A leading expert on Obsessive-Compulsive Disorder who advised actor Leonardo DiCaprio on portraying Howard Hughes in "The Aviator" says the actor himself developed a serious case of the condition.

"There were moments when I was very concerned for Leo," UCLA psychiatry professor Dr. Jeffrey Schwartz told Scotland on Sunday.

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Leonardo DiCaprio portrays Howard Hughes in "The Aviator" (Miramax)

Schwartz, who was called in by director Martin Scorsese to help replicate the characteristics of OCD from which Hughes suffered, says DiCaprio's brain started to malfunction, as he reawakened his own OCD he suffered as a child.

"He let his own mild OCD get worse to play the part," Schwartz told the paper. "There were times when it was getting not so easy for him to control it. By playing Hughes and giving into his own compulsions, Leo induced a more severe form of OCD in himself. There is strong experimental evidence this kind of switch can happen to actors who concentrate so hard on playing OCD sufferers."

"The Aviator" recounts the story of Hughes, one of America's first billionaires, who became a Hollywood movie mogul before plunging into a reclusive lifestyle prompted by obsessional fears of germs and contamination.

"By replicating the compulsions that Hughes suffered from, [DiCaprio] had given in to his own and caused it to get worse," Schwartz said. "If we had taken a scan of Leo's brain we would probably have found he had increased the activity in this part of his brain to the point where it was what we would find in severe OCD."

While poorly understood, OCD is regarded by the medical establishment as a brain disorder that manifests itself in numerous ways, including obsessions involving contamination and symmetry. Those who suffer from it are often found to hoard items or carry out mental routines such as counting.

DiCaprio himself admits his portrayal of Hughes had an impact on his behavior, both during production of the film and afterward.

"I remember my makeup artist and assistant walking me to the set and going, 'Oh God, we're going to need ten minutes to get him there because he has to walk back and step on that thing, touch the door and walk in and out again,'" the actor told the Daily Mirror.

"I let myself do it because I wanted that to come out. I was trying to be that character but it became really bothersome – and it continued way after the filming. I can talk myself through it, whereas Howard Hughes couldn't do that and people with hardcore OCD can't."

"The Aviator" has been nominated for 11 Academy Awards, including Best Picture, Best Director and Best Actor for DiCaprio.