Academic journal article Perception and Psychophysics

On the Role of Otoliths and Semicircular Canals in Spatial Orientation: Dynamics of the Visually Perceived Eye Level during Gondola Centrifugation

Academic journal article Perception and Psychophysics

On the Role of Otoliths and Semicircular Canals in Spatial Orientation: Dynamics of the Visually Perceived Eye Level during Gondola Centrifugation

Article excerpt

The visually perceived eye level (VPEL) was measured during gondola centrifugation. Subjects (N = 11) were seated upright, facing motion in a swing-out gondola. The head was adjusted so that Reid's baseline was tilted 10° anterior end up. The subjects were requested to adjust the position of a small luminous dot so that it was perceived as gravitationally at eye level. In the 1-g environment, the VPEL was a few degrees below the true gravitational eye level (M = -1.75°, SD = 1.90°). After rapid acceleration of the centrifuge to 2 G (vectorial sum of the earth gravity force and the centrifugal force), there was an exponentially increasing depression of the VPEL. The initial value was -6.4° ± 5.2°. During 10 min at 2 G, the VPEL approached an asymptotic value of -24.8° ± 5.4°. The time constant showed a large interindividual variability, ranging from 59 to 1,000 sec (M = 261 sec, median = 147 sec). The findings are discussed, taking into consideration otolith-semicircular-canal interaction, as well as memory functions of the vestibular system.

Virtually all perceiving and thinking has a spatial component. Orientation in space denotes the ability of an individual to perceive the position and motion of his or her own body, as well as of objects in the surroundings, with respect to a frame of reference. The ever-present gravity vector provides a direction to our image of the surroundings. The vestibular receptors of the inner ear play an important role in spatial orientation by sensing the head's position with respect to gravity, as well as angular and linear head movements. This idiothetic information makes the perception of the orientation or movement of visual objects relatively independent of the individual's own motor activities. For the study of spatial orientation, it is often useful to consider a simple quantifiable component, such as the perceived vertical or perceived eye level. How such variables are influenced by stimuli to the sense of balance may then shed light on the relationship between different functions of the vestibular system and our ability to orient in space.

The otolith organs (the utricle and the saccule) sense linear accelerations of the head, as well as the gravity force. The effective stimulus is the gravitoinertial shear force component acting in the plane of the macula (Fernandez, Goldberg, & Abend, 1972; Loe, Tomko, & Werner, 1973). This causes a displacement of the otoconial membrane, relative to the sensory epithelium, which will bend the sensory hairs of the receptor cells. The utricular macula is oriented in approximately the same plane as the lateral (horizontal) semicircular canal (Lindeman, 1969), tilted rostrally upward 30° with respect to Reid's baseline (a line from the external auditory meatus to the inferior margin of the orbit; Corvera, Hallpike, & Schuster, 1958). Thus, when the head is held in its natural upright position, the utricle is tilted rostrally upward by approximately 40°. The saccular macula is vertically oriented in the parasagittal plane (Lindeman, 1969). It is elongated, and the length axis is approximately parallel with the plane of the utricular macula (Corvera et al., 1958; Naganuma, Tokumasu, Okamoto, Hashimoto, & Yamashina, 2001).

For any head position with respect to gravity, there will be a specific pattern of shearing forces acting in the planes of the utricular and saccular maculae. For instance, when the head is tilted backward, there is an increase in the posteriorly directed shear force component acting in the plane of the utricular macula, which changes the activity patterns in neural afferents from different regions of the macula. Such vestibular information contributes to the ability to perceive a stationary visual object as keeping its position regardless of changes in head position with respect to gravity.

Many studies on the influence of the vestibular organs on spatial orientation have employed a small luminous target at a straight-ahead position in front of the test subject. …

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