Conditioned Inhibition from Incomplete Reductions in the Probability of Reinforcement

Abstract

Five experiments, all using appetitive, discrete trial operant conditioning, studied the properties of a stimulus that signalled a reduction in the probability of reward. Discriminations were trained in which reinforcement was available on 100% of trials signalled by a tone, but only on some occasions when the tone was presented simultaneously with a light. The properties of the light were assessed in summation tests with a clicker. The first two experiments established that if the tone-light compound signalled reinforcement on only 25%, 33% or 50% of trials the light acted as a discriminative inhibitor, suppressing responding maintained by the clicker. In these experiments reinforcement had been available on 86.7% of clicker trials during initial training. Experiments 3, 4 and 5 examined further the properties of the light after animals had been trained on a discrimination where reinforcement was available on 50% of trials signalled by the tone-light compound. The light was evaluated in a summation test with a clicker, which had signalled only a 25% or 15% probability of a reinforcement. In this instance, the light did not suppress the responding maintained by the clicker. Instead, the light acted as an excitatory discriminative stimulus to enhance that responding. This dependence of the properties of the light on the reinforcement history of the clicker poses problems for most contemporary models of inhibitory performance.