Download Anteroposterior dynamic balance reactions induced by

J fhwiolog.v
(Puis)
( 1996) 90, 53-62
OElsevier. Paris
Anteroposterior dynamic balance reactions induced
by circular translation of the visual field
A Skverac Cauquil, M Bessou, Ph Dupui, P Bessou
Centre de Rechrrchr Cervenu et Cognition. FucultP de M&de&e
dr Rangwil.
UtGversitP Paul-Sabatirr.
133. route de Nnrbonnr. 31062 Toulo~tse cede.\. France
(Received
16 May 1996: accepted
I I June 19961
Summary - The anteroposterior sway of subjects under conditions of spontaneous dynamic balance on a wobbly platform was measured
during visual stimulation by a visual target executing a circular trajectory in the frontal plane. The target was either a component of the
whole moving visual scene or moving on a stationary background. With the former stimulation, obtained through the use of rotating prismatic
glasses, every point of the visual field appeared to describe a circular trajectory around its real position so that the whole visual field appeared
to be circularly translated, undistorted, inducing a binocular pursuit movement. Under these conditions, stereotyped anteroposterior dynamic
balance reactions synchronous with the position of the stimulus were elicited. The latter stimulation consisted of pursuing a luminous target
describing a trajectory similar to that of the fixation point seen through the rotating prisms on the same, this time stable, visual background.
Although pursuit eye movements were comparable, as demonstrated by electro-oculographic
recordings. no stereotyped equilibration reaction
was induced. It is concluded that the translatory motion of the background image on the retina in the latter experiments contributed to the
body’s stability as well as to the perception of a stable environment
dynamic balance I visual field I eye movements
I proprioception
Introduction
Vision coupled to eye movements provides complex
sensory information
to the central nervous system
(CNS) from both retinal and extraretinal structures.
Gaze-anchoring
onto a target in the environmental
space facilitates the body balance whereas severe reduction in visual acuity disturbs fixation and decreases
subject stability (Brandt et al, 1985). Eye movements,
by modifying
visual information
from retinal and
extraretinal structures, interfere with balance according
to the kind of eye movements used. Brandt er nl(l986)
showed that the ocular pursuit of a single moving target
within a stationary visual environment significantly impairs static balance, contrary to what happens with saccadic eye movements. Schulman et al (I 986) observed
that the time spent by subjects in balance on an unstable
platform was longer during saccades than during
smooth pursuit eye movements. Under each condition,
the image of the background sweeps across the retina
with unequal velocity. Moreover, during a saccade in
normal viewing conditions the blurred background
image is obliterated
(Campbell and Wurtz, 1978)
whereas during pursuit the background is clearly perceived as steady. It can be thought that the slow displacement
of the background
image in pursuit
movement jeopardises the body balance even if the subject perceives the environment as staying stable.
The aim of the present work was to investigate the
part played by the motion of the retinal image of the
background in the balance responses induced by eye
pursuit movements. Two conditions of visual tracking of
a small target were tested, leading to identical eye pursuit
movements. In the first case, the pursued target moved
on a stationary visual background, inducing motion of
the background image on the retina. In the second, the
whole visual field described the same trajectory as the
visual target, resulting in a stationary background image
on the retina. ln these experiments a circular pursuit movement has been induced, both to obtain a movement of
constant velocity, prolonged at will, and to simultaneously
activate horizontal and vertical components of eye motricity known to be controlled by different parts of the
reticular formation (Robinson, 198 1).
Spontaneous dynamic balance was used to enhance
the sensitivity of body stability to visual input: the sensitivity increases when the upright posture is difficult
(Kapteyn, 1972; Amblard and Cremieux, 1976; Bles
et al, 1980) and when the activation level of the central
neuronal network increases (Soechting and Berthoz.
1979). In the present paper, the balance responses were
measured only in the sagittal plane because the reactions are relatively easy to understand on biomechanical
grounds. Indeed they mainly result from flexion and
extension movements that involve both body sides symmetrically. Besides, anteroposterior
balance is more
sensitive to some visual inputs (Severac, 1993; Paulus
et cd, 1994). Part of these data have been presented
before (Bessou ef al. 1995).
