Habituation revisited: an updated and revised description of the behavioral characteristics of habituation.

The most commonly cited descriptions of the behavioral characteristics of habituation come from two papers published almost 40 years ago [Groves, P. M., & Thompson, R. F. (1970). Habituation: A dual-process theory. Psychological Review, 77, 419-450; Thompson, R. F., & Spencer, W. A. (1966). Habituation: A model phenomenon for the study of neuronal substrates of behavior. Psychological Review, 73, 16-43]. 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. One additional characteristic, relating to long-term habituation, was added. This article thus provides a modern summary of the characteristics defining habituation, and can serve as a convenient primer for those whose research involves stimulus repetition.

Sensitization and habituation regulate reinforcer effectiveness.

We argue that sensitization and habituation occur to the sensory properties of reinforcers when those reinforcers are presented repeatedly or for a prolonged time. Sensitization increases, and habituation decreases, the ability of a reinforcer to control behavior. Supporting this argument, the rate of operant responding changes systematically within experimental sessions even when the programmed rate of reinforcement is held constant across the session. These within-session changes in operant responding are produced by repeated delivery of the reinforcer, and their empirical characteristics correspond to the characteristics of behavior undergoing sensitization and habituation. Two characteristics of habituation (dishabituation, stimulus specificity) are particularly useful in separating habituation from alternative explanations. Arguing that habituation occurs to reinforcers expands the domain of habituation. The argument implies that habituation occurs to biologically important, not just to neutral, stimuli. The argument also implies that habituation may be observed in "voluntary" (operant), not just in reflexive, behavior. Expanding the domain of habituation has important implications for understanding operant and classical conditioning. Habituation may also contribute to the regulation of motivated behaviors. Habituation provides a more accurate and a less cumbersome explanation for motivated behaviors than homeostasis. Habituation also has some surprising, and easily testable, implications for the control of motivated behaviors.

Spontaneous recovery and dishabituation of ethanol-reinforced responding in alcohol-preferring rats.

This study examined whether habituation, a decrease in responsiveness to a repeatedly presented stimulus, occurs to ethanol reinforcers in alcohol-preferring (P) rats. Three fundamental properties of habituation were evaluated: generality, spontaneous recovery, and dishabituation. In each experiment, P rats' lever pressing was reinforced by 10% ethanol on a variable-interval 15-s schedule during 50-min sessions. Experiment 1 evaluated the generality of habituation to repeatedly presented stimuli by using ethanol and water reinforcers. Rates of responding were higher for ethanol than they were for water. Additionally, the within-session patterns of responding differed for each reinforcer, suggesting that the pattern of responding was specific to the exact nature of the repeatedly presented reinforcer. Experiment 2 examined spontaneous recovery, an increase in responsiveness to a habituated stimulus when that stimulus is not presented for a time, by separating experimental sessions by 5 min, 2 hr, or 24 hr. Early-session rates of responding during Session 2 were slower than the corresponding rates during Session 1 when sessions were separated by 5 min or 2 hr. Response rates and within-session patterns of responding during Sessions 1 and 2 were similar when sessions were separated by 24 hr. Experiment 3 tested for dishabituation, a restoration of responsiveness following the presentation of an extraneous stimulus, by presenting a tone or a light 24 min and 55 s into the session. Rates of responding temporarily increased after the tone was presented. The results of these experiments support the idea that habituation contributes to the regulation of ethanol consumption.

Regulation of drug taking by sensitization and habituation.

The authors argue that drug taking is an operant behavior that is reinforced by the drug itself. The effectiveness of a drug as a reinforcer is modulated by sensitization and habituation to the drug as it is consumed. According to this model, drug taking stops when habituation reduces the ability of the drug to reinforce its own consumption. Drug taking resumes when spontaneous recovery restores the effectiveness of the drug as a reinforcer. This parsimonious model provides a framework for understanding many findings in the drug literature, including acute and chronic tolerance, the effect of deprivation on consumption, the contextual specificity of tolerance, polydrug abuse, cross-sensitization between stress and drugs, behavioral sensitization, priming, and reinstatement. Although this model cannot explain all aspects of drug taking (e.g., the effect of cognitive manipulations), it has many implications for understanding and controlling human drug consumption and addiction.

Stimulus change dis-habituates operant responding supported by water reinforcers.

The present experiment examined whether habituation contributes to within-session decreases in operant responding for water reinforcers. The experiment asked if this responding can be dis-habituated, a fundamental property of habituated behavior. During baseline, rats' lever pressing was reinforced by water on a variable interval 15-s schedule. During experimental conditions, rats responded on the same schedule and a new stimulus was introduced for 5 min at 15, 30 or 45 min into the 60-min session. The new stimulus was extinction, continuous reinforcement or flashing lights in different conditions. Rate of responding primarily decreased within the session during baseline. Introducing a new stimulus sometimes suppressed (extinction, continuous reinforcement) and sometimes increased (flashing lights) responding while it was in effect. The new stimulus increased responding after it ended and before it was presented in the session. The results are incompatible with the idea that non-habituation satiety factors (e.g., cellular hydration and blood volume) contributed to within-session changes in responding. These satiety factors should increase with increases in consumption, decrease with decreases in consumption and remain constant with constant consumption of water. Nevertheless, all stimulus changes increased operant responding for water. These results support the idea that habituation contributes to within-session decreases in responding for water reinforcers.

