Madeon recently spoke about workng with Gaga in the studio for ARTPOP to Digital Spy at Sony's Xperia Z smartphone launch.
SOURCE: http://www.gigwise.com/news/80368/lady-gaga-producer-madeon-praises-amazing-artpop-studio-sessions
She's great. She's a really, really interesting and nice person. From a personal standpoint it's been hugely admirational, to be like, 'Wow - you're at this level of super, ultimate fame - and there's this aura of superstardom - and you're still an engaging and nice person'. So that was really humbling. Professionally, I think she's amazing. She would come out of a show after singing live for three hours and giving it everything, but just go straight to the studio and record. She would still give everything - even on the demo take! She's really impressive. I learnt a lot because I don't really work with vocalists in real life that often. For my previous collaborations I'd never met them while doing the song. It was really interesting for me to feel the vibe of working together, sitting at the piano and collaborating on ideas right away. I think it's great that my first real experience of that was with Lady GaGa! I did a session with Beth Ditto before, so they were the only two I'd worked with.
SOURCE: http://www.gigwise.com/news/80368/lady-gaga-producer-madeon-praises-amazing-artpop-studio-sessions