54
Materials and methods
Circular translation of the visual field (CTVF) was elicited
by means of goggles (fig IA) fitted with two 13-diopter
Fresnel membrane prisms (Severac et al, 1994). A motor in
each side of the goggles induced the rotation of the goggle
glasses around their optical axis and consequently the rotation of the prisms they supported. Both motors were driven
by a microcomputer. The angular speed of prism rotation
was fixed at 45”/s so that the fixation target described its
apparent circular trajectory in 8 s. All the objects in the visual field of the subjects were seen describing a circle with
a 13-degree diameter (fig lB), so that the apparent circular
trajectory was 90 cm diameter for all the objects situated
in the plane of a wall 3.5 m distant in front of the subjects.
A luminous bulb on the wall was used as a fixation target.
Viewing through the prisms reduced visual acuity by approximately half. The size of the whole visual field including
the binocular field was slightly diminished in the nasal direction and downward.
Circular translation of the visual target alone (CTVT)
was elicited by projecting on the wail in front of the subject
a luminous spot which had about the same diameter and
the same brightness as the fixation target under CTVF conditions. The projector was operated circularly so as to make
the target describe the same trajectory as the fixation point
seen through the rotating prisms, ie a 90 cm diameter circle
(fig lC).
Dynamic balance conditions were induced by using an unstable platform (stabilometer, Bessou et al, 1988) derived
from Freeman platforms designed to develop calf muscle
co-ordination (Freeman ef al, 1965). The device, consisting
of a platform on a segment of cylinder (55 cm radius, 50
cm length) reduces the area of support of the subject on the
ground into a 50-cm segment of line called the ‘pivot’ (fig
1D). The pivot constitutes, at a given instant. the contact of
the system man-platform with the ground. It can be displaced on the ground perpendicularly to its direction by the
subject himself, as a consequence of the platform tilt. The
displacement of the pressure force exerted by the subject
elicits a rotation of the cylinder and thus a platform tilt.
Due to the system design and the low inertia of the stabilometer (small mass: 2 kg relative to the body mass), the
position of the pivot on the ground may be considered as
the instantaneous projection of the centre of pressure. The
instability of the system required from the subject uninterrupted movement of the pivot, ie of the subject support on
the ground, characterising the ‘dynamic balance conditions’.
The dynamic balance conditions could be considered as
spontaneous since no disturbing external force was applied
to the system subject-stabilometer. In all the experiments the
subject stood on the platform so that the pivot moved forwards-backwards
(sagittal balance, fig IE). The resulting
displacement of the pivot on the ground (stabilogram) was
calculated from the measurement of the platform tilt, monitored by an optical encoder linked to a microcomputer with
a dedicated program (sampling frequency 100 Hz. accuracy
7 IO-’ cm). The horizontal displacements of the head in the
sagittal plane were simultaneously recorded using a device
derived from the Wright ataxiameter (197 1) (fig 1D). The
head movements were transmitted by a weighted string (20
g) to a pulley fixed to the shaft of an optical encoder linked
to a microcomputer (sampling frequency 100 Hz, accuracy
2 IO-’ cm). Any forward or backward head displacement of
a subject standing on the stabilometer resulted from the tuming action about the pivot of the gravity force exerted on
the centre of mass. Our method did not offer the opportunity
to assess postural adjustments at intermediate levels between
the head and the pivot. The influence on head posture of
possible moditications of intersegmental joints was inferred
from the comparison between the measured head position
and the theoretical head position, computed from the position of the pivot according to the model of the inverted pendulum. According to these assumptions, a forward head
movement was considered as an overall body flexion (dorsiflexion of the ankle) whereas a backward head movement
was taken as resulting from an extension movement (plantar
flexion of the ankle).
Eye movements were monitored by electro-oculography.
Six adhesive electrodes were stuck on the skin at the edge
of the eye sockets, Two electrodes were used for recording
horizontal movements and four electrodes for vertical movements. Electrodes were connected to a bridge circuit derived
from Remond et al (1957) a system that allowed proper
dissociation of vertical and horizontal tracks. Calibration
consisted in asking the subject to look successively at LEDs
lighted alternately and situated at the extremities of a cross,
IO” away from the centre, right, left, down and up. Amplified signals were recorded on paper for I2 subjects using
an electrostatic trace recorder (Gould ES 1000, frequency
response IO kHz) and collected for five subjects with a
microcomputer at 10 ms sampling rate.
