[Eeglablist] Wrapped Phase Angle from newtimef?

[Philip Michael Zeman]

Hi Zach,

a quick look at your data suggests that you have simply run into a fundamental characteristc of measuring phase angle. Phase angle is a continuous and repeating value. Hence, a static, unchanging, reference point is needed.  There is probably a simple tweak you can make to your algorithm that can 'fix' your issue.  Alternatively, there are some transformations you can make:


See:

http://en.wikipedia.org/wiki/Quaternion

I also came across some information about calculating 'circular means' from circular statistics.  I haven't had a chance to check this out, but you might find it helpful.  I think this was posted on the EEGLab group a while back.
http://www.mathworks.com/matlabcentral/fileexchange/10676-circular-statistics-toolbox-directional-statistics


Regards,

Phil

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Philip Michael Zeman B.Eng, Ph.D.
Applied Brain and Vision Sciences Inc.
Brain Function Analysis for Novel Paradigms and Serious Games
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Email:   pzeman at alumni.uvic.ca
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  ----- Original Message ----- 
  From: Zachary Moran 
  To: eeglablist at sccn.ucsd.edu 
  Sent: Wednesday, April 14, 2010 11:17 PM
  Subject: [Eeglablist] Wrapped Phase Angle from newtimef?


  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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