Various - Harvard Psychological Studies, Volume 1

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TABLE IV.

Illumination. Per Cent. Showing Progress. Binocular. Monocular.

In light. 7.6 % 50 % 50 %
In darkness. 18.3 34.2 65.8

Among judgments made in daylight those series which present
progression are equally distributed between binocular and monocular
vision. When, however, the determinations are of a luminous point in
an otherwise dark field, the preponderance in monocular vision of the
tendency to a progression becomes pronounced. That this is not a
progressive rectification of the judgment, is made evident by the
distribution of the directions of change in the several experimental
conditions shown in the following table:

TABLE V.
Light. Darkness.
Direction of Change. Binocular. Monocular. Binocular. Monocular.
Upward. 50 % 100 % 38.4 % 65.0 %
Downward. 50 00.0 61.6 35.0
Const. Err. -7.70 +11.66 -36.62 -3.38

When the visual field is illuminated the occurrence of progression in
binocular vision is accidental, the percentages being equally
distributed between upward and downward directions. In monocular
vision, on the contrary, the movement is uniformly upward and involves
a progressive increase in error. When the illuminated point is exposed
in an otherwise dark field the progression is preponderatingly
downward in binocular vision and upward in vision with the single eye.
The relation of these changes to phenomena of convergence, and the
tendency to upward rotation in the eyeball has already been stated.
There is indicated, then, in these figures the complication of the
process of relocating the ideal horizon by reference to the sense of
general body position with tendencies to reinstate simply the set of
eye-muscle strains which accompanied the preceding judgment, and the
progressive distortion of the latter by a factor of constant error due
to the mechanical conditions of muscular equilibrium in the resting
eye.

IV.

The influence of this factor is also exhibited when judgments made
with both eyes are compared with those made under conditions of
monocular vision. The latter experiments were carried on in alternate
series with those already described. The figures are given in the
following tables:

TABLE VI.

JUDGMENTS MADE IN DIFFUSED LIGHT.

Observer. Constant Error. Average Deviation. Mean Variation.
_A_ (50) - 28.46 29.04 8.87
_C_ " + 7.54 14.86 8.01
_D_ " + 39.32 43.28 13.83
_E_ " + 50.46 65.26 9.86
_F_ " + 62.30 62.30 1.60
_G_ " 0.00 45.28 9.66
_H_ " + 22.92 79.12 5.07
_I_ " + 14.36 51.96 8.02
_K_ " + 9.26 38.10 9.55
_L_ " - 61.10 61.10 6.36
Average: + 11.66 49.03 8.18

TABLE VII.

JUDGMENTS IN ILLUMINATED POINT.

Observer. Constant Error. Average Deviation. Mean Variation.
_A_ (50) - 38.42 51.96 32.64
_C_ (30) - 29.03 41.23 35.75
_D_ (20) - 30.87 34.07 17.24
_E_ (50) + 65.30 75.86 29.98
_F_ " + 50.74 50.74 5.89
_G_ " + 66.38 88.10 44.98
_H_ " + 65.40 80.76 42.93
_I_ " - 0.02 80.22 47.53
_K_ " - 44.60 52.56 32.93
_L_ " - 71.06 73.30 31.86
Average: - 3.38 62.88 32.17

The plane of vision in judgments made with the right eye alone is
deflected upward from the true horizon to a greater degree than it is
depressed below it in those made with binocular vision, the respective
values of the constant errors being -7'.70 and +11'.66, a difference
of 19'.36. When the field of vision is darkened except for the single
illuminated disc, a similar reversion of sign takes place in the
constant error. With binocular vision the plane of the subjective
horizon is deflected downward through 36'.62 of arc; with monocular
vision it is elevated 3'.38, a difference of 40'.00, or greater than
in the case of judgments made in the lighted room by 20'.64. This
increase is to be expected in consequence of the elimination of those
corrective criteria which the figured visual field presents. The two
eyes do not, of course, function separately in such a case, and the
difference in the two sets of results is undoubtedly due to the
influence of movements in the closed eye upon that which is open; or
rather, to the difference in binocular functioning caused by shutting
off the visual field from one eye. The former expression is justified
in so far as we conceive that the tendency of the closed eye to turn
slightly upward in its socket affects also the direction of regard in
the open eye by attracting toward itself its plane of vision. But if,
as has been pointed out, this elevation of the line of sight in the
closed eye is accompanied by a characteristic change in the process of
binocular convergence, the result cannot be interpreted as a simple
sympathetic response in the open eye to changes taking place in that
which is closed, but is the consequence of a release of convergence
strain secondarily due to this act of closing the eye.