Pilot experiments (Severac, 1993) having shown that the sagittal
balance responses to CTVF or CTVT were independent of the
direction of the target rotation (clockwise or counter-clockwise)
or of the visual conditions (binocular or monocular vision), the
following experiments were performed with counter-clockwise
rotations and under binocular vision.
With local ethics committee approval, experiments were
performed on 64 subjects who gave their informed consent.
In a first set, 32 subjects (12 men and 20 women) underwent
counter-clockwise CTVF, with target motion starting either
on the extreme right of its trajectory (3 o’clock, 16 subjects)
or on the extreme left (9 o’clock, 16 subjects). Since an
analysis of variance did not reveal any difference between
the balance parameters (see below) calculated from measurements in both conditions, their values were subsequently
pooled for statistical analysis. In a second set of experiments,
32 other subjects ( 15 men and 17 women) underwent
counter-clockwise CTVT test, with target motion starting at
55
VISUAL FIELD
E
P
*
h.d.
0.e.
STABILOMETER
<
Fig 1. A. The prismatic glasses. B. CTVF when prisms are rotating counter-clockwise,
the bright spot (dotted circle) and every point of the
visual field (eg dotted cross) describe a circle of the same radius around their real position (plain symbols). C. CTVT when the projector is
rotating, the bright spot (circle) describes a circle on the stable background. D. The recording devices for area of support (stabilometer)
movements: platform with a curved base in contact with the ground through a line. the pivot (p). When the platform tilts, a lever fixed to
an extremity of the platform slides on the ground and rotates around its point of fixation. The rotation is assessed by an optic encoder (o-e).
The ataxiameter is used to measure head linear displacement (hd) in the sagittal plane: string tightened between the head band fixed around
the head and a pulley transmits head displacements to an optic encoder (oe) fixed to the shaft of the pulley. E. Position of the subject to
assess dynamic balance in the anteroposterior plane.
3 o’clock. For both CTVF and CTVT experiments, each
session lasted 50 s and included: a 10-s reference period
(R) during which the prisms or the projected target were
immobile; a 30-s stimulation period (ST) during which either
the prisms or the target rotated and a 10-s post-stimulation
period (P) of target or prism immobility.
Individual records with the same starting point were averaged. The last procedure provided the advantage of enhancing the synchronised oscillations in the recordings and
reducing the masking effects of the more rapid (=I Hz) oscillations that characterise the dynamic balance, since they
are asynchronous from one record to another.
56
Three parameters were measured from head (H) and
pivot (P) recordings: the length of the linear displacement
(LD), HLD and PLD (cm), obtained by summation of absolute values of the displacements between two successive
measurements (Granat et al, 1990); the maximum oscillation
amplitude (MA), HMA and PMA (cm); and the average position (AvP) around which the records oscillate, HAvP and
PAvP (cm). The ST period was subdivided for convenience
of data analysis into three periods of 10 s (ST], ST2, ST3).
The comparison of the effects of CTVF and CTVT on
dynamic balance parameters were statistically assessed by
means of a two-way analysis of variance (ANOVA). One
factor was the factor ‘period’ with repeated measurements
(five levels: R, STI, ST2, ST3, P). The other factor was
the factor ‘test’ (two levels: CTVF, CTVT). P < 0.05
denoted significance (*). For each stimulus condition, the
differences between the periods were assessed by a one-way
ANOVA with repeated measurements: factor period (R,
STl, ST2, ST3, P) and method of contrasts.
Results
Ocular movements and dynamic balance of a
subject during circular translation
of the
fixation point
Figure 2 presents EOG recordings (two upper traces)
and dynamic balance recordings (two lower traces) of
a subject asked to binocularly fixate a target either
through prisms (fig 2A, CTVF) or with the naked eyes
(fig 2B, CTVT). When the target is stationary (R
period) it appears that under both the A and B conditions neither the eye fixation task nor the dynamic body
balance are disturbed. When the target is circularly
translated (ST periods), either on a stationary background (fig 2B, CTVT) or simultaneously
with the
whole background due to a dioptric change (fig 2A,
CTVF), EOG recordings indicated eye pursuit movements were similar under both conditions. The maximal
amplitude of the vertical eye movement signal was
smaller than the horizontal one. This might be due to
a shunt effect of the bridge circuit used to isolate ho+
zontal and vertical movements. At the same time, dynamic balance recordings show some modifications
characterised by the appearance of slow oscillations in
the first two cycles of the target rotation. The oscillations were much more noticeable in CTVF recordings
than in CTVT recordings and their periodicity, calculated
by counting peaks, was found to be close to 8 s.
