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OC ULOMOTOR AND SPAT IAL ORI EN TAT I ON FACTORS
.. ~
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INTRODUCTION
Herschel W. Leibowitz
The importance of emphasizing oculomotor and spatial orientation
derives in part from the history of research in vision. Because one of
the most critical aspects of visual performance is the ability to read,
visual research and tests have been directed toward evaluating and cor-
recting reading ability. There are literally scores of studies in the
literature describing the functional relation between visual acuity,
contrast sensitivity, and luminance. However, the demands on the vis-
ual system in the military context involve much more than reading or
appreciation of contrast. Significant additional factors in the mili-
tary environment include spatial orientation, prolonged exposure to low
ambient illuminances, and concomitant stimulation of the vestibular
system.
A consequence of the multidimensional nature of visual demands in
military operations is that our present understanding, as well as the
tests available for evaluating visual performance, is incomplete. In
particular, lowered illuminance results in significant changes in the
oculomotor adjustments of accomodation and convergence. These changes,
as well as their striking individual differences, have minimal conse-
quences during daylight observations but become critical under low-
luminance observation conditions. It is essential in studying human
performance that our efforts be directed toward the "worst-case" con-
ditions which include twilight and night observations, degraded visual
environments, and observer motion. Not only is performance degraded at
night but illusions, spatial disorientation, and distortions of distance
and motion are also more likely.
The papers in this section serve to define the problem, to document
some encouraging progress that has been made during the past years, and
to provide guidelines for future research. The important message under-
lying these papers is that vision is multidimensional and that it in-
volves different kinds of tasks, particularly recognition and spatial
orientation, with different functional relationships to ambient illu-
minance. It is also clear that to predict visual performance under
demanding conditions, it is also necessary to consider the interactions
among the visual, oculomotor, vestibular, and proprioceptive systems.
Reference has been mace to the fact that the present report repre-
sents the first systematic attempt by the Committee on Vision to survey
the night vision literature since the end of World War II. Even a
83
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84
cursory comparison of the two literatures reveals the increased scope
of the contemporary problem. Meeting this challenge will both increase
our understanding of basic mechanisms and, at the same time, assure us
that recommendations and procedures for military personnel operating in
visually demanding environments can be carried out efficiently and
effectively.
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NORMAL VARIATIONS OF VISUAL ACCOMMODATION AND BIN=UL~ WRGENCE:
,
SOME IMPLICATIONS FOR NIGHT VISION
-
D. Alfred Owens
One of the axioms of vision science, as well as everyday experi-
ence, is that visual performance generally deteriorates under low illu-
mination. Since the earliest psychophysical research, results have
been presented documenting the sensory changes that occur with reduced
illumination, such as diminished color vision, acuity, and contrast
sensitivity. More recently, behavioral research on eye movements has
shown that these sensory changes are coupled with variations in the
efficiency of oculomotor functions, particularly of accommodation and
binocular vergence. Although the sensory processes that guide accommo-
dation and vergence are not yet fully understood, both responses depend
heavily on variables such as spatial contrast and retinal disparity for
accurate and efficient control (e.g., Toates, 1972, 1974~. This infor-
mation is gradually lost with reduced illumination, and, as a result,
the accuracy and operating range of accommodation and vergence progres-
sively decrease. These normal variations of oculomotor behavior con-
tribute to a number of problems of night vision, and they may help to
explain and predict individual differences that are characteristic of
nighttime visual performance.
THE RESTING STATE OF THE EYES
The resting state of the eyes is a key concept for understanding
normal variations of oculomotor behavior. Like the skeletal muscles,
the eye muscles exhibit continued tonic activity in the absence of
stimulation and active movement (Granit, 1970; Burian and van Noorden,
1974~. Contrary to popular traditional theories, however, the eyes'
resting (or tonus) posture does not correspond to optical infinity.
Rather, it typically falls at an intermediate distance, which varies
widely among individuals with normal vision (Leibowitz and Owens, 1978;
Owens 1984~. Although the intermediate resting state has been observed
by several investigators during the past century (Cogan, 1937; Morgan,
1946; Schober, 1954; Weber, cited by Cornelius, 1861), it was not gen-
erally accepted until recently when measurements of accommodation and
vergence under open-loop conditions became available.
