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Humza Tahir, Ian Murray, David Carden, Neil Parry; Sensitivity recovery following a bleach; a dual “smiley” arc stimulus technique for studying abnormal dark adaptation. Invest. Ophthalmol. Vis. Sci. 2013;54(15):2767.
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© ARVO (1962-2015); The Authors (2016-present)
There is much interest in sensitivity recovery under low luminance conditions. The rate of recovery of rod-vision following a bleach is important as it is slowed in the older eye and in many clinical conditions, notably Age-Related Macular Degeneration. It is not known for certain whether this is a pan-retinal effect or if there are localised regions of impaired rod function. To address this a dual arc stimulus was developed that simultaneously samples sensitivity recovery in two retinal locations. Preliminary findings on normal observers are presented.
Arc-shaped white “smiley” stimuli were presented on a black CRT screen at two locations in the inferior visual field. They were 75° segments of annuli with internal and external diameters 5° and 7° (stim 1) or 10° and 12° deg (stim 2). The dynamic range of the CRT was expanded by covering the left half of the screen with a 1.2 log unit neutral density (ND) filter, and the right half with 3.6 log ND. The fixation/stimulus ensemble initially appeared in the left window but shifted rightwards when screen stimulus intensity fell below -1.5 log cd/m2, giving a total filtered luminance range of about 6.5 log units (0.8 to -5.7 log cd/m2). Recovery of sensitivity to the two stimuli was measured simultaneously using method of adjustment by alternately presenting them following a localised bleach of at least 30% using a calibrated photographic flash.
In normal observers the dark adaption process is composed of a cone and a rod stage. When the stimuli are approximately matched for photometric sensitivity their dark adaptation curves coincide for the cone branch of the curve and remain separate for the duration of the ‘S2’ stage which, due to the bleach intensity, can be thought of as rate limited (Lamb and Pugh, IOVS, 2006;47;5138-5152). In some subjects the curves coalesce again for ‘S3’, the third stage of the dark adaptation curve, but not in others indicating a possible dissociation between S2 and S3.
This technique allows the measurement of dark adaptation in multiple retinal locations simultaneously using a new PC-based method. Preliminary results show differences in the tested regions. As this new method places little additional demands on subjects it allows for the easy investigation of potential regional differences in impaired rod function in older and diseased eyes.
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