The effects of pitch tilt on postrotatory nystagmus in cats.

When the head is tilted away from the upright position immediately after termination of earth-vertical axis (EVA) head rotation at a constant velocity, the rotation axis of postrotatory nystagmus (PRN) is gradually directed toward gravity (spatial reorientation). During roll tilt, the extent of the axis shift varies among species. In cats, the reorientation is limited to about 30-45 degrees of head tilt. In the present study, we examined features of PRN during pitch tilt in cats. The animals were rotated about EVA at a constant velocity of 100 degrees /s for 120 s. Within 2 s after stopping EVA rotation, the animals were tilted toward the pitch plane by as much as +/-90 degrees (nose-down or nose-up) in steps of 15 degrees. Eye movements were measured with 3D magnetic search coils. The angle of the PRN plane and its slow phase eye velocity (SPV) were measured. The duration of PRN decreased as pitch tilt increased regardless of whether the direction was nose-up or nose-down. The mean time constant of SPV was significantly longer for nose-up than for nose-down. PRN maintained its horizontal orientation without any vertical drift during pitch tilt, but there was little reorientation toward gravity. That is, no significant torsional component of SPV emerged to reorient the eyes according to gravity. Our results indicate that spatial reorientation depends on head orientation. For the PRN responses after pitch tilts in cats, the interactions of semicircular canal and otolith inputs in the central vestibular system might be vestigial.