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Martina Poletti, Michele Rucci; Dependence of fixational saccades on the visual task and image fading conditions. Journal of Vision 2007;7(9):21. doi: https://doi.org/10.1167/7.9.21.
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Our eyes are constantly in motion, even during visual fixation, when periods of drifts alternate with small fixational saccades. It is known that visual percepts tend to fade when images are stabilized on the retina. However, it has long been debated whether, during natural viewing, fixational saccades have other functions in addition to preventing the visual scene from fading. In this study, we examined the frequency of fixational saccades under different viewing conditions and tasks. In agreement with the results of previous studies, fixational saccades were highly influenced by the visual task. The same subjects performed virtually no fixational saccades while freely scanning a scene but exhibited, instead, frequent saccades when discriminating the orientation of a grating. We also examined the influence of image fading on fixational saccades. Natural images were displayed for 10 s under 4 different fading conditions: (1) Sustained fixation. (2) Contrast fading; the contrast of the image was progressively lowered during presentation. (3) Frequency fading; the image was low-pass filtered at a progressively lower cut-off frequency during presentation. (4) Retinal stabilization. In conditions 2 and 3, the stimulus changed faster in the periphery than at the center of the visual field to closely resemble natural fading. Artificial fading had little effect on fixational saccades. The rate and characteristics of fixational saccades measured in conditions 2 and 3 were identical to those measured under sustained fixation. Unexpectedly, the rate of fixational saccades was significantly lower under retinal stabilization, a condition which enhanced image fading. Furthermore, in control experiments, fixational saccades were unaffected by increments in image contrast and cut-off frequency, which simulated visibility enhancements. These results do not support a causal relationship between image fading and the production of fixational saccades.
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