated dumb-bell the pendulum
exposes two pieces of glass of nearly complementary colors, one after
the other coming opposite the place of exposure, the sensations will
fuse or will not fuse according as the pendulum swings rapidly or
slowly. But now a mean rate of succession can be found such as to let
the first color be seen pure before the second is exposed, and then to
show the second fused with the after-image of the first. Under some
conditions the second will persist after the first has faded, and will
then itself be seen pure. Thus there may be three phases in
consciousness. If the first color exposed is green and the second red,
the phases of sensation will be green, white, and perhaps red. These
phases are felt to be not simultaneous but successive. A modification
of this method is used in the following experiment. (See Fig. 8, Plate
IV.)

_T_ and _I_ here correspond to the cards _T_ and _I_ of Fig. 6.
_T_ consists of a rectangular opening, 9x5 cm., which contains three
pieces of glass, two pieces of green at the ends, each 2.8 cm. wide
and 7 cm. high, and a piece of red glass in the middle 3.4 cm. wide
and only 1.5 cm. high, the space above and below this width being
filled with opaque material. The shape of the image is determined as
before by the hole in _I_, which now, instead of being a dumb-bell, is
merely a rectangular hole 2 cm. wide and 5 cm. high. Exactly as
before, _T_ is fixed in the background and _I_ swings with the
pendulum, the eye moving with it.

The speed of the pendulum must be determined, such that if _I_ lies in
the front groove (Fig. 5, _x_) and the eye is at rest, the image will
clearly show two phases of color when _T_ swings past on the pendulum.
With _T_ and _I_ as described above, a very slow pendulum shows the
image green, red (narrow), and green, in succession. A very fast
pendulum shows only a horizontal straw-yellow band on a green field
(Fig. 8:5). There is but one phase and no feeling of succession.
Between these two rates is one which shows two phases--the first a
green field with a horizontal, reddish-orange band (Fig. 8:3), the
second quickly following, in which the band is straw-yellow (5). It
might be expected that this first phase would be preceded by an
entirely green phase, since green is at first exposed. Such is however
not the case. The straw-yellow of the last phase is of course the
fusion-color of the red and green glasses. It would be gray but that
the two colors are not perfectly complementary. Since the arrangement
of colors in _T_ is bilaterally symmetrical, the successive phases are
the same in whichever direction the pendulum swings.

[Illustration: MONOGRAPH SUPPLEMENT 17. PLATE IV.
Fig. 8.
HOLT ON EYE-MOVEMENT.]

It is desirable to employ the maximum rate of pendulum which will give
the two phases. For this the illumination should be very moderate,
since the brighter it is, the slower must be the pendulum. With the
degree of illumination used in the experiments described, it was found
that the pendulum must fall from a height of only 9.5 deg. of its arc: a
total swing of 19 deg.. The opening of _T_, which is 9 cm. wide, then
swings past the middle point of _I_ in 275[sigma].

Now when the eye moves it must move at this rate. If the eye is 56 cm.
distant from the opening, as in the previous case, the 9 cm. of
exposure are 9 deg. 11' of eye-movement, and we saw above that 9 deg. 11' in
110[sigma] is a very slow rate of movement, according to the best
measurements. Now it is impossible for the eye to move so slowly as 9 deg.
11' in 275[sigma]. If, however, the eye is brought nearer to the
opening, it is clear that the 9 cm. of exposure become more than 9 deg.
11' of eye-movement. Therefore the eye and the fixation-points are so
placed that _EA_ (Fig. 5) = 26 cm. and _PP'_ = 18 cm. The total
eye-movement is thus 38 deg. 11', of which the nine-centimeter distance of
exposure is 19 deg. 38'. Now the eye is found to move very well through
19 deg. 38' in 275[sigma], although, again, this is much more than a
proportionate part of the total time (99.9[sigma]) given by Dodge and
Cline for a movement of the eye through 40 deg.. The eye is in this case
also moving slowly. As before, it is permissible to let the pendulum
run down till it swings too slowly for the eye to move with it; since
any lessened speed of the pendulum only makes the reddish-orange phase
more prominent.

As in the experiment with the dumb-bell, we have also here three
cases: the control, the case of the eye moving, and again a control.

