EEG signal plot varies if all channel data is plotted separately...

I have plot my EEG data with the code given below:

x=load('02SegmentedData.mat');

LC=length(x)

fs=125

TC=[0:LC-1]/fs

%Plotting

figure;subplot (211),plot(TC,x)

xlabel('Time (s)'), ylabel('Amplitude (uV)'), title('Complete EEG data')

hold on;

I got the plot with the code given ablove but it is different from the plot which i got after plotting each channel data seperatley with the code given below:

for k=1:16

subplot(4,4,k),plot(TC,x(:,k))

xlabel('Time (s)'), ylabel('Amplitude (uV)'), title(sprintf('EEG signal of %d channel',k))

hold on;

end

Any help will be highly appreciated.Why i am getting diffenerent amplitude value in these two graph

Answer 1

[Aastha]

Hi @Saima Raza,

I understand that you are observing a discrepancy between the EEG plots generated using the two different code snippets: one where all channels are plotted together and another where each channel is plotted separately.

This difference in appearance is primarily due to axis scaling, not due to any change in the actual amplitude values caused by the "plot" function.

When you use the MATLAB code mentioned below:

figure; subplot(211), plot(TC, x)

all 16 EEG channels are overlaid on the same subplot. As a result, the y-axis automatically scales to encompass the full range of values across all channels, from the minimum amplitude (e.g. -6 µV from channel 1) to the maximum amplitude (e.g. 6 µV from channel 10). This broader range can make individual variations look smaller and compressed.

When you plot each channel separately using the MATLAB code snippet mentioned below:

for k = 1:16
    subplot(4,4,k), plot(TC, x(:,k))
    xlabel('Time (s)'), ylabel('Amplitude (uV)'), title(sprintf('EEG signal of %d channel', k))
    hold on;
end

Each subplot is independently scaled based on the amplitude range of that specific channel. This allows each plot to show more detail and variation within its own amplitude range, making the signals appear more distinct.

So, the perceived differences between the two plots are purely visual due to the automatic scaling behaviour of MATLAB subplots; There is no alteration in the underlying EEG data or amplitude values.

For more information on the "plot" and "subplot" functions, you may refer to the following MathWorks documentation:

- plot: https://www.mathworks.com/help/matlab/ref/plot.html

- subplot: https://www.mathworks.com/help/matlab/ref/subplot.html

I hope this is helpful!