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Residual Torsion Following Ocular Counterroll
aNeurology Department, Zurich University Hospital, Switzerland bOphthalmology Department, Zurich University Hospital, Switzerland
Address for correspondence: A. Palla, M.D., Neurology Department, Zurich University Hospital, Frauenklinikstrasse 26, CH-8091 Zurich, Switzerland. Voice: +41-1-255-5500; fax: +41-1-255-4507. antpalla{at}access.unizh.ch
A recent study on static ocular counterroll suggested the existence of residual torsion (RT): when healthy subjects repositioned their head to the upright position after sustained static tilt, eye position differed from the original ocular torsion measured prior to the static head tilt. Our experiments aimed at further characterizing this phenomenon. Using a three-dimensional motorized turntable, healthy human subjects (n = 8) were rotated quasi-statically (0.05 deg/s2, 2 deg/s velocity plateau reached after 40 s) from the upright position about the naso-occipital axis. Three full whole-body rotations were completed while subjects fixed upon a blinking laser dot straight ahead in otherwise complete darkness. Three-dimensional eye movements were recorded with modified dual search coils (wires exiting inferiorly). Torsional position of the right eye at consecutive upright body positions was analyzed. The torsional eye position before the beginning of the chair rotation was defined as zero torsion. On average, the right eye was intorted by 1.3° or extorted by 2.0° after the first full chair rotation in the clockwise or counterclockwise direction, respectively. These torsional offset values of the right eye did not significantly change after the two subsequent full chair rotations. We conclude that RT observed after static ocular counterroll is the result of static hysteresis, that is, a position lag of the eye, which depends on the direction of head roll. The fact that residual torsion did not further increase after the first rotation cycle emphasizes that RT is a static rather than a dynamic phenomenon.
Key Words: vestibulo-ocular reflex • head roll • head tilt • otolith • eye movements This article has been cited by other articles:
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