Hilbert transform analysis of the mouse scotopic electroretinogram reveals two distinct bursts of oscillatory potentials with progressively dimmer flashes

Purpose: Study the scotopic oscillatory potentials (OPs) in mice over a wide range of flash luminance levels using the Hilbert transform (HT) to extract new features of the high frequency components of the electroretinogram (ERG). Methods: Scotopic ERGs [Intensity: - 6.3 to 0.9 log cd center dot s center dot m(-2); 12 h of dark-adaptation] were obtained from adult mice (C57BL/6; n = 7). The Hilbert transform (HT) was obtained within 3 consecutive frequency bands (65-90 Hz, 90-115 Hz and 115-140 Hz), with OPs being denoised, automatically identified and analyzed. Measurements included: number of OPs, duration of the OP response, surface-under-the-curve (SUC) of the HT envelopes, implicit times, and instantaneous frequency at the HT envelope peak, mean peak time differences (PTD) between the envelopes of each frequency band (measuring their synchrony), correlation coefficient and lag between consecutive HT envelopes, as well as the number of peaks on the HT envelopes. Results: The OP response duration, number of OPs and PTD all peaked for flashes between the level corresponding to the RodVmax (maximal b-wave amplitude of the rod ERG; i.e., the first asymptote of the scotopic luminance-response curve) and K (the flash luminance at which the amplitude of the b-wave is half of that of the RodVmax;), i.e., between -3.9 and -2.4 log cd center dot s center dot m(-2). The correlation between consecutive envelopes is close to 1 at flashes > -1.2 log cd center dot s center dot m(-2), with small lags (min. = 1.93 +/- 0.45 ms at - 1.2 log cd center dot s center dot m(-2)), then gradually drops to 0.81 +/- 0.02 at the dimmest flash intensity (with a max. lag = 14.76 +/- 8.92 ms at - 5.1 log cd center dot s center dot m(-2)). Finally, we found that the single OP burst (i.e., a single HT envelope peak) seen at flash intensities > - 1.2 log cd center dot s center dot m(-2) progressively divided in two (or more) OP bursts (i.e., multiple HT envelope peaks) with gradually dimmer flashes. Conclusions: Our HT method enabled the analysis of the OP response without the subjective interpretation of the experimenter. Analysis of the scotopic OPs at dim flashes with the HT revealed a novel feature of the OP response not yet reported elsewhere, namely: a split of the OP response into two (or more) distinct bursts. Furthermore, the synchrony peak (measured with the PTD) matched the peak in OP response duration between K and RodVmax, suggesting a disorganization (or dephasing) of the retinal signal in ERGs evoked for weaker flashes. The increased synchronization and correlation of the single burst observed for the strongest flashes could suggest an optimization or saturation of the retinal response. We believe that these novel features of the OP components of the ERG went unnoticed given that previous studies did not use weak enough flashes and failed to recognize the added value that time and frequency domain analysis of the ERG (such as what is achieved with the HT) brings to the interpretation (and our understanding) of the retinal response.