The vestibular illusions are divided as per the affected sub-system viz. otolith organ or semicircular canals. Though there are many an illusions, only the common vestibular illusions have been briefly discussed.
Vestibular Illusions due to Otolith Organs
Somatogravic illusions. Due to head movement i.e. up, backwards or forward, the otolith organ also remains up or tilts backwards or forward. From past experience we come to know whether we are facing forward, upward or backward due to different stimulus going from the otolith organ inspite of our eyes closed. In the event of otolith organ being pulled backwards or forwards due to inertial forces acting on it during acceleration or deceleration of the aircraft, one gets the sensation of facing upwards or downwards even though he has not moved his head. Thus, one interprets these feelings as if the nose of the aircraft is going up or down even though it is actually not taking place. These illusions are called somatogravic illusions. During flying, sudden acceleration of the aircraft may give climbing sensation and during deceleration, one may get nose down sensation. Correction of this misperception can rapidly cause an accident. Pilots operating on aircraft carrier need to be particularly aware of this illusion as they typically receive impulses of +3 to +5 Gx during a catapult take-off. Usually this sensation is ignored with experience or wherever adequate visual cues are available. However, when both these are lacking, it may lead to problem in maintaining orientation.
Oculogravic illusion. It is the visual analogue of the somatogravic illusion. This illusion results from the same stimulus conditions as the somatogravic illusion. This illusion results in the perception, that instrument panel has moved upward or downward during acceleration or deceleration respectively.
Vestibular illusions due to Semicircular Canals
Somatogyral Illusion. The basis of this illusion is false sensation of self-rotation caused by inability of the semicircular canals to accurately register sustained motion. During angular acceleration (bank to right or left side), the cupula of the semicircular canals deflect to the side opposite to the rotation due to inertia and thus the sensations from the canal inform the brain the speed and direction of the rotation. However, once uniform velocity is reached, the cupula returns to the resting position, hence no information goes to the brain about the rotation. One may feel one is ‘not rotating’ when actually he continues to rotate at a constant velocity. During deceleration (correction of bank), due to movement of the fluid in the canal, the cupula is now deflected in the opposite direction for sometime till the fluid movement stops. The movement of the cupula again informs the brain regarding rotation in the opposite direction even though he has stopped rotating. Thus one gets disoriented during a roll or a spin i.e. one during uniform velocity period and secondly during deceleration period. All these illusions can take place in the absence of visual information available from the ground or horizon. In addition to this, the individual will be affected by nystagmus due to stimulation of vestibular system, which will adversely affect the visual cues thus leading to a serious situation. Various illusions like graveyard spin, graveyard spiral, turning in opposite direction, reversal of roll are of this nature.
Oculogyral Illusion. It is the visual element of somatogyral illusion. In this, there is a false sensation of the motion of an object viewed by a subject undergoing angular motion. The subject perceives that an object, which is actually stable in front, is rotating in the opposite direction and this confirms the perception of the subject’s own rotation.
Coriollis Illusion. It is an abnormal sensation appreciated by the pilots when more than one set of semicircular canals are stimulated simultaneously. It results in a bizarre sensation or sensation of tumbling in the space. For example, if a pilot in a right-sided roll moves his head forward he gets a sensation of yaw to the left, though actually he is in a roll. Accidents may occur due to this sensation.
The Leans. It is the most common vestibular illusion. This phenomenon is basically an illusion of bank when one is straight and level. The roll rate is below that perceptible by the pilot (sub-threshold bank) as predicted by the Mulder’s constant. If the pilot then notices the bank on the instruments, and abruptly returns to straight and level flight, there will be the misperception that the aircraft is banked in the opposite direction. The pilot resolves this situation by leaning in the opposite direction, and flying the aircraft like that. It can thus be understood that leans are a result of resolved conflict and usually do not result in a mishap.
Pressure Vertigo. Due to sudden changes in the middle ear pressure during sudden ascent or descent a turning sensation may be felt due to vertigo.
- Orientation in Aviation
- Orientation in Aviation: Vision
- Orientation in Aviation: Vestibular Apparatus
- Spatial Disorientation: An Introduction
- Spatial Disorientation – Visual Illusions
- Spatial Disorientation: Prevention
1. Ernsting’s Aviation Medicine. Rainford DJ, Gradwell DP (Editors). 4th Edition. Hodder Arnold, London 2006.
2. Fundamentals of Aerospace Medicine. DeHart RL, Davis JR (Editors). 3rd Edition. Lippincott, Williams & Wilkins, Philadelphia 2002.
3. Human Performance & Limitations – JAA ATPL Theoretical Knowledge Manual. 2nd Edition. Jeppesen GmbH, Frankfurt 2001.
Acknowledgement Image courtesy Wikimedia Commons