Minimal retinal slip is sufficient for peak visual acuity in the fovea

Humans can resolve visual targets beyond the Nyquist limit of photoreceptor sampling, likely because fixational eye movements (FEM) induce optimal retinal signals. The retinal space covered by a visual stimulus and hence the number of photoreceptors involved in a visual task grows with time, as FEM continuously move the retina across the image. Here, we determined the minimum amount of retinal slip required for achieving maximum visual performance in two visual acuity tasks. Using adaptive optics scanning laser ophthalmoscopy-based microstimulation, Tumbling-E and Two-dot stimuli were displayed in the foveola of two experienced and two naive participants. Therefore, resolution and positional acuity were measured and the cone photoreceptors used in the task were made visible. The amount of naturally occurring retinal slip was manipulated by varying stimulus duration (2-600 ms) and by additionally restricting it in one of the viewing conditions using real-time retinal stabilization. As expected, slip amplitudes increased linearly with increasing stimulus duration in all participants, at individual rates. Drift velocity and the covered area varied across participants, both linked to the individuals’ cone density. Across participants, we found maximum acuity between 0.9 and 1.13 of the cone Nyquist limit for resolution and between 0.26 and 0.57 for positional stimuli. Surprisingly, this was achieved after very short presentation times (80 ms), after which thresholds did not improve significantly. Within 80 ms, drift amplitudes ranged from 0.8 - 1.6 arcmin, equivalent to an absolute retinal slip of 1.5 - 2.4 cone diameters, depending on the participant. On average, resolution thresholds were unaffected by retinal stabilization. During short stimulus durations, positional acuity was better when presented retinally stabilized. These results demonstrate that the human visual system can extract spatial information during time frames that do not allow extended motion paths, and that natural motion is not required to reach maximum performance.