Several points of comparison between judgments made with binocular and
with monocular vision remain to be stated. In general, the process of
location is more uncertain when one eye only is used than when both
are employed, but this loss in accuracy is very slight and in many
cases disappears. The loss in accuracy is perhaps also indicated by
the range of variation in the two cases, its limits being for
binocular vision +46'.29 to -56'.70, and for monocular +62'.30 to
-61'.10, an increase of 20'.41. In the darkened room similar relations
are presented. The mean variations are as follows: binocular vision,
31'.42; monocular, 32'.17. Its limits in individual judgments are:
binocular, -1'.62 to -128'.70, monocular, +66'.38 to -71'.06, an
increase of 10'.36. In all ways, then, the difference in accuracy
between the two forms of judgment is extremely small, and the
conclusion may be drawn that those significant factors of judgment
which are independent of the figuration of the visual field are not
connected with the stereoscopic functioning of the two eyes, but such
as are afforded by adjustment in the single eye and its results.

VI.

The experimental conditions were next complicated by the introduction
of abnormal positions of the eyes, head and whole body. The results of
tipping the chin sharply upward or downward and keeping it so fixed
during the process of location are given in the following table, which
is complete for only three observers:

TABLE VIII.

Observer. Upward Rotation. Downward Rotation.
C.E. A.D. M.V. C.E. A.D. M.V.
_L_ (50) +43.98 43.98 5.62 +28.32 28.32 5.02
_K_ (50) -33.72 33.72 71.33 +19.49 19.49 55.22
_L_ (20) -39.10 45.90 33.60 -68.65 69.25 25.20
Average: - 9.61 41.20 36.85 -19.94 39.02 28.48
Normal: -64.14 67.08 33.51

The results of rotating the whole body backward through forty-five and
ninety degrees are given in the following table:

TABLE IX.

Observer. Rotation of 45 deg.. Rotation of 90 deg..
C.E. A.D. M.V. C.E. A.D. M.V.
_B_ (30) + 4.10 24.57 18.56
_D_ (30) +291.03 291.03 61.86
_G_ (50) +266.78 266.78 22.83 +200.16 200.16 11.00
_F_ (60) +116.45 116.45 17.14 - 36.06 36.30 6.29
_J_ (20) +174.30 174.63 30.94
Average: +170.53 174.69 30.66

The errors which appear in these tables are not consistently of the
type presented in the well-known rotation of visual planes
subjectively determined under conditions of abnormal relations of the
head or body in space. When the head is rotated upward on its lateral
horizontal axis the average location of the subjective horizon,
though still depressed below the true objective, is higher than when
rotation takes place in the opposite direction. When the whole body is
rotated backward through 45 deg. a positive displacement of large amount
takes place in the case of all observers. When the rotation extends to
90 deg., the body now reclining horizontally but with the head supported
in a raised position to allow of free vision, an upward displacement
occurs in the case of one of the two observers, and in that of the
other a displacement in the opposite direction. When change of
position takes place in the head only, the mean variation is decidedly
greater if the rotation be upward than if it be downward, its value in
the former case being above, in the latter below that of the normal.
When the whole body is rotated backward through 45 deg. the mean variation
is but slightly greater than under normal conditions; when the
rotation is through 90 deg. it is much less. A part of this reduction is
probably due to training. In general, it may be said that the
disturbance of the normal body relations affects the location of the
subjective horizon, but the specific nature and extent of this
influence is left obscure by these experiments. The ordinary movements
of eyes and head are largely independent of one another, and even when
closed the movements of the eyes do not always symmetrically follow
those of the head. The variations in the two processes have been
measured by Muensterberg and Campbell[1] in reference to a single
condition, namely, the relation of attention to and interest in the
objects observed to the direction of sight in the closed eyes after
movement of the head. But apart from the influence of such secondary
elements of ideational origin, there is reason to believe that the
mere movement of the head from its normal position on the shoulders up
or down, to one side or the other, is accompanied by compensatory
motion of the eyes in an opposite direction, which tends to keep the
axis of vision nearer to the primary position. When the chin is
elevated or depressed, this negative reflex adjustment is more
pronounced and constant than when the movement is from side to side.
In the majority of cases the retrograde movement of the eyes does not
equal the head movement in extent, especially if the latter be
extreme.

