The effects of frequency and tilt on motion sickness induced by optokinetic stimuli

Background Nauseogenicity is determined by exposure time, frequency and acceleration of real motion. Our aim was to determine the nauseogenicity of these parameters for the optokinetic equivalent of OVAR (Off Vertical Axis Rotation). Methods The visual stimulus was a computerised scene of coastline as seen by a 'pilot' at moderate altitude. The scene could be rotated at different frequencies and tilted, just as during OVAR. The projected image subtended 84o at the eyes and subjects performed a concurrent task of detecting occasional visual targets to control attention. Exposures were for 10min or until nausea developed. Design was repeated measures counterbalanced for order. Optokinetic nauseogenicity was determined in three experiments: 0.05, 0.2, 0.8 Hz all at 18o tilt (n=14); 0, 45, 90o tilts all at 0.2Hz (n=12); 18, 36, 54, 72 o tilts all at 0.2Hz (n=24). Results 0.2 Hz was more nauseogenic than lower 0.05 or higher 0.8 Hz frequencies (p<0.01); 18 to circa 45o tilts were most nauseogenic (p<0.01). For all experiments symptoms usually increased over time. Conclusions The nauseogenicity peak around 0.2Hz, and increase with stimulus 'strength' up to circa 45o tilt, are similar for optokinetic and real motion. However, decreasing nauseogenicity with even higher tilt angles suggests that the impact of virtual stimuli is partially 'quarantined', perhaps because they appear obviously 'absurd', and therefore no longer present a sensory conflict that has to be resolved. Introduction Nauseogenicity of real translational motion increases with exposure duration and acceleration and peaks around a frequency of 0.2 Hz (Lawther & Griffin, 1987; O’Hanlon & McCauley, 1974; Golding et al, 2001). Off vertical axis rotation (OVAR) has some similarities to provocative translational motion except that the acceleration stimulus is the gravity vector rotating with respect to the long axis of the head-body, the magnitude being determined by the angle of tilt. The peak frequency for OVAR nauseogenicity has been estimated to be around 0.2 Hz (Miller & Graybiel, 1973) although one experiment suggested a slightly higher peak around 0.3 Hz (Denise et al, 1996). Motion sickness (MS) susceptibility has also been shown to increase dramatically with increasing angle of off-vertical tilt (Miller and Graybiel, 1973). An increase from 2.5 to 25 of tilt decreases the time to onset of moderate nausea from 60 minutes to just 6 minutes. OVAR studies tend not to use angles of tilt beyond 30, partly because of additional technical difficulties but mainly because the stimulus is so extremely nauseogenic. For example with a tilt of 90, 12 out of 20 subjects rotated about earth-horizontal through the z-axis of the body were unable to complete a very short 2 minute exposure due to severe nausea (Correia & Guedry, 1966). This so-called ‘barbecue-spit’