Emily Hollenbeck lived with a deep, recurring depressionshe likened to a black hole, where gravity felt so strong and her limbs so heavy she could barely move. She knew the illness could kill her. Both of her parents had taken their lives. She was willing to try something extreme: Having electrodes implanted in her brain as part of an experimental therapy. Researchers say the treatment —- called deep brain stimulation, or DBS — could eventually help many of the nearly 3 million Americans like her with depression that resists other treatments. It’s approved for conditions such as Parkinson’s di…