The relationship between the decline in cone density and acuity assessed in normal viewing conditions across the central fovea

Even if cone density declines with increasing eccentricity within the 1-degree foveola, acuity across this area is believed to be uniformly constrained by eye optics. Acuity aligns closely with the Nyquist sampling frequency of the cone mosaic only when optical aberrations are corrected. Although the impact of optics across the foveola is uniform, vision, assessed in the presence of physiological optical aberrations, is not uniform within this region. Here we examined the relationship between the rate at which cone density declines and changes in normal acuity across the foveola. Acuity thresholds were determined using an adaptive PEST procedure and a 4AFC visual discrimination task, while subjects' gaze position (N=8) was recorded with a high-precision dDPI eye-tracker to ensure fixation remained within a 5-arcmin region around the center of the display throughout the task. Acuity thresholds were assessed at eccentricities of 0, 10, 15, and 25-arcmin from the fixated point. Subjects’ retinae were imaged at high-resolution with an Adaptive Optics Scanning Laser Ophthalmoscope, and cone density was determined as a function of eccentricity with respect to the preferred retinal locus. We report considerable individual variations in the rate of decline of both acuity and cone density. At all eccentricities, acuity was below the corresponding Nyquist frequency of the cone mosaic. Yet, all subjects showed a substantial drop in acuity (from 20/17 to 20/22 on the Snellen chart) from 0 to 25-arcmin eccentricity, leading to a 33% increase in thresholds. Despite a slight decrease in cone density, for most subjects, visual acuity remained relatively constant from 0 to 10-arcmin. On the other hand, the decline in acuity from 0 to 25-arcmin closely mirrored the rate of cone decline at these eccentricities, suggesting that factors beyond optics play a role in limiting acuity at larger eccentricities in the central fovea.