Reliance on Dynamic Visual Cues for Postural Control: Effect of Age is More Important than Unilateral Vestibular Hypofunction Patrick J. Sparto 1,2,3, Joseph M. Furman 1,2,3, Mark S. Redfern 2,3 Departments of 1 Physical Therapy, 2 Otolaryngology and 3 BioEngineering, 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 < 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. 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. Acknowledgement Discussion References Methods SUBJECT GROUPS: YOUNG UVH: 4 young adults ( years) with unilateral vestibular hypofunction (UVH) OLDER CON: 12 healthy older controls ( years) OLDER UVH: 9 older adults ( years) with UVH Clinical characteristics of the subjects with UVH are provided in Table 1. Figure 1. Subject standing within full field of view optic flow environment. Harness protects subject from falling. Head sway measured using Polhemus Fastrak. Results Table 1. Clinical characteristics of subjects with unilateral vestibular hypofunction 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: Meniere’s 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) Data Analysis: Percentage of responders: Significant sway responses at stimulus frequency identified using statistical method (Percival, 1994, Sparto et al., 2004)  2 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 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 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. 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 4. Effect of surface and optic flow conditions on the magnitude of head sway. Note that SURF, FREQ, and AMP main effects and SURF*FREQ, FREQ*AMP interactions were all significant (p < 0.005) 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 Table 2. Percentage of trials in which there was a significant postural response at the stimulus frequency, and  2 test of association to determine if the proportion of responses differed among the levels of the independent variable. Magnitude of postural responses: RMS of head sway was significantly affected by GROUP, SURF, FREQ, AMP, SURF*FREQ and FREQ*AMP (all p < 0.005) GROUP Effect (Figure 3): Amount of head sway was significantly less in YOUNG UVH compared with OLDER CON and OLDER UVH SURF*FREQ 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. FREQ*AMP 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 This research was supported in part by NIH Grants AG01049, DC05205, and by the Eye and Ear Foundation. 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%. 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. Black FO, Shupert CL, et al. (1989). "Effects of unilateral loss of vestibular function on the vestibulo-ocular reflex and postural control." Annals of Otology, Rhinology & Laryngology 98: Norre ME, Forrez G, et al. (1987). "Vestibular dysfunction causing instability in aged patients." Acta Oto-Laryngologica 104: 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: 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: