[FieldTrip] Wavelet advice

[Peter Goodin]

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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