[1] Muensterberg, H., and Campbell, W.W.: PSYCHOLOGICAL REVIEW,
I., 1894, p. 441.

The origin of such compensatory reactions is connected with the
permanent relations of the whole bodily organism to the important
objects which surround it. The relations of the body to the landscape
are fairly fixed. The objects which it is important to watch lie in a
belt which is roughly on a horizontal plane with the observing eye.
They move or are moved about over the surface of the ground and do not
undergo any large vertical displacement. It is of high importance,
therefore, that the eye should be capable of continuous observation of
such objects through facile response to the stimulus of their visual
appearance and movements, in independence of the orientation of the
head. There are no such determinate spatial relations between body
position and the world of important visual objects in the case of
those animals which are immersed in a free medium; and in the
organization of the fish and the bird, therefore, one should not
expect the development of such free sensory reflexes of the eye in
independence of head movements as we know to be characteristic of the
higher land vertebrates. In both of the former types the eye is fixed
in its socket, movements of the whole head or body becoming the
mechanism of adjustment to new objects of observation. In the
adjustment of the human eye the reflex determination through sensory
stimuli is so facile as to counteract all ordinary movements of the
head, the gaze remaining fixed upon the object through a series of
minute and rapidly repeated sensory reflexes. When the eyes are closed
and no such visual stimuli are presented, similar reflexes take place
in response to the movements of the head, mediated possibly by
sensations connected with changes in position of the planes of the
semicircular canals.

VII.

If eye-strain be a significant element in the process of determining
the subjective horizon, the induction of a new center of muscular
equilibrium by training the eyes to become accustomed to unusual
positions should result in the appearance of characteristic errors of
displacement. In the case of two observers, _A_ and _H_, the eyes were
sharply raised or lowered for eight seconds before giving judgment as
to the position of the illuminated spot, which was exposed at the
moment when the eyes were brought back to the primary position. The
effect of any such vertical rotation is to stretch the antagonistic
set of muscles. It follows that when the eye is rotated in the
contrary direction the condition of equilibrium appears sooner than in
normal vision. In the case of both observers the subjective horizon
was located higher when judgment was made after keeping the eyes
raised, and lower when the line of sight had been depressed. In the
case of only one observer was a quantitative estimation of the error
made, as follows: With preliminary raising of the eyes the location
was +36'.4; with preliminary lowering, -11'.4.

