Abstract
Abstract: :
Purpose: To examine light adaptation and dark adaptation of human rod bipolar cell (RBC) responses in vivo. To test whether the recovery of RBC sensitivity after bleaching is equivalent to the fading of background illumination, and to compare the results with psychophysical recovery of the scotopic visual system. Methods: Ganzfeld ERGs were recorded using a conductive fibre electrode and a dilated pupil, in response to (a) a family of flashes of increasing intensity; (b) dim test flashes presented on a range of background intensities; and (c) dim test flashes presented before, and up to 45 mins after, exposure to intense illumination eliciting bleaches from a few percent to near total. Test flashes and adapting backgrounds were blue (max = 470 nm), while bleaching exposures were white; all were provided by LEDs. In most experiments a very dim blue background (∼4 × 10–4 sc cd m–2) was present, to minimize an early negative signal (STR). Results: In the presence of the very dim blue background, the ERG response to a dim test flash (3 × 10–3 sc cd m–2 s, delivering ∼1 isomerization rod–1) was characterised by a prominent b–wave, peaking at ∼120 ms, which we take to reflect primarily RBC activity. With increasing levels of background illumination the RBC response reached peak earlier and desensitized according to Weber’s Law with I0 ≈ 4 × 10–3 sc cd m–2. Following exposure to a bleach, the RBC response was initially greatly desensitized, and recovered slowly with time. Following a near–total bleach, the sensitivity was unrecordable for ∼15 min, then after ∼22 min half–maximal sensitivity was reached, with full recovery of sensitivity occurring at ∼35 min. After smaller bleaches, recovery commenced sooner. A Crawford transformation was applied to estimate the post–bleach decay of equivalent background intensity, and the results could be fitted, across the range of bleaches, with an ‘S2’ slope of 0.24 log10 min–1. Conclusions: The post–bleach decay of equivalent background intensity in RBCs exhibits a slope consistent with that reported for component S2 of psychophysical dark adaptation in the scotopic visual system.
Keywords: electroretinography: non-clinical • retina: proximal (bipolar, amacrine, and ganglion cells) • retina