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Ramon Iovin, Martina Poletti, Fabrizio Santini, Michele Rucci; Visual discrimination during controlled retinal image motion. Journal of Vision 2007;7(9):591. doi: 10.1167/7.9.591.
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© ARVO (1962-2015); The Authors (2016-present)
Our eyes are constantly in motion, even during visual fixation. In a previous study, we found that fixational eye movements improve the discrimination of high spatial frequency stimuli. Here, we describe the results of two experiments which further support the hypothesis that fixational eye movements constitute an effective sampling strategy by which the visual system enhances the processing of fine spatial detail. In a forced-choice discrimination task, subjects reported the orientation (±45°) of an 11 cycles/deg grating embedded in low-frequency noise. To guarantee normal fixational instability, stimuli were displayed for 1s following an initial saccade. Stimuli were modified in real time by EyeRIS, a custom-developed system which processes the eye movements measured by a DPI eye-tracker to enable precise and flexible control of the spatio-temporal stimulus on the retina. In the first experiment, we examined the effect of selectively stabilizing the stimulus along only one axis. We found that performance was impaired, as under complete retinal stabilization, when retinal image motion was restricted to the axis parallel to the grating, but remained unchanged when motion was allowed on the orthogonal axis. In the second experiment, we reconstructed the retinal image motion experienced during the normal occurrence of fixational eye movements by superimposing recorded eye movement on an otherwise stabilized stimulus. We found that passive exposure to retinal image motion was sufficient to reestablish performance. These results provide further evidence for the causal relationship between fixational modulations of luminance and performance. Discrimination is impaired when fixational input modulations do not convey information about the grating. Conversely, performance is normal when these input modulations contain information about the grating's orientation, regardless of whether the subject actively produced retinal image motion by means of fixational eye movements, or was passively exposed to it under conditions of retinal stabilization.
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