Frequently with EEG data you need to filter the data as a part of the analysis. Here you will see how to implement a Butterworth filter on EEG data and also a plot to see the impact of the filter.

Download the data HERE first and put it in your MATLAB path.

The code to run is HERE and below.

clear all;
close all;
clc;
load('eegData');
samplingRate = 500;
filterOrder = 2;
lowCutoff = 0.1;
highCutoff = 30;
amountOfDataToPlot = 500;
% get some data to plot from before the filter
preFilterData = eegData(1,1:amountOfDataToPlot);
% demean the data for comparison with the filtered data
preFilterData = preFilterData - mean(preFilterData);
% define Butterworth filter
[b,a] = butter(filterOrder,[lowCutoff highCutoff]/(samplingRate/2));
% filter data one channel at a time
for channelCounter = 1:size(eegData,1)
     eegData(channelCounter,:) = filtfilt(b,a,double(eegData(channelCounter,:)));
end
% get some data to plot post filter
postFilterData = eegData(1,1:amountOfDataToPlot);
% plot some stuff
plot([1:1:amountOfDataToPlot],preFilterData,'linewidth',3);
hold on;
plot([1:1:amountOfDataToPlot],postFilterData,'linewidth',3);
hold off;
title('Pre versus Post Filter Data');