[FieldTrip] Wavelet advice

Peter Goodin pgoodin at swin.edu.au
Mon Aug 20 01:20:49 CEST 2012

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.

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 6:58 AM
To: FieldTrip discussion list
Subject: Re: [FieldTrip] Wavelet advice

You can always try variables number of cycles may be start with 3 and then go to 5.
Martinos Center

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.


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