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]
>>
>
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