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In humans, habituation of vestibulo-ocular reflex (VOR) by repeated caloric or rotational stimulation has been well documented. However, less attention has been directed to the effect of habituation on the sensation of self-rotation and little is known about the retention duration of vestibular habituation. To investigate these characteristics, subjects were exposed to ten sessions of angular velocity steps in yaw, with a chair rotating either alternatively in both CW and CCW directions (bidirectional protocol) or always in the same direction (unidirectional protocol), i.e., CW or CCW. The retention of habituation of VOR and sensation of rotation induced by both protocols was studied for a period up to 8 months following the end of the habituation protocols. There was a progressive decline in the VOR peak slow phase velocity and time constant throughout the sessions during both protocols. These parameters then followed an exponential recovery with a time constant of about 1 month. The duration of the sensation of rotation also habituated during repeated angular velocity steps, but it was shorter for both directions of stimulation, including after the unidirectional protocol. Sinusoidal VOR gain was not affected by vestibular habituation to velocity steps, but sinusoidal VOR phase showed an increase in phase lead at 0.02 and 0.04 Hz, which also returned to baseline values within about 1 month. We conclude that vestibular habituation is a long-lasting phenomenon. These results may be helpful for designing and scheduling the protocols for drug studies using crossover design, rehabilitation of balance disorder patients, and for the application of intermittent artificial gravity during space missions. [ABSTRACT FROM AUTHOR]

The effect of stimulus frequency on tilt and translation motion perception was studied during constant velocity off-vertical axis rotation (OVAR), and compared to the effect of stimulus frequency on eye movements. Fourteen healthy subjects were rotated in darkness about their longitudinal axis 10° and 20° off-vertical at 45°/s (0.125 Hz) and 20° off-vertical at 180°/s (0.5 Hz). Perceived motion was evaluated using verbal reports and a joystick capable of recording tilt and translation in both sagittal and lateral planes. Eye movements were also recorded using videography. At the lower frequency, subjects reported the perception of progressing along the edge of a cone, whereas at the higher frequency they had the sensation of progressing along the edge of an upright cylinder. Tilt perception and ocular torsion significantly increased as the tilt angle increased from 10° to 20° at the lower frequency, and then decreased at the higher frequency. The phase lag of ocular torsion increased as a function of frequency, while the phase lag of tilt perception did not change. Horizontal eye movements were small at the lower frequency and showed a phase lead relative to the linear acceleration stimulus. While the phase lead of horizontal eye movements decreased at 0.5 Hz, the phase of translation perception did not vary with stimulus frequency and was similar to the phase of tilt perception during all conditions. A second data set was obtained in 12 subjects to compare motion perception phase when using a simple push-button to indicate nose-up orientation, continuous setting of pitch tilt alone, or continuous setting of tilt and translation in both pitch and roll planes as in the first data set. This set of measurements indicated that in the frequency range studied subjects tend to lead the stimulus when using a push-button task while lagging the stimulus when using a continuous setting of tilt with a joystick. Both amplitude and phase of tilt perception using the joystick were not different whether concentrating on pitch tilt alone or attempting a more complex reporting of tilt and translation in both sagittal and lateral planes. During dynamic linear stimuli in the absence of canal and visual input, a change in stimulus frequency alone elicits similar changes in the amplitude of both self-motion perception and eye movements. However, in contrast to the eye movements, the phase of both perceived tilt and translation motion is not altered by stimulus frequency over this limited range. These results are consistent with the hypothesis that neural processing to distinguish tilt and translation stimuli differs between eye movements and motion perception. [ABSTRACT FROM AUTHOR]

On Earth, eye velocity of horizontal optokinetic nystagmus (OKN) orients to gravito-inertial acceleration (GIA), the sum of linear accelerations acting on the head and body. We determined whether adaptation to microgravity altered this orientation and whether ocular pursuit exhibited similar properties. Eye movements of four astronauts were recorded with three-dimensional video-oculography. Optokinetic stimuli were stripes moving horizontally, vertically, and obliquely at 30°/s. Ocular pursuit was produced by a spot moving horizontally or vertically at 20°/s. Subjects were either stationary or were centrifuged during OKN with 1 or 0.5 g of interaural or dorsoventral centripetal linear acceleration. Average eye position during OKN (the beating field) moved into the quick-phase direction by 10° during lateral and upward field movement in all conditions. The beating field did not shift up during downward OKN on Earth, but there was a strong upward movement of the beating field (9°) during downward OKN in the absence of gravity; this likely represents an adaptation to the lack of a vertical 1-g bias in-flight. The horizontal OKN velocity axis tilted 9° in the roll plane toward the GIA during interaural centrifugation, both on Earth and in space. During oblique OKN, the velocity vector tilted towards the GIA in the roll plane when there was a disparity between the direction of stripe motion and the GIA, but not when the two were aligned. In contrast, dorsoventral acceleration tilted the horizontal OKN velocity vector 6° in pitch away from the GIA. Roll tilts of the horizontal OKN velocity vector toward the GIA during interaural centrifugation are consistent with the orientation properties of velocity storage, but pitch tilts away from the GIA when centrifuged while supine are not. We speculate that visual suppression during OKN may have caused the velocity vector to tilt away from the GIA during dorsoventral centrifugation. Vertical OKN and ocular pursuit did not exhibit orientation toward the GIA in any condition. Static full-body roll tilts and centrifugation generating an equivalent interaural acceleration produced the same tilts in the horizontal OKN velocity before and after flight. Thus, the magnitude of tilt in OKN velocity was dependent on the magnitude of interaural linear acceleration, rather than the tilt of the GIA with regard to the head. These results favor a ‘filter’ model of spatial orientation in which orienting eye movements are proportional to the magnitude of low frequency interaural linear acceleration, rather than models that postulate an internal representation of gravity as the basis for spatial orientation. [ABSTRACT FROM AUTHOR]