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Auris Nasus Larynx 31 (2004) 11–17
Monocular versus binocular vision in postural control
Elina Isotalo a,b,∗ , Zoi Kapoula a,1 , Pierre-Henri Feret c , Karine Gauchon c ,
Françoise Zamfirescu c , Pierre-Marie Gagey d
a
LPPA, Collège de France CNRS, 11 place Marcelin Berthelot, 4 F-75005 Paris, France
Department of Ear, Nose and Throat, Helsinki University Central Hospital, Haartmaninkatu 4E, FIN-00290 Hus, Finland
Centre National Hospitalier d’Ophthalmologie des Quinze-Vingts, service du Pr. L. Laroche, 28 rue de Charenton, 755571 Paris, France
d Institut de Posturologie, avenue de Carbéra, 75012 Paris, France
b
c
Received 2 March 2003; accepted 14 October 2003
Abstract
Objective: In previous studies about the control of posture there have been controversial findings. Our aim was to examine the role of
monocular and binocular vision in controlling posture in quiet stance. Methods: Twenty-eight normal subjects were tested. We used a force
platform in measuring postural stability. In main experiment, postural stability was measured in four conditions: both eyes open (BEO),
dominant eye open (DEO) non-dominant eye open (NDEO), and both eyes closed (BEC). In a further experiment, 11 subjects were tested in
conditions where a vertical prism was placed in front of dominant eye. Prism was strong enough to cause diplopia. Our interest was to see, if
diplopia affected the balance. Results: In main experiment, at level of group the body-sway in any of the three ocular (viewing) conditions did
not differ from each other. At level of individuals, binocular vision was more effective on controlling posture in only half of subjects. In prism
experiment, relative to normal binocular viewing the postural stability was modified in both prism conditions, but there was no difference
between monocular and binocular viewing with prism. Conclusion: In quiet stance and in subjects with perfect binocular vision and stereopsis,
the benefit out of binocular viewing in postural stability is subject-dependent. At the level of group, monocular vision provides equally good
postural stability as binocular vision.
© 2003 Elsevier Ireland Ltd. All rights reserved.
Keywords: Binocular vision; Monocular vision; Balance; Posturography
1. Introduction
The visual system has been shown to be important in
the postural control by means of comparing the area of
body-sway with eyes open and eyes closed. It has been found
that in normal subjects the sway area is 2–3 times larger
with eyes closed than with eyes open [1–3].
Gentaz [4] studied the control of balance in 40 normal
subjects with posturographic tests in monocular conditions
where either dominant or non-dominant eye were covered.
He suggested that there is a preferred eye, the so-called
“postural eye” that allows better postural stability than the
non-preferred eye, and that the postural eye is not necessarily
the dominant eye [4].
∗
Corresponding author. Tel.: +358-9-4711; fax: +358-9-4717-5010.
E-mail addresses: [email protected] (E. Isotalo),
[email protected] (Z. Kapoula).
1 Tel.: +33-1-44-27-16-35; fax: +33-1-44-27-13-82.
In earlier studies, the patients with strabismus have been
found to have reduced postural stability when compared with
normal subjects, even though these patients can have normal visual acuity in each eye [5–7]. In binocular conditions,
patients with strabismus can have diplopia. Ocular misalignment with diplopia or altered ocular proprioceptive signals
could be the cause of reduced postural stability in these patients in visual conditions [7].
Fox [8] has studied, whether postural stability under visual conditions employs mainly monocular visual input or
binocular visual input. He examined the effect of binocular and monocular (with dominant eye open) visual conditions and the non-visual condition on the postural control
in a normal population. The postural stability was better
under binocular vision than under monocular vision. This
benefit, however, was found to be present even in dark,
which was interpreted as evidence against the role of the
binocular vision per se. We decided to study his hypothesis
further.
