BASIC AND CLINICAL ASPECTS OF VERTIGO AND DIZZINESS Effect of Gravity on Vertical Eye Position

There is growing evidence that gravity markedly influences vertical eye position and movements. A new model for the organization of brainstem upgaze pathways is pre-sented in this review. The crossing ventral tegmental tract (CVTT) could be the efferent tract of an “antigravitational ” pathway terminating at the elevator muscle motoneu-rons in the third nerve nuclei and comprising, upstream, the superior vestibular nucleus and y-group, the flocculus, and the otoliths. This pathway functions in parallel to the medial longitudinal fasciculus pathways, which control vertical eye movements made to compensate for all vertical head movements and may also comprise the “gravita-tional ” vestibular pathways, involved in the central reflection of the gravity effect. The CVTT could provide the upgaze system with the supplement of tonic activity required to counteract the gravity effect expressed in the gravitational pathway, being perma-nently modulated according to the static positions of the head (i.e., the instantaneous gravity vector) between a maximal activity in the upright position and a minimal ac-tivity in horizontal positions. Different types of arguments support this new model. The permanent influence of gravity on vertical eye position is strongly suggested b