Changes of mean parameters
of anteroposterior dynamic balance during ocular
pursuit movement
Results of the two-way ANOVA performed on the three
dynamic balance parameters for head and pivot are
summarised in table I. For two parameters of the head
(HMA, HLD) and of the pivot (PMA, PLD) both ‘test’
and ‘period’ effects were significant and showed a significant interaction (P < 0.001). Only the ‘period’ factor
was significant for the parameter AvP (P < 0.001).
Figure 3 gives the mean values (n = 32) of the three
dynamic balance parameters for the five periods of
CTVF and CTVT tests, for the head and for the pivot
recordings. Statistical difference between periods assessed by the method of contrasts is indicated. During
R period, HLD, HMA, PLD, PMA slightly differed
according to the test condition. These parameters statistically increased during ST periods. However, this increase was much larger in the CTVF than in the CTVT
condition. As a summary, fixation of a target while
wearing the unmoving prisms increased moderately the
MA and LD parameters (70%) but when the prisms
were rotating, the parameter increases were 200% and
400%, respectively. During the P period of CTVF tests,
PLD, PMA, HMA were still slightly higher than during
the R period.
Head and pivot underwent a clear forwards displacement (HAvP, PAvP) during ST periods. No significant
difference was found between the effects of CTVF and
CTVT for this parameter. At the end of the visual
Table I. Results of the two-way ANOVA with repeated measurements
ns, non significant.
Balance pammeters
PLD
HLD
PMA
HMA
PAvP
HAvP
Factor
F =
F =
F =
F =
‘test’
66; P < 0.001
143; P < 0.001
90; P < 0.001
99; P < 0.001
F=
1;ns
F = 0.1; ns
performed on the measured dynamic balance parameters.
Factor
Interaction
‘period’
F = 38; P < 0.001
F = 50; P < 0.001
F = 64; P < 0.001
F = 55; P < 0.001
F = 9; P < 0.001
F = 7; P < 0.001
F
F
F
F
=
=
=
=
33;
42;
37;
37;
F=
F =
P
P
P
P
< 0.001
< 0.001
< 0.001
< 0.001
1;ns
0.5; ns
57
CTVF
Q .*.*.*.*.*.*.*..Q
.*.*.*.*.e.*.e.0
%
0
Amplitude (deg)
(deg)
“1
r*,
Left
“1
Left
0
-10
Amplitude
CTVT
B
Right
0
I
1
20
10
I
I
30
1
50
40
Time (s)
-10
I
10
I
20
I
30
Backward
I
I
0
10
20
I
30
0
I
I
40
50
Time (s)
(cm)
(cm)
5-
Forward
Backward
-5 J
5-
Forward
O-1
Stabilogram
_5
Backward
0
10
R
-5
20
ST1
30
ST2
50
40
Time (s)
ST3
P
I
40
I
50
Time (s)
R
ST1
ST2
ST3
P
Fig 2. Vertical and horizontal EOG: four upper traces. Ataxiograms and stabilograms in the anteroposterior plane: four lower traces. Traces
were recorded simultaneously from subject 6 during fixation of the target: immobile (R and P) and describing a circular trajectory, either
illusory (subjects wearing rotating prisms (A) or real (naked eyes, B) (ST). The dots indicate the instantaneous position of the fixation target
during A or B.
58
HEAD
PLD (cm)
Linear
Displacement ( LD )
1
90
80
60
HLD (cm)
40:
-****-
CNF
.
20 .
30
***
*
I***-
7
70
A
-***
PIVOT
50
-
40
r***,
+-A_
F-c * ---- *** ---
30
20
R
B
CNT
ST1
ST2
ST3
R
HLD (cm)
PLD (cm)
20.
20.
-*-
. I--**
0-T
” 5+.-e
R ST1 ST2
Maximal
Amplitude ( MA )
HMA (cm)
A
10
”
ST3
m1
*-I
k**
P
10
IL
I-*
10
0
ST1
ST2
ST3
P
ST1
ST2
ST3
P
I
10 .
