Hi Branka
IF you have very stable scanner a HP cut frequency of 1280s would be
OK, but it would be extremely rare and you would have to convince the
reviewer that this is indeed the case e.g. by using a phantom and
SPMd. The high-pass filter is used to remove low frequency noise
components, which makes the i.i.d. assumption more correct. If your
HP filter does not remove your system noise your residuals will not
be i.i.d. and you will get biased statistics (your p-values will be
too low), so not using a HP filter is not a solution either. The sad
thing is of course that the usual 128s HP filter will remove all of
your signal as well. If your experiment needs to have this many
conditions and they cannot be presented in an event related
experiment you should consider ASL or PET which has different noise
characteristics (i.e. less drift).
Best
Torben
Torben E. Lund
Danish Research Centre for MR
Copenhagen University Hospital
Kettegaard Allé 30
2650 Hvidovre
Denmark
email: [log in to unmask]
webpage: http://www.drcmr.dk
On 22 Mar 2006, at 07:40, Branka Milivojevic wrote:
> Hi all,
> I realise that a number of threads on this topic are available in
> the mailing list archives,
> but I am finding those difficult to understand.
>
> I have 6 conditions, each repeated twice during a run and the
> experimental conditions
> alternate with a fixation only condition. I've collected 4 runs of
> data, alternating the order
> of the conditions. There are only 2 orders: These are as follows:
>
> Order1:
> fix 9 seconds
> condition1 18 seconds
> fix 9 seconds
> condition2 18 seconds
> fix 9 seconds
> condition3 18 seconds
> fix 9 seconds
> condition4 18 seconds
> fix 9 seconds
> condition5 18 seconds
> fix 9 seconds
> condition6 18 seconds
> fix 9 seconds
> condition6 18 seconds
> fix 9 seconds
> condition5 18 seconds
> fix 9 seconds
> condition4 18 seconds
> fix 9 seconds
> condition3 18 seconds
> fix 9 seconds
> condition2 18 seconds
> fix 9 seconds
> condition1 18 seconds
> fix 6 seconds
>
> Order2
> fix 9 seconds
> condition6 18 seconds
> fix 9 seconds
> condition5 18 seconds
> fix 9 seconds
> condition4 18 seconds
> fix 9 seconds
> condition3 18 seconds
> fix 9 seconds
> condition2 18 seconds
> fix 9 seconds
> condition1 18 seconds
> fix 9 seconds
> condition1 18 seconds
> fix 9 seconds
> condition2 18 seconds
> fix 9 seconds
> condition3 18 seconds
> fix 9 seconds
> condition4 18 seconds
> fix 9 seconds
> condition5 18 seconds
> fix 9 seconds
> condition6 18 seconds
> fix 6 seconds
>
> The TR is 3 seconds, and I am analysing the runs as a single
> session. I've looked at the
> frequency graphs in the design matrix, and I estimated the cut-off
> frequency to be 1280
> seconds. This seems quite long. Is this correct? Should I use
> another value or perhaps
> no high pass filter.
>
> I would appreciate any help on this matter.
>
> Cheers,
> Branka
>
>
> Branka Milivojevic
> PhD candidate
> Department of Psychology
> University of Auckland
> Private Bag 92019
> Auckland
> New Zealand
> Ph: (+649) 373 7599 ext. 88636
> Fax: (+649) 3737450
|