fourier analysis and slow spectra

Gerald Pollack gpollack at
Thu Nov 30 09:36:11 EST 1995

In article <49cjnl$hnp at> LEOPOLD at writes:
>From: LEOPOLD at
>Subject: fourier analysis and slow spectra
>Date: 27 Nov 1995 14:59:01 GMT

>In our lab we use the Neuroscan software equipement. For our spontaneous EEG we
>compute the fast fourier transformations.
>The result is a huge amount of (artifical!) theta in our relative power
>spectrum. Who can give an explanation and how to solve the problem.

The problem might be what's known as aliasing. The FFT procedure assigns all 
of the energy in a signal to frequencies between 0 Hz and half the sampling 
frequency (i.e. the digitizing rate of your A-to-D board). If energy is 
present at frequencies greater than half the sampling frequency, then it shows 
up in spectra at lower frequencies. The solution is to use a hardware low-pass 
filter to ensure that there is no signal above half the A-to-D sampling 

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