85
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86
Individual Differences
Typical values of the resting positions of accommodation and ver-
gence are illustrated in Figure 1. These distributions represent
measurements taken in total darknesss, referred to as dark focus and
dark vergence. Both sets of data were obtained from the same sample of
60 college students (Owens and Leibowitz, 1980~. These data reveal two
important and unexpected variations of the resting state of the eyes.
First, both resting states exhibit substantial between-subject varia-
bility. All subjects had ostensibly normal vision by conventional cri-
teria, yet individual dark focus values ranged from about 25 cm to low
hyperopia, and dark vergence values ranged from about 60 cm to infinity.
These large individual differences are not picked up by standard clini-
cal assessment techniques.
The second point, which is not immediately obvious from Figure 1,
is that the two resting postures for a given individual are often quite
different. Although the mean dark focus and mean dark vergence corres-
pond to an intermediate distance (76 and 116 cm, respectively), these
mean values are significantly different. Moreover, within-subject
comparison of the dark vergence and dark focus values illustrated in
Figure 1 showed that they are only weakly correlated (r = 0.32~. These
findings indicate that an individual's vergence and refractive state in
darkness often differ greatly from those obtained under high illumina-
tion, and they imply that knowledge of the resting (dark) state of one
system (accommodation or vergence) does not allow prediction of the
resting state of the other system.
Relation to Clinical Indices
While individual differences of the dark focus and dark vergence
are not detected by standard clinical measures, there is some rather
inconsistent evidence for a relation between the resting states and
clinical indices.
Two investigations reported evidence that the dark focus is
inversely related to ametropia; that is, when far points are equated
optically, hyperopes tend to have a nearer dark focus than myopes
(Maddock et al., 1981; Epstein et al., 19811. A third investigation
reported the opposite relation, i.e., that relative to the far point,
the dark focus of myopes tends to be nearer than that of hyperopes
(Simonelli, 19831. The basis for this discrepancy is not clear, but one
possibility is a difference in the def inition of ametropia. Maddock et
al. (1981) and Epstein et al. ( 1981) used standard clinical ret raction
techniques to determine their subjects' refractive status, while
Simonelli (1983) used measures obtained with a polarized Vernier opto-
meter of the subjects' accommodative response for a distant monocular
target.
Comparisons of the dark vergence to measurements of phoria have
also yielded inconsistent results. One study reported that dark
vergence measures were signif icantly correlated (r = 0.62) with near
phoria but not significantly correlated with distance phoria (Francis
OCR for page 87
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and Owens, 1983~. These differences are probably related to differences
in the stimuli for accommodation and vergence in particular test situa-
tions and to complex synergistic interactions of accommodation and ver-
gence that arise from stimulation of either system.
THE RESTING STATE AND VARIATIONS OF OCULOMOTOR PERFORMANCE
The intermediate resting state concept has helped to clarify nor-
mal variations of oculomotor performance that occur under degraded
stimulus conditions. In general, accommodation and vergence responses
tend to be biased toward the subject's characteristic resting posture,
and these biases increase progressively as stimulation is reduced.
Dark Focus Bias
Johnson (1976) was among the first to report progressive biases of
accommodation with reduced illumination. He used a laser optometer to
measure accommodative responses for a high-contrast target, similar to
a reticle, viewed monocularly in Maxwellian view at four luminance
levels ranging from 0.051 to 51.4 cd/m . His results for four subjects
are presented in Figure 2 as mean accommodative response functions under
the four luminance conditions. Under the highest luminances, all sub-
jects exhibited accurate focusing performance, with response functions
approximating the ideal prediction indicated by the dashed line. As
luminance was reduced, however, accommodation became increasingly myopic
for stimuli nearer than the dark focus. At the lowest luminance, accom
modation remained near the dark focus regardless of stimulus distance.
