Comparison of static ocular counter-roll induced by active and passive head tilt and whole body roll

Background: When humans are roll-tilted around the naso-occipital axis, a compensatory torsional rotation of the eyes in the opposite direction occurs known as ocular counterroll (OCR). This is thought to be mainly a vestibular (otolithic) response. The contributions of cervico-ocular reflexes to OCR are still unknown. Moreover, mainly for technical reasons there are still no standardized OCR tests. Therefore, the aim of this study was (1) to compare OCR in response to active and passive lateral head tilts and whole body roll and (2) to develop a test for the evaluation of otolith function which can be used in the daily clinical routine.

Methods: The eye movements were recorded by three-dimensional video-oculography in ten subjects (mean age 27 years +/- 2 years, 2 males) who were instructed to fixate a small dot in the dark. The head of the subjects was tilted 12.5 degrees and 25 degrees respectively to the left and right under the following three conditions: (1) active head tilt, (2) passive head tilt (induced by a motor-driven device), and (3) whole body roll. Statistical analysis was performed with ANOVA for the two parameters position gain (eye position/head position) and velocity gain (eye velocity/head velocity).

Results: ANOVA showed no significant difference for the mean position and velocity gains under the three conditions tested. The mean gain for position was 0.15 +/- 0.05 in the tilt position of 12.5 degrees and 0.13 +/- 0.03 in the tilt position of 25 degrees. Thus, the position gain decreased with increasing head tilt or body roll. The mean gain for velocity was 0.16 +/- 0.06 in the tilt position of 12.5 degrees and 0.20 +/- 0.06 in the tilt position of 25 degrees. The velocity gains were significantly larger than the gains of position.

Conclusion: There were no significant differences in position gain and velocity gain between active and passive head tilts or whole body roll. From a clinical point of view OCR induced by passive head tilt seemed to be most reliable for testing otolith function and will now be evaluated in different peripheral and central disorders.