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O.A. Mahroo, N. Husain, H. Mehta, A. Patel, T.D. Lamb; Recovery of the Human Cone Electroretinogram A-Wave Following Bleaching Exposures: A Measure of Pigment Regeneration In Vivo . Invest. Ophthalmol. Vis. Sci. 2003;44(13):4161.
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Purpose: To investigate the initial reduction, and time-course of recovery, of the amplitude of the human cone electroretinogram (ERG) a-wave response to dim flashes, following intense light exposures. To test whether these dim-flash measurements reflect the bleaching and regeneration of human cone pigment in vivo. Methods: Ganzfeld ERGs were recorded from two normal subjects using a conductive fibre electrode, in response to repetitive dim red flashes (8 photopic Td s) delivered before and after exposure to a range of intense white lights (intensities ranged from 10,000 to 60,000 ph Td; durations ranged from 5 s to 6 min). A blue LED background (~2000 scotopic Td) was present continually to ensure saturation of the rod system. (The dilated pupil area was ~50 mm2, but an effective area of 20 mm2 has been used to calculate photopic intensities.) The a-wave amplitude was measured 15 ms after each flash, and was corrected for non-linearity of the response-intensity relation. Results: Recovery from all bleaches in both subjects appeared to follow a common time-course, which could be described well by a model based on Michaelis-Menten regeneration of photopigment. The initial rate of recovery following a total bleach was ~0.45 min-1, and the Michaelis constant Km was ~0.15 of the total pigment. The apparent reduction in pigment content after exposures of different duration, and the steady-state level reached at each intensity, could be accounted for using the same parameters, and a fixed photosensitivity. Conclusions: The reduction in, and recovery of, a-wave dim-flash response amplitude appear consistent with bleaching and regeneration, respectively, of cone pigment. As a result cone pigment kinetics can be measured in vivo by an alternative approach to the more usual method of reflection densitometry. Human cone pigment regeneration, assessed this way, is consistent with Michaelis-Menten kinetics. Our findings may explain previous reports of pigment regeneration that deviate from an exponential time-course (e.g. Smith et al., 1983, Vis Res 23: 517).
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