Wavelet analysis reveals dynamics of rat oscillatory potentials

We characterised the dynamics in the oscillatory potentials (OPs) of the rat electroretinogram (ERG) using a continuous complex Morlet wavelet transform. Dark-adapted (>12 h) full field ERG responses were recorded from five anaesthetized (ketamine:xylazine, 60:5 mg/kg) adult Long-Evans rats (10–12 weeks). Five responses were obtained for brief LED flashes (1–4 ms) in a ganzfeld at exposures ranging from −4.2 to 1.58 log cd s m−2. Signals were recorded across a bandwidth of 0.3–1 kHz and digitized at 10 kHz. Morlet wavelets with frequencies between 50 and 250 Hz were correlated with raw ERG signals at 1 ms intervals. The amplitude of the correlation at each time and frequency was given by the modulus of the complex wavelet response. Candidate OPs were identified as local peaks within 10% of the maximum amplitude. As flash exposure increased, the amplitude of the OP response increased, the peak OP occurred earlier, and the peak OP frequency increased. OPs at brighter flashes clustered into two groups, peaking at 50 ms in the 70 and 130 Hz band for moderate intensities, and peaking at 20 ms in the 70 Hz band and 50 ms in the 120 Hz band for the highest intensities (>0 log cd s m−2). These dynamics agree with physiological, pharmacological and clinical studies that suggest several distinct neural mechanisms contribute to OPs. Wavelet analysis reveals important dynamics in OP data that are not evident with traditional analytical approaches.