Individual larvae of the zebrafish mutant belladonna display multiple infantile nystagmus-like waveforms that are influenced by viewing conditions.

PURPOSE. Infantile nystagmus syndrome (INS) is characterized by involuntary eye oscillations that can assume different waveforms. Previous attempts to uncover reasons for the presence of several nystagmus waveforms have not led to a general consensus in the community. Recently, we characterized the belladonna (bel) zebrafish mutant strain, in which INS-like ocular motor abnormalities are caused by misprojection of a variable fraction of optic nerve fibers. Here we studied intrinsic and extrinsic factors influencing the occurrence of different waveforms in bel larvae. METHODS. Eye movements of bel larvae were recorded in the presence of a stationary grating pattern. Waveforms of spontaneous oscillations were grouped in three categories: ‘‘pendular,’’ ‘‘unidirectional jerk,’’ and ‘‘bidirectional jerk,’’ and the occurrences of each category were compared within and between individual larvae. Moreover, the effects of the characteristics of a preceding optokinetic response (OKR), of the field of view, and of the eye orbital position were analyzed. RESULTS. The different waveform categories co-occurred in most individuals. We found waveforms being influenced by the preceding OKR and by the field of view. Moreover, we found different kinds of relationships between orbital position and initiation of a specific waveform, including pendular nystagmus in a more eccentric orbital position, and differences among jerk oscillations regarding the beating direction of the first saccade or waveform amplitude. CONCLUSIONS. Our data suggest that waveform categories in bel larvae do not reflect the severity of the morphological phenotype but rather are influenced by viewing conditions.