We are most aware of our place in the world when about to fall.
[show abstract] [hide abstract]
ABSTRACT: Standing sway can be reduced simply by conscious effort but the extent to which this ability changes with stance conditions is unknown. Here, the influence of stance width and vision upon the ability to voluntarily reduce sway was investigated. 14 subjects were asked to stand either relaxed or still. Three stance conditions (wide/narrow/tandem) were compared, with eyes open or closed. When standing still, subjects successfully reduced body sway by up to 24% (root mean square of lateral trunk velocity), primarily by attenuating their peak sway frequency (0.2-0.4 Hz). Standing still was associated with a mean increase in ankle muscle co-contraction, but the extent of this increase did not correlate with the ability to reduce sway for individual subjects. Within each stance condition, subjects who swayed more when relaxed also displayed the greatest scope for sway reduction when asked to stand still. However, the opposite trend was observed across conditions: as relaxed sway increased, the capacity for sway reduction was reduced. Hence, voluntary control was lowest during tandem stance and greatest with feet apart, an effect augmented by eye closure. The results show that the degree to which sway can be voluntarily modified is not fixed, but reflects the difficulty of the standing task.Gait & posture 10/2009; 31(1):78-81. · 2.58 Impact Factor
Article: Deconstructing acrophobia: physiological and psychological precursors to developing a fear of heights.[show abstract] [hide abstract]
ABSTRACT: Acrophobia is one of the most prevalent phobias, affecting as many as 1 in 20 individuals. Of course, heights often evoke fear in the general population too, and this suggests that acrophobia might actually represent the hypersensitive manifestation of an everyday, rational fear. In this study, we assessed the role of sensory and cognitive variables in acrophobia. Forty-five participants (Mean age 25.07 years, 71% female) were assessed using a booklet with self-reports as well as several behavioral measures. The data analysis consisted in multivariate linear regression using fear of heights as the outcome variable. The regression analyses found that visual field dependence (measured with the rod and frame test), postural control (measured with the Sharpened Romberg Test), space and motion discomfort (measured with the Situational Characteristics Questionnaire), and bodily symptoms (measured with the Bodily Sensation Questionnaire) all serve as strong predictors for fear of heights (Adjusted r(2)=.697, P<.0001). Trait anxiety (measured with the State Trait Anxiety Inventory Form Y-2) was not related with fear of heights, suggesting that this higher order vulnerability factor is not necessary for explaining this particular specific phobia in a large number of individuals. The findings reveal that fear of heights is an expression of a largely sensory phenomena, which can produce strong feelings of discomfort and fear in the otherwise calm individuals. We propose a theory that embraces all these factors and provides new insight into the aetiology and treatment of this prevalent and debilitating fear.Depression and Anxiety 09/2010; 27(9):864-70. · 4.18 Impact Factor
Article: Multisensory origin of the subjective first-person perspective: visual, tactile, and vestibular mechanisms.[show abstract] [hide abstract]
ABSTRACT: In three experiments we investigated the effects of visuo-tactile and visuo-vestibular conflict about the direction of gravity on three aspects of bodily self-consciousness: self-identification, self-location, and the experienced direction of the first-person perspective. Robotic visuo-tactile stimulation was administered to 78 participants in three experiments. Additionally, we presented participants with a virtual body as seen from an elevated and downward-directed perspective while they were lying supine and were therefore receiving vestibular and postural cues about an upward-directed perspective. Under these conditions, we studied the effects of different degrees of visuo-vestibular conflict, repeated measurements during illusion induction, and the relationship to a classical measure of visuo-vestibular integration. Extending earlier findings on experimentally induced changes in bodily self-consciousness, we show that self-identification does not depend on the experienced direction of the first-person perspective, whereas self-location does. Changes in bodily self-consciousness depend on visual gravitational signals. Individual differences in the experienced direction of first-person perspective correlated with individual differences in visuo-vestibular integration. Our data reveal important contributions of visuo-vestibular gravitational cues to bodily self-consciousness. In particular we show that the experienced direction of the first-person perspective depends on the integration of visual, vestibular, and tactile signals, as well as on individual differences in idiosyncratic visuo-vestibular strategies.PLoS ONE 01/2013; 8(4):e61751. · 4.09 Impact Factor
We are most
aware of our
place in the world
when about to fall
A. Bray1, A.Subanandan1, B.
Isableu2, T. Ohlmann1,3, J.F.
