Do you need to define standard abbreviations like "EEG" and "fMRI" in the abstract?
Yes, define your abbreviations as per the instructions. Each reader may be familiar with all, none, or some of the techniques you mention. There is certainly a set of acronyms and terminology which readers in your field will be familiar with, but anything you can do to help your reader understand your material is potentially beneficial.

You can also use the other published abstracts in the same journal as a guide: note how this one defines otherwise common acronyms like EEG (but, as pointed out below, does not define some other terms).

Or, if you're worried about abstract length, avoid discussing every individual technique in the abstract and save it for the body of the manuscript.