Case 1. _T_ swings with the pendulum. _I_ is placed in the front
groove, and the eye looks straight forward without moving. The
pendulum falls from 9.5 deg. at one side, and the illumination is so
adjusted that the phase in which the band is reddish-orange, is
_unmistakably_ perceived before that in which it is straw-yellow. The
appearance must be 3 followed by 5 (Fig. 8).

Case 2. _T_ is fixed in the background, _I_ on the pendulum, and the
phenomena are observed with the eye moving.

Case 3. A repetition of case 1, to make sure that no different
adaptation or fatigue condition of the eye has come in to modify the
appearance of the two successive phases as at first seen.

The possible appearances to the moving eye are closely analogous to
those in the dumb-bell experiment. If the eye moves too soon or too
late, so that it is at rest during the exposure, the image is like _T_
itself (Fig. 8) but somewhat fainter and localized midway between the
points _P_ and _P'_. If the eye moves reflexly at the rate of the
pendulum, the image is of the shape _i_ and shows the two phases (3
followed by 5). It is localized in the middle and appears to move
across the nine-centimeter opening.

A difficulty is met here which was not found in the case of the
dumb-bell. The eye is very liable to come to a full stop on one of the
colored surfaces, and then to move quickly on again to the final
fixation-point. And this happens contrary to the intention of the
subject, and indeed usually without his knowledge. This stopping is
undoubtedly a reflex process, in which the cerebellar mechanism which
tends to hold the fixation on any bright object, asserts itself over
the voluntary movement and arrests the eye on the not moving red or
green surface as the exposure takes place. A comparable phenomenon was
found sometimes in the experiment with the dumb-bell, where an
eye-movement commenced as voluntary would end as a reflex following of
the pendulum. In the present experiment, until the subject is well
trained, the stopping of the eye must be watched by a second person
who looks directly at the eye-ball of the subject during each
movement. The appearances are very varied when the eye stops, but the
typical one is shown in Fig. 8:1. The red strip _AB_ is seldom longer
and often shorter than in the figure. That part of it which is
superposed on the green seldom shows the orange phase, being almost
always of a pure straw-yellow. The localization of these images is
variable. All observations made during movements in which the eye
stops, are of course to be excluded.

If now the eye does not stop midway, and the image is not localized in
the center, the appearance is like either 2, 4, or 5, and is localized
over the final fixation-point. 2 is in all probability the case of the
eye moving very much faster than the pendulum, so that if the movement
is from left to right, the right-hand side of the image is the part
first exposed (by the uncovering of the left-hand side of _T_), which
is carried ahead by the too swift eye-movement and projected in
perception on the right of the later portion. 3 is the case of the eye
moving at very nearly but not quite the rate of the pendulum. The
image which should appear 2 cm. wide (like the opening _i_) appears
about 3 cm. wide. The middle band is regularly straw-yellow, extremely
seldom reddish, and if we could be sure that the eye moves more slowly
than the pendulum, so that the succession of the stimuli is even
slower than in the control, and the red phase is surely given, this
appearance (3) would be good evidence of anaesthesia during which the
reddish-orange phase elapses. It is more likely, however, that the eye
is moving faster than the pendulum, but whether or not so
inconsiderably faster as still to let the disappearance of the reddish
phase be significant of anaesthesia, is not certain until one shall
have made some possible but tedious measurements of the apparent width
of the after-image. Both here and in the following case the _feeling
of succession_, noticeable between the two phases when the eye is at
rest, has _disappeared with the sensation of redness_.

The cases in which 5 is seen are, however, indisputably significant.
The image is apparently of just the height and width of _i_, and there
is not the slightest trace of the reddish-orange phase. The image
flashes out over the final fixation-point, green and straw-yellow,
just as the end-circles of the dumb-bell appeared without their
handle. The rate of succession of the stimuli, green--red--green, on
the retina, is identical with that rate which showed the two phases to
the resting eye: for the pendulum is here moving at the very same
rate, and the eye is moving exactly with the pendulum, as is shown by
the absence of any horizontal elongation of the image seen. The
trained subject seldom sees any other images than 4 and 5, and these
with about equal frequency, although either is often seen in ten or
fifteen consecutive trials. As in the cases of the falsely localized
images and of the handleless dumb-bell, movements of both eyes, as
well as of the head but not the eyes, yield the same phenomena. It is
interesting again to compare the appearance under reflex movement. If
at any time during the experiments the eye is allowed to follow the
pendulum reflexly, the image is at once and invariably seen to pass
through its two phases as it swings past the nine-centimeter opening.

