Rotation rate and duration effects on the somatogyral illusion

INTRODUCTION: Aviation spatial disorientation mishaps remain a concern, especially due to their fatality rate. Some of the most insidious disorientations are due to vestibular stimuli in the absence of visual cues. A category of such disorientations are known as somatogyral illusions. METHODS: To determine the effects of spin rate and duration on the perception of the somatogyral illusion, we examined the subjective response of pilots and non-pilots to rotation around the yaw axis in a flight simulator in a manner that would mimic two vestibular illusions found in flight: the washout of the semi-circular canals following sustained turns, and the illusory counter-rotation following return to straight and level flight. There were 29 subjects (14 pilots) who were seated blindfolded in a flight simulator which accelerated to constant plateau rotation rates of 20, 70, and 120 degrees x s(-1) and then decelerated to stationary; plateaus were 10, 20, or 40 s. Subjects reported 1) the time when the perception of rotation ceased (i.e., the subjective time until washout was reached); 2) the relative magnitude of the counter-rotation experienced; and 3) the time until the perception of counter-rotation ceased. Subjects also manipulated a slider to provide a continuous subjective measure of their experience of rotation. RESULTS: The two time measures increased with increases in both the duration and magnitude of the spin. The increase in perceived washout time with spin rate was non-linear (geometric). There was an interaction between spin duration and spin rate on the experience of illusory counter-rotation magnitude such that at low rates, spin duration had no effect, but its effect increased at faster rates. The time constant of adaptation of the semicircular canals was estimated to be 8.3 s. DISCUSSION: The effects were validated against a model of semicircular canal and cupola adaptation, which predicted the data with high accuracy. Pilots and non-pilots did not differ in their illusory experience.