So far, I have calculated the evoked potentials. However, I would like to see if there is relatively more activity in the theta band wrt the other bands. When I use mne.Evoked.filter, I get a plot which lookes a lot like a sine wave, containing no useful information. Furthermore, the edge regions (time goes from -0.2s to 1s) are highly distorted.
Filtering will always result in edge artifacts, especially for low frequencies like theta (longer filter). To perform analyses on low frequency signal you should epoch your data into longer segments (epochs) than the time period you are interested in.
Also, if you are interested in theta oscillations it is better to perform time-frequency analysis than filter the ERP. ERP contains only time-locked activity, while with time-frequency representation you will be able to see theta even in time periods where it was not phase-aligned across trials. You may want to follow this tutorial for example.
Also make sure to see the many rich tutorials and examples in mne docs. If you have any further problems we use Discourse now: https://mne.discourse.group/