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Kady S Bruce, Bradley R Langston, Wolf Harmening, William Scott Tuten, Austin Roorda, Lawrence Sincich; Single-cell psychophysical sensitivity of weakly reflective cones in normal subjects. Invest. Ophthalmol. Vis. Sci. 2014;55(13):4544.
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Our aim was to assess the relationship between single-cone reflectance and perceptual threshold in the human retina. As adaptive optics scanning laser ophthalmoscopy (AOSLO) moves into clinical use, the reflectance properties of the cone photoreceptors may become a diagnostic tool for retinal disease. However, it is well established that cones in normal subjects exhibit variable reflectance in single AOSLO images, and that this reflectivity varies over time. In normal subjects, we have observed that some cones have persistently poor reflectance over time. We asked whether these low-reflectance cones tested in normal subjects exhibit reduced psychophysical sensitivity.
In 4 subjects with normal color vision, cones were imaged using a multi-wavelength AOSLO with infrared light (842±25 nm). Real-time eye tracking allowed stabilization of a 1.2° patch of retina. We selected two cones, one with poor reflectance (dark) and one with normal reflectance (bright), within the stabilized mosaic for targeted delivery of a briefly flashed 45 arcsec square of 543 nm light. Chromatic offsets were measured and corrected with an image-based method. Increment thresholds were measured using a self-paced, 20-trial Bayesian staircase method. Staircases were interleaved for 3 conditions: a dark cone, a neighboring bright cone, and both cones together. For intermittently dark cones, the cone was tested when dark. Cones selected were not located beneath capillaries, as assessed by vasculature mapping. Thresholds were measured 3 times for each condition in 4 cone pairs located 2.2-3.3° from the fovea.
Persistently dark cones showed measurable psychophysical increment thresholds that were indistinguishable from thresholds derived from bright cones (dark/bright threshold ratio = 0.99±0.16). Thresholds measured when the dark and bright cones were stimulated simultaneously were always lower than that of either single cone, suggesting that dark cone signals contributed to paired-cone thresholds. When intermittently dark cones were tested, thresholds were comparable to those of persistently dark or bright cones.
Poor cone reflectivity does not appear to be correlated with psychophysical measurement of increment threshold. Although most cones reflect light well in AOSLO imaging, the presence of some dark cones may not be a sign of functional disruption in normal subjects.
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