LOS ANGELES — Billionaire media mogul Sumner Redstone is being deposed early afternoon on Thursday by lawyers representing his former girlfriend Manuela Herzer.

Redstone’s mental competency is at the heart of a salacious lawsuit that could determine who controls his 80 percent interests in Viacom and CBS. The stakes are held by Redstone’s National Amusements Inc.

The tape of the interview, being conducted by Greenberg Glusker’s Pierce O’Donnell, won’t be played at the trial but will be heard behind closed doors by the judge presiding over the case.

O’Donnell will get 15 minutes to test the 92-year-old’s mental faculties and then Redstone’s lawyers will get a chance to do follow-up questions.

The week-long trial starts Friday in Los Angeles Superior Court and is aimed at discovering whether the ailing Redstone lacked the mental capacity to replace Herzer with Viacom CEO Philippe Dauman as his healthcare agent last October.

Herzer’s lawyers will argue that Redstone did not have the ability to know what he was doing when Herzer was replaced and thrown out of Redstone’s Beverly Park mansion last October.