" Bad habits can be ingrained in our neurons as easily as good ones. Pascual-Leone observes that “plastic changes may not necessarily represent a behavioral gain for a given subject.” In addition to being “the mechanism for development and learning,” plasticity can be “a cause of pathology.”35 It comes as no surprise that neuroplasticity has been linked to mental afflictions ranging from depression to obsessive-compulsive disorder to tinnitus. The more a sufferer concentrates on his symptoms, the deeper those symptoms are etched into his neural circuits. In the worst cases, the mind essentially trains itself to be sick. "

― Nicholas Carr , The Shallows: What the Internet Is Doing to Our Brains