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 teleosts over the last decades. Although previous work on teleost OKR mostly focused on its horizontal component, mammals are known to perform vertical and torsional OKR in addition to horizontal OKR. In this study, we characterized the vertical optokinetic response (vOKR) in larval zebrafish and compared it with the horizontal OKR (hOKR) and the vertical vestibulo-ocular reflex (vVOR). Our simultaneous camera-based tracking of vertical and horizontal eye positions revealed a distinct vOKR in larval zebrafish, but with a much smaller dynamic range compared with the hOKR and without any quick phases (resetting saccades). When presented with constant roll-rotating visual stimuli, zebrafish exhibited a brief initial vertical eye rotation in the direction of the stimulus, followed by a period without further slow phase response and interspersed with only spontaneous saccades. This behavior contrasts sharply with the periodical occurrence of resetting saccades (quick phases) during hOKR. The initial vertical response is tuned to similar spatial frequencies and angular velocities as the hOKR. We furthermore showed that the vVOR has a much larger vertical dynamic range than the vOKR, demonstrating that the neuronal circuitry itself - and not the oculomotor plant - is the limiting factor. Although 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 visuomotor circuits in teleosts.