Vestibular habituation in student pilots

The dynamics of vestibular nystagmus were measured in 42 military student pilots (age 19 to 21). Their responses were compared with 40 non-flying subjects of similar age who were also fulfilling requirements for the mandatory Swiss military service. The following differences emerged: the time constant of the nystagmus response after vestibular stimulation was shorter (p less than 0.001; t-test) in student pilots, whereas the gain tended to be higher (p less than 0.025). These changes in the response dynamics are attributed to habituation. Student pilots were additionally tested with conflicting visual-vestibular stimulation. Nystagmus response was delayed and attenuated when compared to stimulation in darkness. Under these conditions motion sickness occurred in one third of the subjects. We found no relation between the occurrence of motion sickness and the value of the time constant or gain of vestibular nystagmus. Results show that there is no single "normal" value of vestibular nystagmus. This becomes important when defining "normal" values as opposed to pathological values in vestibular testing.