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Nicole M. Putnam, Pavan Tiruveedhula, Austin Roorda; Characterization Of The Preferred Retinal Locus Of Fixation And The Locus Of Perceived Fixation In Relation To The Photoreceptor Mosaic. Invest. Ophthalmol. Vis. Sci. 2011;52(14):4461.
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© ARVO (1962-2015); The Authors (2016-present)
To use Adaptive Optics Scanning Laser Ophthalmoscopy (AOSLO) to investigate the retinal loci responsible for steady fixation (preferred retinal locus or PRL) and relate it to the precision with which we can correctly identify the direction we are looking relative to briefly flashed stimuli near the foveal center (perceived fixation direction or PFD). Previous studies have investigated these loci, but technical limitations prevented their rigorous correlation with measures of the underlying photoreceptor mosaic.
AOSLO has the ability to simultaneously present a stimulus while recording a video of microscopic structures in the underlying retina such as the cone photoreceptors. For this study we imaged both eyes of three subjects on three different days. All AOSLO imaging and psychophysics were done with 840 nm light, which provided sufficient brightness for psychophysics and enough power for high resolution adaptive optics imaging. We first recorded high-resolution images over a 0.7 deg field to visualize most of the foveal cones. We then performed simultaneous imaging and psychophysics over a 2.1deg field to measure the PRL and PFD. The PRL was measured by recording video while the subject fixated on a small spot that was presented near the center of the scanned field. The PFD was measured using a four alternative forced choice procedure in which the subject had to decide where the stimulus landed with respect to their line of sight.
We were able to resolve cones close to the anatomical center of the fovea in all eyes (within 0.1 degree). The standard deviation for steady fixation at the PRL was measured to be 2.4-6.6 arcminutes, which was expected from the literature. The PRL was not located at the point of maximum cone density and its position was not fixed, sometimes changing from trial to trial, but generally stayed within 1 standard deviation of the PFD, which formed a broad distribution that was about 3 times larger than the PRL on average.
There is not one location within the anatomical fovea responsible for the perception of object direction or one region targeted by the fixation system.
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