Thomas Jarchow

Incremental adaptation to yaw head turns during 30 RPM centrifugation.

A 3-day incremental protocol was conducted with the aim of adapting human subjects to make head movements comfortably during 30 RPM centrifugation. With motion sickness as a potentially limiting factor, the protocol was designed using a quantitative motion sickness model based upon the neural mismatch sensory conflict theory. Centrifuge velocity was incremented from 14 RPM on day 1, to 23 RPM on day 2, to 30 RPM on day 3, with subjects making a total of 42 head movements on each day. Twenty-four subjects completed the experiment with average motion sickness levels below five (out of 20). Four subjects aborted due to motion sickness. Adaptation of non-compensatory vertical nystagmus was observed through an 18% decrease in the vertical aVOR time constant over the 3 days. Subjective intensity ratings for the head movements decreased by approximately 40% over the 3 days, while illusory motion duration decreased by 18%. Feasibility of head movements during 30 RPM rotation was demonstrated with only 3 days of incremental training.

Adaptation of VOR to Coriolis stimulation.

Abstract: The vestibulo‐ocular reflex (VOR) is normally characterized by the gain and phase of slow‐phase velocity (SPV) relative to the stimulus velocity. Although this is perfectly satisfactory for steady‐state sinusoidal oscillations about a single axis, it is less useful when applied to transient responses. The well‐known decay of nystagmus following a step change of head velocity approximately follows a double exponential, with an initial amplitude (A), a long time constant (τ), and an adaptation time constant (τa). We have developed a means of representing the transient response for a complex head velocity stimulus as experienced during high‐speed artificial gravity (AG) experiments. When a subject, lying supine on a rotating horizontal platform, makes a yaw head movement of amplitude θ, the vertical semicircular canals experience a step in angular velocity. The pitch stimulus is equal to the change in the component of the centrifuge angular velocity (ωc) aligned with the interaural axis, and gives rise to a vertical VOR. The magnitude of the step change is ωc sin θ. The SPV is approximated by an exponential decay of amplitude A and single time constant τ, and then normalized relative to this stimulus step. MATLAB scripts filter the raw eye position data to remove noise, blinks, and saccades, differentiate the signal, and remove fast phases to obtain SPV. The amplitude of the fitted SPV exponential is divided by ωc sin θ to obtain the normalized SPV. A and τ are shown to behave differently as subjects adapt to repeated head movements of different amplitudes.