The otolith organs ambiguously transduce linear forces due to both tilt and translation. The ambiguity is thought to be resolved by frequency-selective processing, with low-frequency stimuli conveying tilt, and high-frequency conveying translation. For low frequency or prolonged accelerations along the naso-occipital (NO) axis, this processing results in the sensation of pitch, and a corresponding illusory movement of the horizon (a form of """"""""elevator illusion""""""""). To investigate this illusion in humans, centripetal accelerations produced by a rotating sled were presented to human subjects. Subjects were rotated at 127 /s at zero eccentricity until angular vection and nystagmus ended, and then were slowly oscillated over B140 cm eccentricity (peak centripetal acceleration of 0.25 g) at frequencies between 0.005 and 0.025 Hz, resulting in an effective dynamic displacement of the gravity vector (effective 14 pitch tilt). A monocularly viewed laser spot was projected onto a head-fixed screen in a dark room, and subjects maintained this spot on the perceived horizon using a joystick to control the vertical position. Alternatively, subjects were instructed to keep their eyes on the horizon in complete darkness without the presence of the laser spot, while eye movements were recorded. During stimulation, subjects perceived a tilt and an associated illusory movement of the horizon, and responded by smoothly adjusting the laser spot or gaze vertically to compensate. The responses at lower frequencies were larger than at higher frequencies. Phase usually lagged the stimuli, and more so for the higher frequencies of stimulation. These findings demonstrate a low-pass characteristic of tilt perception and reflex eye movements in response to roll tilt, analogous to torsional responses to roll tilt. A related but more complete assessment of the tilt-translation parsing problem in the LVOR was performed in squirrel monkeys. Both true tilt and translational accelerations were used, over a broad frequency bandwidth (0.01D4.0 Hz). The unique properties of our rotating linear sled were required to accomplish this task. We demonstrated that both horizontal and torsional responses occur in response to both true roll tilt and interanral translation, reflecting the physical ambiguity of otolith response properties (they respond to all forms of acceleration equally). However, the horizontal responses operate with high-pass characteristics while the torsional responses operate with low-pass characteristics. This confirms our contention that the brain parses otolith input into tilt and translational functions through simple frequency-dependent filtering. Modeling efforts have illustrated that all VOR response characteristics can be well simulated by a straightforward linear systems model with simple dynamic elements in a canal-driven AVOR pathway, and two otolith-driven pathways, a high-pass translational LVOR, and a low-path tilt LVOR.

National Institute of Health (NIH)
National Center for Research Resources (NCRR)
Biotechnology Resource Grants (P41)
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