VISUAL DEPENDENCE IN POSTURAL CONTROL AND SPATIAL ORIENTATION Massimo Cenciarini1, Patrick J. Loughlin1,2, Mark S. Redfern1,3, Patrick J. Sparto1,3 Depts. of 1Bioengineering, 2Electrical & Computer Engineering, and 3Otolaryngology University of Pittsburgh, Pittsburgh, Pennsylvania 15261

Objective & Hypothesis: Vision is used for both postural control and spatial orientation Do these two processes share common pathways? Do people that rely more on vision for postural control are also visually dependent for spatial orientation?

Methods Testing dependence on vision for postural control Postural response to visual perturbation Testing dependence on vision for spatial orientation Subjective visual vertical in different visual conditions Correlation between postural response and SVV result

Methods - Protocol Postural Testing Subjects viewed full field optic flow while standing on a modified Neurocom postural platform (Fig. 1A) Eyes open throughout 90 s trials Moving scene stimulus characteristics 60 s sinusoidal moving scene stimulus preceded and followed by 15 s stationary visual scene 16 cm peak-to-peak anterior-posterior (A/P) sinusoidal movement of visual scene at 0.1, 0.25, 0.4, 0.7, or 1 Hz Each frequency condition presented three consecutive times, in randomized triplets Support surface conditions Fixed support surface (first visit) Sway-referenced (second visit)

Methods - Protocol Subjective Visual Vertical testing Subject seated in the dark, 1 m from fluorescent rod (Fig. 1B) Rod (L 40 cm) initial position was ±40º with respect to vertical Background conditions No background (complete dark) Tilted fluorescent frame (90x90 cm) rotated ±28º Rotating disc of random dots (Ф 90 cm) at constant velocity of ±30º/s Subjects were allowed unlimited time to set the rod to vertical In all cases, no more than 30 sec were needed Two tests performed for each SVV condition

Methods - Protocol Figure 1: A. Setup for balance testing. Virtual reality environment to generate optic flow. B. Subjective visual vertical setup and background conditions. Top figure: subject being tested. Bottom figures: examples of background conditions.

Methods - Analysis Postural response to moving visual scene COP position data in the A/P direction was low-pass filtered (2 Hz cut-off) and down-sampled (5 Hz) Total root mean square (RMS) of COP during 60 s moving scene was calculated to characterize the amount of sway in response to visual perturbation Repeated measures ANOVA used to determine if RMS of COP was affected by surface condition and frequency of optic flow

Methods - Analysis SVV analysis Magnitude of the final rod position, or SVV, was measured to characterize dependence on vision for spatial orientation Repeated measures ANOVA was used to determine if SVV was affected by background condition

Results Postural response to moving visual scene RMS values were higher for the sway-referenced surface than for the fixed surface Fixed: 0.46 ± 0.28 cm (mean ± SD) Sway-referenced: 1.76 ± 0.80 cm (Fig. 2A) No significant difference was found among RMS values obtained at different stimulus frequencies for the fixed surface (p > 0.11) Significant effect of the stimulus frequency was found for the sway-referenced surface (p < 0.05). However, post-hoc analysis did not reveal any difference between each of the frequencies and the other four stimulus frequencies (Fig. 2A).

Methods - Protocol Figure 2: A. Mean value of RMS of COP at each stimulus frequency. Top-left: RMS for fixed surface. Top-right: RMS for sway-referenced surface.

Results SVV responses The effect of background condition was significant (p < 0.01); post-hoc analysis revealed that each background condition was different from the other two conditions (Fig. 2B) Average SVV deviation for no background condition was 1.0 ± 0.7º (mean ± SD) Average SVV deviation for frame condition was 4.9 ± 4.7º Average SVV deviation for disc condition was 8.2 ± 4.0º

Figure 2: B. Mean of SVV magnitude for each background condition. Methods - Protocol Figure 2: B. Mean of SVV magnitude for each background condition.

Results Correlation analysis results Mean value of RMS of COP across repetitions showed no significant correlation with mean value of magnitude of final rod position for both no-background and tilted frame conditions (Tab. 1) Table 1: Summary of results from correlation analysis between total RMS of COP and SVV magnitude for all surface and SVV background conditions. Correlation coefficient (r) and p-value are reported (Npts = 80).

Results Correlation analysis results Mean RMS value showed a positive correlation with mean magnitude of the final rod position for both fixed (r = 0.46) and sway-referenced (r = 0.58) surface conditions (p < 0.001) (Fig. 3)

Results Figure 3: Mean total RMS vs SVV magnitude for the disc condition. Left plot: RMS data for fixed surface. Right plot: RMS data for sway-referenced surface.

Comments from the ARO MWM The poster received a good acceptance at the meeting. It was surprising the no correlation was found between frame SVV and sway data. Maybe the two different scenes (full FOV and central FOV) don’t help for correlation of the responses. Would be possible to combine one of the two and run both experiments with one setup? It will be interesting to test this protocol on vestibular patients.

Suggestions from the ARO MWM What if we normalize the COP data by the height? “Angle”, which should be a good index of sway. What about using real scenarios instead of the checkered pattern to provide optic flow? Some think that real objects are more “provocative” that unrealistic scenes. Something to deal with perception and consciousness about the stimulation, therefore we should also look at the repetition and see if there are any changes over time in the trial sequence (Habituation).

My questions about this study Is it possible to test a A/P postural sway similarly to the SVV protocol? Possible postural testing protocol: Static visual cues: scene of a forward/backward tilt at different angles (fixed and sway-ref) Dynamic visual cues: since the SVV has a constant spinning velocity, would be possible to have a peripheral spinning scenario to see how the subjects sway would vary in response to it? It would be interesting if we could quantify the influence of static and dynamic cues in different environmental conditions, and this is also important for modeling purposes.

Any Comments? Please feel free to give me a feedback about the material I just showed you. Thanks!!