VESTIBULO-OCULAR CONTROL IS EVOLUTIONARILY CONSERVED

Clear vision requires gaze-stabilizing eye movements, including the vestibulo-ocular reflex (VOR). VOR is evolutionarily conserved and is already present in the oldest extant vertebrate, the lamprey. However, the characteristics of the lamprey VOR, the dynamic and static gains during rotations about all axes (yaw, roll and pitch), remain unclear. To investigate these aspects systematically, we manufactured a custom-made rotating platform which delivers well-controlled mechanical vestibular stimuli while synchronously recording lamprey eye movements. Recordings were done in darkness using infrared cameras. Animals were rotated about all axes at velocities of 2, 5, 10, 20, 50 and 80 deg/s and accelerations of 10, 250 and 800 deg/s2. Preliminary results showed that the dynamic gain in roll plane was approximately unity at high velocities (50, 80 deg/s) and accelerations (250, 800 deg/s2), and was accompanied by an overshoot in eye velocity at peak rotation velocity. In contrast, dynamic gain was reduced at lower velocities (2, 5, 10, 20 deg/s) and acceleration (10 deg/s2). A similar pattern was observed for static gain in roll. Dynamic and static gains in yaw plane were lower than roll and pitch. However, the response increased progressively with higher velocities and accelerations. These findings demonstrate that the lamprey VOR exhibits axis- and stimulus-dependent tuning, revealing conserved yet specialized mechanisms of gaze stabilization reflecting early evolutionary adaptations of vertebrate vestibulo-ocular control. Additionally, spontaneous low-amplitude saccadic eye movements occurred in lampreys in steady-state in light and dark, suggesting the presence of microsaccades in lampreys, as in mammals.