[Eeglablist] Is ITC biased by trial numbers?

Zara Bergström zmb25 at cam.ac.uk
Fri Nov 19 08:50:27 PST 2010

Dear EEG experts,

is the ITC measure as implemented by EEGLAB biased towards lower trial 
numbers (i.e. higher itc when fewer trials are used in the computation, as 
some measures of phase coherence supposedly are), and if so, how do you 
deal with that issue when comparing conditions with different trial 
numbers? Do you think it is appropriate to compute baseline corrected ITC, 
which might help?

I analysed epoched datasets (-1-2s using default pre-stimulus baseline for 
ERSP) with wavelets using newtimef to get complex ITC values (using the 
default 'phasecoher' option), converted the ITC to real numbers between 0-1 
(absitc=sqrt(real(itc).^2+imag(itc).^2)), and averaged these into 
participant x condition x time x frequency ITC matrices for use in group 
level statistics.

The attached line plot shows grand average (24 subjects) ITC averaged 
across the alpha band (8-12 hz) for four conditions. The condition with the 
fewest average trial numbers (red line) has significantly higher ITC than 
the other conditions throughout the trial, even before stimulus onset, 
which cannot be explained by psychological factors since these conditions 
were presented randomly intermixed. If I however were to subtract the 
average baseline period ITC from the post-stimulus data, it seems that the 
difference would disappear. Would that be an appropriate step to take here?

Do you think this pattern is caused by a trial number bias, or have I done 
something wrong in the analysis pipeline?

Any thoughts would be very much appreciated.

Thanks very much for you time,

Zara Bergstrom
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