Rats' preferences in the suboptimal choice procedure: Evaluating the impact of reinforcement probability and conditioned inhibitors

Previous research has shown that pigeons and other birds display a strong and consistent preference for an alternative of reinforcement that presents stimuli that allow to discriminate whether a reinforcer will be delivered or not, even when its probability of reinforcement is lower than that of another alternative without those stimuli. In contrast, most of the studies performed with rats report that they show the opposite preference, choosing the alternative with higher probability of reinforcement. To explain these opposite preferences, it has been proposed that rats and pigeons have a differential sensitivity to the conditioned inhibition that emerges from the stimulus that predicts non-reinforcement: While it does not have an impact in pigeons, it strongly influences rats' preferences. Alternatively, it was recently proposed that there is not a fundamental difference in the behavior of rats and pigeons, but that the procedure employed to evaluate each of these species has generated the difference; in particular, it was proposed that both species prefer the discriminative alternative when the discriminative stimuli have incentive salience. Two recent studies provide support for each of these hypotheses, so that the available evidence does not allow to distinguish between them. In the present report, we present three studies that systematically explore the influence of the procedural differences between the studies with discrepant results. The obtained results provide support for the following ideas: a) there is a fundamental difference between pigeons and rats in their choice behavior in the "suboptimal choice procedure", b) considering the incentive salience of the discriminative stimuli does not resolve it, and c) rats' optimality is a consistent phenomenon, which resists manipulations in reinforcement probabilities and the absence of conditioned inhibitors in the discriminative alternative.