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 -----UrsprÃ¼ngliche 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):366â€“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