FILTERS

Introduction

Filters in EEG are used to try and remove non-cortical activity from the reading frame. We know that the brain produces waves on a spectrum from lower frequency delta waves, to higher frequency gamma waves. We find that for the purpose of EEG, waveforms from 1-70 Hz reflect cortical activity and everything else is generally background noise. In order to filter these extracortical frequencies out, we use high frequency filters (HFF) and low frequency filters (LFF) to create a “Band pass” filter that effectively gets rid of frequencies less than 1 Hz, and greater than 70 Hz.

High Frequency Filter (HFF)

The high frequency filter is named as such because it filters out high frequencies from the EEG. If it is filtering out high frequencies, that would also mean that it is allowing lower frequencies to make their way into the EEG. This is where the high frequency filter gets its other commonly used name, the Low-Pass filter. The default setting for most EEG systems is a High Frequency filter set at 70 Hz, meaning that it is trying to filter out waveforms that are greater than 70 Hz.

Low Frequency Filter (LFF)

The low frequency filter gets its name from the fact that it filters out low frequencies and stops them from making their way into the EEG. If the low frequency filter is filtering out lower frequencies, it can be assumed that it is allowing in faster frequencies and thus derives its other commonly used name, the High-Pass filter. The default setting for most EEG systems, is a Low Frequency filter at 1 Hz, meaning that it is trying to filter out waveforms that are less than 1 Hz.

Band-Pass Filter

We’ve established that the High Frequency filter filters out high frequency waveforms and that the Low Frequency filter filters out low frequency waveforms. When we put these filters together, we get what is called the Band-Pass filter, as it only allows a “Band’ of frequencies between to be seen on the EEG in default while attenuating waveforms that are unlikely to be from cortical activity. A band-pass filter is essentially just the combination of the Low Frequency Filter and High Frequency Filter, and is what is used in every EEG recording.

Introduction to the 70/30 Rule

The 70/30 rule says that if the frequency of a waveform matches the filter setting I.E. (70 Hz waveform and 70 Hz High Frequency Filter setting) the waveform will be attenuated by 30% and you will be able to see the remaining 70%. Using this rule can help us decide how much a waveform will be attenuated by certain filter settings, which will be a high yield concept come test day.