These data show that the operating range of accommodation gradually
collapses toward the dark focus, resulting in functional presbyopia, as
illumination is reduced through low photopic to mesopic levels. This
tendency is also found with more complex natural stimuli. In one study,
Leibowitz and Owens (1975a) measured the refractive state of 30 subjects
while they viewed total darkness and a distant outdoor scene at three
luminances ( full daylight and with light reduced 1. 95 and 4.2 log units
by neural-density filters). The results are illustrated in Figure 3 as
scatter diagrams comparing individual dark focus values with the sub-
jects' accom~r.odative responses for the outdoor scene. In this case,
accurate focusing responses would yield data falling along the horizon-
tal line at 0 diopter~ (D), and accommmodative responses equivalent to
the sub jects' dark focus would yield data falling along the diagonal
theoretical line. Consistent with Johnson's (1976) results, as lumi-
nance was reduced from daylight to an approximation of nautical twi-
light, accommodative responses shifted progressively toward the sub-
jects' characteristic dark focus values. As a result, subjects with
a relatively near dark focus became increasingly myopic for the distant
scene, while these with a relatively distant dark focus showed little
change in refraction.
It is important to note that biases of accommodation toward the
dark focus are not conf ined to conditions of low illumination. Indeed,
OCR for page 89
89
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FIGURE 2 Accommodative response functions of four subjects for a mono-
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0.051 cd/m2. Individual dark focus values are indicated by the black
arrows. As luminance decreases, accommodative responses are increas-
ingly iased toward the subject' s dark focus, producing anomalous ref. rac-
tive errors for targets farther ~ night myopia) and nearer ~ night hyper-
opia) than the resting posture. Source: Johnson ( 1976~ .
OCR for page 90
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FIGURE 3 Scatter diagrams illustrating the relationship between indi-
v~dual differences in the dark focus and night myopia. Each diagram
compares the sub jects' dark focus with their accommodative responses
for a distant outdoor scene at the indicated light level. As luminance
decreased from daylight to nautical twilight, focusing responses shifted
toward the dark focus, indicated by the diagonal theoretical lines.
Thus, subjects with a near dark focus exhibited correspondingly greater
twilight and night myopia. Source: Le~bowitz and Owens ( 1975a) .
OCR for page 91
91
they appear under a wide variety of commonly occurring viewing condi-
tions, for example, (1) when viewing a bright empty field (Whiteside,
1952; Westheimer, 1957; Leibowitz and Owens, 1975b); (2) when using
optical instruments (Hennessy, 1975) or viewing through small pupils
(Hennessy et al., 1976~; (3) when viewing distant objects through an
intervening surface or screen positioned near the distance of the sub-
ject's dark focus (i.e., the Mandelbaum effect) (Owens, 1979~; and
(4) when reading text presented by projection devices (Kintz and
Bowker, 1982) or a video display terminal (Murch, 1982~.
Dark Vergence Bias
Although studied less extensively, analogous biases of vergence
toward the subject's dark vergence posture appear when fusional stimr
uli are reduced. This was first demonstrated by Ivanoff and Bourdy
(1954), who found increasing fixation disparity (i.e., vergence errors)
as the luminance of a peripheral binocular stimulus decreased to sco-
topic levels. The pattern of fixation disparity varied with the sub-
ject and the distance of the target. In general, convergent fixation
disparities were found with a distant target, and divergent fixation
disparities were found with a near target. The magnitude of these
fixation errors varied among subjects, however, with some subjects
showing a greater bias toward near fixation (i.e., larger convergent
fixation disparity), while others showed a greater bias toward far
fixation (lee., larger divergent fixation disparity!. This finding
foreshadowed later evidence for individual differences in dark ver-
gence (Figure 11.
More recently, Francis and Owens (1983) found that changes in
vergence accuracy are related to the subject's dark vergence posture.
Subjects viewed dim binocular stimuli presented over a range of dis-
tances, with retinal eccentricities ranging from 2 to 8 degrees. With
increasing retinal eccentricity, vergence responses were progressively
biased toward the individual's dark vergence distance, resulting in
convergent errors for farther targets and divergent errors for nearer
targets. Thus, as fusional stimulation is degraded, the vergence sys-
tem is biased toward its resting state in much the same manner as the
accommodative system.
Summary
Research conducted mostly during the past decade has led to a new
conception of the behavior of accommodation and vergence. In opposi-
tion to traditional views fundamental to modern clinical practice,
vergence and accommodation do not relax at the far point of their
operating range. Rather, they typically rest at an intermediate dis-
tance, which averages about 1 m and varies widely among normal sub-
jects. When visual stimulation is degraded, as with reduced illumi-
nation, accommodation and vergence responses are progressively biased
toward their respective resting postures. These biases produce
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130
performance that frequently accompany changes in level of illumination
is possible.
