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David Williams, Robin Sharma, Soon Keen Cheong, Matthew Keller, Jennifer Hunter, Christina Schwarz; Two photon ophthalmoscopy reveals a new oddity of S cones. Journal of Vision 2019;19(8):39. doi: 10.1167/19.8.39.
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Two-photon excited fluorescence (TPEF) ophthalmoscopy provides novel images of retinal structure and function, such as the first images of single ganglion cells and tracking intermediate stages of the visual cycle in the primate eye. TPEF uses femtosecond infrared pulses to image fluorophores such as retinol and NADH that cannot be observed with conventional fluorescence in the living eye because their excitation spectra lie in the ultraviolet, outside the eye’s spectral transmittance. Before TPEF can be used in humans, improved safety standards for the ultrashort pulses used in TPEF must be established. We have previously established conditions using a two-photon adaptive optics scanning light ophthalmoscope in the living macaque eye that do not produce detectable signs of damage. At light levels that were 40 times higher (retinal radiant exposures of 856 J/cm2), however, permanent selective S cone damage was observed without corresponding effects in the other receptor classes. Immediately after the exposure, S cones emitted about 2.6 times less TPEF and showed an altered TPEF time course. Several weeks after the initial exposure, the S cone outer segments had disappeared. Rods migrated in the peripheral retina and cones near the fovea to fill gaps vacated by S cones. The susceptibility of S cones to damage by ultrashort pulses is surprising because the infrared light used in TPEF is expected to be far more strongly absorbed in the other receptor classes than the S cones, adding to the long list of ways S cones differ from their M and L cone neighbors.
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