Compensatory cycloversion in response to stimulus rotation assessed by binocular video-oculography
H. D. Schworm1,2, T. Pansell1, J. Ygge1, G. Lennerstrand1
Purpose: There is no doubt about the existence of ocular counter-rolling compensatory to head tilt. The purpose of the present study was to find out whether compensatory cycloversion can be evoked by tilting of the stimulus while the head being fixed in upright position.
Methods: In 10 healthy subjects, binocular cycloduction was assessed using the so-called three-dimensional video-oculography (3D-VOG, SMI, Germany) during stepwise tilting of a photographic picture consisting of multiple spatial clues and a superimposed grid pattern. The stimulus was rotated around a central fixation spot by 15, 30 and 45 degrees both to the right and to the left. During the experiment, the subject's head was immobilized by means of a stable chin rest, a bite bar, and a neck rest.
Results: Tilting of the stimulus induced a true stable compensatory binocular torsional eye movement towards the same direction as the stimulus. In all subjects, compensatory cycloversion increased with the amount of stimulus rotation. The average amplitude of cycloversion at a stimulus tilt of 45 degrees was 2 to 3 degrees. Independent of the order of the paradigm, the response to the first direction of stimulus rotation (e.g. to the right) evoked less compensatory cycloversion than that to the second one (e.g. to the left).
Conclusion: Our study demonstrated that compensatory cycloversion in response to rotation of the stimulus during fixed head position occurs. Compared to ocular counter-rolling evoked by head tilt, the response to stimulus rotation was smaller and compensated for only a minor portion of the tilt. It remains unclear why the response was more pronounced in the second part of the paradigm than in the first. Our results suggest an influence on compensatory cycloversion not only by the vestibulo-ocular reflex but also by visual input.
1Karolinska Institutet, St. Eriks Eye Hospital, 11282 Stockholm, Sweden; 2University Eye Hospital, Mathildenstr. 8, 80336 Munich, Germany