In addition to the static aspects of stimulation discussed above,
optokinetic stimulation has long been appreciated as a source of eye
movement control in the service of gaze stability (ter Braak, 1936;
Dichgans, 1977), with the capability of driving the eyes around all
three axes (Crone, 1975~. Optokinetic patterns are highly functional
in normal environments, serving to match eye and image velocity, but
they also serve as an updating system for the vestibular ocular res-
ponse (VOR). Since the VOR is a feed-forward system, error correction
takes place via an external optokinetic loop, a pursuit system loop,
or both (Miles and Lisberger, 1981; Ebenholtz, in press). Accordingly,
the consequences of poor image quality for the several systems subser-
ving gaze stabilization appear to be considerable. Furthermore, an
interesting observation is that optokinetic expansion-contraction
patterns may have specialized motion-detecting channels (Beverley and
Regan, 1980) and also may drive accommodation and convergence (Ittelson
and Ames, 1950; Kruger and Pola, 1985~. Although in the latter case
the relation between image quality and stimulus adequacy remains to be
established, the functional value of kinetic expansion patterns may
lie in their capability to signal changes in distance over a range of
distances well beyond the several meters usually assigned to accommo-
dation and convergence.
PRINC IPAL SOURCES OF OCULOMOTOR-ElASED ILLUSI ONS
While it may be granted that static and kinetic sources of optical
stimulation play a significant role in the control of eye movements,
if the oculomotor control system did not contribute to perception,
there would be few implications to be drawn for the understanding of
visual illusions. In fact, the conscious registration of disjunctive
and conjugate eye positions is capable of signaling distance, direc-
tion, target orientation (Ebenholtz, in press) and, in general, the
three-dimensional spatial path of a moving target. Consequently, the
adequacy of registration of the position and trajectory of the eye
determines the likelihood of oculomotor-based spatial illusions. In
this respect, perhaps the single most important heuristic principle is
that reflexive movements do not tend to be registered in consciousness,
whereas voluntarily initiated eye movements do lead to an awareness of
gaze direction and path. A typical example in the first instance is
found when unregistered after nystagmus causes the movement of retinal
images to be interpreted, not as to eye movements but to actual move-
ment of the surrounding environment. Table 1 lists relatively common
modes of eye movement control according to the extent of the volitional
component.
Given these two categories of eye movement control, illusions of
target position and movement path result from one of two sets of condi-
tions. These are (1) maintaining fixation in the presence of perturba-
tions of a reflexive nature (Post and Leibowitz, 1985; Whiteside et
al., 1965y, and (2) maintaining fixation after set points in adaptive
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131
TABLE 1 Eye Movement States with Dominant Reflexive and Volitional
Components
Reflexive Initiation
Vestibuloocular and obolith driven
Optokinetic and optostatic Vergence
Volitional Initiation
Fixation and pursuit
oculomotor systems have been altered (Ebenholtz, in press). Since vol-
untary movements may be associated with ,-efference (Merton, 1972) and
since the latter is known to increase muscle spindle gain (~hitteridge,
1959), it seems reasonable to propose that increased sentience of the
volitional states results from the competitive action of the reflexive
eye movements. It is thus out of the need to balance voluntary against
automatic modes of eye movement control that illusions are produced.
On the other hand, competing patterns of control provided by stationary
backgrounds (Yee et al., 1983; Raymond et al., 1984b) are either not
present or are represented with poor image quality during eye movements
in darkness. Consequently, muscle spindle gain can be assumed to be
lower during darkness than under full illumination conditions because
of reduced y-efferent stimulation. Hence, lower sentience or awareness
of eye position and eye trajectory (and, hence, of target movement path)
is to be expected in darkness.
IMPLICATIONS FOR RESEARCH, SELECTION, AND TWINING
At dim illumination levels, the contrast between surface edges and
backgrounds and between a figure and the ground diminish. At the same
time accommodation to targets falls toward the resting level (Johnson,
1976~. The net effect of these processes is to bias the composition of
retinal images in night vision in favor of low spatial frequencies, as
if one were scanning an environment through low-band-pass filters.
