LinkedIn profile. According to the profile, Yamanaka has "designed PSP and PS3 User Interface all by myself." Wow. Not even a little bit of help?
He's not only in charge of making the UI for PlayStation owners. "I am also in charge of localization of the on-screen messages." So next time you get a PSN message, or you unlock a Trophy, remember that Yamanaka is the guy responsible for what they say.