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When the eye is stimulated by a flickering light, the electroretinogram (ERG) and other electrophysiological responses in the visual pathway often exhibit period doubling. This phenomenon is manifested as an alternation in the shape of the response waveform from cycle to cycle, and also as spectral components at the half-fundamental frequency (F/2) and its odd multiples. Although period doubling has been described in humans as well as in other animals, its features in the rodent flicker ERG have not been characterized. We investigated the properties of period doubling in the rat cone flicker ERG elicited with full field, sinusoidal photic stimuli. Period doubling was observed when the temporal frequency of the stimulus was in the range of 20 to 30 Hz. The F/2 component of the Fourier spectrum of the ERG was more pronounced than its odd harmonics. The magnitude of the cycle-to-cycle variation in amplitude differed depending on whether measurements were based on peak-to-trough or trough-to-peak amplitudes, owing to the relative phase relationship between F/2 and F as a function of stimulus frequency. The frequency-response characteristics of period doubling varied with stimulus contrast, such that reducing the contrast shifted the peak F/2 amplitude to a lower stimulus frequency. Period doubling was evident in rat eyes in which PDA was administered intravitreally, indicating that the phenomenon can occur independently of OFF-pathway activity in the rat retina. The period doubling properties we observed in the flicker ERG response of the rat cone system provide constraints on the nature of the nonlinear feedback mechanism presumed to underlie the period doubling phenomenon.

The electroretinogram (ERG) provides a noninvasive, objective measure of retinal function, and is one of the most widely used diagnostic tools in the study of visual disorders. Although rodents are often used in the study of retinal disease, the properties of the flicker ERG of the rodent retina have not been fully characterized. Here, we show that the fundamental response of the rat ERG to sine-wave flicker exhibited a low-pass pattern in the frequency range from 2 to 30 Hz, whereas the second harmonic (F2) showed a more complex frequency response relation. The F2 component represented only a small fraction of the ERG response at low temporal frequencies (below 12 Hz), but it made a substantial contribution to responses at high frequencies. The contrast-response relation was linear when tested with a low-frequency (6 Hz) stimulus, but saturated in response to a high-frequency (20 Hz) stimulus. After intravitreal injection of L-AP4, a specific blocker of the retinal ON pathway, the flicker responses elicited by either 6- or 20-Hz stimuli were greatly reduced in amplitude, whereas only a very slight enhancement was seen after the application of PDA, a drug that blocks retinal OFF-pathway activity. Based on the observed differences in the degree of non-linearity, and contrast-response properties of the rat flicker ERG at low and high frequencies, as well as the pharmacological results, we postulate that sustained and transient ON bipolar cells generate the flicker ERG responses elicited at low and high temporal frequencies, respectively.

The electroretinogram (ERG) affords a quantitative, objective, and noninvasive method by which to examine light-evoked neuronal activity, and is commonly used to study the functional integrity of normal and diseased retinas (Fishman, 2001; Frishman, 2006; Peachey and Ball, 2003). ERG elicited by a periodic stimulus (flicker ERG) is a useful tool to investigate temporal property of retinal circuitry, especially cone pathway in the retina (Alexander et al., 2005; Burns et al., 1992; Kondo and Sieving, 2001; Qian et al., 2010; Qian et al., 2008). Two kinds of light stimuli are commonly used for flicker ERG recordings: sinewave modulated light and a pulse light flickers. Sine-wave modulated light, due to its simplicity in spectrum component, is often used to characterize frequency-response relationship of ERG (Alexander et al., 2003; Burns et al., 1992; Kondo and Sieving, 2001; Krishna et al., 2002; Qian et al., 2008; Viswanathan et al., 2002). On the other hand, pulse light flicker can easily be implemented technically, and has long been applied for clinical use (Audo et al., 2008; Garry et al., 2010; Jacobs et al., 1996). Indeed, 30 Hz pulse flicker under light-adapted condition is recommended by the International Society for Clinical Electrophysiology of Vision (ISCEV) for evaluating activity of the cone system in human patients (Marmor et al., 2009). For a pulse train consisting of a series of flashes with intensity I, duration d, and period P, the Fourier transform in the frequency domain is given by the expression