[FieldTrip] Wavelet advice
"Jörn M. Horschig"
jm.horschig at donders.ru.nl
Tue Aug 21 13:24:21 CEST 2012
Hey Peter,
when doing what Sheraz suggested you should, however, keep in mind that
the edges are computed using data outside your epoch, i.e. possible
muscle artifacts and other stuff (e.g. stimulation) will corrupt the
frequency estimation if not properly accounted for. Make sure to note
that when interpreting your data.
Best,
Jörn
On 8/20/2012 5:18 AM, Peter Goodin wrote:
> Thanks for that! Much appreciated.
> ------------------------------------------------------------------------
> *From:* fieldtrip-bounces at science.ru.nl
> [fieldtrip-bounces at science.ru.nl] on behalf of Sheraz Khan
> [sherrykhan78 at gmail.com]
> *Sent:* Monday, 20 August 2012 11:29 AM
> *To:* FieldTrip discussion list
> *Subject:* Re: [FieldTrip] Wavelet advice
>
> You can not avoid the border effects they comes from the convolution.
>
> Only possible solution which I sometimes use when the epochs are
> shorter due to the design, is to do the wavelet decomposition on
> the continuous data, and then epoch it, this will avoid the borders
> from the epoch.
>
> Sheraz
> Martinos Center
> MGH/MIT/Harvard
>
> On Sun, Aug 19, 2012 at 7:20 PM, Peter Goodin <pgoodin at swin.edu.au
> <mailto:pgoodin at swin.edu.au>> wrote:
>
> Thanks for all the replies so far.
>
> Re-reading my previous question I appear to have been a bit vague
> to the overall problem.
>
> The problem is I've used wavelets widths starting from 1 and
> increased to the default 7, but only a value of 1 gives me any
> usable data around 4Hz.
>
> What confuses me is that Kaplan et al, 2011
> (http://www.plosbiology.org/article/info%3Adoi%2F10.1371%2Fjournal.pbio.1001267)
> were able to get theta band activity using epochs of 1s (200ms pre
> 800ms post) without apparent boundary effects at a width of 5, but
> when I tried the same thing (including zero padding), the results
> were not usable.
>
> Again, any advice and explanations as to why 1. a value of 1 is
> not usable and 2. why I may be getting these boundary effects
> would be greatly appreciated.
>
> Peter
> ------------------------------------------------------------------------
> *From:* fieldtrip-bounces at science.ru.nl
> <mailto:fieldtrip-bounces at science.ru.nl>
> [fieldtrip-bounces at science.ru.nl
> <mailto:fieldtrip-bounces at science.ru.nl>] on behalf of Sheraz Khan
> [sherrykhan78 at gmail.com <mailto:sherrykhan78 at gmail.com>]
> *Sent:* Monday, 20 August 2012 6:58 AM
> *To:* FieldTrip discussion list
> *Subject:* Re: [FieldTrip] Wavelet advice
>
> Hi,
> You can always try variables number of cycles may be start with 3
> and then go to 5.
> Sheraz
> Martinos Center
> MGH/MIT/Harvard
>
> On Aug 19, 2012 7:17 AM, "Peter Goodin" <pgoodin at swin.edu.au
> <mailto:pgoodin at swin.edu.au>> wrote:
>
> Hi Fieldtrip list,
>
> I'm attempting to use wavelets to analyse some data collected
> with a neuromag system (sample rate of 500Hz, pre-trigger
> period is 200ms, post is 1.6 seconds) but am new to time
> frequency analysis. I'm interested in lower frequency bins (~4
> to 20 Hz). The config settings I've been playing with are as
> follows:
>
> cfg.method = 'wavelet';
> cfg.channel = 'MEG';
> cfg.keeptrials = 'yes';
> cfg.foi = [4:1:20]
> cfg.toi = [-.1:.01:1.5]
> cfg.width = x
>
> Using a default width of 7, I'm getting large boundary effects
> leading to a period of ~300ms (400 - 700ms post trigger) of
> calculated data at 4Hz. I've played with the width and have
> found that a width of 1 gives only a small amount of boundary
> effect at the extreme edges and shows a large increase in the
> frequencies I'm interested in at a time point which
> corresponds quite nicely with the ERF data also analysed.
>
> Having read the Tallon-Baudry (1999) article, material from
> the tutorials and items from the mailing list, I understand
> that by using a wavelet width of 1 I've biased my results
> towards higher temporal vs. spectral resolution, but
> considering the low range of frequencies I'm interest in, is
> this a problem? Does using a low width with my cfg.foi set as
> it is just lead to a large amount of redundant data?
>
> Advice on this subject would be greatly appreciated.
>
> Peter.
>
>
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--
Jörn M. Horschig
PhD Student
Donders Institute for Brain, Cognition and Behaviour
Centre for Cognitive Neuroimaging
Radboud University Nijmegen
Neuronal Oscillations Group
FieldTrip Development Team
P.O. Box 9101
NL-6500 HB Nijmegen
The Netherlands
Contact:
E-Mail: jm.horschig at donders.ru.nl
Tel: +31-(0)24-36-68493
Web: http://www.ru.nl/donders
Visiting address:
Trigon, room 2.30
Kapittelweg 29
NL-6525 EN Nijmegen
The Netherlands
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