Fixational instability impedes visually-guided behaviors in patients with amblyopia

Coordinated binocular fixation is required for single perception of objects at high resolution. During fixation, fixational eye movements (FEMs) are not only crucial for enhancing visual acuity, but also for optimizing goal-directed behaviors. Compromised FEMs result in visual impairments, e.g., in visual acuity, CSF, diplopia, etc. Such failures are detrimental for tasks that require fine spatial vision (i.e., visual search). Although the frequency of FEMs increases significantly with increasing search demand tasks, it is not clear how abnormal FEMs contribute to visually-guided behaviors in complex visual search. In addition, with larger fixational instabilities (i.e., amblyopia), it is not clear whether the two eyes behave similarly in a conjugate fashion. Here, we set out to determine: 1) the relationship between dominant (fellow) and non-dominant (amblyopic) eyes across various oculomotor characteristics, and 2) the role of FEMs on subsequent visually-guided behavior during visual search. Neurotypical and amblyopic participants viewed naturalistic stimuli – a filtered version of “Where’s Waldo” – while performing a visual search task with varying levels of crowding. Amblyopic participants showed slower FEMs and saccade latencies with the amblyopic eye compared to the fellow eye, but neurotypicals only showed the same trend on FEMs. In addition, fixational instability was different between the two eyes in both populations. Furthermore, larger fixational instability was significantly correlated with a decrease in the first saccade landing accuracy at the peripheral target in both groups. Importantly, saccade landing accuracy improved with a reduction in crowding at the target. While fixational instability was not correlated with reaction times (RTs), decrease in crowding also decreased RTs in both populations. Binocular viewing significantly reduced the difference in fixational instability between the two eyes in both neurotypical and amblyopic participants. Our work demonstrates that fixational instability and binocular viewing conditions play an important role in subsequent oculomotor behavior.