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Janis Intoy, Michele Rucci; The Role of Microsaccades in the Snellen Acuity Test. Journal of Vision 2017;17(10):920. doi: https://doi.org/10.1167/17.10.920.
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Standard tests of visual acuity are commonly taken to measure limits in visual resolution imposed by the optical and physiological properties of the eye and retina. However, previous studies have shown that performance in high-acuity tasks also depends on fixational eye movements (FEM), the small eye movements that humans perform even when attending to a selected location. FEM contribute both by transforming spatial information into temporal transients [Rucci et al., 2007] and by precisely positioning a preferred retinal locus on the stimulus of interest [Ko et al., 2010]. These previous findings suggest that the results of standard tests of visual acuity also depend upon oculomotor activity. Here we investigated the function of microsaccades in the Snellen eye-chart test, the most common test of visual acuity. We recorded eye movements in human observers as they identified the optotypes of the 20/20 line of a tumbling-E eye chart designed to fit within the foveola, the small region (~1o in diameter) of the retina with the highest cone density. Eye movements were recorded with a Dual Purkinje Image eye-tracker, a system with high spatial and temporal resolution. We show that microsaccades are tuned to the task during the Snellen test. Microsaccades are smaller and slightly less frequent than during sustained fixation and are used to precisely shift gaze across optotypes. When optotypes were stabilized on the retina by counteracting the consequences of eye movements, their size had to be increased by 0.15 logmar to recover the normal level of performance. These findings demonstrate that the outcome of a Snellen test incorporates contributions from fixational eye movements and that visual acuity deficits may originate from poor oculomotor control.
Meeting abstract presented at VSS 2017
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