Reliance on Dynamic Visual Cues for Postural Control:

1
Reliance on Dynamic Visual Cues for Postural
Control Effect of Age is More Important than
Unilateral Vestibular Hypofunction Patrick J.
Sparto1,2,3, Joseph M. Furman1,2,3, Mark S.
Redfern2,3 Departments of 1Physical Therapy,
2Otolaryngology and 3BioEngineering, University
of Pittsburgh, Pittsburgh, PA
Abstract Background Young adult subjects with
unilateral vestibular hypofunction (UVH) and
healthy older adults demonstrate greater
visually-induced sway than healthy young adult
control subjects. The purpose of this study was
to determine if the combination of advanced age
and UVH synergistically results in even greater
reliance on dynamic visual cues. Methods Three
groups of subjects were tested four young adults
(28-39 years) and nine older adults (58-80 years)
with a history of functionally compensated UVH,
and 12 older controls. Twelve of the thirteen
subjects with UVH had a unilaterally absent
caloric response 1 had 67 loss. Subjects viewed
sinusoidal anterior-posterior optic flow in a
full field of view environment while standing on
both a fixed and a sway-referenced platform. The
optic flow consisted of nine trials each using a
single frequency (0.1, 0.25, or 0.4 Hz) and a
single amplitude (4, 8 or 12 cm).
Anterior-posterior head sway was digitized at 20
Hz. Data were processed using a phaseless digital
bandpass filter centered at the stimulus
frequency of the optic flow. The root-mean-square
(RMS) of the head sway was used to measure
postural response. Results Analysis of Variance
revealed a significant effect of age on the
magnitude of sway in response to optic flow (p
0.003) older subjects with and without UVH
swayed more than young adults with UVH. There
was no difference between older controls and
older subjects with UVH. Additional significant
effects were found for surface condition, and
frequency and amplitude of optic flow stimulus (p
lt 0.001). Conclusion Age was a greater
determinant of visually-induced sway than the
presence of UVH. Reliance on dynamic visual cues
for balance in older subjects with a history of
UVH may depend, in part, on the degree of
functional compensation.
Results
Table 1. Clinical characteristics of subjects
with unilateral vestibular hypofunction

• Identification of significant responses
• In 84 of the trials, a postural response was
  elicited at the optic flow stimulus frequency
  (Figure 2, left)
• In 16 of the trials, no discernible response
  was observed at the stimulus frequency (Figure 2,
  right)

Figure 3. Individual (open symbol) and mean
(closed symbol) postural responses for the three
subject groups, averaged over all 18 conditions.
Overall group effect was significant (p 0.003).
Significance level of post-hoc comparison between
subject groups listed under labels. Older UVH
subjects designated with an correspond with
subjects U07 and U14 in Table 1.
AN acoustic neuroma DHI Dizziness Handicap
Inventory, 0 no handicap EVAR Asym. Earth
Vertical Axis Rotation Asymmetry at 0.02 and 0.04
Hz Abnormal defined as greater than 2 SD above
mean from control subjects MD Menieres
Disease PVL Unspecified peripheral vestibular
lesion PF Perilymphatic fistula Positional
Nystag. Abnormal defined as nystagmus greater
than 3 deg/sec in at least one position SSC DH
Semicircular Canal Dehiscence

• Procedures
• Subjects viewed 90 s of anterior-posterior (A-P)
  optic flow in a full horizontal field of view
  environment, while standing on a Neurocom
  (Clackamas, OR) force platform (Figure 1)
• Full factorial design 2 surface X 9 optic flow
  conditions
• Surface conditions (SURF) Fixed and
  Sway-referenced
• Optic flow conditions 3 amplitude (AMP 4, 8,
  and 12 cm) X 3 frequency (FREQ 0.1, 0.25, and
  0.4 Hz) sinusoidal stimuli
• A-P head sway measured using Polhemus Fastrak
  (Colchester, VT)

Figure 2. Postural responses to 0.25 Hz, 4 cm
optic flow stimuli for 2 subjects who stood on a
sway-referenced platform. Significant response
detected for Subject 9, but not Subject 7.

