Otolith orientation and downbeat nystagmus in the normal cat

Upward drift of the eyes in darkness, influenced by whole body orientation, was studied in 12 cats using electromagnetic search coil and electro-oculographic techniques. Animals were positioned stationary with respect to gravity with 0° tilt ('upright') or rolled 90° ('on side'), pitched 90° ('on nose' or 'on tail'), or inverted 180° ('upside down'). A downbeat quick-phase nystagmus (slow-phase upward in the cat's orbit) was measured, varying in magnitude with angle of tilt (0.21°/s at 0° tilt; 4.14°/s at 180° tilt). The drift was not present in the light. Upward eye velocities over a range of body orientations in darkness suggest a systematic drive to the eyes which increases with tilt away from upright. The relationship of this behavior to previous models of angular velocity estimation by an otolith-driven central mechanism is discussed.