0385-8146/$ – see front matter © 2003 Elsevier Ireland Ltd. All rights reserved.
doi:10.1016/j.anl.2003.10.001
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E. Isotalo et al. / Auris Nasus Larynx 31 (2004) 11–17
Fox [8] also made an additional experiment about the
effect of proprioceptive information (standard and tandem
Romberg conditions) in the control of posture in binocular
and monocular conditions in light. The standard Romberg
was more stable than the tandem Romberg in different conditions. The tandem Romberg condition had little effect on
body-sway in the binocular condition. But, in monocular
condition the body-sway was increased. So, the binocular
vision had a stabilizing effect. The tandem Romberg posture amplified the visual influence, since it attenuated the
usefulness of proprioceptive input and reflexes on postural
equilibrium.
In a recent paper, Guerraz et al. [9] studied the effect of
motion parallax and binocular versus monocular vision in
the control of posture in eight subjects. They found that the
effect of motion parallax on body-sway was of monocular
origin. In their study, monocular viewing was studied only
with the dominant eye, as in the study of Fox [8].
In a recent study of Kunkel et al. [10], the efficacy of
visual stabilization of posture was investigated with different
spatial frequencies of a visual stimulus. They found that none
of the stimuli in monocular condition diminished the sway
parameters as effectively as unrestricted binocular vision did.
Thus, previous studies in normal subjects provide rather
controversial answers to the question of the importance of
monocular or binocular vision in controlling posture. However, the methods used have varied remarkably in those studies cited above. All of them bring some aspects related to
the question of the role of binocular/monocular vision in the
control of posture.
The purpose of our study was to re-examine the role of
monocular versus binocular vision in postural control. We
extended the study by examining the monocular condition
also with the non-dominant eye open in addition to the condition with dominant eye open. We designed a further experiment where we used a prism to test directly the effect
of disruption of binocular vision on postural stability.
2. Subjects and methods
2.1. Subjects and preliminary tests
Twenty-eight subjects participated in this study. All subjects were healthy without any neurological, otoneurological
or ophthalmological symptoms. Informed consents according to the Helsinki Declaration were given.
Preliminary examinations were done to all the subjects
participating in the study. They included measures of visual
acuity, binocular vision (TNO test for stereoacuity, Wirt and
Lang tests), and fusional disparity-vergence (bar-prism test).
Eye dominance was determined with subjective tests (pointing task, fixating via a hole task) and with the Jampolsky 4
prism-diopters test based on the observation of movements
of the eyes, when the prism was placed or removed in front of
each eye. Normality of the controlling of balance was eval-
uated with the Unterberger/Fukuda stepping test. Eighteen
out of the 28 subjects could be considered healthy according to the preliminary tests. Nine subjects had pathological
findings in the preliminary examinations, particularly in the
tests assessing the control of balance, and one subject had a
disparity of 1 cm in the length of the legs.
2.2. Posturography
We used a force platform (standards by Association Francaise de Posturologie (AFP) [11]; produced by Dynatronic,
Céreste, France) to measure the postural stability (principle
of strain gauge; [11]). The alteration of center point of force
(CFP) was measured during a 50 s period. The equipment
contained also an AD converter of 12 bits. Resolution of the
measures was 0.001 m, and the frequency response of the
platform was more than 10 times the highest frequency studied (the exact frequency response was not assessed). The parameter analyzed in each condition was the surface area of
CFPs [12]. CFP and its changes were assessed by the force
platform during the testing with the sampling frequency of
5 Hz, and after the test the recorded results were converted
to the surface area of CFPs [12]. The surface area of the
CFPs was assessed so that 90% of the CFPs were inside an
ellipsoid [12]. The force platform equipment was computerized. Normal reference values for these measurements are:
39–210 mm2 for the eyes open condition, and 79–638 mm2
for the eyes closed condition [13,14]. The subject stood in
a standard Romberg condition on the force platform In all
viewing conditions in light, the subject was asked to fixate
on a cross in front of the eyes in a distance of 90 cm.
