Date of Award

Spring 1-1-2019

Document Type

Thesis

Degree Name

Master of Science (MS)

Department

Aerospace Engineering Sciences

First Advisor

Torin K. Clark

Second Advisor

Allison P. Anderson

Third Advisor

Zachary P. Kilpatrick

Abstract

Vestibular perceptual thresholds quantify how small of a self-motion a human is able to perceive as one direction versus the other reliably. A sensitive measure of vestibular function, thresholds have direct functional relevance.

While thresholds in translation and rotation have been extensively characterized and variation in thresholds between different axes (e.g., up/down vs. left/right) established there had been limited investigation of tilt thresholds in different axes using modern psychophysical techniques. Therefore, in phase 1 of this study, we quantify pitch tilt thresholds and compare to those in roll tilt in the same group of subjects, across a range of stimulus frequencies (i.e., 1/motion duration). In phase 2 of this study, we hypothesize anatomical asymmetries in pitch tilt may lead to directional asymmetries (i.e., differing sensitivity for tilts forward versus backward), which we investigate at one frequency using modern threshold approaches (assuming no asymmetry) and comparing resulting fits with those obtained from the hybrid dual sigma (asymmetry) model proposed by Roditi and Crane (Roditi and Crane 2012).

Our Tilt-Translation Sled device (without the translation activated) was used to create whole-body tilt motions to a seated subject in the dark, assessing pitch tilt and roll tilt thresholds in separate sessions. Subjects reported motion direction (left or right for roll tilt; forward or backward for pitch tilt) in a forced-choice, direction-recognition task and confidence level of their selection (between 50 and 100% at increments of 5).

In phase 1, ten subjects performed blocks of 200 trials for each tilt axis (roll or pitch) and stimulus frequency (0.15, 0.2, 0.5 or 1 Hz) presented in a counterbalanced order. As previously observed for roll, tilt angle thresholds increased at lower frequencies but stabilized around 0.15-0.2 Hz. Pitch tilt thresholds, across each of the frequencies we tested, were observed to be similar to, but slightly higher than, roll tilt thresholds. Specifically, the geometric mean threshold for pitch tilt (versus roll tilt) was 1.66° (1.50°) for 0.15 Hz, 1.61° (1.46°) for 0.2 Hz, 0.99° (0.96°) for 0.5 Hz, and 0.51° (0.47°) for 1 Hz. In phase 2, four subjects performed a total of 2000 trials for pitch tilt at 1 Hz. Substantial directional asymmetries were identified in one of four subjects (with better sensitivity for backward tilts), while two were highly symmetric and one less so.

To our knowledge this is the first study to quantify pitch tilt thresholds across a range of frequencies, providing a comparative baseline of healthy subjects as well as reporting the presence of directional asymmetries in pitch tilt at 1 Hz. Understanding tilt thresholds across directions and frequencies, as well as the possibility for asymmetries, is essential for those with clinical balance impairments (e.g., elderly) and healthy individuals in unique balance environments (e.g., astronauts), alike.

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