[Eeglablist] ERSP and ITC measures as revealed in ERPimage plots

[Tineke Grent - 't Jong]

Hi all,

I have some questions about the output of the ERSP and ITC values in the ERPimage plots that I can't find appropriate answers to. Here are the questions:

1) What does the number, specified in dB, plotted above the baseline of the ERSPs actually reflects? I understand that it is the output of a log-conversion (10log(power^2) to be precise) of the absolute baseline powerlevel (at least that is what I get from the tutorial). However, how can it reflect an ABSOLUTE value? By convention, dB is used to express a RATIO between two values. Shouldn't it be relative to something else? And what is it that it is compared to?

2) If the ITC is not significant, does this then automatically indicate that all (remaining) activity in the plotted frequency range is non-phaselocked? How do I have to read this? Does an overall non-significant ITC value of 0.2, for example, means that only 20% of the data shows some sort of phase-locking (although seemingly not significantly different between trials), whereas 80% of the data is non-phaselocked? How do I know whether that non-phaselocked activity is actually significant? 

3) How can I combine all the information provided by ERPimage in a meaningful way? Say I want to know what percentage of the data (or how much of the data in terms of amplitude values) is non-phaselocked and thus not reflected in the ERPs. Using ERPimage information, how would I get an estimate of this non-phaselocked information, contributing to the effect I am looking for in a particular experiment? Or do I need to adopt a totally different approach to get to that question? If yes, what approach would provide me with an answer to this question?

Any input on any of these questions would be greatly appreciated ;-). 

Tineke Grent-'t-Jong