Subtracting mean of columns from corresponding columns in 3d matrix

Hi!

I am trying to use a for loop in a function to subtract the mean value (baseline) of column 1:51 for 64 rows, and subtract this value from the corresponding columns. The matrix (epochs_CG) is 64x250x586. So for example, for epochs_CG (1, 1:51, 1) the baseline = mean (epochs_CG (1, 1:51, 1)), and the baseline corrected epoch for that channel (row) is epochs_CG(1, 1:51, 1) - baseline.

However, I need to do this calculation in a loop so it can subtract the baseline from the for all the rows and for all the different epochs, but I just can't seem to get it right. Here is my function so far; Somehow, I'm

function epochs = baselineCorr(epochs, startT)
    Fs = 250;                                   % Sampling Frequency (Hz)
    Ts = 1/Fs;                                  % Sampling Interval (s)                             
    startIdx = floor(startT / (Ts * 1E+3));     % Start Index
    
for i = 1:length(epochs) 
    baseline = mean(epochs(:,51+startIdx:51,i),2);
    epochs(:,51+startIdx:51,:) = epochs(:,51+startIdx:51,:) - baseline  ;
end

This does not seem to be doing what i want it to do :P Would appreciate any help!

Answer 1

[Alex Mcaulley]

Try this, is the result the expected one?

epochs = rand([64,250,586]);
col = 51;
baseline = mean(mean(epochs(:,1:col,:),1),2);
epochs(:,1:col,:) = epochs(:,1:col,:)-repmat(baseline,size(epochs,1),col,1);

[User48765]

function epochs = baselineCorr(epochs, startT)
    Fs = 250;                                   % Sampling Frequency (Hz)
    Ts = 1/Fs;                                  % Sampling Interval (s)                             
    startIdx = floor(startT / (Ts * 1E+3));     % Start Index
    
    baseline = mean(epochs(:,51+startIdx:51,1),2);
    epochs(:,51+startIdx:51,:) = epochs(:,51+startIdx:51,:)- repmat(baseline, size(epochs,1),51,1); 
end

Hi, thank you! It worked fine when i ran your code by itself. When I implemented your code into my function however, I got this error: "Array dimensions must match for binary array op".

Did I implement it wrong maybe?

[Alex Mcaulley]

what is the value of startIdx? If it is positive (51+startIdx:51) is empty.

Anyway, there is an error in this line:

baseline = mean(epochs(:,51+startIdx:51,1),2);

It should be

baseline = mean(mean(epochs(:,51+startIdx:51,:),1),2);

[User48765]

thank you, I got it working!