Do you need to define standard abbreviations like "EEG" and "fMRI" in the abstract? I am preparing to send my first paper to a springer journal (journal of computational neuroscience), in the instruction author there is an instruction say Abbreviations should be defined at first mention and used consistently thereafter. I think that the abbreviations in the abstract are known for the neuroscience community. Do I have to define them first?    

abstract

Based on clinical data collected using different brain imaging and recording techniques (fMRI, CT, PET, EEG, MEG, NRIS,...),
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.