0
P
I
R
PMA (cm)
12
CTVF
6
6
4
0
R
HMA (cm)
61
B
-***-
ST1
ST2
ST3
P
ST2
ST3
P
** -
R
ST1 ST2
PMA (cm)
-***
7
-t*
-*-Y
Average
Position ( AvP
CNF
ST1
HAvP (cm) Forward
)
-
***
R
ST3
P
ST1
ST2
ST3
P
PAvP (cm) Forward
7
1.5
1.5
1
1
.5
.5
0
-***-r
***-
0
R
ST1
ST2
ST3
P
HAVP (cm) Forward
R
1.5
***-
CNT
.
1-
R
ST1
ST1
ST2
ST3
P
ST3
P
PAvP (cm) Forward
-**_
B
-I
CNT
R
A
k
I-*
l **
2
ST2
ST3
P
-**-
v****-
R
***
ST1
-
ST2
Fig 3. Means and standard deviations of sagittal balance parameters for 10-s periods: before (R), during (STs) and after (P) counter-clockwise
target rotation. STs, tracking periods while wearing prisms (A) or naked eye (B) conditions. Significance of difference between two periods:
*P < 0.05; **P < 0.01: ***P < 0.001. Reference position (O), head and stabilometer fixed horizontal.
A
Amplitude (cm)
CTVF
Ataxiagram
0
5
0
/
Stabilogram
Time (s)
B
Amplitude (cm)
CTVT
5
Forward
, ,
,
,
, ,
,
,
,
I
,
,
Ataxiagram
Stabilogram
-5
3ackward
I
I
I
I
I
,
I
,
, ,
,
I
, ,
10
20
I
I
I
, ,
,,‘I
I
t
I
I
I
,
I
,
30
I
,
I
40
-
I
50
Time (s)
Fig 4. Average (n = 16 subjects) sagittal ataxiagram and stabilogram (continuous curves) before (R), during (STs) and after (P) counter-clockwise
target rotation starting at 3 o’clock due to prism rotation (A) or projector rotation (B). Dotted lines. mean-standard deviation. Dots. target
position every 2 s (l/4 period). Crosses, theoretical position of the head for a rigid body,
60
stimulation neither the head nor the pivot returned to
their initial position.
Averaged
stabi~ograms
and ataxiograms
Figure 4 illustrates the averaged recordings under the
CTVF (A, IZ = 16) and under the CTVT condition (B,
II = 16) with the same starting point: 3 o’clock. Under
both conditions, continuous traces show mean ataxiogram and stabilogram while the intervals between a
continuous trace and the corresponding dotted trace
give the negative standard deviation from the mean.
Standard deviations are approximately of the same amplitude in A and B.
During R periods, under both conditions the stabilogram and the ataxiogram were almost flat. During
the ST periods, modulation of the traces by circular
translation of the target was observed. Under CTVF
conditions the peak-to-peak deflections were large:
about 7 cm for the pivot and 8 cm for the head. Under
CTVT conditions they were small, approximately 1.5
cm in both curves, and in any case not greater than
the standard deviation in these points (approximately
2.5 cm). The dynamic balance responses were stereotyped: head and pivot were at the backward maximum
of their displacement when the gaze through prisms
was deviated upward and conversely they were at their
forward maximum when the gaze was downward.
The amplitude of synchronous oscillations of the
head was of greater amplitude than that of the pivot.
It is worth noting that the theoretical values (indicated
by crosses) of the head position computed from pivot
position with the hypothesis of a rigid body are very
close to the actual measured values. This suggests that
the head stabilisation by angulation changes of the
lower body segments could be considered as ineflicient.
At the end of the circular translation of the target,
the return of head and pivot traces to the zero reference
level was longer in CTVF than in CTVT tests for which
sagittal dynamic balance was little disturbed.
Discussion
The present work provides evidence that during visual
pursuit of the target describing a circular translation,
the movement of the background with the target (as
in CTVF) is a factor of strong body instability and of
synchronous oscillations in sagittal dynamic balance.
The similar eye pursuit movements with a stationary
background (as in CTVT) appear to be responsible for
some instability, though for a lesser degree, which is in
agreement with the results of Schulmann et al (1986).
The experimental situations differed in the stimulus
used to obtain the pursuit movements. The stimulus
used in CTVT conditions (a small bright spot moving
on a stationary
background)
is a typical pursuit
stimulus. The stimulus used in CTVF conditions (a
bright spot simultaneously
moving with the background) is not so common a stimulus. However, it has
been used by .Kowler et al (1984) to obtain smooth
eye movements.
