Extreme motion environments can induce loss of visual acuity, motion sickness, and spatial disorientation. Understanding how human sensory integration of acceleration stimuli affects spatial awareness will improve models of spatial disorientation and mishap analysis. While there are numerous studies describing vestibular semicircular canal responses to angular acceleration, less is known about vestibular otolith responses to linear acceleration. This gap in knowledge is important to resolve, since seasickness and airsickness are highly dependent on the predominant frequency of a linear acceleration stimulus.

With controlled laboratory off-vertical axis rotation (OVAR), gaze reflexes respond to low-frequency (e.g., 0.05 Hz) stimulation as if the observer is tilted, while high-frequency OVAR (e.g., 0.55 Hz) creates an illusion of translating sideways. In conjunction with this effect, low-frequency OVAR is also known to cause eye torsion that attenuates the predominance of horizontal eye movements. The purpose of this work was to evaluate the mid-range OVAR frequency (0.25 Hz) at which there is no clear predominance of tilt or translation response. The reason for evaluating this range of motion is to determine if it is the frequency at which maximum motion sickness may be observed. The finding that vestibular gaze reflexes become altered at the same frequency where OVAR becomes most sickening will have important implications for defining human tolerance in extreme motion environments.

Data were successfully collected from 10 subjects, each of whom completed three separate trials at sequences of low, medium, and high OVAR spin rates. The results of these tests revealed no significant change in horizontal and torsional eye movements between the low OVAR spin frequency of 0.03 Hz and the predicted crossover point of 0.25 Hz; however, there did appear to be a trend toward reduction of horizontal eye movement when the high OVAR rate of 0.55 Hz was compared with the low (0.03 Hz) and medium (0.25 Hz) rates. Based upon the collected data, a revised crossover rate of 0.42 Hz was extrapolated as the most probable spin frequency for inducing gaze reflex changes with the potential for triggering motion sickness.

A primary goal of this work was to address gaps in knowledge and determine whether tilt perceptions, like eye movement responses, tend to transition to translation perceptions at higher frequencies of OVAR. When the OVAR frequency was increased from slow to fast, some subjects reported more translation and less tilt, which suggests there is a point at which perceptual and reflexive responses transition. It is possible that this is also the point at which sensory conflict and motion sickness are most likely to occur. Based on these findings, researchers have postulated that people can be made sick in the least amount of time at the frequency at which it is most difficult for the vestibular system to resolve between tilt and translation stimuli.

Following several modifications generated by completion of an initial pilot study, the original hypothesis was modified and redefined for the purpose of examining how gaze stabilization responses become altered at low, medium, and high spin rates with 30-degree offset OVAR. The revised hypothesis proposed that eye torsion would occur at an OVAR frequency of 0.05 Hz, transition to horizontal eye movement would begin at 0.25 Hz, and horizontal eye movement would dominate at a frequency of 0.55 Hz.

The results from this effort may have an impact on design considerations for future military research and training devices, since it would appear from the data that the most sensory incompatible spin rates may be at a much higher rate than originally predicted.

This work was done by Frederick R. Patterson and Joseph F. Chandler of the Naval Aerospace Medical Research Laboratory. ONR-0022

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Effect of Acceleration Frequency on Spatial Orientation Mechanisms

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This article first appeared in the October, 2013 issue of Defense Tech Briefs Magazine.

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