Visual resolution decreases rapidly outside of the foveal center. The anatomical and physiological basis for this reduction is unclear. We used simultaneous adaptive optics imaging and psychophysical testing to measure cone spacing and resolution across the fovea, and found that resolution was limited by cone spacing only at the foveal center. Immediately outside of the center, resolution was worse than cone spacing predicted and better matched the sampling limit of midget retinal ganglion cells.
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We thank K. Grieve for her assistance with data collection and P. Tiruveedhula for his help on software development. This work was supported by the National Science Foundation Science and Technology Center for Adaptive Optics under cooperative agreement AST-9876783 managed by the University of California, Santa Cruz and by National Institutes of Health grant EY014375.
AOSLO video showing retinal imagery and stimulus delivery. This video shows a tumbling E stimulus being delivered to the retina of observer S3. Cone photoreceptors appear as bright circles arranged in a roughly triangular lattice pattern. The motion of the retinal mosaic is due to normal fixational eye movements. This video has been processed in the following way: sinusoidal distortion caused by raster scanning was removed10,12,16; the aspect ratio was corrected to be 1:1; the video was cropped to be 0.75° × 0.75°. Although the stimulus appears quite sharp, this is due to the fact that the stimulus is delivered by modulating the imaging beam to be off. What the observer sees is a stimulus that is blurred by diffraction and any residual high-order optical aberrations that exist after adaptive optics correction.
Stabilized AOSLO video. Video 2 shows a stabilized version of video 1. This video was stabilized using custom algorithms20 and illustrates how the normal fixational movements of the eye cause the stimulus to move across several photoreceptors over the course of a one second trial (30 frames). Stabilized videos such as these were averaged to produce the high signal to noise ratio images images which were used to build continuous maps of the photoreceptor mosaic for all individuals across test locations. Motion traces obtained through stabilization were used to determine the precise location of stimuli presented to the retina for resolution testing and for creating stimulation maps such as the one shown in Fig. 1.
Animation showing modeled cone-stimulus interaction. This animation illustrates how cone models were used to examine the interaction between the stimulus and the photoreceptor mosaic. This video shows the cone-stimulus interaction that occurred during the trial shown in videos 1 & 2. This animation only shows a subsection of the area shown in videos 1 and 2; scale bar shows size relations. The left panel shows how the convolved stimulus moved across the cone mosaic, while the right panel shows the simulated cone interactions. The stimulus edges are enhanced (brightened) in the left panel to clearly illustrate where the contrast in the image fell to 50% of maximum. The color bar indicates the relative level of stimulation integrated over the course of the presentation for each cone. The change in color from blue to red as cones are stimulated over the course of the presentation illustrates how cone stimulation maps such as Fig. 1 were created.
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Nature Medicine (2015)