Effects of partial vs consistent reward in noncontingent pairings of stimuli with reward: A noncontingently produced positive contrast effect

Rats received pairings of two stimuli with reward noncontingently in the Skinner box. During noncontingent pairings, the bar was immobilized. For Group CC 100% of the presentations of both stimuli were rewarded (S ₁ ±, S ₂ ±), for Group PP 50% of the presentations of each stimulus were rewarded (S ₁, ±, S ₂±), and for Group PC one stimulus was followed by reward on 50% of its presentations, while the second stimulus was followed by reward on 100% of its presentations (S ₁ ±, S ₂ ±). A fourth group received the stimuli and reward nonpaired. In a subsequent rewarded test phase, the response facilitating effects of the stimuli were evaluated. In the test phase all groups that received reward paired with S ₁, and S ₂ performed better in the presence of S ₁ and S ₂ than the group for which the stimuli were not paired with reward. For groups that received the stimuli paired with reward, a difference due to schedule of reward occurred when schedule of reward was varied within Ss (Group PC), but not when varied between Ss (Group PP vs Group CC). The specific form of this finding was that Group PC's performance in the presence of S ₂ ± was more vigorous than its performance in the presence of S ₁ ± and was more vigorous than the performance of Groups PP and CC to S ₂. Group PC's performance to S ₁ ± did not differ from that of Groups PP and CC to S ₁.