A reviewer has, quite rightly, questioned our use of the standard hrf when comparing two groups of elderly participants in an event related study. It has been noted that the hrf is "similar in amplitude and form across groups of young and elderly adults....however the elderly adults show greater voxelwise noise" Huettel et al 2001.
What is the best way to model the hrf? Adding and temporal and dispersion derivatives, would not, I think, allow estimation of activation strength. I initially assumed that using 6 basis functions would allow me to model the hrf, but I'm unsure how to proceed from there. Does the beta value for each basis function need to be divided by the constant term before summing?
My final aim is to extract the equivalent of "beta" values from an roi (activation strength), to compare with some behavioural measures.
Thanks in advance,
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