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B. G. Jeffrey, L. M. Renner, M. Neuringer; Restricted Field ERG Measurement of Central Retinal Dark Adaptation in Monkeys With Macular Drusen. Invest. Ophthalmol. Vis. Sci. 2010;51(13):5567.
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In humans, slowed dark adaptation (DA), measured psychophysically, is correlated with age and early AMD. The restricted field ERG (rfERG) isolates the rod photoreceptor response from a 40o field of the central retina1. Here we developed a rfERG method for measuring DA and examined central DA in aged monkeys with macular drusen.
Full field ERG (ffERG) and rfERG measures of rod phototransduction and DA were made in 14 rhesus monkeys aged 22-30 yrs (ca. 66-90 human yrs). Retinal color photographs were graded for drusen as either none/mild (N=6) or moderate/severe (N=8). For rfERGs, a reversible ophthalmoscope was used to align the macula with the Ganzfeld opening; the flash subtended 40o on the retina at the 30 cm recording distance. For DA, ERGs were recorded to paired flashes (4.4 log sc td-s with 1.2 - 2 sec separation) every 2 mins after a 100% bleach. Rod isolated ERGs were obtained by subtracting the second flash response, generated by the cones (ffERG) or cones+scatter (rfERG) from the first flash. Derived parameters were the time constant of DA recovery (τDA) and the time to 50% full recovery (T50). Rod phototransduction was assessed from a P3 model fit to ERG a-waves recorded to bright flashes (2.1 to 4.8 log sc td-s) presented alone or 2 sec after a conditioning flash. Rod-isolated ERGs were obtained as for DA.
Central τDA was significantly correlated with age (p<0.01). For all monkeys combined, central τDA and T50 (mean±SE; 3.8 ± 0.2 min; 18.9 ± 0.6 min) were significantly longer (p<0.03) than full field values (τDA = 3.3 ± 0.2 min; T50 = 16.7 ± 0.8 min). In monkeys with moderate/severe drusen the delay in central field T50 relative to full-field T50 was greater than in monkeys with no/mild drusen (22% vs. 7%, p<0.07). Maximal rod response (RmaxP3) and rod sensitivity (S) were reduced by 70% in the central field but did not differ between drusen groups.
These results suggest that central field DA measured with the rfERG can provide a specific and sensitive measure of early changes in central retinal function associated with aging and drusen in the monkey.1Nusinowitz et al (1995) J Opt Soc Am A 12:2259-66
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