2.3. The main experiment—standard conditions in light
Four conditions were tested: a condition with both eyes
open (BEO), dominant or non-dominant eye open (DEO and
NDEO respectively), and with both eyes closed (BEC). Each
condition was done twice, and the order of the conditions
was randomized according to the Latin square method. The
mean of the results in each condition was used in the statistical analysis. All 28 subjects performed these conditions.
They were divided into two groups: the 18 subjects that had
normal values in the preliminary tests and the 10 subjects
with pathological findings that were excluded from the further analysis.
2.4. Further experiments with vertical prism
By this experiment, our aim was to evaluate the relevance
of binocular and monocular vision in the control of posture,
when the visual input is disturbed by the prism. Eleven of
the normal subjects were tested in conditions with a vertical prism placed in front of the dominant eye. This experiment followed the first one after a short pause. At first, the
prism (5 D based down Frenzel prism; large enough to cause
the disruption of binocular fusion and, hence, diplopia) was
E. Isotalo et al. / Auris Nasus Larynx 31 (2004) 11–17
placed in front of the dominant eye, the other eye was covered, and the postural stability was assessed (DEOp). This
condition was done twice (the mean of the two measurements was used in the statistical analysis). Then, the subjects performed twice the condition with both eyes open and
the prism placed in front of the dominant eye (BEOp) (the
mean of the two measurements was used in the statistical
analysis). After this, the prism was removed, and the normal
binocular viewing condition was assessed (BEO), and also
the normal monocular viewing condition with dominant eye
open (DEO).
An additional experiment was done on a different day with
the prism test, in order to see, if the order of the conditions
had any effect on the posturographic results. Ten subjects
were tested in two different days (in total from 4 to 5 times).
The three conditions were: BEOp, BEO, and DEOp. BEO,
i.e. normal binocular viewing with the prism removed, was
always done between the prism conditions, in order to avoid
prism adaptation effects. At different times the test began
with either DEOp or BEOp condition. The results from this
additional experiment will be reported in the text.
2.5. Statistical analysis
In the main experiment, the one-way ANOVA was used
to evaluate the effect of non-visual and different visual conditions (BEO, DEO, NDEO, and BEC) on the control of
posture at the level of the group of subjects.
In the experiment with vertical prism, a two-way ANOVA
was used to evaluate: (1) the effect of prism (with and without prism), (2) the effect of ocular condition (binocular versus monocular with dominant eye viewing), (3) the interaction between prism and ocular conditions. The additional
prism experiment, in which the order of ocular conditions
was counterbalanced, was analyzed with repeated measures
ANOVA to evaluate the repeatability of the results.
SPSS for Windows 9.0 was used for statistical analysis.
3. Results
3.1. The main experiment, standard light conditions
The results of the four conditions (BEO, BEC, DEO and
NDEO) for each subject are shown in Table 1. All individual values for all conditions are within the normal ranges
[13,14]. There was a large variability of individual performance. Seventeen of the eighteen normal subjects showed
better stability in the BEO than in the BEC condition.
Our main interest was first in the comparison of BEO
condition with the DEO and NDEO conditions. The BEO
and NDEO conditions differed significantly/tended to differ
out (P < 0.01, <0.05, respectively) from the BEC condition.
The DEO and BEC conditions did not differ from each other
(P = 0.072). The BEO, DEO and NDEO conditions did not
differ statistically significantly from each other. Thus, the
13
only group effect was that the surface area of body-sway
was higher in BEC condition compared to that of all visual
conditions, which is a classical finding.
3.2. The experiments with prism
In the main prism experiment, in DEOp the insertion
of the prism in front of the dominant eye increased the
body-sway area for all of the subjects. Under BEOp, all the
subjects experienced diplopia, in which the horizontal segment of the fixational cross was seen double because the
image was displaced upward by the prism. When comparing body-sway area in the BEOp condition with the BEO
condition, the body-sway area increased in 7 out of the 11
subjects. This could still be comparable with a monocular
effect of the prism. Increased body-sway area in the BEOp
condition in respect to both BEO and DEOp conditions was
observed only in 2 of the subjects (subjects 8 and 13). Only
these subjects might have been to some extent affected by
the diplopia caused by the prism rather than by a monocular
effect of the prism, even though this effect was not statistically significant at the individual level with repeated measures ANOVA or paired samples t-test. For the rest of the
subjects, the differences between individual values of BEO
and BEOp were smaller than the differences between BEO
and DEOp (Table 2).