The frequent and unmistakable appearance of this band of straw-yellow
on a non-elongated green field _without the previous phase in which
the band is reddish-orange_, although this latter was unmistakable
when the same stimulation was given to the eye at rest, is
authenticated by eight subjects. _This appearance, together with that
of the handleless dumb-bell, is submitted as a demonstration that
during voluntary movements of the eyes, and probably of the head as
well, there is a moment in which stimulations are not transmitted from
the retina to the cerebral cortex, that is, a moment of central
anaesthesia_. The reason for saying 'and _probably_ of the head as
well,' is that although the phenomena described are gotten equally
well from movements of the head, yet it is not perfectly certain that
when the head moves the eyes do not also move slightly within the
head, even when the attempt is made to keep them fixed.

Most of the criticisms which apply to this last experiment apply to
that with the dumb-bell and have already been answered. There is one
however which, while applying to that other, more particularly applies
here. It would be, that these after-images are too brief and
indistinct to be carefully observed, so that judgments as to their
shape, size, and color are not valid evidence. This is a perfectly
sensible criticism, and a person thoroughly convinced of its force
should repeat the experiments and decide for himself what reliance he
will place on the judgments he is able to make. The writer and those
of the subjects who are most trained in optical experiments find the
judgments so simple and easily made as not to be open to doubt.

In the first place, it should be remembered that only those cases are
counted in which the movement was so timed that the image was seen in
direct vision, that is, was given on or very near the fovea. In such
cases a nice discrimination of the shape and color of the images is
easily possible.

Secondly, the judgments are in no case quantitative, that is, they in
no case depend on an estimate of the absolute size of any part of the
image. At most the proportions are estimated. In the case of the
dumb-bell the question is, Has the figure a handle? The other
question, Are the end-circles horizontally elongated? has not to be
answered with mathematical accuracy. It is enough if the end-circles
are approximately round, or indeed are narrower than 9 cm.
horizontally, for at even that low degree of concentration the handle
was still visible to the resting eye. Again, in the experiment with
the color-phases, only two questions are essential to identify the
appearance 5: Does the horizontal yellow band extend quite to both
edges of the image? and, Is there certainly no trace of red or orange
to be seen? The first question does not require a quantitative
judgment, but merely one as to whether there is any green visible to
the right or left of the yellow strip. Both are therefore strictly
questions of quality. And the two are sufficient to identify
appearance 5, for if no red or orange is visible, images 1, 2, and 3
are excluded; and if no green lies to the right or left of the yellow
band, image 4 is excluded. Thus if one is to make the somewhat
superficial distinction between qualitative and quantitative
judgments, the judgments here required are qualitative. Moreover, the
subjects make these judgments unhesitatingly.

Finally, the method of making judgments on after-images is not new in
psychology. Lamansky's well-known determination of the rate of
eye-movements[22] depends on the possibility of counting accurately
the number of dots in a row of after-images. A very much bolder
assumption is made by Guillery[23] in another measurement of the rate
of eye-movements. A trapezoidal image was generated on the moving
retina, and the after-image of this was projected on to a plane
bearing a scale of lines inclining at various angles. On this the
degree of inclination of one side of the after-image was read off, and
thence the speed of the eye-movement was calculated. In spite of the
boldness of this method, a careful reading of Guillery's first article
cited above will leave no doubt as to its reliability, and the
accuracy of discrimination possible on these after-images.

[22] Lamansky, S., (Pflueger's) Archiv f. d. gesammte
Physiologie, 1869, II., S. 418.

[23] Guillery, (Pflueger's) Archiv f. d. ges. Physiologie, 1898,
LXXI., S. 607; and 1898, LXXIII., S. 87.

As to judgments on the color and color-phases of after-images, there
is ample precedent in the researches of von Helmholtz, Hering, Hess,
von Kries, Hamaker, and Munk. It is therefore justifiable to assume
the possibility of making accurately the four simple judgments of
shape and color described above, which are essential to the two proofs
of anaesthesia.

