A dementor rose slowly from the box, its hooded face turned toward Harry, one glistening, scabbed hand gripping its cloak. The lamps around the classroom flickered and went out. The dementor stepped from the box and started to sweep silently toward Harry, drawing a deep, rattling breath. A wave of piercing cold broke over him— "Expecto patromun!" Harry yelled. "Expecto patronum! Expecto—" But the classroom and the dementor were dissolving Harry was falling again through thick white fog, and his mother's voice was louder than ever, echoing inside his head—"Not Harry! Not Harry! Please—I'll do anything—"
This passage in Chapter Twelve describes the sensation produced by dementors, and it contributes to the intensive psychological element of this book. Lupin as a werewolf has learned to reconcile and control his dual natures of kind, competent teacher with that of a savage, flesh-hungry werewolf. Harry, as the most susceptible to the effects of dementors (namely depression) is here learning through Lupin's instruction how to control himself under this influence. J.K. Rowling places all of the elements of depression within this effect. The dementors cause darkness and coldness to take over a room. They cause the victim to be aware of nothing in his surroundings except for his or her own intense fear. They bring terrible thoughts and memories to the surface. The difficult cure against a Dementor is a shield of happy thoughts, and a simpler cure is eating chocolate. This is one of many instances in which Rowling imbues her characters with traits of worldly problems and their cures.