The absence of so-called compensatory ocular countertorsion: the response of the eyes to head tilt.

OBJECTIVES: To show that so-called compensatory ocular countertorsion (static ocular counterrolling) does not exist and to describe the torsional eye movements that occur while the head is tilting.
METHODS: Two miniature video cameras, a fiberoptic light source, and a fixation target were suspended from a headband. The cameras, fixation target, and light source moved in synchrony with the head. One camera videorecorded iris and conjunctival landmarks, and the other recorded head movement and position. The video frames were digitized and analyzed using computer algorithms.
RESULTS: The eyes showed no compensatory ocular countertorsion in any stabilized head tilt position. During head tilt, periodic torsional eye movements occurred. These movements included a tonic counterlag followed by a saccadic forward torsion that rotated the eyes prior to the head; this was followed by a saccadic countertorsion that realigned and synchronized the eyes with the head, bringing the eyes and head into equilibrium.
CONCLUSIONS: Compensatory ocular countertorsion does not exist. Torsion occurs only during head tilt. The eyes are oriented to the brain and not to the horizon. When the head stabilizes in any tilted position, the retinas assume the same dynamic state of equilibrium with the brain that they assume in every other position. The main function of the oblique muscles is to stabilize the retinas in relation to the brain in all held head positions and directions of gaze. During head tilt, the oblique muscles produce involuntary torsional movements of small amplitude (up to approximately 10 degrees ) that appear to anticipate the final head position and prepare the eyes for obtaining retinal correspondence. These torsional movements may protect the retinas by dampening the effect of the head movement on this sensitive tissue.