• The number of responses depended on subject GROUP
  and FREQ (Table 2)
• Older adults with and without UVH had more
  responses at the stimulus frequency, compared
  with young adults with UVH
• The number of responses increased as FREQ
  increased

Figure 4. Effect of surface and optic flow
conditions on the magnitude of head sway. Note
that SURF, FREQ, and AMP main effects and
SURFFREQ, FREQAMP interactions were all
significant (p lt 0.005)
Introduction The human postural control system
can be very adaptable. A loss of function in one
sensory system contributing to balance can lead
to compensation by other systems. Therefore, a
person with a vestibular disorder is believed to
be able to use visual and proprioceptive
information to a greater extent to stabilize
posture. (Black et al., 1989) There may be
differences in the ability of older and younger
subjects to compensate for the loss of vestibular
function. (Norre et al., 1987) Thus,
understanding the impact of age on how sensory
information is used in balance for patients with
vestibular disease is important for developing
specific strategies for rehabilitation and
thereby reducing the risk of secondary injuries
due to falling. The purpose of this study was to
examine how older persons with vestibular disease
utilize visual and somatosensory information to
maintain balance.
Discussion
Table 2. Percentage of trials in which there was
a significant postural response at the stimulus
frequency, and c2 test of association to
determine if the proportion of responses differed
among the levels of the independent variable.

• The primary finding in this study was that
  reliance on dynamic visual cues was more
  dependent on age than on the presence of
  unilateral vestibular hypofunction.
• Although we did not detect a difference between
  older control subjects and older persons with
  unilateral vestibular hypofunction, the presence
  of a unilateral vestibular disorder in
  combination with advanced age may further
  increase reliance on visual cues
• There was a cluster of six older UVH subjects
  that had greater sway than the older control
  subjects.
• Two of the older UVH subjects (labeled by in
  Table 1 and Figure 3) had substantially less sway
  than the other older UVH subjects. Clinical
  characteristics revealed that these 2 subjects
  may have had greater functional compensation 1)
  they were the least symptomatic of the older
  subjects, as defined by the DHI, and 2) they had
  the highest gait speed.
• We performed a stepwise linear regression with
  RMS sway as the dependent variable and age and
  disease status as independent variables. Age
  explained 25 of the variance in the RMS sway,
  and disease status explained another 15.

• Magnitude of postural responses
• RMS of head sway was significantly affected by
  GROUP, SURF, FREQ, AMP, SURFFREQ and FREQAMP
  (all p lt 0.005)
• GROUP Effect (Figure 3) Amount of head sway was
  significantly less in YOUNG UVH compared with
  OLDER CON and OLDER UVH
• SURFFREQ interaction (Figure 4, left)
  Magnitude of head sway is level across
  frequencies on fixed platform, but decreases as a
  function of frequency on sway-referenced platform
  
• Overall decrease in head sway as a function of
  frequency heavily influenced by surface
  conditions
• Head sway is 3 times greater on sway-referenced
  platform compared with fixed platform.
• FREQAMP interaction (Figure 4, right)
  Magnitude of head sway increased as the optic
  flow amplitude became greater at 0.25 and 0.4 Hz,
  but not at 0.1 Hz
• Head sway increases as optic flow amplitude
  becomes larger

Figure 1. Subject standing within full field of
view optic flow environment. Harness protects
subject from falling. Head sway measured using
Polhemus Fastrak.

• Methods
• SUBJECT GROUPS
• YOUNG UVH 4 young adults (33 5 years) with
  unilateral vestibular hypofunction (UVH)
• OLDER CON 12 healthy older controls (69 7
  years)
• OLDER UVH 9 older adults (69 6 years) with
  UVH
• Clinical characteristics of the subjects with
  UVH are provided in Table 1.

• Data Analysis Percentage of responders
• Significant sway responses at stimulus frequency
  identified using statistical method (Percival,
  1994, Sparto et al., 2004)
• c2 used to determine significant associations
  between subject GROUP, SURF, AMP, and FREQ and
  number of significant sway responses
• Data Analysis Magnitude of postural responses
• Head sway filtered using a digital phaseless
  bandpass filter, with passband equal to stimulus
  frequency 0.05 Hz
• Magnitude of sway was computed from the
  root-mean-square (RMS) of bandpass filtered head
  sway
• Repeated measures ANOVA used to examine effects
  of GROUP, SURF, AMP, and FREQ, and 2-way
  interactions on head sway

References
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vestibulo-ocular reflex and postural control."
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884-9. Norre ME, Forrez G, et al. (1987).
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aged patients." Acta Oto-Laryngologica 104
50-5. Percival DB (1994). Spectral Analysis of
Univariate and Bivariate Time Series. Statistical
Methods for Physical Science. JL Stanford and SB
Vardeman. New York, Academic Press
313-48. Sparto PJ, Jasko JG, et al. (2004).
"Detecting postural responses to sinusoidal
sensory inputs a statistical approach." IEEE
Transactions on Neural Systems Rehabilitation
Engineering 12 360-366.
Acknowledgement
This research was supported in part by NIH Grants
AG01049, DC05205, and by the Eye and Ear
Foundation.