Spatial Orientation: The Basics, Part II

 

By Dick Leland, President of NASTARSM Center

In the last installment of Spatial Orientation, (available in the NASTAR Newsletter Archive; please click the link to the left) we examined the physiology of the visual system. This month we take a look at the vestibular system.

The vestibular system consists of the semicircular canals and the Otolith organs, which are located in the bony labyrinth (a hollow area of the temporal bone), commonly referred to as the inner ear. These structures are responsible for balance, postural stability, and ocular tracking. Information perceived by the vestibular system is processed by the preconscious brain; therefore, it is not subject to validation. Flight illusions attributable to the semicircular canals include somatogyral illusions (eg, leans, graveyard spin, graveyard spiral), coriolis, nystagmus, and the oculogyral illusions. Illusions attributable to the Otolith organ are somatogravic (eg, pitch up / pitch down, inversion illusion, G-excess effect) and oculogravic illusions.

Semicircular Canals

When the pilot is seated in the cockpit, the semicircular canals are roughly aligned with the aircraft axes of yaw, pitch, and roll and will sense angular accelerations that are greater than 1.5 to2.0 degrees per second. The canals are filled with endolymph, a fluid that moves in response to angular accelerations. The movement in the endolymph causes a structure called the cupula to deviate, which in turn causes the hair cells or cilia to move. The movement of the cilia is interpreted by the preconscious brain as angular acceleration.

Otolith Organs

The Otolith organs consist of structures called the utricle and saccule and are located in a bony cavern of the inner ear called the vestibule. Lining the bottom of the utricle and the medial wall of the saccule are the macula, which are small patches of hair cells. These hair cells project into the Otolithic membrane, a gelatinous membrane that contains calcium crystals called Otoliths. This arrangement allows the Otolith organs to respond to linear accelerations. Normally, the Otolith organ uses gravity to detect changes in head position. For example, when a person rotates their head forward, gravity causes the Otolithic membrane to move forward from its resting position. This in turn causes the hair cells to bend, which is interpreted by the preconscious brain as forward head position. Similar processes occur for aft and side to side tilts. However, in a variable-G environment, especially in the absence of adequate visual cues, Otolith-organ movement due to changes in acceleration may be erroneously interpreted as changes in head position.

Next month: Spatial Disorientation Defined
 
 

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