[Eeglablist] Compare the head plots

[David Groppe]

Hi Ming,
  Testing for differences between two topographies is actually 
surprisingly tricky.  Conventional tests can have an inflated Type I error 
rate when the the magnitudes of the two topographies are inequal, which is 
often the case.  This happens when hypothesis tests do not adequately 
compensate for the noisier estimate of the smaller topography.
  This problem is discussed in detail and illustrated with simulated data
in the following paper:

 Urbach, T. P., & Kutas, M. (2002). The intractability of scaling scalp 
distributions to infer neuroelectric sources. Psychophysiology, 39(6), 
791-808. 

for the most conventional technique for testing for ERP topography 
differences, an ANOVA after topography normalization.
  I've tested a couple other permutation based tests of topography 
differences:

Galan, L., Biscay, R., Rodriguez, J. L., Perez-Abalo, M. C., & Rodriguez, 
R. (1997). Testing topographic differences between event related brain 
potentials by using non-parametric combinations of permutation tests. 
Electroencephalography and Clinical Neurophysiology, 102(3), 240-247. 

Greenblatt, R. E., & Pflieger, M. E. (2004). Randomization-based 
hypothesis testing from event-related data. Brain Topography, 16(4), 
225-232. 

and found that they can also have inflated Type I error rates when 
topography magnitudes differ.
  I have matlab code for the former of these on my website:
http://www.cogsci.ucsd.edu/~dgroppe/eeglab.html

  But if you use this code, you should qualify any significant differences
you find with the caveat that the Type I error rate is possibly higher
than your chosen alpha level.  And it would help if you reported the 
number of participants who showed the specific type of topography 
difference (e.g., larger over the left hemisphere in one condition than 
another) to give a sense of how reliable the *specific* topography 
difference is.
    -David


On Wed, 23 May 2007, Arnaud Delorme wrote:

> Dear Ming,
> 
> > I wonder if somebody have experience on comparing two headplots.  For
> > example, one headplot is plotted using average trial data for 1 condition
> > (the EEG response of recognizing something) and one headplot is plotted
> > using average trial data for the opposite condition (the EEG response of
> > not recognizing something).  How can I compare these two and highlight
> > their difference?  Can I just do a simple subtraction and use topoplot
> > function to plot out the difference?  Any suggestions?
> 
> Yes, performing a subtraction sounds like a good solution. If you create 
> a study and you have several subjects, EEGLAB can also compute 
> parametric and non-parametric statistics on the difference (it is not 
> yet able to show the difference though).
> 
> Best,
> 
> Arno
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