Removing mains interference from the mfERG by applying a post-processing digital notch filter: for the good or the bad?

Purpose Ideally, the multifocal electroretinogram (mfERG) is recorded without noticeable intrusion of mains interference. However, sometimes contamination is difficult to avoid. A post-processing digital notch filter can help to recover the retinal response even in severe cases of mains interference. While a digital filter can be designed to have little to no impact on peak times, filtering out mains interference also removes the retinal signal content of the same frequency, which may result in a change of amplitude. The present study addressed this issue in the standard first order kernel mfERG. Methods In 24 recordings from routine exams with no perceivable mains interference, the effects of 50-Hz and 60-Hz non-causal digital notch filters on amplitude and peak time were assessed. Furthermore, the effect of filtering on contaminated traces was demonstrated and simulated mains interference was used to provide an example of nonlinear superposition of retinal signal and mains interference. Results mfERG amplitudes were reduced by 0%–15% (median 6%) with the 50-Hz filter and remained virtually unaffected with the 60-Hz filter. Simulations illustrate that spurious high-frequency components can occur in the filtered signal if a strongly contaminated signal is clipped due to a limited input range of the analog-to-digital converter. Conclusion The application of a 50-Hz digital notch filter to mfERG traces causes a mild amplitude reduction which will not normally affect the clinical interpretation of the data. The situation is even more favorable with a 60-Hz digital notch filter. Caution is necessary if the assumption of linear additivity of retinal signal and mains interference is violated.

Interference Removal with an Improved Incremental Estimation Filter

Mains interference in digitized electrocardiographic signals is often removed with a nonlinear filter, the so-called incremental estimaton filter. A prerequisite for proper functioning of the filter is that the variation of the signal is considerably slower than that of the disturbing interference. This condition will often not be true, especially during wave deflections. As a result, the filter will not fully remove the interference when present, or, alternatively, it may generate a sine wave in the absence of interference. These effects are analyzed and different solutions proposed, one of which is the use of new, very simple filters based on the incremental estimation technique.