We are most aware of our place in the world when about to fall.

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Magazine
R609
Correspondence
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
balancing activity.
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
improved.
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
perception [7,8].
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
seconds.
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

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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
balancing/standing (0.73)
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) [9].
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
Supplemental data are available
at http://www.current-
biology.com/cgi/content/full/14/1
5/R609/DC1/
References
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.
18, 624–644.
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
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6. Wickens, C.D. (1992). Engineering
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(Harper Collins NY), pp. 417.
7. Ernst, M.O., and Banks, M.S.
(2002). Humans integrate visual
and haptic information in a
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Nature 415, 429–433.
8. Battaglia, P.W., Jacobs, R.A., and
Aslin, R.N. (2003). Bayesian
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9. Baily, N.T.J. (1973). Statistical
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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: m.gresty@imperial.ac.uk
Current Biology Vol 14 No 15
R610
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
–3 –2–10123
Frame effect
(deg)
Beam
balancing
y = 0.976x - 0.128
–3 –1135
Frame effect (deg)
Stand at ease
Frame effect (Deg)
Stand at ease
Frame effect
(deg)
Sitting
–3
–2
–1
0
1
2
3
–3
–2
–1
0
1
2
3
4
5
6
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