The chickens were fed a standard poultry ration and were trained to peck the center key for immediate food reward. Complex patterns followed after a bird had learned to peck a key for a food reward. Each daily session lasted for 15 min. Since motivation for food reward depended upon hunger, the birds were kept without food for 12 h prior to their 15-min daily sessions. Following the 15-min session the birds were returned to the home cage where feed and water were available ad libitum. The birds quickly mastered this procedure, and so after several pecking sessions they completely stopped working for a food reward, knowing that they would receive ad libitum feed when they were returned to their home cage. Under indefinite or irregular presentation of feed in the home cage, the birds immediately returned to solving the visual discrimination problems presented in the research cages.