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Hi Mark, Christian-
 
good to hear from you!
Thanks for thge comments: somehow, I had read 
"set either sigma to 0 to skip that filter"
and that's where my confusion came from.
Wrt to matching the hp in FSL / SPM, http://www.fmrib.ox.ac.uk/analysis/techrep/tr04ss1/tr04ss1/node5.html seems to suggest that a given cutoff by hp-FSL corresponds to almost twice the value of a hp-cos (SPM). Or do I read that wrong?
Thanks + best regards-
Andreas
 
 

	-----Ursprüngliche Nachricht----- 
	Von: Christian F. Beckmann [mailto:[log in to unmask]] 
	Gesendet: Do 07.10.2004 09:38 
	An: [log in to unmask] 
	Cc: 
	Betreff: Re: [FSL] hp/lp-filtering
	
	

	Hi Andreas,
	
	I belive that SPM still uses a cosine-basis set for hp filtering. In
	FSL, the hp filter is implemented via a Gaussian weighted least squares
	line fit, see
	
	Marchini, J. and Ripley, B. (2000). A new statistical approach to
	detecting significant activation in functional MRI. NeuroImage,
	12(4):366–380
	
	for details. Essentially, a sliding window is moved across the time
	series and within this window a straight line is fitted through the
	data points using a least-squares criterion (but weighted by a Gaussian
	wrt to distance from the centre of the window. The window itself is
	then moved one data point and the process is repeated. As a final
	result, this technique creates a low-frequency time series which
	'follows' the main time series. When subtracted from the data, only
	higher frequencies remain. Wrt translating between packages: a
	hp-filter cutoff value should be comparable (i.e. the same hp filter
	cutoff should actually remove similar frequencies form the
	time-series).
	
	cheers
	christian
	
	On 7 Oct 2004, at 08:22, Mark Jenkinson wrote:
	
	> Hi Andreas,
	>
	> I'm afraid I don't know how the SPM filter works so I can't
	> comment on that, but the use of -1 in the call from melodic
	> just means that that filtering is to be ignored.  In the
	> help for ip it says:
	>
	> -t <hp_sigma> <lp_sigma> : Bandpass temporal filtering; nonlinear
	> highpass and Gaussian linear lowpass (with sigmas in volumes, not
	> seconds); set either sigma<0 to skip that filter
	>
	> So it is the use of sigma<0 to skip the filter which is being used.
	>
	> Hope that helps a little,
	>        Mark
	>
	>
	> On 7 Oct 2004, at 07:38, Andreas Bartsch wrote:
	>
	>> Hi folks;),
	>>
	>> there seems to be a fundamental difference of the hp-filtering as
	>> implemented in FSL & SPM. Could someone please elaborate a bit on the
	>> details? Is there a simple rule of thumb how a given hp-filter cutoff
	>> (in [secs]) translates into a corresponding value of the other
	>> package? Or is there a more profound difference in the characteristics
	>> of the filtered signal?
	>> Also: ip offers the option to low-pass filter data. By default,
	>> melodic seems call it at "-1". Is the kernel specified in negative
	>> values?
	>> Thanks + best regards-
	>> Andreas
	>>
	>>
	>
	>
	--
	  Christian F. Beckmann
	  Oxford University Centre for Functional
	  Magnetic Resonance Imaging of the Brain,
	  John Radcliffe Hospital, Headington, Oxford OX3 9DU, UK
	  Email: [log in to unmask] - http://www.fmrib.ox.ac.uk/~beckmann/
	  Phone: +44(0)1865 222782 Fax: +44(0)1865 222717