fourier analysis and slow spectra

[Dag Stenberg]

LEOPOLD at mip.paed.uni-muenchen.de wrote:
} 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.
} christian Leopold, Munich

Have you looket out for a) signal contamination, b) aliasing?
Maybe you should give more details.