Preserved nodal number effects under equal reinforcement.

The present set of experiments tested the hypothesis that the nodal number effects observed in previous studies of stimulus equivalence were due to the confounding factor of training structure that resulted in unequal reinforcement across trial types. In Experiment 1, two 5-member equivalence classes were trained across equal and unequal reinforcement conditions, both with and without a limited hold. A significant nodal effect, as measured by response speed, was found in the equal reinforcement, no-limited-hold condition. In Experiment 2, two 6-member equivalence classes were trained in equal and unequal reinforcement conditions without limited hold. In a transfer-of-function test, clear nodal effects were observed in the equal reinforcement condition. Experiment 3 replicated and extended the findings of Experiments 1 and 2 with an increased number of baseline training trials. The results of the present study suggest that the effects of nodal number are independent of differential reinforcement. Furthermore, a transfer-of-function test was most sensitive to nodal effects, response speed was the next most sensitive measure, and response accuracy was the least sensitive measure of nodal effects.