Stereopsis outweighs gravity in the control of the eyes

The eyes are controlled by multiple brain circuits, some phylogenetically o ld and some new, whose aims may conflict. Old otolith reflexes counterroll the eyes when the head tilts relative to gravity. Newer vergence mechanisms coordinate the eyes to aid stereoptic vision. We show that counterroll hin ders stereopsis, weakly when you look into the distance but strongly when y ou look near. The resolution of this conflict is that counterroll virtually vanishes when monkeys look close, i.e., stereopsis overrides gravity-drive n reflexes but only on near gaze. This balance between gyroscopic and stere optic mechanisms explains many other puzzling features of primate gaze cont rol, such as the weakness of our otolith-ocular reflexes even during far vi ewing and the strange geometry of the primate counterpitch reflex, which ro lls the eyes clockwise when monkeys look leftward while their heads are tip ped nose up, but rolls them counterclockwise when the monkeys look rightwar d, and reverses this pattern when the head is tipped nose down.