Since these images continue to play an important role in the control of
oculomotor performance, however, it seems likely that important changes
will occur in the efficiency of oculomotor control in night vision.
There appears, however, to be a gap in knowledge of the potential inter-
actions between a visual system in photopic, mesopic, and scotopic
states of dark adaptation and various aspects of oculomotor control,
such as pursuit tracking capability, peak optokinetic response, and the
ability to maintain foveal fixation when in conflict with vestibular
reflexes or peripheral optokinetic stimulation. On this last point,
there is some recent evidence to indicate that a high-spatial-frequency,
head-fixed target is critical to the suppression of optokinetic nystag-
mus (Howard and Ohmi, 1984), but the role of levels of dark adaptation
remains to be examined systematically.
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132
As more is learned about the relation between the dark-adapted eye
and oculomotor control, it may be advantageous to select aircrew on the
basis of characteristics of their contrast sensitivity function. If
detection and tracking of low-spatial-frequency images is regarded as
an important performance characteristic, then screening for high sens-
itivity to low-spatial-frequency targets would be appropriate.
It is worth emphasizing, perhaps, that in a moving observer, target
fixation, i.e., acquisition, and tracking occur frequently under condi-
tions in which the VOR has been triggered. Under these conditions, the
suppression of VOR by the foveal pursuit system is the indispensable
mechanism underlying both target acquisition and localization. It is
reasonable to suppose, therefore, that the larger the bandwidth of one's
pursuit system frequency-response function (prior to cutoff), the more
efficient the pursuit system will be in canceling perturbations emanat-
ing from vestibular stimulation. Since individuals differ in their
pursuit system cutoff frequency, the possibility of screening for high
cutoff frequencies should be considered.
Similar considerations should be given to the development of
training procedures to enhance the efficiency of pursuit eye movements
made in the context of vestibular stimulation (i.e., during and imme-
diately after). Such training should take place with gravitoinertial
force vectors approaching those encountered in actual flight.
Analogous selection and training procedures should also be explored
to examine the foveal pursuit system when it is perturbed by competing
optokinetic patterns. Oculomotor performance data also are needed to
know whether individuals without adaptive control systems (vergence,
accommodation, VOR) suffer a higher than normal frequency of asthenopic
and motion sickness signs and symptoms. Individuals should be tested
for adaptive effects in dark focus, vergence, and VOR.
Finally, and most importantly, there is the implication from this
analysis of oculomotor function that full advantage should be taken of
the current fund of knowledge to maximize night vision performance
among aircrew members. To do so would entail as a long-term objective
the establishment of a computerized visual function data base to make
possible retrospective studies relating visual and oculomotor function
to actual performance measures. Such a data base should cover a broad
array of visual and oculomotor parameters thought to be relevant to
various aspects of flight performance. The following is a sample of
such a set of parameters:
1. dark focus,
2. resting vergence, and
3. temporal dark adaptation functions.
The following parameters should be measured at mesopic and scotopic
levels:
4. peak OKN for a target of known spatial frequency and contrast,
5. pursuit cutoff frequency for a foveal target of known spatial
frequency and contrast,
contrast sensitivity function,
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133
7. pursuit suppression of the VOR,
8. pursuit suppression of OKN,
9. adaptability of the gain of the VOR, and
10. adaptability of the resting focus and resting vergence.
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GENERAL DI SCUSSI ON
_
HARVEY: This is one for Bob Post. It has to do with flow patterns
in movement. Is there a controversy today about the effects of fixa-
tion when you're not f ixating in the direction of movement, where the
flow pattern originates from? Or as I understand it, the focus of a
flow pattern of motion shifts when you look at a direction different
from the motion, and that then can give rise to very strange signals
of which way your body is actually moving in space.
POST: I don't know if there's a big controversy. In a paper by
Regan and Beverley, there is a demonstration that the focus of expansion
of a flow pattern corresponds to where fixation is rather than the point
of collision, which you're headed toward. This contradicts the previous
position of Gibson.
