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F Naarendorp, J Akula; Rod-Driven B-Waves of the Rat Electroretinogram Isolated on Dim and Moderately Intense Backgrounds . Invest. Ophthalmol. Vis. Sci. 2002;43(13):1818.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: The rod bipolar - AII amacrine cell pathway in the mammalian retina is highly sensitive but saturates in response to relatively weak backgrounds. We explored in the rat whether small rod-driven b-waves isolated on moderate backgrounds are generated in an alternative rod pathway. Methods: Corneal ERGs were recorded with standard methods in the dark and 2.5 seconds after onset of a background stimulus. A flash producing ∼ 0.4 R* rod-1 evoked a nearly linear b-wave response from the dark-adapted eye. Results: (A) Small b-waves were recorded on adapting backgrounds, IB, producing about 1.0 - 2,000 R* rod-1 s-1. Up to an IB value of ∼ 80 R* rod-1 s-1, the flash sensitivity, SF, decreased in a Weber-like fashion. The implicit time, t, shortened according to the relation: t = c IB -b, where c is a constant; b was ∼ 0.1. On brighter backgrounds (IB ≷ 200 R* rod-1 s-1), SF decreased further, however, t remained mostly unchanged. (B) The latency of the b-waves evoked with equal intensity flashes was hardly affected when IB < 80 R* rod-1 s-1. The b-wave latency shortened when IB produced ∼ 200 R* rod-1 s-1. Upon further light adaptation no significant change was seen. The Robson-Frishman model (Vis. Neurosci., 1995) of the initial phase of the dark-adapted rod bipolar response could be fitted, without modification, to intensity-scaled responses obtained on dim backgrounds but not to those recorded on brighter backgrounds. (C) The small b-waves were normalized to unity at the peak. Responses recorded on backgrounds producing < 80 R* rod-1 s-1, were fitted with single exponential functions all having a time constant, tau0.5, of ∼ 9 ms. A similar result was obtained for responses isolated on brighter backgrounds (≷ 200 R* rod-1 s-1); the single tau0.5 value was ∼ 5ms. Conclusion: Upon saturation of the rod bipolar cell, a fast-rising rod-driven signal is generated in an alternative pathway involving perhaps second and/or third order neurons of the cone pathway.
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