[Eeglablist] Wrapped Phase Angle from newtimef?

Bradley Voytek bradley.voytek at gmail.com
Sat Apr 17 14:57:26 PDT 2010


Zachary:

Unwrapping the phase won't really give you what you're looking for
(though the derivative of the unwrapped phase can give you useful
information, such as "instantaneous" frequency).

First of all, I strongly support Dr. Zeman's recommendation of the
circular statistics toolbox. It's very easy to use and quite clear.

If I understand you correctly, you'd like to relate phase to EEG
variables, which I assume means amplitude, power, or latency of the of
the ERP or time-frequency variables.

I'd suggest that you take your phase time-series, such as the one you
plotted, and then extract the instantaneous phase at every time point
of interest (e.g., peak P300 amplitude). Then you can plot the
clustering of those phases in a circular plot. From that you can
extract the mean phase angle and vector length (from 0 to 1) which
would represent the preferred phase and coherence, respectively, at
your P300 peaks. This would be done using the circ_mean and circ_r
commands within the circular statistics toolbox. You can also computer
the significance of the clustering against the assumption of a random
distribution using circ_rtest, for example.

Then you can also extract the P300 peaks themselves, separately.

This would give you two vectors: P300 peak amplitude and phases at
those amplitudes. Now let's say you want to see if the two are
correlated. Obviously because you're phase data are circular, a linear
correlation isn't the correct way to do this. But you *can* correlate
one circular and one linear variable using the circ_corrcl command.

Anyway, good luck! I hope this is helpful.

::brad

On Wed, Apr 14, 2010 at 23:17, Zachary Moran <zdmoran at gmail.com> wrote:
> Hi Everyone,
>
> I'm currently investigating the relationship between phase angle and other
> EEG variables.  In order to get phase angle, I've been taking the angle(ITC)
> in Matlab which gives me phase angle estimates from -pi to pi radians.
> However, when I plot this data against time I find discontinuities at pi and
> -pi (see attached jpeg for an example), which has become problematic in
> using it as a predictor variable.
>
> Does this suggest that the phase information is "wrapped", and, if so, does
> Matlab's "unwrap(phase_angle_data)" function seem appropriate to anyone else
> as a means of making my phase angle data better suited as a predictor?
> Alternatively, I had initially wanted to constrain phase to between 0 and
> 2pi radians - does anyone have any advice for how to make this conversion?
> Any thoughts or ideas would be very greatly appreciated.
>
> Many thanks!
>
> Zach
>
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