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That a subject responds at a particular rate in the presence of one stimulus and at a different rate in the presence of other stimuli on the same dimension has been taken as indicative of the acquisition of dimensional stimulus control over the subject's behavior. Theories accounting for discrimination-derived phenomena in animals have been advanced by Pavlov (1927) and by Spence (1937). The Spencian model assumes that if a response is followed by a reinforcement, "the excitatory tendencies of the immediate stimulus components are reinforced or strengthened..." (Spence, 1936, p. 273). Furthermore, if an instrumental response leads to nonreinforcement, then the tendency to respond to the stimulus components present on that trial are weakened.