Behavior of the human translational vestibulo-ocular reflex during simultaneous head translation and rotation.

Background: During head translations, vestibular eye movements are ∼ 60% of those required to hold the line of sight on target but, during translation of the orbits due to head rotation about an eccentric axis, the eyes are held %eye on target.

Objective: To resolve this paradoxical behavior of vestibulo-ocular reflexes.

Methods: Subjects sat on a moving platform viewing a near target and were: (1) rotated en bloc in yaw about a vertical axis centered on the head at 1 Hz; (2) rotated with their head displaced ∼ 10 cm anterior (eccentric rotation) at 1 Hz; (3) translated along the inter-aural axis at 1.9 Hz; (4) rotated with the head centered at 1 Hz while they were translated along the inter-aural axis at 1.9 Hz. We calculated compensation ratio (CR): Eye velocity/eye velocity geometrically required to hold the eye on target.

Results: During yaw, mean CR was 0.88 and during eccentric rotation CR was 0.93. During translation at 1.9 Hz, CR was 0.65. During combined rotation at 1.0 Hz and translation at 1.9 Hz, CR was 0.81 for head rotations and 0.74 for head translations.

Conclusions: Translations of the orbits due to head rotation are better compensated for than translations of the orbits due to head translation. These different behaviors may be determined by context, the important difference being whether the subject is moving through the environment.