Visual functions of fixational eye movements
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Our eyes are always in motion. Even when we attempt to maintain steady gaze, fixational eye movements, which include ocular drift and microscopic saccades (known as microsaccades or fixational saccades) continually shift the projection of the visual scene onto the retina. Despite much progress during the last decade, many aspects regarding the visual function of these movements remain unknown. This dissertation describes four studies in which fixational eye movements were recorded by means of a high resolution eye tracker to provide answers to the following fundamental questions: (1) What is the visual input to the retina? Data from a large group of untrained observers show that the precision of fixation varies widely across individuals. Furthermore, they show that the line of sight normally covers a much larger area and ocular drift is much faster than commonly assumed. (2) What triggers fixational saccades? Data obtained with a new method for measuring the fixation error clarify that both spatial and temporal factors contribute to the generation of microsaccades. During intended fixation on a point, fixational saccades typically correct for foveation errors, but microsaccades also occur at very small fixation displacements after a prolonged period of time. (3) Are microsaccades controlled in the same way as saccades? The results of adaptation experiments show that microsaccades undergo plastic changes similar to those exhibited by larger saccades, strengthening the hypothesis that microsaccades and saccades are mediated by the same neural structures. (4) What are the mechanisms of visual encoding during fixational eye movements? We examined whether humans are capable of extracting spatial information made available by fixational eye movements in the form of temporal modulations. Results of experiments with precise control of retinal stimulation show that, in the absence of explicit spatial information, fixational modulations are sufficient to establish accurate spatial representation. These results advance the field in multiple important ways. They show that fixational eye movements are under tight oculomotor control and that they contribute to the processing of visual information.