Lucid-dreaming advocate Stephen LaBerge has outlined a possible reason for how dreams are formulated and why nightmares occur with a high frequency. To LaBerge, a dream starts with an individual thought or scene, such as walking down a dimly lit street. Since dreams are not predetermined, the brain responds to the situation by either thinking a good thought or a bad thought, and the dream framework follows from there. Since the prominence of bad thoughts in dreams is higher than good, the dream will proceed to be a nightmare. Recurring posttraumatic stress disorder nightmares in which traumas are re-experienced respond well to a technique called imagery rehearsal.