The use of a second-order schedule to measure feeding motivation in the pig

Abstract

Feeding motivation is often measured using operant conditioning, however, most of the schedules used alter the underlying level of motivation or ‘arouse’ an individual by repeatedly presenting small quantities of food. A solution to these problems could lie in the use of a second-order schedule of food reinforcement, which ultimately delivers food reinforcement, but is composed of a number of sub-schedules which terminate in the presentation of a conditioned stimulus (CS). The action of this CS facilitates responding at very high ratios without extinction and could allow the assessment of feeding motivation in the period preceding the first food reinforcement. This paper reports on two experiments which aimed to investigate if pigs would respond for food delivered under a second-order schedule, and furthermore how the CS gained control over behaviour. The results from Experiment 1 indicated that pigs will respond for food delivered under a second-order schedule, but was unable to show that the CS reinforced operant responding. A refined methodology solved this problem in Experiment 2, and it was shown that stimulus overshadowing and a relatively mild schedule could have diminished the effectiveness of the CS during Experiment 1. Behavioural observations suggested that the CS increased the expectation of good delivery because it caused individuals both to monitor and investigate the reinforcement trough. Therefore, it was concluded that a second-order schedule would allow feeding motivation to be measured in the period of responding preceding the first food reinforcement, and therefore in the absence of food ingestion.