Fixational eye movements improve visual performance at the sampling limit.

PURPOSE: We explored whether fixational movements of the eye in the human visual system might enhance visual acuity, by effectively scanning the retinal image with its sparse receptive fields, rather than taking a static snapshot of the retinal image with a fixed sampling array.

METHODS: We used an adaptive optics scanning laser ophthalmoscope to test orientation discrimination performance in three normal subjects while simultaneously imaging the underlying cone photoreceptor mosaic. Imaging and psychophysics were done with 840-nm light over a 1.2 deg field at about 1 degree from the foveal center. Stimuli were AO-corrected, negative-contrast tumbling-E optotypes presented randomly at one of four orientations. Stimulus size was held constant, with a gap opening that yielded ~40% correct discrimination performance determined in a preceding acuity experiment (0.57, 0.61, 0.74 arcmin of visual angle for the subjects). At that size and retinal eccentricity, the gap opening was below the retina's Nyquist sampling limit (0.94, 0.90, 0.85 arcmin). Stimulus presentation duration was 750 msec, trial progression was self-paced. Stimuli were presented either retinally stabilized by locking the stimulus to the underlying photoreceptor mosaic, or unstabilized, i.e. habitual retinal image motion was allowed to occur. Discrimination performance is reported based on 500 trials for each viewing condition, all pseudorandomly interleaved.

RESULTS: For all subjects, discrimination performance was significantly worse under stabilized than unstabilized conditions. Performance reductions were 33.6, 13.4, and 21.1% (p = 0.015, 0.046, 0.017, binomial z-test) for each subject.

CONCLUSIONS: Performance in the finest visual acuity tasks for retinal images that are at or below the Nyquist sampling limit seem to be enhanced by the retinal image motion caused by natural fixational eye movements. These findings suggest that fixational eye movements are used to facilitate a form of 'super-resolution' that surpasses the photoreceptor sampling resolution. Meeting abstract presented at VSS 2015.