The relation of multiple-schedule behavioral contrast to deprivation, time in session, and within-session changes in responding.

Pigeons' keypecking was reinforced by food on baseline schedules of multiple variable interval (VI) x VI x and on contrast schedules of multiple VI x VI y. Deprivation of food was varied by maintaining subjects at 75%, 85%, and 95% (+/- 2%) of their free-feeding weights. Positive and negative behavioral contrast were observed. The size of the contrast was not systematically altered by changes in deprivation. Positive and negative contrast were both larger later in the session than they were earlier. Within-session decreases in responding were steeper for the baseline than for the contrast schedules for positive contrast. Within-session decreases were steeper for the contrast than for the baseline schedules for negative contrast. These results were predicted by the idea that different amounts of habituation to the reinforcer during the baseline and contrast schedules contribute to behavioral contrast. The results show that contrast occurs under conditions that reduce the effect of the following component. The results support the assumption that positive and negative contrast are produced by symmetrical theoretical variables.

Varying reinforcer duration produces behavioral interactions during multiple schedules.

The experiments tested the idea that changes in habituation to the reinforcer contribute to behavioral interactions during multiple schedules. This idea predicts that changing an aspect of the reinforcer should disrupt habituation and produce an interaction. Pigeons and rats responded on multiple variable interval variable interval schedules. Introducing variability into the duration of reinforcers in one component increased response rates in both components when the schedules provided high, but not low, rates of reinforcement. The increases in constant-component response rates grew larger as the session progressed. Within-session decreases in responding were smaller when the other component provided variable-, rather than fixed-, duration reinforcers. These results are consistent with the idea that changes in habituation to the reinforcer contribute to behavioral interactions. They help to explain why interactions do not occur for some subjects under conditions that produce them for others. Finally, the results question the assumption that induction and behavioral contrast are always produced by different theoretical mechanisms.

Extinguished operant responding shows stimulus specificity.

The experiment tested for stimulus specificity in extinguished operant responding. Eight pigeons pecked keys for food reinforcers delivered by a variable interval (VI) 60-s schedule. The key was illuminated with red light during some sessions and white light during others. Then, responding was placed on extinction. During some sessions of extinction, the color of the key light remained constant throughout the session (red or white). During other sessions the color changed at 30 min into the session (red to white or white to red). Response rate increased after the change of key color in extinction. If it is assumed that key color is part of the stimulus to which subjects habituate, then these results are consistent with McSweeney and Swindell's [J. Gen. Psychol. 129 (2002) 364] suggestion that responding declines in extinction partly because subjects habituate to the stimuli that support conditioned responding. Habituation is relatively specific to the exact nature of the stimulus presented. Therefore, changes in the stimulus violate stimulus specificity and restore habituated responding. The results are also consistent with other theories that attribute extinction to a reduction of stimulus control [e.g., Psychol. Bull. 114 (1993) 80; J. Exp. Psychol.: Anim. Behav. Process. 16 (1990) 235], but considerations such as parsimony and testability favor the habituation hypothesis over these theories.

Within-session rates of responding when reinforcer magnitude is changed within the session.

In the present experiment, the authors investigated the idea that within-session changes in operant response rates occur because subjects sensitize and then habituate to the reinforcer. If that is true, then altering an aspect of the reinforcer within the session should alter the observed within-session responding. The authors tested that idea by having rats press a lever for 2 food-pellet reinforcers delivered by a variable-interval 120-s schedule during 60-min baseline sessions. In treatment conditions, the magnitude of the reinforcer was halved (1 pellet) or doubled (4 pellets) 10, 20, 30, 40, or 50 min into the session. That magnitude of reinforcement then remained in effect for the rest of the session. Altering reinforcer magnitude altered the rates of responding within the session in a fashion consistent with the habituation explanation, that is, response rates increased, relative to baseline, when the magnitude of reinforcement was increased. They decreased when the magnitude was decreased. Those results were seemingly inconsistent with the competing idea that within-session decreases in responding rates are produced by satiation.

Dynamic changes in reinforcer effectiveness: satiation and habituation have different implications for theory and practice.

