EFFECTS OF DISCRETE‐TRIAL AND FREE‐OPERANT PROCEDURES ON THE ACQUISITION AND MAINTENANCE OF SUCCESSIVE DISCRIMINATION IN RATS

Rats were trained on a successive discrete‐trial discrimination between two tonal stimuli to examine the effects of availability of a lever during intertrial intervals. In the discrete‐trial condition, in which a lever was removed from the chamber during intertrial intervals, 10‐s trials were initiated by the presentation of both discriminative stimulus and lever. In the free‐operant condition, in which a lever was present during both trials and intertrial intervals, 10‐s trials were initiated only by the presentation of a discriminative stimulus. Experiment 1 employed 50‐s intertrial intervals and demonstrated that discriminative performances were acquired faster and maintained better in the free‐operant conditions than in the discrete‐trial conditions. Experiment 2 employed 5‐s intertrial intervals and showed that poor discriminative performances in the discrete‐trial conditions were improved. These results indicate that the presentation of a lever to start a trial can overshadow or mask the control by a discriminative stimulus and thereby obstruct the acquisition and maintenance of discriminative performances. Furthermore, the overshadowing or masking effects are strengthened as a function of the duration of intertrial intervals.