Prolonged reduction of motion sickness sensitivity by visual-vestibular interaction

Abstract

The angular vestibulo-ocular reflex (aVOR) and optokinetic nystagmus (OKN) were elicited simultaneously at low frequencies to study effects of habituation of the velocity storage time constant in the vestibular system on motion sickness. Twenty-nine subjects, eleven of whom were susceptible to motion sickness from common transportation, were habituated by sinusoidal rotation at 0.017 Hz at peak velocities from 5 to 20°/s, while they watched a full-field OKN stimulus. The OKN stripes rotated in the same direction and at the same frequency as the subjects, but at a higher velocity. This produced an OKN opposite in direction to the aVOR response. Motion sickness sensitivity was evaluated with off-vertical axis rotation (OVAR) and by the response to transportation before and after 5 days of visual-vestibular habituation. Habituation did not induce motion sickness or change the aVOR gains, but it shortened the vestibular time constants in all subjects. This greatly reduced motion sickness produced by OVAR and sensitivity to common transport in the motion susceptible subjects, which persisted for up to 18 weeks. Two motion susceptible subjects who only had aVOR/OKN habituation without being tested with OVAR also became asymptomatic. Normal subjects who were not habituated had no reduction in either their aVOR time constants or motion sickness sensitivity. The opposing aVOR/OKN stimulation, which has not been studied before, was well tolerated, and for the first time was an effective technique for rapid and prolonged habituation of motion sickness without exposure to drugs or other nauseating habituation stimuli.

Figures

Fig. 1

Schematic representation of the experimental protocol. Subjects received the first OVAR test (OVAR 1) at the start of the experiment. One week later, they had a second OVAR test (OVAR 2). In the third week, they received 5 days of OKN/aVOR habituation. They were then tested again with OVAR in the third (OVAR 3) and fourth weeks (OVAR 4)

Fig. 2

Cancellation of the post-rotatory aVOR response by OKN. a Post-rotatory aVOR in response to 20°/s rotational velocity. b OKN and OKAN from stimulation at 20°/s. c Rotation in light at 20°/s with a stop in darkness. Note the cancellation of the post-rotatory aVOR by the OKAN after Lights Off/Chair Stop in c

Fig. 3

a Experimental setup for the cancellation of the aVOR with OKN. The chair and OKN drum were oscillated concentrically at a frequency (f) of 0.017 Hz. The OKN had a phase lead to chair rotation of ϕ degrees. b aVOR response showing a phase advance. The aVOR slow phase eye velocities were fit with a sine function (green line) that had an amplitude of 14°/s and a phase advance of 0.5 radians (28.6°). c Visual-vestibular interaction. The expected aVOR (green line) was opposed by the OKN (blue line)

Fig. 4

Horizontal aVOR time constants (a, b) and gains (c, d) before and after each day of habituation over 5 days. The vertical bars show ± 1 SD. a There was a reduction in time constant at the end of each day of habituation in the normal group that returned to a lower level on the following day. b The susceptible subjects had a large initial fall in time constant that was further reduced during the week. c, d The aVOR gains were unaffected by the habituation. The gains were higher in the susceptible subjects (P = 0.007, unpaired Student’s t test)

Fig. 5

Mean aVOR gains of normal and susceptible subjects over the test period. The gains of the susceptible subjects (red line) were greater than those of the normal subjects (P = 0.007, unpaired Student’s t test). The error bars (±1 SD) are shifted to the right or left to avoid overlap

Fig. 6

Change in aVOR time constants and motion sickness sensitivity (MSS) as a function of OVAR tests, as well as the effect of habituation on the time constant and MSS for normal subjects (a, e), susceptible subjects (b, f), and normal subjects without habituation (c, g). No change in time constant of the aVOR was produced by OVAR (c). Changes only occurred if there was a period of habituation (a, b). Concurrently, there was a reduction in MSS after habituation (e, f). The vertical bars show ± 1 SD

Fig. 7

Average motion sickness scores during transportation before and after habituation. There was a significant reduction in motion sickness among motion susceptible subjects that persisted for 4 weeks in 10 subjects, and for 18 weeks in 8 of the 11 subjects