Is it possible to be a paid RA in a math or computer science lab in order to be immersed in the academic research context? I'm currently a research assistant in a neuroscience lab at MIT and I'm considering pursuing a PhD in math or theoretical computer science. The RAs in my lab who want to eventually become neuroscientists are in a perfect place - a large, collaborative environment where they get paid to work closely with neuroscientists and learn hands on what the entire research process is like in that field before committing to a 5 year grad program. 

I want an environment as close to this as possible in math or theoretical computer science to solidify my belief that I would be happy pursuing research in these areas. Is this kind of opportunity available in these fields? What is the closest I can get? I did an REU program in undergrad and loved it, but want more if it's out there. 

Thank you so much for your advice, this is a great community.
Opportunities to work in specific labs are abundant, but you need to start up a conversation with the professors who run individual labs or research groups to see what is available at your school.  Whether or not you can get funding for a position will vary per lab/RG.  It is generally more difficult to get a funded position in a math or theoretical computer science research lab because without experience, there isn't much you can do to justify getting paid (whereas in an experimental lab you can at least help out with the menial work that is inherent to experimental labs).

My suggestion: make appointments to see professors or lab directors in the labs or groups you might like to work in.  Plan on working for free for at least a year, in order to prove your worth.  If that means taking a second job elsewhere, so be it.  Be prepared to discuss why you think you will make a valuable team member to include a discussion of your long-term goals.  This should be more than "I might want to work in theoretical computer science."  Do some pre-reading of recent papers that have come out of the lab, and know something about the professor's background.

Unfortunately, jumping into a theoretical research group probably isn't the best way to test the waters for a particular field (esp. without a good background already), so you may get more worth out of continuing to take courses in those disciplines you find interesting, and doing a good deal of reading on your own.