When the illuminated button is exposed in a darkened room and is
fixated by the observer, it undergoes a variety of changes in apparent
position due to unconscious shifting of the point of regard, the
change in local relations of the retinal stimulation being erroneously
attributed to movements in the object. These movements were not of
frequent enough occurrence to form the basis of conclusions as to the
position at which the eyes tended to come to a state of rest. The
number reported was forty-two, and the movement observed was rather a
wandering than an approximation toward a definite position of
equilibrium. The spot very rarely presented the appearance of sidewise
floating, but this may have been the result of a preconception on the
part of the observer rather than an indication of a lessened liability
to movements in a horizontal plane. Objective movements in the latter
direction the observer knew to be impossible, while vertical
displacements were expected. Any violent movement of the head or eyes
dispelled the impression of floating at once. The phenomenon appeared
only when the illuminated spot had been fixated for an appreciable
period of time. Its occurrence appears to be due to a fatigue process
in consequence of which the mechanism becomes insensible to slight
changes resulting from releases among the tensions upon which constant
fixation depends. When the insensitiveness of fatigue is avoided by a
slow continuous change in the position of the illuminated spot, no
such wandering of the eye from its original point of regard occurs,
and the spot does not float. The rate at which such objective
movements may take place without awareness on the part of the observer
is surprisingly great. Here the fatigue due to sustained fixation is
obviated by the series of rapid and slight sensory reflexes which take
place; these have the effect of keeping unchanged the retinal
relations of the image cast by the illuminated spot, and being
undiscriminated in the consciousness of the observer the position of
the point of regard is apprehended by him as stationary. The
biological importance of such facile and unconscious adjustment of the
mechanism of vision to the moving object needs no emphasis; but the
relation of these obscure movements of the eyes to the process of
determining the plane of the subjective horizon should be pointed out.
The sense of horizontality in the axes of vision is a transient
experience, inner conviction being at its highest in the first moments
of perception and declining so characteristically from this maximum
that in almost every case the individual judgment long dwelt upon is
unsatisfactory to the observer. This change I conceive to be a
secondary phenomenon due to the appearance of the visual wanderings
already described.

VIII.

The influence of sensory reflexes in the eye upon the process of
visual orientation was next taken up in connection with two specific
types of stimulation. At top and bottom of the vertical screen were
arranged dark lanterns consisting of electric bulbs enclosed in
blackened boxes, the fronts of which were covered with a series of
sheets of white tissue-paper, by which the light was decentralized and
reduced in intensity, and of blue glass, by which the yellow quality
of the light was neutralized. Either of these lanterns could be
illuminated at will by the pressure of a button. All other
experimental conditions remained unchanged. The observers were
directed to pay no special regard to these lights, and the reports
show that in almost every case they had no conscious relation to the
judgment. The results are presented in the following table:

TABLE X.

Light Below. Light Above.
Observer. Const.Err. Av.Dev. M.Var. Const.Err. Av.Dev. M.Var.
_C_ (40) +156.37 156.37 19.67 +169.85 169.85 19.22
_D_ (20) + 39.30 43.30 17.95 + 46.65 47.35 15.41
_F_ (30) + 19.47 19.47 8.83 + 58.37 58.37 7.83
_G_ (50) + 66.11 112.76 14.65 +117.86 117.86 13.10
_H_ (30) -147.63 147.63 21.07 -105.30 105.30 30.31
_J_ (20) + 1.90 31.95 22.33 + 44.40 44.40 20.55
Average: + 22.59 85.28 17.42 + 55.30 90.52 17.74

The eye is uniformly attracted toward the light and the location of
the disk correspondingly elevated or depressed. The amount of
displacement which appears is relatively large. It will be found to
vary with the intensity, extent and distance of the illuminated
surfaces introduced. There can be little doubt that the practical
judgments of life are likewise affected by the distribution of light
intensities, and possibly also of significant objects, above and below
the horizon belt. Every brilliant object attracts the eye toward
itself; and the horizon beneath a low sun or moon will be found to be
located higher than in a clouded sky. The upper half of the ordinary
field of view--the clear sky--is undiversified and unimportant; the
lower half is full of objects and has significance. We should probably
be right in attributing to these characteristic differences a share in
the production of the negative error of judgment which appears in
judgments made in daylight. The introduction of such supplementary
stimuli appears to have little effect upon the regularity of the
series of judgments, the values of the mean variations being
relatively low: 17'.42 with light below, 17'.74 with it above.

IX.

In the final series of experiments the influence of limiting visual
planes upon the determination of the subjective horizon was taken up.
It had been noticed by Dr. Muensterberg in the course of travel in hill
country that a curious negative displacement of the subjective horizon
took place when one looked across a downward slope to a distant cliff,
the altitude (in relation to the observer's own standpoint) of
specific points on the wall of rock being largely overestimated.
Attributing the illusion to a reconstruction of the sensory data upon
an erroneous interpretation of the objective relations of the
temporary plane of the landscape, Dr. Muensterberg later made a series
of rough experiments by stretching an inclined cord from the eye
downward to a lower point on an opposite wall and estimating the
height above its termination of that point which appeared to be on a
level with the observing eye. He found an illusion present similar to
the case of an extended slope of country.

