Postural Sway in a Virtual Environment in Patients With

Unilateral Peripheral Vestibular Lesions

Susan L. Whitney, PhD, PT, NCS, ATCPatrick J. Sparto, PhD, PT

Kathryn E. Brown, MS, PT, NCSMark S. Redfern, PhD

Joseph M. Furman, MD, PhDDepartments of Physical Therapy, Otolaryngology and

BioengineeringUniversity of Pittsburgh

Acknowledgements: Eye and Ear Foundation, NIDCD Grants: DC05205 & DC05372

Introduction• Vestibular compensation adjusts for

abnormalities in the vestibulo-ocular reflex (VOR) and postural stability seen acutely following unilateral peripheral vestibular lesions (UPVL).• Long term visual dependence of

individuals with UPVL has not been fully examined.• The ability to receive cues from peripheral

vision in these individuals has not been studied.

Purpose• The purpose of this pilot study was to begin

to answer the question of how visual scenes affect postural control after vestibular dysfunction, using a subset of all possible variables available in virtual environments.• To do so we started with the following goals: – assess the visual motion sensitivity of patients

with chronic UPVL’s as compared to individuals with normal vestibular function.

– determine the amount of subjective discomfort each subject perceived.

Characteristics of the visual scenes that can be varied

• Movement of the scene• Stereoscopic vs. monoscopic vision • Binocular vs. monocular vision• Height of the focus of vision• Size of the virtual world on the screen• Central vs. peripheral motion• Complexity of the scene• Brightness

Experimental Design

• Independent variables:– Subject group (patient vs. control)– Field of View (FOV)– Frequency of tunnel movement

• Dependent variables:– Amount of sway– Subjective Units of Discomfort (SUDs)

rating

Experimental Conditions• Each session had an initial and final “quiet”

trial with a blank screen in a darkened room.

• Each subject was tested under 3 different FOV conditions.– Full vision.– Peripheral vision only (central 30º occluded).– Central vision only (central 30º).

• Each subject was also tested under 2 different frequencies of visual scene movement.– 0.1 Hz movement.– 0.25 Hz movement.

Methods• A visual stimulus of an infinitely long

tunnel with checkered walls was displayed in a virtual environment display facility (BNAVE).• Subjects stood barefoot for 80 seconds

while viewing sinusoidal movements of the virtual tunnel.– RMS velocity was 1.2 m/s.

• Sixty seconds of movement were preceded and followed by 10 seconds of no tunnel movement.

Methods• Subjects stood on force platform with their

feet comfortably apart, wearing a harness to prevent a fall.• The subject’s head movement was

measured using an electromagnetic sensor affixed to an adjustable plastic headband. • Eye movement was monitored to ensure

that eyes were viewing the screen.• All data were collected with the room

darkened.

Methods• Subjects were asked to rate their

Subjective Units of Discomfort (SUDs) prior to the start and after each trial.• The subject was asked to rate their

level of anxiety on a scale of 0 to 100 with 100 being the greatest.

Patient Group

• Twelve former patients all of whom were s/p vestibular nerve resection were recruited to participate.– 10 secondary to acoustic neuroma.– 2 secondary to Meniere’s disease.•Gender: 5 males, 7 females.• Age: Mean: 49 ± 10; Range: 31 - 65.

Patient Group• Time since unilateral peripheral

vestibular loss:– Range: 10 – 72 mos.–Mean: 39 ± 22 mos.• Highly functional individuals.– Activities-specific Balance Confidence:

Mean: 88 ± 11; Range: 67 - 100.– Dizziness Handicap Inventory: Mean: 10

± 11; Range: 0 - 26.

Control Group

• Twelve gender and age (± 2 years) matched controls were recruited to participate.• All exhibited normal vestibular

laboratory testing.

Data Analysis

• Anterior-Posterior head position was sampled at 20 Hz.• Data was processed with a phaseless

digital bandpass filter.– Filter was 0.1 (± .05) or 0.25 (± .05) Hz.

• RMS sway was computed for the full 60 sec of the moving visual stimulus.

Pt 2, 0.10 Hz, Peripheral Vision

-6

-4

-2

0

2

4

6

0 10 20 30 40 50 60 70 80Time (s)

Hea

d Po

sitio

n (c

m)

Tunn

el P

ositi

on (m

)

Head PositionTunnel Position

Pt 2, 0.10 Hz, Central Vision

-6

-4

-2

0

2

4

6

0 20 40 60 80Time (s)

Hea

d Po

sitio

n (c

m)

Tunn

el P

ositi

on (m

)

Head PositionTunnel Position

Effect of Field of View on Sway at 0.1 Hz

0.798

0.3200.515

0.756

0.2830.572

00.20.40.60.8

11.21.41.61.8

Full Peripheral Central

Field of View

RM

S Sw

ay (c

m)

Control Patient

Effect of Field of View on Sway at 0.25 Hz

0.346 0.386 0.2090.493 0.478 0.304

0.00.20.40.60.81.01.21.41.61.8

Full Peripheral Central

Field of View

RMS

Sway

(cm

)

ControlPatient

Effect of Field of View on Subjective Units of Discomfort at 0.1 Hz

11 1424 21 12

50102030405060708090

100

Full Peripheral Central

Field of View

Subj

ectiv

e U

nits

of

Dis

com

fort

ControlPatient

Effect of Field of View on Subjective Units of Discomfort at 0.25 Hz

13 1319 233 90

102030405060708090

100

Full Peripheral Central

Field of View

Subj

ectiv

e U

nits

of

Dis

com

fort

Control Patient

Conclusions• There is no statistically significant

difference between the patient and control groups.– Patients did not appear to be more

visually dependent than controls. Why? • Too challenging for both groups?• Not challenging enough to discriminate

between the groups?• Patients were well-compensated.

Summary• Central FOV results in less visual

motion-induced sway in both people without disease and in patients with chronic UPVL.• FOV significantly influences SUDs

scores in both groups.• Lower frequency of visual scene

movement results in greater sway in both people without disease and in patients with chronic UPVL.