Golding1,4and M.A. Gresty1,3*
When making orientational
judgements, such as aligning
picture frames or positioning for a
golf swing, we maneuver rather
than remaining immobile. This
observation is at odds with many
psychophysical studies of spatial
orientation. Accurate perception
of spatial orientation is of
greatest importance when it is
imperative to maintain precarious
balance. It has been proposed
[1,2] that, when balancing with
respect to the gravito-inertial
force vector (GIF), a subject gains
information about orientation
through actions and reactions
(the ‘dynamics of balance’). This
guides the maintenance of
equilibrium. We give the first
experimental evidence that
perception of the direction of the
GIF does improve in a situation
demanding a high level of
In the classic test of perception
of visual orientation, a subject
attempts to align a rod to the
Earth vertical. If the rod is
surrounded by a tilted rectangular
frame to give misleading cues to
verticality, observers tend to set
the rod tilted in the direction of
frame tilt [3,4]. This deviation
from the true vertical is called the
‘frame effect’ and indicates the
preferential dependence on visual
cues. To test the ‘dynamics of
balance’ hypothesis, we
compared subjects’ abilities to
perform the rod and rod and
frame tests (RFT) while standing
at ease and while balancing on a
narrow beam (Figure 1). When
they balanced on the beam, our
subjects set a rod within a tilted
frame 27% more accurately to
vertical (Supplemental Data). The
accuracy of ‘rod alone’ settings,
without a frame, was also
This suggests that information
from the ‘dynamics of balance’
improves the perception of
orientation, underlining the
aphorism that ‘we are most aware
of our orientation when about to
fall’. As a corollary, the
preferential dependence on a
particular sensory channel can be
modified by an orientation
challenge. The precariousness
provides the imperative to focus
on establishing correct vertical
orientation. Distraction or down-
regulation of attention by the
tilted frame, while retaining focus
on the rod, could also reduce the
impact of the frame tilt on the
perception of orientation [5,6].
However, as ‘rod alone’ settings
also improved with balancing,
distraction of attention from the
frame cannot be the only reason
for the improved accuracy.
It would seem paradoxical that
the perception of the Earth
vertical improved while the
postural sway increased during
balancing, because the efferent
and sensory feedback signals
during complex balancing
movements could be noisy and
difficult to interpret. However,
incorporation into perceptual
estimates of verticality, together
with visual cues, in a Bayesian
combination of probabilities
might be a way of refining
To determine whether
perception is influenced by the
dynamics of balance we tested if
RFT errors improved with an
increase in balancing activity,
from sitting, to standing, to
balancing on a beam. The
subjects (12 males, 16 females,
average age 26.4 years, ranging
from 20 to 54 years) set the rod to
the perceived vertical position,
with a frame present as well as
with the rod alone visible (Figure
1). In each trial, with the subject’s
eyes closed, the experimenter
positioned the frame tilted to left
or right by 28° and/or set the rod
in left or right tilt to a random
angle of 25°–35° from the Earth
vertical. The subjects opened
their eyes, adjusted the rod to
Earth vertical with a hand control,
then re-closed their eyes. While
on the beam, subjects held a
safety handrail, let go and
balanced while adjusting the rod
to vertical, then regained hold of
the handrail. Conditions were
assigned according to William’s
Latin Squares balanced for order.
For each sitting, standing or
balancing condition, subjects
performed 8 rod settings.
Individual rod settings took 5–6
As visual vertical estimates are
usually obtained seated, a control
study was performed to evaluate
the comparative effect of stance
on the rod and frame effect (RFE).
Subjects (8 males, 13 females,
average age 29.2 years (21–55
years)) performed rod settings
against a tilted frame, sitting and
standing (Figure 1). The order of
presentation alternated between
subjects and individual trials were
conducted as for standing/beam-
balancing described above.
In the main and control
experiments, one third of the
subjects set the rod tilted in the
direction of the frame tilt
(Supplemental Data), whereas
one quarter of subjects set the
rod tilted in the opposite
direction, which we refer to as a
‘negative frame effect’. The
negative effect may be
overcompensation for misleading
frame tilt. The subject knows she
is mislead by the frame tilt, but
cannot estimate by how much, as
she has difficulties in transferring
to non-visual cues for orientation.
For the main experiment, the
regression slope of RFT
balancing was significantly less
than unity (0.73 with 95%
confidence intervals of
0.56–0.88). This shows that RFT
estimates during balancing were
less than during standing. A
repeated measures analysis of
variance (ANOVA) on RFT
settings during standing at ease
versus balancing and with ‘rod
alone’ versus ‘rod with frame’
was performed on the absolute
mean values. Subjects who, when
standing at ease, had ‘rod alone’
or RFT settings of less than 0.3°
from upright were excluded,
leaving 19 subjects. Tilts of less
than 0.3° are less than the
absolute threshold of tilt
detection with our apparatus and
were excluded to reduce
variance and facilitate the
sensitivity of the ANOVA. There
was a significant reduction of
33% in RFT error when balancing
was compared to standing and a
marginal reduction in ‘rod alone’
error (variance ratio: F = 7.9;
degrees of freedom: df = 1,18;
p = 0.012; see Supplemental
Data). There were no interactions.
