^Freud, Siegmund. Beyond the pleasure principle. New York: Liveright. 1950.
^ 2.02.12.22.32.4Schultz W. Neuronal reward and decision signals: from theories to data. Physiological Reviews. 2015, 95 (3): 853–951. doi:10.1152/physrev.00023.2014. PMC 4491543. PMID 26109341. Rewards induce approach behavior, also called appetitive or preparatory behavior, and consummatory behavior. ... Thus any stimulus, object, event, activity, or situation that has the potential to make us approach and consume it is by definition a reward. ... Rewards can also be intrinsic to behavior (31, 546, 547). They contrast with extrinsic rewards that provide motivation for behavior and constitute the essence of operant behavior in laboratory tests.