In one study, I use 1-minute recordings with participants in a resting state. After treating possible artifacts, I divide the recording into 2-second windows. Then, I apply the "Coherence NxN [2021]" process to the generated window series, using the "Magnitude-squared Coherence" option.
I configure the remaining parameters based on reference studies. However, I have doubts about the process output. Should I select the "Save individual results..." option, which will generate multiple files to later calculate the average, or the "Average cross-spectra..." option, which will generate a single file?
Another point, is it possible to calculate the average coherence values for a frequency range? For example, Theta (4 - 7Hz)... When exporting the file, it asks me to choose a specific frequency (e.g., 5.08Hz). Is it possible to obtain average values for the entire range?
"Save individual results..." This option will compute one estimation of MSC for each epoch, using the data only for that epoch, using N windows (N depend on the window length and percentage of overlap). This option can be used to evaluate any change in coherence across epochs.
"Average cross-spectra..." This option will compute one estimation of MSC for all the epochs, using the data from all the epochs, using N windows × number-of-epochs, thus it is a better estimation of MSC.
The average of MSC results from Option1 are not the same as the MSC obtained with Option 2 as MSC is not linear.
The Coherence NxN [2021] processes uses the Fourier transform to compute the PSD and cross-spectrum, to later compute MSC. Thus, MSC has the same resolution than FT. To obtain MSC in a band range, a different approach can be take, by filtering data and using Hilbert transform to compute the T-F representation and compute the PSD and cross-spectrum from that T-F representation. More information on these differences here: Lagged coherence - #3 by Raymundo.Cassani
In a not-too-far update, the coherence process GUI will be updated to include both (Fourier transform and Hilbert (or wavelet) transform) approaches
Yes, with the process Frequency > Group in time or frequency bands
So the process mentioned above, splitting a 1-minute file into 2-second blocks, applying MSC Coherence, saving it as 'Average cross-spectrum,' and then averaging the results in frequency bands, is a usual process. Can I use it for all participants and statistical analysis, right?
Another question: The parameter 'Overlap for PSD estimation' set to 50% is sufficient for a recording with 19 channels and dozens of 2-second windows? Some articles indicate 75% as the value.
The average of MSC results from Option1 are not the same as the MSC obtained with Option 2 as MSC is not linear.