Probably only news to me, but when going through the synopsis for Shutter Island I found it to be remarkably similar to that of Shock Corridor.
U.S. Marshal Teddy Daniels is assigned to investigate the disappearance of a patient from Boston's Shutter Island Ashecliffe Hospital. Teddy's investigating skills soon provide a promising lead, but the hospital refuses him access to records he suspects would break the case wide open. As Teddy is ever closer to the end of the case he begins to doubt everything - his memory, his partner, even his own sanity.
Daily Globe Johnny Barrett aims to win the Pulitzer Prize for solving the murder of the inmate Slone, stabbed in the kitchen of a mental hospital with a butcher knife and witnessed by three insane interns. With the support of his boss and manager of the newspaper Swanson and the orientation of the psychiatrist Dr. Fong, Johnny simulates an incestuous situation with his stripper girlfriend Cathy, who is totally against the idea, and is sent to the institution. While being submitted to a mental treatment along the weeks, he approaches to the three witnesses, waiting for a moment of sanity to interview them. The former soldier Stuart tells him that the killer wore white pants; the former university black student Trent tells him that it was an attendant; and the former physicist Dr. Boden tells him the name of the killer. However, Johnny finds the price he pays for his award may cost him his sanity.
Samuel Fuller, the director of Shock Corridor was an inspiration for Martin Scorsese so I wouldn't be surprised if this is a nod to Fuller's work.