In the interaural direction, linear acceleration is loaded during lateral translational motion, and gravity acceleration is loaded during lateral tilting movement. The otolith is a sensory organ that responds to linear acceleration, i.e., the two acceleration types. Otolith induces otolith-ocular reflex. Otolith-ocular reflex is a reflex that produces compensatory eye movements in response to head movement. The eye movements are equal in amplitude and speed to the head movement, but in the opposite direction. According to Einstein‘s equivalence principle, the two acceleration types cannot be discriminated. However, the frontal-eyed animals such as humans and monkeys are typically able to discriminate the two acceleration types perceptually, and act in space accordingly. My study revealed that humans use two types of otolith-ocular reflexes that control the gaze to ensure stable perception of the environment during movement. One type of otolith-ocular reflex induces horizontal eye movement during lateral translational motion and the other type induces ocular counter rolling (OCR) during lateral tilting movement.
My study also revealed that lateral-eyed animals such as mice only exhibited one type of otolith-ocular reflex, OCR, both during translational linear motion and during tilting movement.
The ancestor of modern primates, frontal-eyed shoshonius, evolved from oldest primates, lateral-eyed purgatorius. The progressive frontalization of the eyes resulted in overlap of the left and right visual fields, which led to stereopsis that enabled the perception of depth in the overlapped narrow visual field. Because clear vision during translational motion in humans and monkeys requires stabilization of gaze in space via eye movement to compensate for translational motion, the primitive otolith-ocular reflex, i.e., OCR evolved into an additional type of otolith-ocular reflex that compensates for translational motion, i.e., horizontal eye movement by combining signals from multiple sensory pathways, such as semi-circular canals and the visual system.