Uvula-nodulus and gravity direction (a study on vertical optokinetic-oculomotor functions)

Some characteristics of vertical optokinetic nystagmus and after-nystagmus, in particular, its up/down gain asymmetry, are known to be influenced by gravity direction, which is basically perceived by otolith organs. However, how such an information of gravity direction is processed within the central nervous system is not well understood, even though the role of cerebellar uvula and nodulus in vertical eye movements has been identified for some time. Accordingly, in this study, vertical optokinetic nystagmus and after-nystagmus were studied in squirrel monkeys both in the upright and side-lateral positions (different gravity direction to the head), before and after uvulonodulectomy. When the optokinetic stimulus was given, the slow phase eye velocity of optokinetic nystagmus was significantly higher in the side-lateral position than that obtained in the upright position. This difference disappeared after uvulonodulectomy. When the initial slow phase eye velocity of optokinetic after-nystagmus was compared, the one obtained in the side-lateral position was significantly higher than that acquired in the upright position. This difference also disappeared after the uvulonodular ablation. Thus, in this study of vertical optokinetic nystagmus and after-nystagmus, the role of uvula and nodulus in relaying the information of gravity direction was confirmed.