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A. Raghuram, K.R. Alexander; Properties of Synchronous Period Doubling in the Flicker ERG of The Cone System . Invest. Ophthalmol. Vis. Sci. 2006;47(13):1664.
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© ARVO (1962-2015); The Authors (2016-present)
The full–field flicker ERG is commonly used to evaluate dysfunction of the cone system in retinal diseases. It has been observed that the flicker ERG can show a cyclic variation in amplitude, termed synchronous period doubling, that is thought to result from a nonlinear feedback signal involving bipolar cells and photoreceptors together with lateral neural coupling that induces response synchrony (Crevier & Meister, 1998). The purpose of this study was to identify the specific stimulus conditions that elicit period doubling in the human flicker ERG in order to gain a better understanding of the underlying mechanism.
Full–field ERGs were recorded from visually normal subjects using an Espion stimulating and recording system. The stimulus was full–field sinusoidal flicker presented at temporal frequencies ranging from 31.25 to 83.33 Hz. The rod system was typically desensitized with a dim short–wavelength adapting field. Stimulus wavelengths included 468, 512 and 632 nm, which were photopically matched using heterochromatic ERG flicker photometry.
Period doubling was observed in the ERG waveform as an alternation in response amplitude from cycle to cycle, which began on the same cycle on each trial for a given stimulus condition. From spectral analysis of the waveform, period doubling was manifested as harmonics of a frequency that was 1/2 the stimulus fundamental frequency (F/2, the subharmonic). The amplitude of the component that was three times the frequency of the subharmonic (3F/2) was used as the measure of period doubling. A region of prominent period doubling was observed at temporal frequencies near 36 Hz. Additional regions of period doubling occurred at higher temporal frequencies, depending on the mean luminance. The amplitude of period doubling varied non–monotonically with stimulus contrast. Of particular interest, the amplitude of period doubling was wavelength–dependent, such that at high stimulus contrasts, period doubling was prominent for a 512–nm stimulus but was not above the noise level for a 632–nm stimulus. However, period doubling was present under conditions that isolated the long–wavelength (L) cone system.
Synchronous period doubling is a replicable feature of the full–field cone flicker ERG. However, the manifestation of period doubling depends in a complex manner on stimulus mean luminance, contrast, and wavelength, consistent with the operation of a nonlinear dynamic process.
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