Vertical Optokinetic Eye Movements in the Larval Zebrafish

The optokinetic response (OKR), a reflex enabling stable visual processing by minimizing retinal slip, has been well characterized in fish over the last decades, leading to insights and a better understanding of the underlying neuronal control and circuitries. However, only the horizontal component of the OKR has been investigated so far, although the optokinetic response is not limited to the horizontal plane, as it has also been observed vertically and even torsionally in other species. In this study, we characterize the vertical optokinetic response (vOKR) in larval zebrafish and compare it to the horizontal OKR (hOKR) and the vertical vestibulo-ocular reflex (vVOR). Two custom-built experimental setups, allowed for controlled vestibular stimulation alongside precise eye tracking and simultaneous recording of both horizontal and vertical eye movements during visual stimulation. Our findings reveal a distinct vOKR in larval zebrafish, but with a much smaller dynamic range compared to the hOKR and without any quick phases (resetting saccades). When presented with constant roll-rotating visual stimuli, zebrafish exhibit a brief initial vertical eye rotation in the direction of the stimulus, followed by a period with no further rotation but interspersed with only spontaneous saccades. This behavior contrasts sharply with the periodical occurrence of resetting saccades during hOKR. Despite its limited dynamic range, the initial vertical response is tuned to similar spatial frequencies and angular velocities as the hOKR. Intriguingly, zebrafish are capable of large vertical eye movements during the vVOR, suggesting that the restricted dynamic range of the vOKR is not due to inherent motor limitations. While it is unclear whether the observed differences in vertical versus horizontal optokinetic control have an adaptive value for zebrafish, the identified differences are drastic and informative for further studies on oculomotor circuits.