Process in which the frequency or predictability of a behavioral response is increased through reinforcement (i.e., a stimulus or a reward for the desired response). Classical, or respondent, conditioning, which involves stimulus substitution, is based on the work of Ivan Pavlov, who conditioned dogs by ringing a bell each time the aroma of food was presented. Eventually the dogs salivated when the bell rang, even if no food odour was present; salivation was thus the conditioned response. In instrumental, or operant, conditioning, a spontaneous (operant) behaviour is either rewarded (reinforced) or punished. When rewarded, a behaviour increases in frequency; when punished, it decreases. Operant conditioning was studied in detail by B.F. Skinner.