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Somatogyral illusions during flight: effect of the post-roll illusion on pilot’s control behaviour


Nooij,  SAE
Max Planck Institute for Biological Cybernetics, Max Planck Society;

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Nooij, S. (2010). Somatogyral illusions during flight: effect of the post-roll illusion on pilot’s control behaviour. Talk presented at XXVI. Bárány Society Meeting. Reykjavik, Iceland.

Background: Due to the high-pass characteristics of the semicircular canals, the rotation sensation dies out during constant velocity rotation, and a sensation of rotation in the opposite direction arises when the rotation stops. This phenomenon is known as the somatogyral illusion. A model of the semicircular canal dynamics predicts that the magnitude of this illusion depends on the amount of per-rotary response-decay. This has been extensively studied for rotations about the vertical yaw axis, where the rate of decay is relatively low (due to velocity storage). In roll – where the sense of counter rotation is referred to as the post-roll illusion – the situation is different. Due to the absence of velocity storage the response-decay is faster, and noticeable after-effects may occur after shorter movements than in yaw. This has implications for flying, where short roll movements occur frequently. When unnoticed, the post-roll illusion may trigger the pilot to give erroneous control inputs leading to excessive aircraft bank. Objectives: We hypothesized that the post-roll illusion is determined by both the duration and angular rate of roll motion. We investigated this by studying the effect of different roll stimuli on the control inputs of pilots who actively stabilized the aircraft bank in a movingbase spatial disorientation trainer. Methods: In flight roll manoeuvres are often coordinated, meaning that the gravito-inertial vector is always aligned with the pilot, and the graviceptors do not provide roll cues. When aircraft roll is simulated by simply tilting a simulator relative to gravity this will give the sensation of uncoordinated flight, or sideslip. For that reason, we tilted the simulator backward as to orient the subject in a supine position with his roll-axis being earth vertical. In this situation, simulator rollwas independent of gravity, simulating coordinated flight. Subjects (n = 15) were exposed to six different motion profiles (reference condition, 10◦/s for 12 s, 30◦/s for 2 and 6s, 60◦/s for 2 and 6s) and were instructed to “hold attitude” following the roll movement, without any visual attitude reference. In other words: they had to cancel all perceived cabin motion following the roll movement. Three simulation conditions were investigated. In the first two, subjects were either blindfolded (BLIND) or viewed the interior of the cockpit (COCKPIT; no outside view or instruments) and roll motion was automated. In the third condition (LEAD) subjects actively performed the roll motion by following a lead aircraft that disappeared in the fog (no visual attitude information) after the desiredmovement. The subjects’ control input and simulator movement following each roll movement were recorded. Results: In general, subjects corrected for the perceived counter-rotation by inducing a roll in the same direction as the preceding movement. Effects were smallest in the BLIND condition and largest in the LEAD condition where the pilot was in the loop. The effect increased with roll rate and duration. These results reflected the semicircular canal dynamics and were in accordance with a semicircular canal based motion perception model. Conclusion: The results indicate that the post-roll illusion affected the pilot’s ability to maintain a stable attitude following a roll movement when visual attitude cues are absent. Although the effect is largest with sustained rolling motion, it is also present in shorter movements lasting only 2s. As far as we know, this was the first successful attempt to reproduce the post-roll illusion in a ground-based spatial disorientation trainer. Such demonstration may be useful in demonstrating this effect to student pilots.