Cue reactivity is a learning explanation of relapse that is used as an explanation for the relapse of smoking. Cue reactivity is the idea that a person associates specific moods, situations or environmental factors (smoking-related cues) with the rewarding effects of nicotine, and these cues can trigger a relapse. Years of smoking will have led to a conditioned association between things like the sight and smell of cigarettes and the reinforcing effects of nicotine. Nicotine is the unconditioned stimulus (UCS) and the pleasure caused by the increase in dopamine levels is the unconditioned response (UCR). However, following this increase in levels, the brain will try to lower the dopamine back to normal. Any stimulus that is associated with nicotine entering the brain (e.g. the smell of cigarette smoke) will change from a neutral stimulus (NS) to a conditioned stimulus