window size and overlapping in pwelch function of matlab for PSD evaluations

Question

Could anyone please suggest ideal Window size and overlapping samples for pwelch function in Matlab. I have several 200 ms EEG signals with sampling rate 1000 (signal length or number of samples = 200) to evaluate spectral power. By default pwelch uses hamming window and divides the data into 8 segments with 50% overlap. Are these default values okay for a signal with only 200 samples? The default values are working fine and its giving me a PSD plot. I want to make sure that what I am doing is conceptually correct and if someone could suggest any better way of doing this? This is a study of ERP response to stimuli in a 200 ms time duration. I want to compare the spectral powers in different frequency bands.

Thanks for the help!

Answer 1

Considering your time window is only 200 ms, I would suggest using pwelch with a single 200 ms window and no overlap. The frequency precision of the underlying Fourier transform is related to the length of the data segment used. Specifically, the spacing between Fourier Transform bins is related to the length of the data segment as (Fs / N) where Fs is the sampling rate of the data, and N is the length of the segment. So for example with a sampling rate of 1000 Hz and a 200 ms data segment, the effective resolution of your frequency transform will be 4 Hz. (1000 / 200).

see also: https://electronics.stackexchange.com/questions/12407/what-is-the-relation-between-fft-length-and-frequency-resolution

If you were analyzing a longer section of data (say, a few seconds), then using pwelch with overlapping windows would be more appropriate.

A good introduction to frequency based analyses is the text 'Analyzing Neural Time Series Data' by M.X. Cohen.