Decreased Key Pecking in Response to Reward Uncertainty Followed by Surprising Delay Extension in Pigeons
Abstract
The Pavlovian autoshaping paradigm has often been used to assess the behavioral effects of reward omission on behavior. We trained pigeons to receive a food reward (unconditioned stimulus or UCS) following illumination of a response key (conditioned stimulus or CS). In Experiment 1, one group of pigeons was trained with two 100% predictive CS-UCS associations (reward certainty) and another group with two 25% predictive CS-UCS associations (reward uncertainty) for 12 sessions. In both groups, the two CS durations were 8 s. Then, in each group, the duration of one CS remained unchanged and that of the other CS was suddenly extended from 8 to 24 s for 6 sessions. In Experiment 2, some experienced individuals (from Experiment 1) and naïve individuals formed two groups trained with a 24-s CS throughout for 18 sessions. Our results show that pigeons (a) pecked less at the uncertain than the certain CS, (b) decreased and then increased CS-pecking after extending CS duration, especially in the certainty condition, (c) were unresponsive to the 24-s CS in the absence of previous experience, and (d) decreased their response rate close to the end of a trial irrespective of the reinforcement condition, CS duration, and amount of training. These results are discussed in relation to several theoretical frameworks.