Temporal transfer and nonlinearity properties of turtle erg: Tuning by temperature, pharmacology, and light intensity

Abstract

ERG impulse response, amplitude and phase temporal spectral transfer functions, and nonlinearities were measured in turtle retina under different retinal temperatures, pharmacological treatments, and light intensities.b-Wave amplitude is strongly temperature dependent; amplitudes of thea-wave and slow P-III are less sensitive. Their time courses all slow markedly as temperature decreases. ERG amplitude transfer function is bimodal bandpass with narrow low-frequency peak (below 1 Hz) and broader mid-frequency peak (5 Hz at 8°C). Both peaks broaden and their frequency increases (low, 1 Hz; mid, 15 Hz, at 23°C) as temperature increases. Phase transfer function slope decreases (from −70°/Hz at 8°C to −25°/Hz at 23°C) as temperature increases. Nonlinear properties of ERG at high input intensity are modelled by a quadratic nonlinearity, low-pass prefilter with cutoff above 12.4 Hz, and low-pass postfilter broadly peaked at 6–10 Hz with cutoff above 20 Hz. For low input intensity, ERG exhibits linear properties with low-pass filtering sharply cut off above 6 Hz.a-Wave and slow P-III were isolated by aspartate treatment; depolarizing bipolar cell activity was examined using ethanol/GABA treatment of retina. High-frequency components, including broad mid-frequency peak, were attenuated and low-frequency components were enhanced with aspartate. Transfer function narrows and peaks at a lower frequency with ethanol/GABA.