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SPM  February 2011

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Subject:

Re: Bandpass filtering with wide bands in spm8: Why is there a problem?

From:

Vladimir Litvak <[log in to unmask]>

Reply-To:

Vladimir Litvak <[log in to unmask]>

Date:

Wed, 2 Feb 2011 15:45:17 +0000

Content-Type:

text/plain

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That's really strange and should not happen. Perhaps there 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]
>>>
>>
>

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