The first experiments of this group repeated those just described. The
previous mechanical conditions were varied only by the introduction of
a slender cord which was stretched from just below the eyes to the
bottom of the vertical screen. Full results were obtained from only
two observers, which are given in the following table:

TABLE XI.

Observer. Const. Err. Av. Dev. Mean Var. Exp. Conds.

_C_ (30) +123.92 123.92 11.94 Cord present and
_G_ (30) +66.47 66.47 15.56 consciously referred to.
_C_ (30) +126.90 126.90 6.31 Cord not present.
_G_ (30) +83.20 83.20 6.31
_C_ (30) +126.93 126.93 6.39 Cord present but not
_G_ (30) +86.63 86.63 9.40 consciously referred to.

Averages. I +95.19 95.19 13.75
" II +105.05 105.05 6.31
" III +106.78 106.78 7.89

The effect of introducing such an objective plane of reference is
twofold: the mean variation is increased, and the plane of the
subjective horizon is displaced downwards. First, then, it acts as a
simple factor of disturbance; it distracts from those habitual
adjustments upon which the accuracy of the judgment depends. Secondly,
it enters as a source of constant error into the determination of the
subjective horizon, which is attracted toward this new objective
plane. In the third section of the table are given the results of
judgments made in the presence of such a plane but without conscious
reference to it.[2] The figures here are of intermediate value in the
case of the mean variation and of slightly greater value than the
first in that of the constant error. In other words, the introduction
of such a plane cannot be wholly overlooked, though it may be greatly
abstracted from.

[2] In the preceding experiments the cord was definitely to be taken
into account in making the judgment. The method of so doing was by
running the eye back and forth over the cord preliminary to
determining the location of the point.

The single cord was next replaced by a plane of blackened wood six
inches wide and extending from the observer to the vertical screen.
This strip was arranged in two ways: first, from the observer's chin
to the bottom of the screen, and secondly, from the feet of the
observer to a point on the screen a short distance below the plane of
the objective horizon. The individual and average results are given in
the following table:

TABLE XII.

Observer. Descending Plane. Ascending Plane.

_A._ (10) +18.80 18.80 5.24 +35.10 35.10 8.27
_E._ (20) +79.30 79.30 11.56 +131.67 131.67 12.07
_H._ (10) -37.50 37.50 16.80 -46.90 46.90 7.90
_K._ (30) +71.40 71.40 12.85 +48.05 48.05 5.11
Average: +33.00 51.75 11.61 +41.95 65.43 8.34

The introduction of a descending plane lowers the apparent horizon;
that of an ascending plane elevates it. The general disturbance of
judgment appears distinctly greater in the case of a downward than in
that of an upward incline.

The results of a third variation of the experimental conditions may be
presented at once. In it the location of the subjective horizon under
normal conditions was compared with the results of adjustments made
when the screen bearing the white disc was rotated backward from the
observer through an angle of varying magnitude. The averages for each
of the two subjects are as follows:

TABLE XIII.

Observer Const. Err. Av. Dev. Mean Var. Rotation.
_F_ (20) +130.50 130.50 3.20 20 deg.
" " +115.50 115.50 1.10 50 deg.
_J_ (20) +443.10 443.10 9.47 45 deg.

These experiments were carried on in the presence of the definitely
figured visual field of the lighted room, and the observers were
conscious of taking these permanent features into account as
correctives in making their judgments. Before proceeding, this defect
was remedied as far as possible by enclosing the apparatus of
experimentation, including the observer, between two walls of black
fabric. Nothing was to be seen but these two walls, and the inclined
plane which terminated the observer's view. The position of the screen
remained constant at an inclination of 45 deg.. The upper bounding lines
of the enclosing walls, on the contrary, were adjusted in three
different relations to the plane of the gravity horizon. In the first
arrangement these lines were horizontal; in the second the ends next
to the observer were depressed five degrees; while in the final
arrangement these ends were elevated through a like angular distance.