Date sent: Tue, 29 Oct 2002 14:52:49 +1100
Send reply to: FSL - FMRIB's Software Library <[log in to unmask]>
From: Mark Jenkinson <[log in to unmask]>
Subject: Re: [FSL] FEAToutput, ANALYZE hdr and HRF parameters.
To: [log in to unmask]
Hi there!
Mark,thank you very much for your reply/comments.
When I look at the values in filtered_func_data and compare them to those of the
original data, I notice that they differ by a factor of almost 1000 :I open the file in
MATLAB.I also visualize it in that environment, so I can see that the images are not
'scrambled', and basically look the similar to those I feed in FSL -but they have better
contrast, as they should!.I have tested my 'getANALYZE' routine independently and it
seems to be reading the data correctly. Still this difference in mean puzzles me and I
was wondering if it is something reasonable (ie simply the result of the filtering) or
whether I should look more into it.
Concerning the low-pass filtering,you raised an issue that I would like to revisit,
because I think I have probably misunderstood something here (in regards to both how
FSL works and how it contrasts with SPM):
I equate the process of convolving the data with the HRF to low-pass filtering.From
what you mentioned in your posting I gather that this is not so.
'Taking it from the begginning' then, am I correct in the following statements ?:
In FSL:
Eessentially what happens is that the design matrix gets convolved with the HRF - and
that, plus the high pass filteringon both sides of the GLM, is all the smoothing that takes
place in the temporal domain.
In SPM:
There is the added step of applying the low pass filter on both sides of the
GLM(signal+design matrix), what they call 'coloring',I suppose.
If these statements are correct, I am a little confused about the HRF convolution step -
isn't that in a certain sense a low pass filter in itself, and if that is the case, wouldn't
SPM be 'filtering twice' the design matrix?
Cheers !
Charis.
Charidimos Tzagarakis MD
Brain Sciences Center and University of Minnesota.
Address:
Brain Sciences Center (11B)
Minneapolis VA Medical Center
One Veterans Drive
55417 Minneapolis MN
USA
Tel:612-725-2000 ext 1756
email: [log in to unmask]
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