BEDELL: Dr. Ebenholtz, I understood you to say that there was not
an internal signal for reflex eye movements, whereas there was one for
voluntary eye movements. An alternative possibility seems to be that
there is an internal signal for both kinds of eye movements, but for an
involuntary eye movement, say a vestibular-ocular movement, there's
also simultaneously a signal of the head movement, which would tend to
cancel out the signal of eye movement. Can you distinguish between
those two things?
EBENHOLT2: I'd like to add a third hypothesis as well. I have been
wondering along with others, for a long time, what in the world muscle
spindles are doing in ocular muscles? And so it seems to me that I
would like to pose a hypothesis that under reflexive control you simply
are lacking the gamma efferent stimulation. And as a consequence, gain
is low but is probably registered nonetheless. I am sure that some
part of the brain has that recorded but that the gain is so low that
conscious centers are not aware of reflexive eye movements. So I think
that we have then the possibility of several nice hypotheses that now
need to be tested.
COPENHAGEN: I had a question about plasticity in the system. It
almost seems as though we're approaching Heisenberg's uncertainty prin-
ciple where you can get changes in the vestibular-ocular reflex in 10
minutes, so by testing it you're automatically changing it. We can see
reorganization in the somatosensory cortex in a period of a few minutes.
Would you like to comment on that?
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137
EBENHOLTZ: I have no doubt that, depending on the starting point
of the VOR, certain test procedures may indeed alter the very gain that
you want to measure. The problem is complicated because the changes in
gain, and also changes in phase that have been induced in VOR, decay at
differential rates. Now if you imagine that these gain changes are
varying from day to day and from time to time, depending upon the tasks
that we have been involved in before getting to the laboratory, then
you have differential decays of changes in VOR and phase gain back
perhaps to some normal level. So that if you really want to measure
VOR with some certainty you probably have to take the measurements
after a period of isolation to begin with. At least we have a handle
on the variance that one finds in these measures. It's in the very
plasticity itself.
MACLEOD: I'm curious about this plasticity you're discussing.
Specifically, I'm wondering if anything is known about whether this
adaptation can be situation contingent. For example, when a spectacle
wearer adapts to an inappropriate perturbation of his VOR input, can he
eventually learn to deal with the situation in such a way that when he
takes them off, the system instantly readjusts? It sounds like a
trivial question, but I think it has significant implications for how
sophisticated an adaptation process is: whether it simply accumulates
vestibular and retinal information and makes a blind correlation or
whether it makes use of every sort of cue that is available.
EBENHOLTZ: I think it is a very important question. People inter-
ested in development of simulators--where anyone in the simulator will
make certain head movements at the time when his eye is on the target--
are also very interested in whether these things transfer. Now I know
that you can induce changes simultaneously in the VOR and in different
directions. For example, from the VOR, changes that I've investigated
in my laboratory, one obtains the illusory movement when the head moves
in the horizontal direction. It is a very specific adaptive shift. If
you move your head up and down (nodding) you do not get illusory move-
ment unless you adapted initially also in the vertical plane. So we
know that the nature of the plasticity, especially in the VOR, is very
highly specific.
Can you control the changes in VOR, or perhaps other ocular-motor
systems, through cognitive links? There are one or two studies in the
literature of prism adaptation that seem to suggest that simply the
prism frames were enough to trigger the effect, but I remain rather
skeptical. I think rather what is triggering it are head movements.
I think you have to get into the system with a particular phase and
frequency of head movement before you can trigger it. But it is cer-
tainly an open question as to whether we can train multiple adaptations
simultaneously and simply trigger them by using the top-gown paradigm.
We need some more research on this question.
JOHNSON: I have a question for either Hersch or Jim [Sheehy] about
trying to come up with rapid, simple, cark-focus corrections for low-
luminance situations. I '11 just play devil's advocate here for a
second. Why worry about the dark focus at all? Why worry about the
theoretical implications? Why not just go out and define a performance
task, fit the observer with a series of lenses, and find the corrections
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for which performance is optimal? You circumvent all the problems of
trying to make measurements of oculomotor responses, developing instru-
mentation, and then translating the results into something that again
gets back to performance.
LEIBOWITZ: That is what we were trying to do with the field
studies. We did not use any instruments, but rather had subjects sit
in the car and rotate the square-wave grating (by means of a motor)
illuminated by the automobile headlights until it could be resolved.
