Optimal behavior by rats in a choice task is associated to a persistent conditioned inhibition effect.

When given a choice between an alternative with a low probability of reinforcement and discriminative stimuli, and another with a higher probability of reinforcement and non-discriminative stimuli, pigeons show a clear preference for the former but rats clearly prefer the later. It has been reported that pigeon's suboptimal choice is associated to a diminishing effect of the stimulus correlated with non-reinforcement. In the present paper, we explored the possibility that rats' optimal choice is more strongly influenced than pigeons' by the stimulus associated to non-reinforcement and that the effects of it do not dissipate during training. We trained rats to choose between an alternative with 0.50 probability of reinforcement and discriminative stimuli, and an alternative with 0.75 probability of reinforcement and non-discriminative stimuli. We replicated the strong preference for the optimal alternative. Then, after several sessions of training, we presented summation trials in which both the stimulus associated to reinforcement and the stimulus associated to non-reinforcement were simultaneously presented. The results showed that the stimulus associated to non-reinforcement exerted a strong effect on choice, and, more importantly, that it did not seem to dissipate across training. These results suggest that the strong difference found between pigeons and rats in the suboptimal choice procedure is potentially related to differences in the impact of conditioned inhibitors.

Rats are optimal in a choice task in which pigeons are not.

In an extensive list of studies, it has been found that pigeons prefer an alternative associated with discriminative stimuli over another associated with non-discriminative stimuli, even when the probability of reinforcement is higher in the latter. This behavior has been named "suboptimal choice". In the present experiment, we evaluated whether rats, another widely studied species within the Experimental Analysis of Behavior, also shows this behavior. We systematically replicated the procedure employed with pigeons, and found that rats are not suboptimal, i.e., they prefer the non-discriminative alternative associated with .5 probability of reinforcement, over the discriminative alternative associated with .2 probability of reinforcement. This effect occurred even though rats discriminated the contingencies of reinforcement associated with each stimulus, suggesting that rats' optimal choice was driven by the overall probability of reinforcement of each alternative. Different procedural details are offered as possibilities for explaining this apparent inter-species difference.