Associative mechanisms involved in specific Pavlovian-to-instrumental transfer in human learning tasks

Four experiments compared the effect of forward and backward conditioning procedures on the ability of conditioned stimuli (CS) to elevate instrumental responding in a Pavlovian-to-instrumental transfer (PIT) task. Two responses were each trained with one distinct outcome (R₁->O₁, R₂->O₂), either concurrently (Experiment 1) or separately (Experiments 2, 3 and 4). Then, in Experiments 1 and 2, four CSs were either followed or preceded by one outcome (A->O₁, B->O_2, O₁->C, O₂->D). In Experiment 3, each CS was preceded and followed by an outcome: for one group of participants, both outcomes were identical (e.g., O₁->A->O₁, O₂->B->O₂), but for the other, they were different (e.g., O₁->A->O₂, O₂->B->O₁). In Experiment 4, two CSs were preceded and followed by identical outcomes, and two CSs by different outcomes. In the PIT tests, participants performed R₁ and R₂ in the presence and absence of the CSs. In Experiments 1 and 2, only the CSs followed by outcomes in Pavlovian training elevated responding. In Experiments 3 and 4, all the CSs elevated responding but based on the outcome that followed them in training. These results support the stimulus-outcome-response (S-O-R) mechanism of specific PIT, according to which CSs elevate responding via activation of its associated outcome representation.