What was surprising was that in almost every case a correction ba see on
the dark focus of accommodation degraded acuity, which, in turn, led us
to question our night myopia correction procedure. In practice, there
is another consideration because during driving, the problem of seeing
small stimuli, which are most affected by refractive errors, is not very
great. Although contrast is certainly critical during night driving,
there are very few stimuli subtending small visual angles that are cri-
tical during nighttime driving. The implication of these data is to
question the basis for night vision corrections based simply on the
dark focus. Regarding your suggestion, one could follow the procedure
practiced by some people when they buy glasses. They just go into a
store and try them on. I am confident we can do better than this, but
in any event, as you suggest, the final criterion must be performance
in the relevant real-world situation.
KINNEY: I had another question along the same lines. Did I under-
stand you to say that the test in the field was 16 cycles/degree? And
did you use any ether one?
LEIBOW~TZ: The target, when oriented vertically, presented a
15-cycle/degree square wave. The procedure was to start at a random
set point and rotate until the observer could just discern the stripes.
he then measured the angle subtending by the target and calculated the
threshold in cycles per degree. Contrast of the target, which was i~lu-
minated by standard automobile headlights, was 77 percent.
KINNEY: Have you tried lower spatial frequencies? I would think
that woula be more applicable to the laboratory studies, that this would
be a major difference between the two, if the dark focus does not work
so well for the higher ones.
SHEEHY: Bell, in the laboratory we tried to number different spa-
tial frequencies. The f irst one we started with was a 4-cycle/degree
pattern, and we found absolutely no effect except that people were less
sensitive. So with that we switched to ~ 16- and then 24-cycle/degree
pattern, both sinusoids. Again, they were adjusting the contrast of
that pattern while wearing their corrections. The results were iden-
tical.
MONACO: Fred [Owens!, Her sch had mentioned something about the
dark retinoscopy. I was wondering whether you might want to elaborate
a little on that in terms of its being an effective tool for defining
the dark focus value. Is there ongoing work with that?
OWENS: To my knowledge, there are a couple of optometrists in pri-
vate practice who use dark retinoscopy to prescribe for night myopia
corrections for those patients who complain. Dark retinoscopy is just
standard static retinoscopy without a fixation target. Now, theory has
always held that it is essential to control accommodation; the standard
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techniques are either by cycloplegia or by having the patient fixate a
distant acuity chart during the retinoscopic exam. We were surprised
that a retinoscope beam is not an effective stimulus for accommodation,
that when a patient is fixating the retinoscope beam without any other
visible contours, their accommodation returns to the individual's dark-
focus state. So it is possible to measure with a clinical instrument
that's readily available. That's the advantage of dark retinoscopy.
I don't think we have sufficient data to indicate how effective this
technique would be for wide-scale screening.
I'd like to take the opportunity to come back to a point that
Dr. Johnson raised a few moments ago about "why measure the resting
state anyway?" There may be a couple of good reasons. One is that
for individuals who recognize their problems with night vision, we
found measurement of their dark focus to be quite effective in coming
up with a correction that helps them--and without going out in the
field and doing a sort of empirical "trial-by-error" determination.
Perhaps a better argument, though, is the general importance of ocu-
lomotor bonus for visual performance under a wide range of conditions.
We have seen evidence that the dark focus predicts the best correction
for empty-field myopia, and it is probably the best predictor for very
dark conditions. Other data indicate that visual problems associated
with the use of optical instruments and with fatigue from near work
depend on the individual's resting state. For example, individuals who
have a near-dark focus and near-aark vergence are less susceptible to
fatigue from near work. I suspect this is related to the sort of plas-
t~city that Dr. Ebenholtz was talking about. Symptoms of visual fatigue
appear to be related to changes of oculomotor tonus of the kind that
Dr. Ebenholtz discussed; the problems might be avoided by setting the
working distance to correspond to the individual's resting posture.
My point is that we may be able to predict and correct a number of
different problems on the basis of the resting state, which would be
preferable to separate attempts to solve each problem empirically.
The night-driving situation, in particular, may be problematic--because
it involves a number of interacting factors, such as variations in low-
level response gain and vergence-accommodation synergy.
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Representative terms from entire chapter:
dark vergence
