Dear everyone,

I would like to rerun an SPM analysis with polynomial regressors accounting for slow frequencies instead of SPM's default high-pass filter. As there are different options (e.g. Legendre ones) I would like to rely on some established method, which made me come to AFNI. Unfortunately I have no experience with AFNI, thus a few questions. My session would be < 20 min, suggesting an order of 8 based on 1 +int(dur/150 s).
1) Is this commonly applied or are there instances in one would want to turn to a lower order (and/or turn to other polynomials)? For example in SPM the default cut-off value 1/128 Hz for the Discrete Cosine Transform is appropriate for designs in which there's sufficient signal within the fast frequencies, but one might want to adjust the cut-off in case of long block designs with long intervals between two blocks of the same type in which one would also expect signal in the low frequency range. This should also hold for the polynomial regressors.
2) Are there any additional calculations on the polynomial regressors before they enter the design matrix and/or at a latter step performed automatically in AFNI but possibly not in SPM (e.g. orthogonalization of the regressors, well, Legendre polynomials should be orthogonal anyway)?

Thank you very much!

Best regards,

Helmut