[FieldTrip] Phase Slope Index: How to choose bandwidth

[Hähnke, Daniel]

Hi all,

since only a few people seem to be using the PSI, I went into the code to see what it actually does. I would like to point out that there are some issues with the current implementation if you’re using logarithmically spaced frequency axes.

The parameter cfg.bandwidth (in Hz) is used to calculate the number of frequency bins that is used to calculate the PSI. This calculation, however, is only done once using the frequency from the lowest frequency bin. If the chosen bandwidth is not an integer multiple of a frequency bin step, this computation leads to a changed bandwidth.
If your frequency axis is not evenly spaced this leads to various bandwidths across the frequency axis, since the code only uses a fixed number of frequency bins across which to compute the PSI.

I think it would be useful to change the cfg.bandwidth input from Hz to number of bins. Also, people should be aware of the fact that the PSI value is correlated with the bandwidth. So, if you use a logarithmically spaced frequency axis, you should normalize your PSI values by the bandwidth corresponding to each frequency bin.

Best,

Daniel

> On 01.03.2016, at 11:51, Hähnke, Daniel <daniel.haehnke at tum.de> wrote:
> 
> Dear FieldTrip users,
> 
> I currently want to double-check the results I got from a non-parametric Granger-causality  analysis with another measure of directed communication, phase slope index.
> However, after having read Nolte et al.’s paper from 2008 that introduced the PSI, I still don’t understand how to best choose the parameter cfg.bandwidth. 
> 
> I tried out different values and got something qualitatively very similar to my Granger results. But this is rather fitting the parameter to the Granger results and kind of undermines my purpose of double-checking.
> 
> Furthermore, I am using a logarithmic (to base 2) frequency axis. Consequently, my frequency bins are not evenly spaced. I have the feeling that I would rather need different bandwidths for the individual frequency bins.
> 
> Does anyone know how to objectively choose the bandwidth parameter?
> 
> Best,
> 
> Daniel
> --
> Daniel Hähnke
> PhD student
> 
> Technische Universität München
> Institute of Neuroscience
> Translational NeuroCognition Laboratory
> Biedersteiner Straße 29, Bau 601
> 80802 Munich
> Germany
> 
> Email: daniel.haehnke at tum.de
> Phone: +49 89 4140 3356
> 
> 
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