Effect of off-vertical tilt and macular ablation on postrotatory nystagmus in the squirrel monkey.

The slow phase eye velocity (SPEV) and duration of post-rotatory nystagmus (PRN) were studied in squirrel monkeys (Saimiri sciureus) after a ramp speed rotation (0-200 degrees/sec, with 1 degree/sec2 angular acceleration). When the results were compared between straight upright vertical rotation, 9 degrees tilt rotation, and 18 degrees tilt rotation, faster decay both in SPEV and in duration was found in the tilt rotation situations. Difference in nystagmic decay curves by tilting rotation axis could be from the convergence of macula-semicircular canal inputs. Subsequently bilateral macular ablation (two-stage) was performed. The difference in nystagmus decay curves between three different rotations was reduced; therefore, the change of gravity direction perceived through gravity receptors other than macular endorgans was minimal and did not produce a difference in three different rotations.