Eye movements and motion perception induced by off-vertical axis rotation (OVAR) at small angles of tilt after spaceflight

Translational otolith-ocular reflex during off-vertical axis rotation in humans

Two characteristics of otolith-ocular responses - linear vestibulo-ocular reflex and vergence - were examined during constant velocity off-vertical axis rotation (OVAR) in the dark. Sixteen subjects were rotated about their longitudinal axis when tilted 30° relative to the direction of gravity. Rotational velocities were 36 and 288/s corresponding to frequencies of 0.1 and 0.8Hz, respectively. Subjects were asked to imagine stationary targets located at 0.5m, 1m, and 2m in the straight-ahead direction. Binocular eye movements were recorded in the dark using infrared videography. The modulation of horizontal slow phase velocity during OVAR was larger at 0.8Hz than at 0.1Hz, and the modulation at the high frequency was larger for the near target than for the mid and far targets. These characteristics confirm that the horizontal slow phase velocity during yaw OVAR represents a translational otolith-ocular reflex in response to acceleration along the inter-aural axis that is dependent on imagined fixation distance.

Motion sickness induced by otolith stimulation is correlated with otolith-induced eye movements

This article addresses the relationships between motion sickness (MS) and three-dimensional (3D) ocular responses during otolith stimulation. A group of 19 healthy subjects was tested for motion sickness during a 16 min otolith stimulation induced by off-vertical axis rotation (OVAR) (constant velocity 60 degrees /s, frequency 0.16 Hz). For each subject, the MS induced during the session was quantified, and based on this quantification, the subjects were divided into two groups of less susceptible (MS-), and more susceptible (MS+) subjects. The angular eye velocity induced by the otolith stimulation was analyzed in order to identify a possible correlation between susceptibility to MS and 3D eye velocity. The main results show that: (1) MS significantly correlates in a multiple regression with several components of the horizontal vestibular eye movements i.e. positively with the velocity modulation (P<0.01) and bias (P<0.05) of the otolith ocular reflex and negatively with the time constant of the vestibulo-ocular reflex (P<0.01) and (2) the length of the resultant 3D eye velocity vector is significantly larger in the MS+ as compared with the MS- group. Based on these results we suggest that the CNS, including the velocity storage mechanism, reconstructs an eye velocity vector modulated by head position whose length might predict MS occurrence during OVAR.

Tilt and translation motion perception during off-vertical axis rotation

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 degrees and 20 degrees off-vertical at 45 degrees /s (0.125 Hz) and 20 degrees off-vertical at 180 degrees /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 degrees to 20 degrees 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.

[Results of videonystagmographic (VNG) analysis in vestibular post-traumatic pathology]

From February 1998 to July 1999, 569 patients affected by vestibular disorders--following a whiplash injury and/or a cranial trauma--were studied. The neuro-otological examination included a tonal audiometry, brain stem auditory evoked potentials, clinical and instrumental vestibular tests (caloric test, rotatory test, assessment of visually guided eye movements). The study of eye movements was conducted with the support of the Ulmer video-oculographical system. The aim of the research was to analyse, both from a qualitative and a quantitative perspective, the vestibular and clinical findings. Particular emphasis was laid on the incidence of the "up-beating" nystagmus in different categories of patients, all supported by a statistical study. As a result, an etiopathogenetical hypothesis of the nystagmus was advanced, based on altered otolithic and cervical inputs. At the same time, the importance of the video-nystagmographical system was stressed, to the extent that it leads to a standardised and objective analysis, fundamental for clinical and forensic aspects.

Alteration of eye movements and motion perception in microgravity

This review article summarizes the results of space research on eye movements and subjective perception during vestibular stimulation. Inflight and postflight changes in reflex eye movements gain are described for head angular rotation (yaw, pitch, and roll), linear acceleration, off-vertical axis rotation, and optokinetic stimulation. There is evidence that changes in eye movements in microgravity primarily occur for head movements in pitch or roll which normally stimulate the otolith organs on Earth, but the data are not conclusive. The relationship between the eye movements gain and self-motion perception remains to be determined. We advocate the use of a human on- and off-axis rotator combined with the measurements of both tri-dimensional eye movement and perceptual response as a method to systematically investigate the adaptive changes in vestibular function to microgravity.