The most commonly cited descriptions of the behavioral characteristics of habituation come from two papers published almost 40 years ago (Thompson and Spencer, 1966;Groves and Thompson, 1970). In August 2007, the authors of this review, who study habituation in a wide range of species and paradigms, met to discuss their work on habituation and to revisit and refine the characteristics of habituation. This review offers a re-evaluation of the characteristics of habituation in light of these discussions. We made substantial changes to only a few of the characteristics, usually to add new information and expand upon the description rather than to substantially alter the original point.In the 20 th century, great progress was made in understanding the behavioral characteristics of habituation. A landmark paper published by Thompson and Spencer in 1966 clarified the definition of habituation, synthesized the research to date and presented a list of nine behavioral characteristics of habituation that appeared to be common in all organisms studied The history of habituation and the historical context of Thompson & Spencer's (1966) distillation are reviewed more fully in an article by Thompson (2009) that is included in this issue. This list was repeated and expanded upon by Groves and Thompson in 1970. These two papers are now citation classics and are considered to be the authorities on the characteristics of habituation. In August 2007, a group of 15 researchers (the authors of this review) who study habituation in a wide range of species and paradigms met to revisit these characteristics and refine them based on the 40 years of research since Thompson and Spencer 1966. The descriptions and characteristics from 1966 have held up remarkably well, and the revisions we have made to them were often for clarity rather than content. We made substantial changes to only a few of the characteristics, usually to add new information and expand upon the description rather than to substantially alter the original point. We restricted ourselves to an analysis of habituation; there was insufficient time for detailed discussions of the other form of non-associative learning "sensitization." Thus this review is restricted to our discussions of habituation and dishabituation (as it relates directly to habituation).Many people will be surprised to learn that, although habituation is termed "the simplest form of learning" and is well studied behaviorally, remarkably little is known about the neural mechanisms underlying habituation. Researchers who work on this form of learning believe that because habituation allows animals to filter out irrelevant stimuli and focus selectively on

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.

Rats pressed levers for Noyes pellets or keys for sweetened condensed milk reinforcers delivered by multiple schedules. Session length and baseline rates of reinforcement were varied in two experiments. Rates of responding increased during the early part of the session and then decreased for both responses and reinforcers, as weIl as for all subjects and values ofthe independent variables. Changes in response rates across the session sometimes exceeded 500%. Response rates peaked approximately 20 min after the beginning of the session, regardless of session duration, when subjects responded on a multiple variable intervall-min variable intervall-min schedule. The function was flatter for longer sessions than it was for shorter sessions. The function was flatter, more symmetrical, and peaked later for lower rates ofreinforcement than for higher rates of reinforcement. The function appeared early in training, and further experience moved and reduced its peak. Variables related to reinforcement exerted more control over some aspects ofthis function than did variables related to responding. These within-session patterns ofresponding may have fundamental implications for experimental design and theorizing. 160Absolute rate of responding is one of the primary dependent variables in operant psychology. Given its importance, it is surprising that little is known about the way in which response rates change across experimental sessions.Preliminary infonnation on this topic was provided by McSweeney, Hatfield, and Allen (1990). In their study, rats pressed levers for Noyes pellets or pressed keys for sweetened condensed milk delivered by a multiple variable interval l-min variable interval l-min (multiple VI l-min VI l-min) schedule. The components ofthe multiple schedule alternated every 5 min. Rates of responding increased for the first 20 min of the 6O-min experimental session and then decreased. This bitonic function occurred for both responses and both reinforcers, as well as for each subject. The change in response rate was large. For example, the rate of keypressing averaged 15.7 and 27.5 responses per minute for the first and last 5 min of the session, respectively. It averaged 70.0 responses per minute at its peak.The bitonic functions reported by McSweeney et al. have many implications. First, they imply that the rate of responding averaged across the session-one of the primary dependent variables in operant psychologymasks large and reliable regularities in animal behavior, at least in some cases. Therefore, this molar measure must This research was panially supported by NIMH Grant MH 42466. The author wishes to thanklohn Hinson for his comments on an early version of Ibis manuscript and Cam Melville for bis help in conducting the experiments. Reprints may be obtained from F.

for their comments on a draft of this article. We also thank John M. Hinson for his help with Fxjuation 1.

When the procedure is held constant within an experimental session, responding often changes systematically within that session. Many of these within-session changes in responding cannot be dismissed as learning curves or by-products of satiation. They have been observed in studies of positive reinforcement, avoidance, punishment, extinction, discrimination, delayed matching to sample, concept formation, maze and alley running, and laboratory analogues of foraging, as well as in the unconditioned substrates of conditioned behavior. When aversive stimuli are used, responding usually increases early in the session. When positive reinforcers are used, responding changes in a variety of ways, including increasing, decreasing, and bitonic functions. Both strong and minimal reinforcement procedures produce within-session decreases in positively reinforced behavior. Within-session changes in responding have substantial theoretical and methodological implications for research in conditioning.