Sensitization–habituation may occur during operant conditioning.

Abstract

Operant response rates often change within experimental sessions, sometimes increasing and then decreasing. The authors attribute these changes to sensitization and habituation to aspects of the experimental situation presented repeatedly (e.g., reinforcers) or for a prolonged time (e.g., the experimental enclosure). They describe several empirical similarities between sensitization-habituation and within-session changes in operant responding. They argue that many alternative explanations for within-session changes in operant responding can be dismissed. They also examine some implications of linking the literatures on habituation and operant responding. Because responding follows a similar pattern in several other cases (e.g., human vigilance, classical conditioning, and unconditioned responding), 2 relatively simple processes may be responsible for the temporal patterning of behavior in a wide variety of situations. We observed large changes in response rate within experimental sessions when subjects (e.g., rats) responded on operant conditioning procedures (e.g., McSweeney, Hatfield, & Allen, 1990). In many of our experiments, response rates increased to a peak and then decreased. In other experiments, response rates increased without decreasing or decreased without increasing. Figure 1 contains an example of each of these types of changes. The top represents results for rats pressing levers on multiple variable interval (VI) 60-s VI 60-s schedules; the middle, the results for rats pressing levers on a VI 15-s schedule; and the bottom, the results for pigeons pecking keys on a variable ratio (VR) 15 schedule. Each graph represents the proportion of total-session responses during successive 5-min intervals in the session. Throughout this article, we calculated proportions by dividing the number of responses during a 5-min interval by the total number of responses during the session. Although within-session changes in operant responding have been observed in the past (e.g., McSweeney & Roll, 1993), these changes have been treated as problems to control by procedures, such as giving warmup trials (e.g., Hodos & Bonbright, 1972) or time to adapt to the apparatus (e.g., Papini & Overmier, 1985), rather than as phenomena to study. Further consideration suggests that within-session changes deserve study in