Abstract
PurposeIdeally, 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.MethodsIn 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.ResultsmfERG 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.ConclusionThe 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.
Highlights
When recording a multifocal electroretinogram, care should be taken to minimize the intrusion of mains interference [1]
In all patients assessed for the present study, the dominant spectral components of the multifocal electroretinogram (mfERG) were in a lower frequency range than the mains frequency
Note that the effect of a 60-Hz notch filter on the retinal response can be assessed despite the data having been collected in a region with 50-Hz mains frequency
Summary
When recording a multifocal electroretinogram (mfERG), care should be taken to minimize the intrusion of mains interference [1]. For instance with young children or with frail patients, delays in the examination procedure can be detrimental to the patient’s ability to cope with the measurement. In such cases, one might have to accept a suboptimal quality of the recorded data. Excessive mains interference may be removed by means of a notch filter. Alternative approaches have been proposed (e.g., by Fisher et al [4]), a notch filter has the advantage of being very simple and easy to implement
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
