kara bagot: so, as often as we possibly can. >> cooper: dr.tor on that $300 million n.i.h. study. her team is scanning teenager's brains as they follow instagram, the most popular social media app. when we met 18-year-old roxy shimp, she was about to participate in dr. bagot's study. how much time do you actually spend on screens? >> roxy shimp: i check my phone pretty regularly, i'd say. >> cooper: what's pretty regularly? >> shimp: every, at least ten to 20 minutes. >> cooper: is that a conservative estimate? >> shimp: probably. >> cooper: she can't take her phone into the m.r.i. because of the powerful magnets in the machine, so a mirror has been placed above her face to allow her to look across the room at a movie screen displaying images from her instagram account. this way, dr. bagot can see exactly which parts of the brain's reward system are most active while using social media. >> cooper: so you can actually see a part of the brain light up when you're feeling good. >> bagot: yes, in the scanner. >> cooper: in the scanner. based on he

carol bagot is studying children's brains as they follow instagram, the most popular social media appet 18-year-old roxie shimp, she was about to join the study. >> how long are you on it? >> every 15 minutes, probably. >> reporter: she can't take her phone inside the mri because of the magnets in the screen. so is she is allowed to look at a screen displaying images of her instagram account. that way dr. bagot can see which parts of the brain system are active when looking at social media. >> so you can actually see parts of the brain light up when you're feeling good. >> yes. in the scanner. >> in the scanner. >> yes. >> reporter: dr. bagot is one of the scientists who believe screen time releases dopamine which has a role in desire. >> so you're likely to act compulsively instead of checking yourself. >> you want to check on it to keep getting -- >> the good feelings. >> lisa damour is a cbs news contributor and is here with more on this. lisa, good morning. >> good morning. >> we won't know conclusive studies for quite a while, but do you know anything so far? >> here's what we do

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