Hi
Glad you enjoyed the course. My advise is to keep components in if you
don't have a clear interpretation as these being noise components. The
~.1Hz effects do look like cardiac maps but personally I would not
remove these - sptailly localised noise like this is not problematic
unless the areas involved overlap areas of expected signal. Components
3/4 looks very much like residual head motion in Z direction, that's
the kind of effect that should be removed using the de-noising approach.
Wrt component 2: this 2nd peak in the power spectrum looks like the
1st harmonic frequency to me - not necessarily related to double-
responses, you'd already see such an effect even with a single
response due to the fact that the response is not perfectly
sinusoidal. I'd check the peri-stimulus plots after using flobs to see
what's going on
cheers
Christian
On 7 Jul 2008, at 16:14, Heather Urry wrote:
> Hello FMRIB et al:
>
> First, thanks FMRIB for such a great course in Brisbane week before
> last--
> I learned a lot that will definitely enhance my use of FSL and
> Freesurfer.
> Plus, it was fun to meet you in person-- you're all much younger
> than I
> would've guessed! (which is not meant to suggest you sound stodgy
> and old
> via email... just wise!)
>
> Second, as a result of the course, I've been exploring using MELODIC
> at
> the single-subject level for the purpose of denoising my data prior to
> FEAT. Many of the components are obvious noise, and I have little
> worries
> about getting rid of them. Others, however, are less clearcut, and
> so I
> was hoping for some input, if anyone is willing. I made a MELODIC
> report
> available here: http://www.tufts.edu/~hurry01/ebbl/report/
> 00index.html.
> The components I'm specifically wondering about are 1, 5, 6, 7, 11,
> 13,
> 20, and 30. Components 6, 20 and 30 (and maybe 11) look to me perhaps
> like aliased cardiac signals, but I'm not sure. All of these
> questionable
> components have very fast signals with a similar power spectrum, with
> peaks at about .091146 and .109375 Hz. I would lean towards removing
> them, but maybe that would be bad?!
>
> The design I'm using is an event-related design whose fundamental
> frequency is .046875 Hz, which is nicely apparent in component 2. In
> that
> component's power spectrum, you can also see a small peak at .09375
> Hz,
> not surprising since in a sense there are at least two "events" per
> trial
> (e.g., picture appears, instruction provided 4 seconds later, then
> on to
> making a rating), and you can see in component 2 how many of the
> responses
> have little double peaks. In any case, to the extent that more than
> one
> BOLD response is generated in each trial, the task frequency could be
> double or even triple .046875 Hz. Clearly I don't want to remove
> components that contain purely task-related variation...
>
> I'd love some input from those of you who are accustomed to judging
> these
> pICA components for denoising. Thanks in advance!
>
> Cheers,
>
> Heather
>
> P.S. If there is in fact reason to believe that I have double-
> responses
> in my signal, what would be the best way to model that at the first
> level? I'm currently using the default flobs to model the canonical
> HRF
> with a temporal derivative and one that gets at dispersion as well. I
> suspect this won't capture the variation of interest... Perhaps I
> should
> generate a double-humped canonical HRF?
>
_______________________________________________
Christian F. Beckmann, DPhil
Senior Lecturer, Clinical Neuroscience Department
Division of Neuroscience and Mental Health
Imperial College London
Hammersmith Hospital - London W12 0NN
Tel.: +44 (0) 208 383 3722 --- Fax: +44 (0) 208 383 2029
Email: [log in to unmask]
http://www.imperial.ac.uk/medicine/people/c.beckmann/
Senior Research Fellow, FMRIB Centre
University of Oxford
JR Hospital - Oxford OX3 9DU
Tel.: +44 (0) 1865 222551 --- Fax: +44 (0) 1865 222717
Email: [log in to unmask]
http://www.fmrib.ox.ac.uk/~beckmann
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