How to define the knee frequency for separating low- and high-frequency slopes in PSD?

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1

Hello everyone,
I am currently attempting various approaches to analyze the differences in the 1/f slope between two groups.

I would like to divide the slopes into low-frequency and high-frequency components based on the point where the knee occurs.

My expectation was that the knee frequency in the PSD would be around 1.3 (20 Hz). Therefore, I planned to calculate and analyze the slopes for 1–20 Hz and 20–50 Hz separately.

However, the average value of the knee frequency I obtained from each PSD, calculated as "knee^(1/slope)", turned out to be around 1.11 (13 Hz), which was much earlier than I had expected. I double-checked using kernel density estimation, but found no error.

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My questions are as follows:

  1. Does the calculated knee frequency truly reflect the actual bending point in the PSD? If this value is not accurate, would it be acceptable to simply define the bending point visually at 20 Hz?

  2. I have not found any papers that aperiodic fitting only the 1–15 Hz range (or below 15 Hz). Is this because the fitting range is too narrow to produce reliable results?

  3. Are there alternative methods available for determining the bending point in the PSD?

Thank you in advance to anyone who can provide insights.

2

For these questions 1 and 3. It's not clear how the knee frequency was computed.
Did you check that the knee frequency that you encounter corresponds to the knee frequency reported in the Specparam (FOOOF) parameters? There it is given already in [Hz]. Also note that the knee frequency sometime is close to 0 Hz, so the the PSD did not bend.

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https://neuroimage.usc.edu/brainstorm/Tutorials/Fooof#Accessing_FOOOF_model_parameters

Seems likely, as 1-15 Hz range will not even comprise the entire beta band, that is a commonly analyzed band in brain recordings