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What do offset and drift represent in an fMRI signal?

I found an MVPA for brain fMRI toolbox online. There is one thing I cannot understand. The toolbox uses:

X = [ones(numTimeStampPerRun,1) [1:numTimeStampPerRun]']; % account for both offset and drift

to account for both offset and drift. I cannot understand what "offset" and "drift" represent in fMRI signal? Why does the toolbox use this format to account for both offset and drift?

Solution

  • My guess is that they are detrending the signal by regressing out x. In this case ones part is the offset, and 1: numTimeStampPerRun is the linear trend.

    You can always just use detrend in MATLAB though.

    EDIT: more explicitly, the linear fit has the form:

    x(t) = a*t + b*1

    where t = 1:numTimeStampPerRun.