[Eeglablist] How ERSP, ITC comparisons are computed?

[Arnaud Delorme]

The comparison for ERSP is a simple difference of log-power values after 
common baseline subtraction (common subtraction if 2 conditions).

To compute statistical difference, each time-frequency point is 
considered independently. Say A(f,t) is the ensemble (across trials) of 
time-frequency decomposition for condition A at frequency f and time t 
(noted simply A below) and B(f,t) is the ensemble of time-frequency 
decomposition for condition B (f and t are fixed in the whole 
procedure). C is the union of time-frequency decomposition trials A and 
B. We take a randomly elements in C (each element is only taken once) to 
construct two permutation ensemble A' and B' (A' being the same size as 
A and B' the same size as B but both A' and B' usually contains elements 
both from A and B). We then recompute the difference between the average 
power of trials in A' and the average power of trials in B'.

We repeat the procedure in between 200 and 5000 times to build a 
distribution of difference power values under the hypothesis that A and 
B are drawn from the same population of trials (thus the original power 
difference should lie within the permutation distribution). If it does 
not lie within this distribution (for instance it is the tail of the 
distribution - lower 2.5% or the upper 2.5% - then we can say that we 
reject the Null hypothesis at 5%). It is easier to explain with graphics.

The same is performed for ITC.

Hope this helps,

Arno

Soyoung Kwon wrote:
 > Dear all,
 > 
 > I wonder how ERSP and ITC comparisons are computed while using newtimef.
 > 
 > Is ERSP comparison merely a mathematical difference between two 
different conditions?
 > Or is it statistically calculated?
 > If so, please let me know the simple protocol of statistical method.
 > Equation is also welcome.
 > 
 > Simliar question is arising with ITC comparisons.
 > 
 > Please give me any advice regarding ERSP, ITC and differences of these.
 > 
 > Thanks in advance.
 > 
 > 
 > Soyoung Kwon