Effect of macular ablation on vertical optokinetic nystagmus in the squirrel monkey.

After macular ablation in squirrel monkeys, a significant improvement of the slow-phase eye speed of vertical optokinetic nystagmus (VOKN) was found. This findings is in contrast to the result of our previous study in which horizontal optokinetic nystagmus (HOKN) was not improved following macular ablation. The different influence of macular ablation on HOKN and VOKN could be due to the directional characteristics of vestibular and visual interaction relative to the direction of gravity and to the difference in horizontal and vertical vestibulo-oculomotor neural pathways, including vestibular end-organs. The vertical directional dominance which was observed preoperatively was similarly observed in the postablative status; the upward slow-phase nystagmus was easier to provoke than the downward nystagmus by application of an identical stimulus.