[FieldTrip] Wavelet advice

[Peter Goodin]

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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