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N. Tanimoto, P. Humphries, M. Biel, F. Mueller, M. W. Seeliger; Assessment of the Absolute Excitatory Level of the Retina by Direct-Current ERG. Invest. Ophthalmol. Vis. Sci. 2009;50(13):4759.
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Conventional ERGs are usually recorded with a non-zero lower cutoff-frequency of the amplifiers to avoid certain artefacts. However, the improved signal quality is associated with the loss of information about the absolute baseline of recordings, as a zero response is obtained in all cases the signal baseline stays constant for a sufficient amount of time. In other words, it is impossible to tell from a conventional ERG whether a zero signal is obtained under conditions of maximal or no excitation of the visual system. Here, we examine the value of the DC-ERG (featuring a lower cutoff-frequency of zero) in the assessment of the absolute excitatory level of the retina.
Wild-type and transgenic mice with specific functional characteristics (Cnga3-/-, pure rod function; rho-/-, pure cone function; Hcn1-/-, prolonged photoresponses) were examined with Ganzfeld DC-ERG. Scotopic flicker frequency series (0.5-30 Hz) were recorded with different stimulus intensities to obtain responses at different levels of baseline excitation. To disclose the transition from the resting state to the steady state conditions, flicker stimuli with a defined onset were used. In addition, the interchangeability of excitation by repetitive stimulation and constant light was verified with a light step.
In conventional scotopic steady-state flicker ERG, amplitudes decline down to zero with increasing stimulus frequencies. However, DC-ERG revealed that the limiting factor was the baseline level of excitation, since further stimuli could only generate signal changes between the baseline and the maximally achievable level. In other words, if the baseline is high, even a very strong stimulus can only generate a small response amplitude. Hcn1-/- mice feature prolonged photoresponses which is associated with abnormally high baseline excitation. Consequently, we found a substantial reduction in flicker fusion frequency, as the remaining dynamic range for signal changes was reduced already at relatively low stimulus frequencies.
The absolute excitatory level of the retina is important to know as it determines the ability to respond to further stimulation. The DC-ERG has proven to be a valuable tool to assess this status.
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