Fixational eye movements across vertebrates: comparative dynamics, physiology, and perception.

During visual fixation, human eyes are never still. Instead, they constantly produce involuntary "fixational eye movements." Fixational eye movements overcome neural adaptation and prevent visual fading: thus they are an important tool to understand how the brain makes the environment visible. The last decade has seen a growing interest in the analysis of fixational eye movements in humans and primates, as well as in their perceptual and physiological consequences. However, no comprehensive comparison of fixational eye movements across species has been offered. Here we review five decades of fixational eye movement studies in non-human vertebrates, and we discuss the existing evidence concerning their physiological and perceptual effects. We also provide a table that summarizes the physical parameters of the different types of fixational eye movements described in non-human vertebrates.