Torsional nystagmus and otolith dumping effects investigated by head circumduction.

Abstract

This study introduces a novel paradigm to induce the torsional vestibulo-ocular reflex (VOR) using head circumduction-a circular motion of the head combining extension, lateral rotation, and flexion of the neck. The aims were to 1) evaluate the reliability of this maneuver in generating torsional nystagmus and 2) explore the impact of postrotational head tilt on induced VOR responses. Fourteen healthy participants (age = 27.3 ± 5.3 yr) were tested using a three-dimensional (3-D) eye-tracker to record eye movements induced by head circumduction, performed at a frequency of 0.75 Hz. Participants were recorded on stopping in either a head-up or head-down condition. All participants showed robust postrotational torsional nystagmus. However, this was significantly shorter during head up (average duration = 10.7 ± 2.4 s, with a time constant of 4.1 ± 1.1 s) compared with head down (average duration = 15.7 ± 3.7 s, with a time constant of 7.2 ± 2.5 s; P = 0.0001). Vertical nystagmus was also observed in most participants, which was either disconjugate or overtly skewed. The shortening of torsional nystagmus duration and time constant in the head-up position supports 1) a role for the velocity storage mechanism in the torsional VOR (which was previously disputed) and 2) the existence of otolith dumping effects in the torsional VOR. In addition, the vertical ocular findings during the stopping response confirm that skewed eye movements can be generated by vertical semicircular canal activity. Our findings support the feasibility of head circumduction as a simple method for assessing semicircular and otolith effects on the torsional VOR.NEW & NOTEWORTHY Of all the triaxial vestibulo-ocular reflexes, torsional is the least studied for the need of heavy-duty rotational equipment. We introduce a simple self-paced paradigm to study the torsional vestibulo-ocular reflex-head circumduction, which produced in all participants stopping vestibular torsional nystagmus when circumduction was stopped. Forward head tilting significantly prolonged the postrotational torsional nystagmus duration-an "otolith dumping effect" that suggests, for the first time in humans, a role of the velocity storage mechanism in the torsional vestibulo-ocular reflex.