REINFORCER VALUE MAY CHANGE WITHIN EXPERIMENTAL SESSIONS

Large and systematic changes in response rates often occur within sess ions during operant conditioning procedures. In the present experiment , we asked whether the value of the reinforcer that supports respondin g also changes within sessions. Pigeons pecked a key for mixed grain a vailable throughout the session. Occasionally, wheat was also provided for pecking a second key. The ratio of the rates of responding for mi xed grain and wheat, a frequently used measure of relative reinforcer value, changed significantly within sessions when mixed grain was prov ided at high, but not an low, rates. Habituation to the reinforcer pro vides the most likely explanation for these changes in reinforcer valu e. Eventually, habituation may provide a unified explanation for the w ithin-session changes in behavior that occur when many species of subj ects respond on a wide variety of tasks.