During translation of the visual field the subjects
never experienced self-motion in the opposite direction
and the scene itself never appeared to stop moving,
two characteristic features of visually-induced
vection
(Mach, 1875; Hehnholtz, 1896). This differs from the
roll vection elicited by visual scene rotation around the
observer’s line of sight (Dichgans and Brandt, 1978;
van Asten et al. 1988). Although visual acuity underwent some reduction in CTVF, the target was conspicuous, even on the structured background. From
subjects’ reports, the whole visual field, including the
spot and the background, was always distinctly seen
describing a circular trajectory. EOG recordings confirmed that eye movements elicited by the illusory displacements of the target were similar to those elicited by
real target displacement (CTVT). Maximal amplitudes of
horizontal and vertical signals were always comparable
for each subject in CTVF and CTVT conditions.
The similarity of eye movements in both cases
allowed us to evaluate the role of retinal information
in balance effects. In CTVF the large synchronous oscillations in body motion cannot be considered as retinal in origin since there is no significant motion
information provided on the retina except for the small
fluctuations in velocity error required for the pursuit
task. Motion information is necessary to initiate but
not to maintain the pursuit since a mechanism of prediction is involved (Barnes and Asselman, 1991). That
suggests that the information from the oculomotor system plays a determinant role in somatic responses to
the visual pursuit task whether this information comes
from the extraocular muscle afferents (Cooper et al,
1955) or from neural oculomotor centres through corollary discharge or efferent copy (MC Closkey, 1981).
A relationship between the contraction of extraocular muscles and postural muscles has been proposed
by Gagey (1988). Roll et al (1991) showed that vibration of the inferior recti, equivalent to a muscle lengthening
and consequently
to an eye movement
upwards, made subjects fall backwards. This is congruent with our results since, because of the platform
configuration (see Materials and methods) it may be
ascertained that an upward movement elicits primarily
an extensor response (tilt backwards) whereas, in contrast, a downward movement of the visual target elicits
61
a flexor response (tilt forwards). So the eye movements
could induce, without any motion retinal afferents, a
signal responsible for the dynamic balance response.
The present results suggest that under CTVT conditions, the simultaneous presence, along with the retinal
stimulus initiating the pursuit task of retinal stimuli giving
rise to information about motion in the opposite direction,
suppressed the large synchronised responses induced by
the ocular motor system. The motion of the image on
the peripheral retina was also responsible for the suppression of motion perception of the visual field. Thus,
there could be a relationship between visual motion perception and body stabilisation. In CTVF, the large synchronous oscillations that accompany the perception of
visual field motion could be termed ‘postural pursuit’.
Gregory (1975) has emphasised that the brain picks out
features of the visual patterns that combine with its internal hypothesis to provide an adapted program of action.
So the activities that require visual pursuit of a vertically
moving target must be facilitated, by reducing synchronous oscillations of dynamic balance in the sag&al
plane when the visual field is stationary and by enhancing
these oscillations when the visual field is vertically moving in synchrony with the target. It is worth noting that
such a pattern could be involved during locomotion. Twice
in the step cycle the head of a subject describes in a frontal
plane an elliptical translation movement with vertical axis
(Steindler, 1964) and consequently the eyes of the subject
looking at the goal towards which he walks describe an
elliptical translation in the opposite direction. During the
step period between midstance and toe-off the head is
moving downwards and the eyes are moving upwards.
At that time, all plantar flexor muscles are active. Between
heel-on and mid stance when the head is moving upwards
and the eyes downwards, the dorsiflexor muscles of the
ankle are active (Cailliet, 1976).
To summarise, in dynamic balance the activity of
the pursuit oculomotor system can elicit general somatic responses in the body sagittal plane. The postural
responses are enhanced when ambient retinal motion information is missing and counteracted when retinal motion information is present evidencing retino-extraretinal
interaction in the dynamic balance regulation.
Acknowledgments
This work was supported by grants from CNES (Centre National d’gtudes Spatiales) and Fondation pour la Recherche
MCdicale. The authors wish to thank Dr CR Barnes for his
helpful comments
on the manuscript
and J Lamaison
for
building the software used in this study.
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