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e is a different reason that would be hard for me to pinpoint if I can't reproduce the problem here. Vladimir On Wed, Feb 2, 2011 at 3:35 PM, Michael Spezio <[log in to unmask]> wrote: > Hi Vladimir, > > I tried the <2 Hz highpass setting for the sampling rate for the tutorial > data, which is 2048 Hz. But I also tried first downsampling to 500 Hz, then > applying <2 Hz filtering, and it also failed for that sampling rate. Both > times, filtering was applied to continuous data rather than to epoched data. > > Perhaps I will try the IIR filter via the batch interface. > > Best, > > Michael > > > Michael L. Spezio, Ph.D. > Assistant Professor of Psychology > Department of Psychology > Scripps College > Claremont, CA > [log in to unmask] > 909.607.0914 > > and > > Visiting Associate Scientist > Psychology & Neuroscience > Division of Humanities & Social Sciences > Caltech > Pasadena, CA 91125 > [log in to unmask] >>>> Vladimir Litvak <[log in to unmask]> 02/02/11 7:26 AM >>> > It's probably an SP toolbox issue, but I have never seen an example of > this happening for any setting. It usually only happens for some > settings. What is your sampling rate? Is your data epoched or > continuous? If it's epoched perhaps your epochs are not long enough? > If it's continuous I'd try changing the filter order or using an IIR > filter (you can do it if you use batch interface to filtering). Or > perhaps downsampling event to a slightly different rate will help. > > Vladimir > > On Wed, Feb 2, 2011 at 3:16 PM, Michael Spezio > <[log in to unmask]> wrote: >> Great, thanks for this very helpful response, Vladimir. >> >> I have noticed that even with using just highpass filtering, any setting >> below 2 Hz results in the data corruption issues. If this is an issue with >> the SP toolbox, then the Mathworks should be contacted to fix it. If it's >> a >> problem with SPM8, then may there be an upgrade in the future? >> >> Best, >> >> Michael >> >> >> Michael L. Spezio, Ph.D. >> Assistant Professor of Psychology >> Department of Psychology >> Scripps College >> Claremont, CA >> [log in to unmask] >> 909.607.0914 >> >> and >> >> Visiting Associate Scientist >> Psychology & Neuroscience >> Division of Humanities & Social Sciences >> Caltech >> Pasadena, CA 91125 >> [log in to unmask] >>>>> Vladimir Litvak <[log in to unmask]> 02/02/11 3:19 AM >>> >> Dear Michael, >> >> On Wed, Feb 2, 2011 at 7:02 AM, Michael Spezio >> <[log in to unmask]> wrote: >>> Hi Vladimir, >>> >>> I saw in your post 041502, dated 2010-05-07, that for some reason >>> bandpass >>> filtering with wide bands (e.g., 0.1 to 30 Hz) elicits bad performance by >>> the Matlab SP Toolbox and causes displays of the subsequent data to fail. >>> >>> Can you specify why this is occurring? It doesn't seem to make sense >>> given >>> that wide bandpass filters are used in signal processing routinely. >>> >> >> It's not a display issue but actual data corruption issue. There are >> numerical stability problems that I have since also observed in >> high-pass filters for some combinations of filter settings and >> sampling rate. These problems are common for wide bandpass filters, >> especially for ones with low cut-off close to DC (e.g. 0.1-40 Hz). >> Therefore, I recommended that people use separate high-pass and >> low-pass in these cases. In the next SPM8 update there is a change in >> the code that detects automatically when a filter is unstable and >> gives an error so people will not have to figure it out later when >> their display crashes. >> >>> Also, for the Multimodal Face tutorial, why isn't filtering done prior to >>> downsampling? Downsampling, especially to 200 Hz or lower, should only be >>> done after filtering out higher frequency contaminating signals, measured >>> at >>> a high sampling rate (500 Hz or above), to avoid aliasing from muscle >>> signals at frequencies of 90 Hz to 200 Hz. Can you help me understand >>> what >>> is going on with the bandpass processing and why filtering is left out of >>> the tutorial? >>> >> >> Aliasing can occur when downsampling is done by simple decimation in >> the time domain (e.g. taking every other sample). SP toolbox >> downsampling routine is smarter than that and it pre-filters data to >> avoid aliasing so it is not necessary to do it explicitly. In the >> absence of SP toolbox we use or own routine that downsamples in the >> frequency domain by truncating the DFT coefficients. This way there is >> also no aliasing. The only problem that can occur is that if there are >> large DC offsets in the data, low-pass filter will cause ringing at >> the edges. Therefore I'd suggest to apply high-pass filtering before >> downsampling especially for epoched data. In the case of the faces >> tutorial this is not a problem because downsampling is done on >> continuous data (for EEG) or on baseline-corrected data (for MEG). >> >> Best, >> >> Vladimir >> >>> Thanks so much. >>> >>> Best, >>> >>> Michael >>> >>> Michael L. Spezio, Ph.D. >>> Assistant Professor of Psychology >>> Department of Psychology >>> Scripps College >>> Claremont, CA >>> [log in to unmask] >>> 909.607.0914 >>> >>> and >>> >>> Visiting Associate Scientist >>> Psychology & Neuroscience >>> Division of Humanities & Social Sciences >>> Caltech >>> Pasadena, CA 91125 >>> [log in to unmask] >>> >> >