Purpose. The human dark-adapted a-wave has been fitted by models based on rod photoreceptor parameters (Hood & Birch, 1990; Breton et. al, 1994). We wondered whether removing post-photoreceptor responses would affect the a-wave parameter fits after blocking ON-bipolar activity with amino,4-phosphono-butyric acid (APB) or blocking OFF-bipolar and horizontal cells with cis-2,3-piperidine-dicarboxylic acid (PDA). Methods. The dark adapted ERG was elicited over a 5 log range using bright xenon flashes with 4.98.105 sc-td-sec maximum intensity in a Ganzfeld bowl. Four Macaca fuscata and fascicularis monkeys were studied. ERGs were recorded before and after 0.1ml intravitreal injections of 40mM APB or 200mM PDA. Responses were normalized by the maximal a-wave amplitudes and were fitted to EXP(-αΦ(T-δ)2) by least squares, with α and δ as free variables. In the model, α is the amplification constant, δ is cascade delay, Φ is photoisomerizations per rod per flash and T is time. Fits for all animals used the same conversion factor K (which includes rod dimension, funneling, optical density, transmissivity, quantal efficiency, etc.) of 8.6 photoisomerizations per rod per sc-td-sec (developed for the normal human eye). Results. In control responses, α was consistent for each animal in recordings done on different days. The mean value of α was 7.2 and was linear up to 104 isomerizations and then decreased as Φ increased, similar to human. α showed 0.8 log spread among the four animals, which could be due to variations in the physical parameters in K. After APB or PDA, no change was observed in the intensity dependence or the value of α. Conclusions. The results in monkey controls correspond well with human. The a-wave model fit was not affected by removing post-photoreceptor cellular responses. Eliminating b-wave contamination by APB did not affect the value obtained for the amplification constant. If there were horizontal cell feedback, removal by PDA also did not affect the fit. These results suggest that (1) the dark-adapted a-wave is determined by the photoreceptor transaction process, and (2) that post-photoreceptor components do not contribute to the shape of the normalized a-wave.
|Original language||English (US)|
|Journal||Investigative Ophthalmology and Visual Science|
|State||Published - Feb 15 1996|
ASJC Scopus subject areas
- Sensory Systems
- Cellular and Molecular Neuroscience