I'm thinking about the averaging and smoothing of data from a group of subjects, and how far an individual's own average frequencies could differ from the group's. The perspective of the investigator is to arrive at generalizations. If I want to entrain an individual's theta frequencies, for instance, the individual's strongest response could be at a different frequency from the group's experimental mean theta frequency.
Will depend on the method being used (fft will be noisier than psd, wavelets will be far more accurate than both), and whether or not you are using absolute or relative power. Individual differences might be spurred on by many comorbid or chemical factors, so it will be worth to assess these before jumping to conclusions.
Individuals frequency spectra will differ in peak frequencies and amplitude due to pure biological differences such as volume conduction (different skull thicknesses will result in different observed EEG amplitudes, for instance) and head size (larger heads may be correlated with lower alpha rhythms; Nunez et al., 1978). A consistent empirical result is that frequency peak location changes based on cognitive deficits. This may be due to white matter connectivity differences (Nunez et al., 2015).
Usually we classify EEG into distinct frequency bands (delta, theta, alpha, beta, mu, and gamma; see attached book chapter). An individual's resting alpha peak is usually in the 8 to 11 Hz range. Although these ranges are more of an empirical finding. For instance, we observed a normal cognitive individual with his largest resting alpha peak at 7 Hz yesterday. I would be interested to see a large-sample individual differences study of resting theta and alpha peaks, I could not find one.
http://www.sciencedirect.com/science/article/pii/0013469478903097
Article EEG functional connectivity, axon delays and white matter disease
Chapter Electroencephalography (EEG): neurophysics, experimental met...
Interesting possible correlation, even if weak, between cortical surface (head size) and alpha peak frequency. The raw data from most EEG investigations should have the individual values, though it may be unavailable partly due to individually identifying elements of the data. It's a different approach to the data, and it would be quite an effort to "clean" individual records of personal identification or other investigative artifacts and make it available, but I believe it would have a great deal of value. Thanks for the insights! I'll read the EEG...signal processing chapter.
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