Evaluation of the vestibulo-ocular reflex in head-tilt mutant mice

The vestibulo-ocular reflex (VOR) is an involuntary reflex essential for maintaining clear vision and balance during head movement. It stabilizes gaze by generating compensatory eye movements in the opposite direction to head movement, ensuring that the visual image is stable on the retina. Maintaining visual stability leads to enhanced postural control and balance, which are crucial for coordinating daily activities, such as exercise and driving. The two types of balance organs, semicircular canals and otolith organs, are involved in the VOR; however, how they interact to contribute to the VOR remains unclear. This study aimed to investigate the interrelationship between semicircular canals and otolith organs in terms of balance function by performing semicircular canal and otolith organ function tests using head-tilt (Het) mutant mice that lack otoconia. Linear VOR results indicated that eye movements induced by linear acceleration stimuli were not observed in Het mutant mice, regardless of the degree of gravitational acceleration. Angular VOR was observed in Het mutant mice at 1 and 2 Hz, but a reduced VOR gain was observed at 0.3 Hz. In wild-type mice, both the linear and angular VOR were normal. The present study demonstrated that in Het mutant mice, the angular VOR decreased after 0.3-Hz semicircular canal stimulation, but normal responses were observed after 1- and 2-Hz semicircular canal stimulation. These results suggest that otolith organs are closely involved in the angular VOR during slow rotational stimulation.