Amid Bruce Willis' nearly three-year battle with dementia , the actor's brain is "failing him," and his ability to communicate is fading, according to his wife Emma Heming Willis .

"Bruce is still very mobile. Bruce is in really great health overall, you know," Heming Willis told ABC News' Diane Sawyer in the ABC special "Emma & Bruce Willis: The Unexpected Journey," a preview of which aired Tuesday on " Good Morning America ." "It's just his brain that is failing him."

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