Ocular counter-rolling during head tilt

When tilting the head towards the shoulder there are numerous mechanisms influencing on the generation and maintenance of ocular counter-roll. Each of these mechanisms has its own temporal and spatial characteristics which influences on the counter-rolled eye position. We described a fast transient anti-compensatory torsion movement (torsion peak) as the first response to the head tilting which most certainly has its origin in the vestibular organ and the utricular maculae. Superimposed on this torsion peak we described nystagmus beats with the fast phase directed in the same direction as the head tilt movement suggesting a paralleled activation of the vertical semicircular canals. Synchronous with the torsion peak a fast vertical vergence eye movement was seen. The vertical vergence was always with left eye over right eye in the rightward head tilt and always with right eye over left eye the leftward head tilts, thus inducing a physiological skew deviation. The aetiology for this fast vertical vergence response is currently unknown but could probably be explained by a similar vestibular mechanism as for the torsion peak. A substantial amount of ocular counter-rolling (OCR) was a consistent finding in all subjects and test conditions during static head tilt. The OCR increased with head tilt but the relative compensation to the amount of head tilt decreased. For example, a 15° head tilt induced 3° OCR which corresponds to a gain of 0.20. A 30° head tilt induced an even larger OCR (5°) which corresponds to a gain of 0.16. A consistent finding was an OCR disconjugacy of the right and left eye which increased with head tilt. For instance, a rightward head tilt induced a larger ex-cyclo of the left eye than incyclo of the right eye leading to an ex-cyclovergence. The underlying mechanism might be the increased saccular impact in extreme head tilts. Sacculus has been suggested to induce disconjugate OCR while utriculus is thought to induce conjugate OCR. The static head tilt induced a vertical disconjugacy (i.e. vertical vergence) that increased with the head tilt. The direction of the eye position was however not consistent. Some subjects demonstrated a right eye over left eye position in the rightward head tilts while others demonstrated the opposite outcome. This might be explained by a difference in the ocular visual and torsional axes. To maintain binocularity the eyes are forced into vergence and depending on the position of the axes the direction of the vergence movement will differ. When holding the head in a tilted position the torsional position is found to drift. Initially this drift was directed away from the reference position thus increasing the amount of OCR. After a minute the drift was found to change direction and heading towards the reference position, thus decreasing the OCR. The OCR increasing drift might be explained by a decline in the leftward utricular discharge, induced by macula inertia during the initial interaural translation, in favour of the rightward utricular gravitational discharge. The OCR decreasing drift might be explained by an adaptation of the utricular maculae. Other possible explanations might be a leaky neural integrator or a vestibular memory loss which could induce the OCR decreasing drift found during a sustained head tilt.