that is hard.
The only general advise I can give you is not to filter out any
frequencies that are of interest to you (typically in the range of the
power spectrum of the hemodynamic response) by taking a long enough
cutoff cycle, but set it short enough to filter out most signals that
can be consider low frequency noise. The latter is *very* much dependent
on your scanner, sequence, analysis, subject and so on. So run a power
spectrum on a timeseries acquired when the subjects is resting, and see
for yourself.
For your dominant task frequency run a power spectrum on the regressors
modelling your task.
OK, a rule of thumb: for block designs I would say take the longest
cycle with signal changes that correspond to your task rythm, multiply
that by 2 and use that. For a start. But really, verify with your BOLD
fMRI power spectrum.
It is very well possible your spectrum of interest is different, and you
have to deal with other low frequency noise patterns as compared to others.
Cheers,
Bas
Op 02-09-11 14:04, Anders Eklund schreef:
> 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
>>
>>
--
--------------------------------------------------
Dr. S.F.W. Neggers
Division of Brain Research
Rudolf Magnus Institute for Neuroscience
Utrecht University Medical Center
Visiting : Heidelberglaan 100, 3584 CX Utrecht
Room B.01.1.03
Mail : Huispost B01.206, P.O. Box 85500
3508 GA Utrecht, the Netherlands
Tel : +31 (0)88 7559609
Fax : +31 (0)88 7555443
E-mail : [log in to unmask]
Web : http://www.neuromri.nl/people/bas-neggers
: http://www.neuralnavigator.com (CEO)
--------------------------------------------------
------------------------------------------------------------------------------
De informatie opgenomen in dit bericht kan vertrouwelijk zijn en is
uitsluitend bestemd voor de geadresseerde. Indien u dit bericht onterecht
ontvangt, wordt u verzocht de inhoud niet te gebruiken en de afzender direct
te informeren door het bericht te retourneren. Het Universitair Medisch
Centrum Utrecht is een publiekrechtelijke rechtspersoon in de zin van de W.H.W.
(Wet Hoger Onderwijs en Wetenschappelijk Onderzoek) en staat geregistreerd bij
de Kamer van Koophandel voor Midden-Nederland onder nr. 30244197.
Denk s.v.p aan het milieu voor u deze e-mail afdrukt.
------------------------------------------------------------------------------
This message may contain confidential information and is intended exclusively
for the addressee. If you receive this message unintentionally, please do not
use the contents but notify the sender immediately by return e-mail. University
Medical Center Utrecht is a legal person by public law and is registered at
the Chamber of Commerce for Midden-Nederland under no. 30244197.
Please consider the environment before printing this e-mail.
|