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J. Rha, R.S. Jonnal, Y. Zhang, D.T. Miller; Rapid Fluctuation in the Reflectance of Single Cones and Its Dependence on Photopigment Bleaching . Invest. Ophthalmol. Vis. Sci. 2005;46(13):3546.
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Purpose: Dynamic fluctuation in the reflectance of single cones has been reported to occur over periods of minutes to many hours in the living eye. The cause of these fluctuations is unknown, although a relationship to disc shedding has been suggested. Current interpretation of these fluctuations, however, implicitly assumes the cone reflectance is stable over shorter periods (secs), which has not been reported. In light of this shortcoming, the reflectance of single cones with various initial bleached states was monitored over a period of a few seconds using the Indiana adaptive optics (AO) retina camera. Methods: The AO system dynamically corrected the most significant aberrations across a 6 mm pupil. For the purposes of imaging and bleaching cones, a 670 nm light source illuminated a 1.5 deg retinal patch at 1.4 deg ecc. Cone images of 2 msec duration were acquired at 24 Hz over 4 sec intervals. The 24 Hz strobing light also acted as a bleaching source providing 78x106 trolands and bleaching 99% of the cone photopigment after 2 sec. Two experiments were conducted. First, cones in the 1.5 deg patch were initially bleached to levels of 100%, 75%, 50%, and 0% and then immediately imaged over a 4 sec interval to monitor cone reflectance as described above. The second experiment was identical to the first except bleached and unbleached cone patches were imaged simultaneously to avoid unwanted variance that might have arisen in experiment one. Fluctuations in the retinal reflectance were assessed at both the level of individual cones and across large patches containing many cones. Results: In both experiments, the reflectance from large patches of cones roughly followed the conventional bleaching time course with maximum reflectance being reached in about 2 sec for an initially unbleached retina. The reflectance of many of the single cones, however, noticeably deviated from this time course, oscillating, often significantly, with a typical period of about 1 sec. The temporal RMS of the reflectance for each pixel in the cone videos vividly revealed that fluctuations were localized to cone apertures rather than gaps between cones. The number of cones that fluctuated in a given patch of retina depended on the bleaching state, with fully regenerated cones providing the most variation. Conclusions: Contrary to the conventional time course for photopigment bleaching, the reflectance of some single cones was observed to fluctuate, often significantly, over periods of about 1 sec. Although we do not know the source of this new phenomenon, it appears localized to light exiting individual cones and dependent on the cone bleaching state.
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