For the whole group analyzed with two-way ANOVA, the
effect of prism factor on the body-sway area was statistically
significant (P < 0.005), whereas the effect of ocular factor
(both eyes open, dominant eye open) was not (P = 0.652),
and neither was the effect of the interaction of these two
factors (P = 0.171).
In BEOp (=121 ± 62 mm2 ) body-sway area was very
similar as in BEO (114 ± 63 mm2 ). The monocular viewing
with the prism produced larger value of body-sway area
(DEOp = 118 ± 39 mm2 ) than the DEO condition without
the prism (75 ± 23 mm2 ), and the difference showed a trend
to be statistically significant (P < 0.05 at the individual
level, with repeated measure ANOVA and paired samples
t-test).
In testing the repeatability of the test (with repeated measures ANOVA) in an additional experiment, no significant
differences were observed in the body-sway areas of different measurement times in the BEO, DEOp and BEOp conditions. All individual values were within the normal range
[13,14]. Thus, postural stability measured during the use of
prisms was repeatable and didn’t show any bias in respect
to the order of the testing conditions. And, thus the results
of the main prism experiment shown in Table 2 can be considered repetitive.
4. Discussion
The present study provides evidence that in quiet stance
conditions, binocular vision per se in controlling postural
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E. Isotalo et al. / Auris Nasus Larynx 31 (2004) 11–17
Table 1
Individual data of the 18 normal subjects
Postural performance (mean body-sway area in mm2 ) for the standard light conditions (BEO: both eyes open, BEC: both eyes closed, DEO: dominant
eye open, and NDEO: non-dominant eye open). Group means and ± standard deviations (S.D.) are shown at the bottom of the table. R: right-eye, L:
left-eye. One-way ANOVA between different visual conditions. P-values of significance, NS: non-significant.
stability is not a major factor, even for normal subjects with
perfect binocular vision and a high degree of stereopsis.
This evidence is based on following findings: (1) In standard
light conditions, only half of the normal subjects showed
better stability under binocular viewing condition than under monocular viewing, and there was no group effect on
controlling posture in the three ocular conditions. (2) When
a vertical prism was placed in front of one eye, binocu-
lar vision was disrupted, and all subjects reported diplopia.
Nevertheless, the body-sway area increased only slightly in
this condition and for only seven of the eleven subjects. In
contrast, under monocular viewing with the prism, the majority of the subjects showed larger body-sway than under
BEOp. A vertical disparity created by a prism of 2 D corresponds to about 1◦ and is fusible, whereas it is impossible
to fuse the disparity created by a prism of 5 D [15]. The use
E. Isotalo et al. / Auris Nasus Larynx 31 (2004) 11–17
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Table 2
Individual data and group means of the eleven normal subjects tested in the prism conditions
Postural performance in BEO: standard binocular viewing without the prism, BEOp: both eyes open, with prism in front of the dominant eye, DEOp:
dominant eye open with prism in front of it, DEO: dominant eye open without prism. Postural performance as mean body-sway area in mm2 . Two-way
ANOVA with the factor prism (with and without), the factor ocular condition (BEO and DEO), and the interaction between the two factors are shown.
P-values of significance, NS: non-significant.
of a powerful prism (5 D) aimed to disrupt binocular fusion.
The prism experiment provides additional evidence against
a major influence of the binocular vision and stereopsis on
postural control in quiet stance. (3) Even in dark (in a control
study not reported here; confirming the related observations
of Fox [8]), the postural stability tended to be better when
both eyes were open than when both eyes were covered—i.e.
this supports the proposition that binocular vision may not
have a major role in postural control during quiet stance.
