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Chiara Listorti, Martina Poletti, Michele Rucci; Non-uniform vision within the fovea might explain microsaccade production. Journal of Vision 2011;11(11):561. doi: 10.1167/11.11.561.
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We recently reported that, during execution of a visuomotor task, microsaccades redirect the preferred retinal location between stimuli at distances smaller than the size of the fovea (Ko et al., Nature Neurosci. 2010). A possible explanation for this finding is that vision is not uniform across the fovea, and microsaccades are used to relocate the stimulus on the optimal retinal region for the given task. Although visual acuity is commonly assumed to remain constant within the fovea, anatomical examinations of the foveal region have in fact revealed changes in neuronal characteristics. Here we studied visual discrimination with stimuli located at different eccentricities within the fovea. In a forced-choice discrimination task, subjects reported if two successively presented gratings had the same or opposite orientation. Gratings were viewed through a 5′ × 20′ rectangular aperture, which was centered at 4′, 9′ or 14′ from the preferred retinal location and displayed first to its left and then to its right. Stimuli were either observed normally or under retinal stabilization. In this latter condition, retinal image motion was eliminated so that the stimulus remained at a fixed retinal eccentricity. Under retinal stabilization, discrimination performance at all eccentricities should be approximately equal if vision was homogeneous. In contrast, we found that discrimination percentages with stimuli at 9′ and 14′ were significantly lower than at 4′. Percentages were instead comparable at all eccentricities in the normal condition, when eye movements normally moved the retinal stimulus. In this condition, microsaccades systematically relocated the gaze from one grating to the other significantly improving performance at 9′ and 14′. These results suggest that vision is not uniform across the fovea and that, in highly demanding task, the need exists for precisely relocating the stimulus within the foveal region.
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