[Eeglablist] anti-aliasing with downsampling?

[Li, Lucia M]

Dear EEGlab users & experts,

I was hoping someone might be able to shed some light how why downsampling my data appears to introduce many frequencies into it.

http://s8.postimg.org/ah6vqvaut/Screen_Shot_2015_10_30_at_10_59_35.png

I have acquired some EEG data at 5000Hz (figure 2). I acquired this with Brain Products and exported the data without doing anything to it, to use in eeglab.
I then lowpass filtered it at 40Hz in eeglab (using the Tools --> filter data --> basic FIR filter (new, default)) (figure 5).
I then downsampled it to 250Hz (Tools --> change sampling rate) and get the resultant spectral plot (figure 6).

I was wondering:
a) are these extra frequencies indicative of anti-aliasing? If not, what might they indicate?
b) why am I still getting anti-aliasing effects if I downsampled after a low pass filter?

Many thanks in advance for your help!
Kind regards,
Lucia


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