But, however, attention state may differ in these light and
dark conditions and may be one cause of this effect [16].
Under dynamic conditions (in perturbed stance), vision
per se and binocular vision have most likely a major role
in the control of balance in control subjects and in patients
[17–20] in contrast to non-perturbed stance in present study.
4.1. Findings of this study in relation to previous studies
on quiet stance
Fox [8] found that binocular viewing most often attenuates the body-sway more than monocular viewing in differ-
ent visual fields. Nevertheless, he also found that in dark the
body-sway was lower when both eyes were open than when
they were closed in the standard Romberg condition. However, in the tandem Romberg condition the body-sway in
binocular and monocular conditions in dark was larger than
in the eyes closed condition in dark. He draw the conclusion
that the benefit of keeping both eyes open in the control
of posture is not due to binocular vision or to stereopsis,
but to other mechanisms such as tonicity. We found that
better postural stability under binocular viewing than under
monocular viewing is not a rule among the normal population in quiet stance. We also confirm the finding of Fox [8]
that the benefit of “binocular viewing” is present even in
dark, in quiet stance and in standard Romberg position (confirmatory study not reported in the results). The findings of
Fox [8] and ours contradict the findings of Paulus et al. [21]
who did not find any difference between the body-sway with
conditions of both eyes open or both eyes closed in dark.
The difference between the studies might be due to the fact
that Paulus et al. [21] made modifications to the platform
in order to reduce the somatosensory contribution to the
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E. Isotalo et al. / Auris Nasus Larynx 31 (2004) 11–17
control of posture and to enhance the contribution of the
visual and vestibular systems.
The results of Guerraz et al. [9] are in line with ours in
the fact that, at least, in certain conditions the better postural stability under binocular viewing than under monocular
viewing is not a rule among normal subjects. In the study
of Guerraz et al. [9], the visual cues consisted of a set of
two LEDs mounted in a piece of perspex. The motion parallax reduced the body-sway both in binocular and monocular
viewing conditions, and Guerraz et al. [9] postulated that
the effect of motion parallax on body-sway was of monocular origin, since it was observed both with monocular and
binocular vision. They interpreted that these results support
the existence of two modes of visual detection of body-sway,
afferent (retinal slippage) and efferent (extra-retinal or eye
movement based). Guerraz et al. [9] had a slab of foam rubber under the feet of the subject in all conditions, in order
to reduce the proprioceptive contribution to the control of
posture and to enhance the contribution of the visual and
vestibular systems. This may explain some of the differences
of results that were also found between these two studies.
Kunkel et al. [10] found, that binocular vision diminished
the sway more effectively than any of the monocular vision conditions. Nevertheless, the binocular condition and
different monocular conditions were not comparable with
each other. Their aim of their study was different from ours,
and they did not evaluate the possible differences between
binocular and monocular conditions with the different stimuli conditions.
These are, however, just speculations that need to be
tested.
In summary, previous studies addressed the question of
benefit of binocular vision in postural stability and presented contradictory results [8–10]. In the present study
using the same posturography techniques and the same
population, occluding one eye or placing a prism in front
of one eye were used to test how these procedures affect
postural stability. Findings didn’t show any convincing benefit of binocular vision compared to monocular vision in
quiet stance. The results force us to reconcile the existing
controversial findings in the literature and to strengthen
the conclusion that the effect of binocular viewing on
postural stability in quiet stance is subject-dependent.
Postural stability is a complex entity controlled by multisensory system, and it is a subject to large individual
differences. For instance, there is great individual variation in the relative dependence on either visual information or on the spinal ascending input in controlling
posture [25,26]. Similarly, there might be individual variation in the dependence upon binocular or monocular
vision.