V. SUMMARY AND COROLLARIES OF THE EXPERIMENTS, AND A PARTIAL,
PHYSIOLOGICAL INTERPRETATION OF THE CENTRAL ANAESTHESIA.

We have now to sum up the facts given by the experiments. The fact of
central anaesthesia during voluntary movement is supported by two
experimental proofs, aside from a number of random observations which
seem to require this anaesthesia for their explanation. The first proof
is that if an image of the shape of a dumb-bell is given to the retina
during an eye-movement, and in such a way that the handle of the
image, while positively above the threshold of perception, is yet of
brief enough duration to fade completely before the end of the
movement, it then happens that both ends of the dumb-bell are seen but
the handle not at all. The fact of its having been properly given to
the retina is made certain by the presence of the now disconnected
ends.

The second proof is that, similarly, if during an eye-movement two
stimulations of different colors are given to the retina, superposed
and at such intensity and rate of succession as would show to the
resting eye two successive phases of color (in the case taken,
reddish-orange and straw-yellow), it then happens that the first
phase, which runs its course and is supplanted by the second before
the movement is over, is not perceived at all. The first phase was
certainly given, because the conditions of the experiment require the
orange to be given if the straw-yellow is, since the straw-yellow
which is seen can be produced only by the addition of green to the
orange which is not seen.

These two phenomena seem inevitably to demonstrate a moment during
which a process on the retina, of sufficient duration and intensity
ordinarily to determine a corresponding conscious state, is
nevertheless prevented from doing so. One inclines to imagine a
retraction of dendrites, which breaks the connection between the
central end of the optic nerve and the occipital centers of vision.

The fact of anaesthesia demonstrated, other phenomena are now available
with further information. From the phenomena of the 'falsely
localized' images it follows that at least in voluntary eye-movements
of considerable arc (30 deg. or more), the anaesthesia commences
appreciably later than the movement. The falsely localized streak is
not generated before the eye moves, but is yet seen before the
correctly localized streak, as is shown by the relative intensities of
the two. The anaesthesia must intervene between the two appearances.
The conjecture of Schwarz, that the fainter streak is but a second
appearance of the stronger, is undoubtedly right.

We know too that the anaesthesia depends on a mechanism central of the
retina, for stimulations are received during movement but not
transmitted to consciousness till afterward. This would be further
shown if it should be found that movements of the head, no less than
those of the eyes, condition the anaesthesia. As before said, it is not
certain that the eyes do not move slightly in the head while the head
moves. The movement of the eyes must then be very slight, and the
anaesthesia correspondingly either brief or discontinuous. Whereas, the
phenomena are the same when the head moves 90 deg. as when the eyes move
that amount. It seems probable, then, that voluntary movements of the
head do equally condition the anaesthesia.

We have seen, too, that in reflex eye-or head-movements no anaesthesia
is so far to be demonstrated. The closeness with which the eye follows
the unexpected gyrations of a slowly waving rush-light, proves that
the reflex movement is produced by a succession of brief impulses
(probably from the cerebellum), each one of which carries the eye
through only a very short distance. It is an interesting question,
whether there is an instant of anaesthesia for each one of these
involuntary innervations--an instant too brief to be revealed by the
experimental conditions employed above. The seeming continuity of the
sensation during reflex movement would of course not argue against
such successive instants of anaesthesia, since no discontinuity of
vision during voluntary movement is noticeable, although a relatively
long moment of anaesthesia actually intervenes.

But decidedly the most interesting detail about the anaesthesia is that
shown by the extreme liability of the eye to stop reflexly on the red
or the green light, in the second experiment with the pendulum.
Suppose the eye to be moving from _P_ to _P'_ (Fig. 5); the
anaesthesia, although beginning later than the movement, is present
when the eye reaches _O_, while it is between _O_ and _N_, that is,
during the anaesthetic moment, that the eye is reflexly caught and held
by the light. This proves again that the anaesthesia is not retinal,
but it proves very much more; namely, that _the retinal stimulation is
transmitted to those lower centers which mediate reflex movements, at
the very instant during which it is cut off from the higher, conscious
centers_. The great frequency with which the eye would stop midway in
its movements, both in the second pendulum-experiment and in the
repetition of Dodge's perimeter-test, was very annoying at the time,
and the observation cannot be questioned. The fact of the habitual
reflex regulation of voluntary movements is otherwise undisputed.
Exner[24] mentions a variety of similar instances. Also, with the
moving dumb-bell, as has been mentioned, the eye having begun a
voluntary sweep would often be caught by the moving image and carried
on thereafter reflexly with the pendulum. These observations hang
together, and prove a connection between the retina and the reflex
centers even while that between the retina and the conscious centers
is cut off.