A subsequent matched pair t-test
between RFT settings during
standing compared to balancing
yielded a significant reduction in
error (n = 19, t = 2.6, r = 0.672,
p = 0.001), showing that the
ameliorating effect of balance on
RFT error is robust (Supplemental
Data). ‘Rod alone’ setting error
was improved by balancing for
the 13 subjects who had the
larger errors (>0.7°) when
standing (t = 2.4, p = 0.034).
There was no difference in the
mean frame effect between
subjects performing the RFT
standing at ease and sitting down
(Figure 1). The slope of the
regression was significantly
smaller than that of the
sitting/standing (0.98) regression.
This shows the specific
ameliorating effect of balancing
(Student’s t-value: t = 2.42;
df = 44; p = 0.02; 2 tailed) .
Sway at the level of platform,
hip and shoulders increased by
200–600% during beam
balancing, whereas head sway
increased by only 60%, hence
the tactic was preferentially to
maintain the head relatively
stable in space (Supplemental
Data). There were no
relationships between changes in
sway measures from standing at
ease to balancing and changes in
the frame effect.
Supplemental data are available
1. Stoffregen, T.A., and Riccio, G.E.
(1988). An ecological theory of
orientation and the vestibular
system. Psychol. Rev. 95, 3–14.
2. Riccio, G.E., Martin, E.J., and
Stoffregen, T.A. (1992). The role of
balance dynamics in the active
perception of orientation. J. Exp.
Psychol. Hum. Percept. Perform.
3. Witkin, H.A., and Ash, S.E. (1948a).
Studies in space orientation III. J.
Exp. Psychol. 38, 603–614.
4. Witkin, H.A., and Ash, S.E. (1948b).
Studies in space orientation IV. J
Exp Psychol. 38, 762–782.
5. Leibowitz, H.W., and Dichgans, J.
(1980). The ambient visual system
and spatial orientation. I Spatial
orientation in flight, Current
problems. Spatial orientation in
flight: Current problems. Conf.
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AGARD/NATO, B4, 1–4.
6. Wickens, C.D. (1992). Engineering
Psychology and Human
Performance, Second Edition.
(Harper Collins NY), pp. 417.
7. Ernst, M.O., and Banks, M.S.
(2002). Humans integrate visual
and haptic information in a
statistically optimal fashion.
Nature 415, 429–433.
8. Battaglia, P.W., Jacobs, R.A., and
Aslin, R.N. (2003). Bayesian
integration of visual and auditory
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Opt. Soc. Am. A Opt. Image Sci.
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9. Baily, N.T.J. (1973). Statistical
Methods in Biology (English
Universities Press), pp. 183–184.
1Academic Department of Neuro-
Otology, Division of Neuroscience &
Psychological Medicine, Imperial
College London, London W6 8RF, UK.
2UFR STAPS, Centre de Recherche en
Sciences du Sport (CRESS E.A 1609).
Bâtiment 335, Université Paris-Sud XI,
91 405 ORSAY CEDEX, France.
3Laboratoire de Psychologie
Expérimentale, CNRS, UMR 5105,
Université Pierre Mendès France,
Grenoble 2, BP 47, Grenoble 38040
Cedex 09, France.
4Department of Psychology, University
of Westminster, London W1B 2UW,
UK. *E-mail: email@example.com
Current Biology Vol 14 No 15
Figure 1. Comparison of estimate of visual vertical seen against a tilted frame for three
levels of postural change.
(A–C) Subject facing a rod and square frame subtending 40° at 1.41 m: (A) sitting;
(B) standing ‘at ease’; (C) Standing on the beam ‘balancing’. The beam was 86 cm long,
9 cm high and 4.5 cm wide and was mounted on a force platform transducing dis-
placements of the center of pressure along the antero-posterior and lateral axes. Only
the rod and frame periphery were illuminated in the experiment and the room was in
darkness. For (B) and (C), head position in roll was measured using a Fastrak (Polhe-
musTM). Single axis gyroscopes (Silicon Sensing Systems TM) attached to the subject
measured angular velocity in roll at (C7) and (L1). (D) Comparisons of Frame Effect for
standing at ease and beam balancing (R2= 0.78, n = 28) (E) Control experiment of
standing at ease and sitting (R2= 0.944, n = 21).
y = 0.726x + 0.027
y = 0.976x - 0.128
Frame effect (deg)
Stand at ease
Frame effect (Deg)
Stand at ease