Power Line interference with different Sampling frequency I've been playing around with EEG data from an online database. 

If the power line interference (PLI) is at 50Hz and sampling frequency is 64Hz, then according to the Nyquist theorem, the 50Hz PLI should be aliased with 14Hz. 

But according to the database header file, the signal from EEG channel is processed by some prefilter, which is a bandpass filter with passband 0.5Hz - 32Hz. 

So in the end, the raw EEG data I get from the database, is analog signal sampled at 64Hz and bandpass filtered in 0.5Hz - 32Hz range. 

So my question is, do I have to design a notch filter to remove the PLI at 50Hz if the signal has already been bandpass filtered (32 Hz above removed.)
That depends if the bandpass filter was applied before or after sampling. If it was before sampling (i.e. in analog domain) it worked as an anti-aliasing filter and you dont need to filter out the PL. If it was applied in digital domain, you'd need to suppress it, because then the aliasing would occur. Have a look at the FFT of your signal to see if there is a peak or not at 14Hz, which could come from the PL.

(I guess the filter was applied in analog domain, because it does not make much sense to do it after sampling, since the high cutoff is already the Nyquist frequency.)