[24] Exner, Sigmund, 'Entwurf zu einer physiologischen
Erklaerung der psychischen Erscheinungen,' Leipzig und Wien,
1894, S. 124-129.

But shall we suppose that the 'connection' between the retina and the
conscious centers is cut off during the central anaesthesia? All that
the facts prove is that the centers are at that time not conscious. It
would be at present an unwarrantable assumption to make, that these
centers are therefore disconnected from the retina, at the optic
thalami, the superior quadrigeminal bodies, or wheresoever. On broad
psychological grounds the action-theory of Muensterberg[25] has
proposed the hypothesis that cerebral centers fail to mediate
consciousness not merely when no stimulations are transmitted to them,
but rather when the stimulations transmitted are not able to pass
through and out. The stimulation arouses consciousness when it finds a
ready discharge. And indeed, in this particular case, while we have no
other grounds for supposing stimulations _to_ the visual centers to be
cut off, we do have other grounds for supposing that egress _from_
these cells would be impeded.

[25] Muensterberg, Hugo, 'Grundzuege der Psychologie,' Leipzig,
1900, S. 525-561.

The occipital centers which mediate sensations of color are of course
most closely associated with those other centers (probably the
parietal) which receive sensations from the eye-muscles and which,
therefore, mediate sensations which furnish space and position to the
sensations of mere color. Now it is these occipital centers, mediators
of light-sensations merely, which the experiments have shown most
specially to be anaesthetic. The discharge of such centers means
particularly the passage of excitations on to the parietal
localization-centers. There are doubtless other outlets, but these are
the chief group. The movements, for instance, which activity of these
cells produces, are first of all eye-movements, which have to be
_directly_ produced (according to our present psychophysical
conceptions) by discharges from the centers of eye-muscle sensation.
The principal direction of discharge, then, from the color-centers is
toward the localization-centers.

Now the experiment with falsely and correctly localized after-images
proves that before the anaesthesia all localization is with reference
to the point of departure, while afterwards it is with reference to
the final fixation-point. The transition is abrupt. During the
anaesthesia, then, the mechanism of localization is suffering a
readjustment. It is proved that during this interval of readjustment
in the centers of eye-muscle sensation the way is closed to oncoming
discharges from the color-centers; but it is certain that any such
discharge, during this complicated process of readjustment, would take
the localization-centres by surprise, as it were, and might
conceivably result in untoward eye-movements highly prejudicial to the
safety of the individual as a whole. The much more probable event is
the following:

Although Schwarz suggests that the moment between seeing the false and
seeing the correct after-image is the moment that consciousness is
taken up with 'innervation-feelings' of the eye-movement, this is
impossible, since the innervation-feelings (using the word in the only
permissible sense of remembered muscle-sensations) must _precede_ the
movement, whereas even the first-seen, falsely localized streak is not
generated till the movement commences. But we do have to suppose that
during the visual anaesthesia, muscle-sensations of _present_ movement
are streaming to consciousness, to form the basis of the new
post-motum localization. And these would have to go to those very
centers mentioned above, the localization-centers or eye-muscle
sensation centers. One may well suppose that these incoming currents
so raise the tension of these centers that for the moment no discharge
can take place thither from other parts of the brain, among which are
the centers for color-sensations. The word 'tension' is of course a
figure, but it expresses the familiar idea that centers which are in
process of receiving peripheral stimulations, radiate that energy
_to_ other parts of the brain (according to the neural dispositions),
and probably do not for the time being receive communications
therefrom, since those other parts are now less strongly excited. It
is, therefore, most probable that during the incoming of the
eye-muscle sensations the centers for color are in fact not able to
discharge through their usual channels toward the localization-centers,
since the tension in that direction is too high. If, now, their other
channels of discharge are too few or too little used to come into
question, the action-theory would find in this a simple explanation of
the visual anaesthesia.