CONTRIBUTION OF VESTIBULAR NERVE IRREGULAR AFFERENTS TO VIEWING DISTANCE-RELATED CHANGES IN THE VESTIBULOOCULAR REFLEX

The contribution of irregular vestibular afferents to viewing distance -related changes in the angular vestibule-ocular reflex (AVOR) and com bined angular and linear VOR (CVOR) was studied in squirrel monkeys tr ained to fixate earth-stationary targets that were near (10 cm) and di stant (90-170 cm) from their eyes. Perilymphatic anodal galvanic curre nts were used to reversibly silence irregular vestibular afferents for periods of 4-5 s during the AVOR and CVOR evoked by 0.5- to 4-Hz sinu soidal rotations (6-20 degrees/s peak velocity) or 250-400 degrees/s(2 ) acceleration steps. The direction and magnitude of linear translatio n were changed by positioning the monkeys at different distances off t he axis of turntable rotation. The effects of irregular afferent galva nic ablation (GA) on viewing distance-related changes in the AVOR were studied in four animals. Viewing distance-related changes in the AVOR could not always be evoked and were frequently small in amplitude. GA reduced viewing distance-related change in the AVOR by an average of 64% when it was present. Thus vestibular irregular afferents appear to play an important and necessary role in viewing distance-related chan ges in the AVOR - on those occasions when the changes occur. Viewing d istance-related changes in the CVOR were large and reliably evoked. GA had very little effect on the gain or phase of viewing distance-relat ed changes in the CVOR, although the viewing distance-related CVOR res ponses of individual central vestibular neurons were affected. We conc lude that irregular afferents probably contribute to central signal pr ocessing related to both the AVOR and the CVOR, but the signals carrie d by these afferents are only essential for viewing distance-related c hanges in AVOR.