Simply, note that if you decrease your cutoff frequency to 40s or 60s, you remove your experimental effect.
Actually the order of the DCT is (in matlab notation)
order = floor ( 2*N*TR/d_cut + 1 )
where N = number of scans, TR = time resolution, d_cut = cutoff_period
Hence, little changes in the cutoff period could not actually change the order of the DCT
I would consider longer cutoff period for the boxcar design with 60s than for the 40s. Let's say 256s?
As Bas suggested, 'Have a look at the relevant chapters in Statistical Parametrical Mapping (the book). It's all there.' Page 123.
Cesar
On Sep 2, 2011, at 2:04 PM, Anders Eklund wrote:
> Thank you, are there any general pointers for how to set the cutoff?
>
> Should the cutoff frequency be different for a boxcar design with periods of 40 seconds compared to a boxcar design with periods of 60 seconds?
>
> /Anders
>
> S.F.W. Neggers skrev 2011-09-01 10:54:
>> Op 02-09-11 00:05, Anders Eklund schreef:
>>> Hello,
>>>
>>> what kind of detrending is performed by default for single subject fMRI analysis in SPM? I can't see any settings for detrending. Is the highpass filtering done instead of (linear, quadratic, cubic) detrending?
>> Yes. Discrete Cosine Transformation regressors are 'added' (not shown) to your design matrix with a wavelength up to the cutoff frequency you enter for high pass filtering. There is no reason (imho) to assume this works less well then the polynomial description you suggest. I say 'added' as in fact the entire GLM equation is filtered this way, but that is roughly equivalent.
>>
>> Have a look at the relevant chapters in Statistical Parametrical Mapping (the book). It's all there.
>>
>> Cheers,
>>
>> Bas
>>>
>>> /Anders
>>
>>
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