The natural incentives that shape behavior reach the central circuitry of motivation trans-synaptically, via the five senses, whereas the laboratory rewards of intracranial stimulation or drug injections activate reward circuitry directly, bypassing peripheral sensory pathways. The unsensed incentives of brain stimulation and intracranial drug injections thus give us tools to identify reward circuit elements within the associational portions of the CNS. Such studies have implicated the mesolimbic dopamine system and several of its afferents and efferents in motivational function. Comparisons of natural and laboratory incentives suggest hypotheses as to why some habits become compulsive and give insights into the roles of reinforcement and of prediction of reinforcement in habit formation.