Wavelet analysis of electroretinograms and its clinical application

The electroretinogram (ERG) is a powerful clinical technique used in clinics around the world to diagnose and monitor the health of the retina. The ERG is the record of the electrical activity of the retina in response to light. Under scotopic conditions (for low light levels, approximately less than 10 scotopic trolands), the human ERG in response to a flash of white light has a specific form: an initial negative deflection called the a-wave, followed by a positive deflection called the b-wave which then falls back to zero. Superimposed on the b-waves are a few high frequency oscillatory potentials. Daubechies D4 wavelets were used to do an eight scale multiresolution analysis of these waves. The most important wavelet coefficients that contribute to the various components of the ERG were identified using the following procedure: (i) Contribution of each wavelet coefficient to the L¹-norm of the wave is determined; (ii) Only coefficients which contribute at least 2.5% of the maximum contribution are retained; (iii) From this reduced set only those coefficients that contribute to signals within 2 standard deviation from the norm are retained as the best basis; (iv) ERG signals were reconstructed using these best basis; (v) The coefficients that contribute to different components of the ERG were directly read from scalograms of these reconstructed waves. Best basis were obtained for normal ERGs as well as ERGs obtained from patients with Rod-cone dystrophy and Duchenne muscular dystrophy. The differences in the basis sets are used to hypothesize the differences in the physiological mechanisms involved in the production of these pathological ERGs --Abstract, page iii