Fixational eye movements in normal and pathological vision

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Abstract

Most of our visual experience is driven by the eye movements we produce while we fixate our gaze. In a sense, our visual system thus has a built-in contradiction: when we direct our gaze at an object of interest, our eyes are never still. Therefore the perception, physiology, and computational modeling of fixational eye movements is critical to our understanding of vision in general, and also to the understanding of the neural computations that work to overcome neural adaptation in normal subjects as well as in clinical patients. Moreover, because we are not aware of our fixational eye movements, they can also help us understand the underpinnings of visual awareness. Research in the field of fixational eye movements faded in importance for several decades during the late 20th century. However, new electrophysiological and psychophysical data have now rejuvenated the field. The last decade has brought significant advances to our understanding of the neuronal and perceptual effects of fixational eye movements, with crucial implications for neural coding, visual awareness, and perception in normal and pathological vision. This chapter will review the type of neural activity generated by fixational eye movements at different levels in the visual system, as well as the importance of fixational eye movements for visual perception in normal vision and in visual disease. Special attention will be given to microsaccades, the fastest and largest type of fixational eye movement.

Section snippets

Fixational eye movements in normal vision

Eye movements during fixation are necessary to overcome loss of vision due to adaptive neural mechanisms that normalize responses across neurons in the face of unchanging or uniform visual stimulation. Thus, the goal of oculomotor fixational mechanisms may not be retinal stabilization, but rather controlled image motion adjusted so as to overcome adaptation in an optimal fashion for visual processing (Skavenski et al., 1979). In the early 1950s, it was shown that all eye movements could be

Abnormal eye movements during fixation

Impaired fixational eye movements are observed in patients with a variety of central and peripheral pathologies (Shakhnovich and Thomas, 1974, Shakhnovich and Thomas, 1977; Ciuffreda and Tannen, 1995). Although we spend about 80% percent of our waking lives fixating our gaze, the contribution of impaired fixational eye movements to vision loss is generally overlooked. This gap in knowledge has prevented the field from developing new treatments and early diagnostic tools to ameliorate those

Conclusions

Approximately 80% of our visual experience happens during fixation. During the other 20% of the time we are virtually blind, due to saccadic suppression mechanisms. Therefore, understanding the neural and perceptual effects of fixational eye movements is crucial to understanding vision.

Fixational eye movements were first measured in the 1950s, but sometime during the 1970s, the field arrived at an impasse due to difficulties in data collection, discrepancies in results by different

Abbreviations

    LGN

    lateral geniculate nucleus of the thalamus

    RF

    receptive field

    V1

    primary visual cortex

Acknowledgments

Drs. R. John Leigh, Stephen Macknik, and Xoana Troncoso read the manuscript and made helpful comments. I am very grateful to Dr. R. John Leigh for graciously providing Fig. 9, Fig. 10, and for his insights and discussion of slow saccades, square-wave jerks, and many other concepts addressed here. Thanks also to Dr. David Sparks for helpful discussion and for pointing me to the Van Gisbergen and colleagues’ studies on the generation of microsaccades. Thomas Dyar and Dr. Xoana Troncoso helped

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