Spatial Organization of the Maculo-Ocular Reflex of the Rat: Responses During Off-Vertical Axis Rotation

Pigmented, head restrained rats were rotated on a turntable about a tilted axis (off-vertical axis rotation; OVAR) in darkness. Evoked eye movements in the horizontal, vertical and torsional planes were recorded simultaneously with a dual search coil in a magnetic field, horizontal response components of both eyes were recorded with a coil on either eye. OVAR resulted in a persisting horizontal, unidirectional ocular nystagmus, compensatory in direction for the rotation of head in space. Superimposed upon this nystagmus were slower cyclic responses of the eye in the vertical and torsional movement planes, that were tightly phase locked with changing head positions in space: ocular depression/elevation with right ear up/down and ocular intorsion/extorsion with nose up/down. Simultaneous recordings of horizontal response components from both eyes revealed phase and gain differences between the horizontal movement components of both eyes, that resulted in a cyclic modulation of the vergence angle. Convergence of the lines of sight during nose up and divergence during nose down, adequate compensatory responses in light for changes in the viewing distance, were actually observed in darkness. Thus the utricular maculo-ocular reflex takes part of the visual consequences of a translational gaze shift into account. It reduces expected retinal disparities by appropriate and rapid vertical, torsional and vergence response components in the same way as canal-ocular reflexes 'compensate' for direction and velocity of expected retinal image slip during head rotation.