Summation of food and shock based responding

In a multiphase experiment, dogs first received discriminative, discretetrial, barrier-jumping training with two tones (S^D, S^Δ) in a shuttle box reinforced by either shock avoidance (Group I) or by food (Group II). Then the dogs were trained on free-operant barrier-jumping reinforced by the qualitatively opposite reinforcer—food in Group I and shock avoidance in Group II. Finally, test presentations of the tone stimuli were superimposed on the free-operant behavior. The tone S^Ds markedly facilitated responding in all animals. This experiment demonstrates a summation of responding maintained by shock avoidance and food reinforcement and casts doubt on explanations of conditioned suppression outcomes that appeal solely to incompatible motivational states within the organism.