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O.A. R. Mahroo, N.A. Moran, J.S. Kenkre, T.D. Lamb; Rapid Recovery of Current in Human Cones Following Bleaching Exposures . Invest. Ophthalmol. Vis. Sci. 2005;46(13):4633.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: To investigate the speed of recovery of circulating current in human cone photoreceptors in vivo upon extinction of intense steady illumination. Methods:Ganzfeld ERGs were recorded, using a conductive fibre electrode, from two subjects, whose pupils were dilated. The subject was exposed to a background of ∼2000 cd m–2, provided by 12 intense white LEDs, for 2–3 min. The background was then switched off for a brief period (<1 s) during which a bright flash was delivered at an interval from 10 to 600 ms after background extinction. Several hundred cycles of extinction and flash delivery were performed (every 3–4 s), to test different intervals and to average out noise in the recordings. In addition, the ERG response to background extinction was measured. A dim rod–saturating blue background was always present. Results: Comparison of bright–flash responses obtained during the intense background and during rod–saturating illumination indicated that ∼50% of the cone circulating current remained in the background at steady state. Separate dim–flash experiments established the bleach level as ∼90%. Following background extinction, the bright–flash response amplitude recovered fully within 100 ms. At earlier times, it was necessary to subtract the ERG ‘background off’ response, and this procedure indicated the current to have recovered substantially within 15 ms. The early part of the ‘background off’ response was well–described by a biophysical model of transduction, and indicated that recovery of cone current began within 5–10 ms and was substantial by 15 ms. Conclusions: Human cone circulating current recovers within 100 ms, and possibly within 15 ms, at cessation of an intense steady exposure that bleaches ∼90% of the cone pigment. This is ∼5,000 times faster than human cone pigment regeneration (Mahroo & Lamb, 2004, J Physiol 554: 417) and ∼50,000 times faster than recovery of human rod current (Thomas & Lamb, 1999, J Physiol 518: 479) after a similar bleach. Thus, cone current appears to be immune to the products of bleaching.
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