4.2. Proprioception of extra-ocular muscles and postural
stability
4.3. Implications of strabismus for postural control in
quiet stance
We do not fully understand why some subjects are more
stable with both eyes open. It cannot be entirely attributed
to the visual input. Perhaps, tonicity of extra-ocular muscles and active force–torques underlying the steady-state primary gaze position, as well as proprioceptive signals from
the extra-ocular muscles, play a role and are more important than binocular vision per se. The proprioception of the
extra-ocular muscles has been shown to affect the reflexes
of balance and the control of posture [22,23], and Eber et al.
[23] have proposed the existence of an oculo-oculogyric reflex. According to them, proprioceptive afferents from the
extra-ocular muscles are situated in the trigeminal nerve, and
project to the trigeminal nucleus. From there, the pathway
has connections to the cerebellum, reticular formation and
vestibular nuclei. From vestibular nuclei the pathway continues to spinal motor neurons and extra-ocular efferents. Proprioceptive inputs of extra-ocular muscles may also project
centrally via the ocular motor nerves [24].
Alternatively, having both eyes open per se in dark could
exert some type of contextual influence and mental set capable of acting on the postural stability; postural stability
would be better just because the eyes are open, even though,
no vision can be used.
Earlier posturographic studies of patients with strabismus,
as they lack binocular vision and stereopsis, showed no difference in their postural stability under both eyes open and
both eyes closed conditions. They were called postural-blind
subjects [27]. Also, the strabismic young patients have significantly larger body-sway with eyes open in the light than
the control subjects [5,6]. The concept of postural-blindness
could be understood as no use of vision at all in the control of posture. That could result from: (1) abnormality in
the tonicity of extra-ocular muscles or abnormality in the
vestibulo-oculo-postural connections or (2) from the lack of
binocular vision.
Because even in the subjects with perfect binocular vision
there is no major benefit of the stereopsis in quiet stance,
we believe that it is the former explanation that is more
plausible for the decrease of postural control in strabismic
patients rather than the lack of stereopsis.
4.2.1. Postural eye and eye dominance
Half of the subjects in our study were found to control
postural stability better with one eye viewing in quiet stance
(see Table 1). These findings are in line with a prior study
of Gentaz [4] that introduced the term “postural eye”. The
postural eye is not necessarily the dominant eye (Table 1).
5. Conclusion
In quiet stance, among the normal population with perfect
binocular vision and stereopsis, only some of the subjects
E. Isotalo et al. / Auris Nasus Larynx 31 (2004) 11–17
seem to use binocular viewing to obtain optimal postural
stability. With a number of subjects, postural stability with
one eye viewing is as good as or even better than with both
eyes viewing. Furthermore, the benefit of binocular viewing, present for some subjects may not be due to binocular
vision per se or stereopsis. Tonicity of extra-ocular muscles
and its influence on the vestibulo-postural connections could
be responsible for the lesser body-sway in these subjects in
binocular viewing condition than in monocular viewing condition. For example, muscular hypotonicity even when both
eyes are open could account for impaired postural stability
in strabismic patients [5,6,27], who have normal vision but
abnormal alignment. The effect of the prism on postural control seems to be due to its effect on tonicity of muscles rather
than its effect on binocular fusion. These findings are limited,
however, to the quiet stance conditions studied here. Dynamic postural control is more dependent on binocular vision
[17–20].
Acknowledgements
E. Isotalo was supported by the Maud Kuistila Foundation (Finland), the Fondation pour la Recherche Medicale
(France) and the Otorhinolaryngologic Foundation (Finland). Special thanks to Professor Ilmari Pyykkö (M.D.,
Ph.D.) and Hilla Levo (M.D., Ph.D.) for valuable comments
about the manuscript, and to Timo Pessi (Ph.D., statistician)
for valuable advice concerning statistical procedures. Revision of the English language of this manuscript by Pekka
Rikkonen (M.D., Ph.D.) is highly appreciated. The experiments were performed in Hôpital Saint-Antoine, Paris and
Hôpital Quinze-Vingts, Paris.
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