Reinforcers lose their effectiveness when they are presented repeatedly. Early researchers labeled this loss of effectiveness as satiation without conducting an experimental analysis. When such an analysis is conducted, habituation provides a more precise and empirically accurate label for the changes in reinforcer effectiveness. This paper reviews some of the data that suggest that habituation occurs to repeatedly presented reinforcers. It also argues that habituation has surprisingly different implications than satiation for theory and practice in behavior analysis. For example, postulating that habituation occurs to repeatedly presented reinforcers suggests ways for maintaining the strength of an existing reinforcer and for weakening the strength of a problematic reinforcer that differ from those implied by an account in terms of satiation. An habituation account may also lead to different ways of conceptualizing the regulation of behavior. For example, habituation may be a single-process contributor to the termination of behaviors that are usually attributed to satiation (e.g., ingestive behaviors such as eating and drinking), fatigue (e.g., energetic behaviors such as running), the waning of attention (e.g., cognitive behaviors such as studying), and pharmacodynamic factors (e.g., drug taking).

Habituation of salivation and motivated responding for food in children.

Repeated presentation of food cues results in habituation in adults, as demonstrated by a decrement in salivary responding that is reversed by presenting a new food cue in adults. Food reinforced behavior in animals shows the same pattern of responding, with a decrease in responding to obtain the food, followed by a recovery of responding when a new food is presented. The present study assessed whether children would show the same pattern of a decrement of food reinforced responding followed by recovery of responding when a new food is presented for both salivation and food reinforcement tasks. Subjects were assigned to one of two groups that differed in the trial that the new food stimulus was presented to ensure recovery was specific to the introduction of the new food stimulus. In the salivation task, subjects were provided repeated olfactory presentations of a cheeseburger with apple pie as the new food stimulus, while in the food reinforcement task subjects worked for the opportunity to consume a cheeseburger, followed by the opportunity to work for consumption of apple pie. Subjects in both groups showed a decrement in salivary and food reinforced responding to repeated food cues followed by immediate recovery of responding on the trial when a new food was presented. Subjects increased their energy intake by over 30% in the food reinforcement task when a new food was presented. These results are consistent with the general process theory of motivation that suggests that changes in food reinforced responding may be due in part to habituation.

The effect of rate of reinforcement and time in session on preference for variability.

Pigeons pecked keys on concurrent-chains schedules that provided a variable interval 30-sec schedule in the initial link. One terminal link provided reinforcers in a fixed manner; the other provided reinforcers in a variable manner with the same arithmetic mean as the fixed alternative. In Experiment 1, the terminal links provided fixed and variable interval schedules. In Experiment 2, the terminal links provided reinforcers after a fixed or a variable delay following the response that produced them. In Experiment 3, the terminal links provided reinforcers that were fixed or variable in size. Rate of reinforcement was varied by changing the scheduled interreinforcer interval in the terminal link from 5 to 225 sec. The subjects usually preferred the variable option in Experiments 1 and 2 but differed in preference in Experiment 3. The preference for variability was usually stronger for lower (longer terminal links) than for higher (shorter terminal links) rates of reinforcement. Preference did not change systematically with time in the session. Some aspects of these results are inconsistent with explanations for the preference for variability in terms of scaling factors, scalar expectancy theory, risk-sensitive models of optimal foraging theory, and habituation to the reinforcer. Initial-link response rates also changed within sessions when the schedules provided high, but not low, rates of reinforcement. Within-session changes in responding were similar for the two initial links. These similarities imply that habituation to the reinforcer is represented differently in theories of choice than are other variables related to reinforcement.

Dishabituation with component transitions may contribute to the interactions observed during multiple schedules.

Rates of responding by rats were usually higher during the variable interval (VI) 30-s component of a multiple VI 30-s fixed interval (FI) 30-s schedule than during the same component of a multiple VI 30-s VI 30-s schedule (Experiment 1). Response rates were also usually higher during the FI 30-s component of a multiple VI 30-s FI 30-s schedule than during the same component of a multiple FI 30-s FI 30-s schedule (Experiment 2). The differences in response rates were not observed when the components provided VI or FI 120-s schedules. These results were predicted by the idea that differences in habituation to the reinforcer between multiple schedules contribute to behavioral interactions, such as behavioral contrast. However, differences in habituation were not apparent in the within-session patterns of responding. Finding differences in response rates in both experiments violates widely-held assumptions about behavioral interactions, including that behavioral contrast does not occur for rats and that improving the conditions of reinforcement decreases, rather than increases, response rate in the alternative component.

Dynamic changes in reinforcer effectiveness: theoretical, methodological, and practical implications for applied research.

Reinforcers lose their effectiveness when they are presented repeatedly. Traditionally, this loss of effectiveness has been labeled satiation. However, recent evidence suggests that habituation provides a more accurate and useful description. The characteristics of behavior undergoing satiation differ for different stimuli (e.g., food, water), and these characteristics have not been identified for the noningestive reinforcers often used by applied behavior analysts (e.g., praise, attention). As a result, the term satiation provides little guidance for either maintaining or reducing the effectiveness of reinforcers. In contrast, the characteristics of behavior undergoing habituation are well known and are relatively general across species and stimuli. These characteristics provide specific and novel guidance about how to maintain or reduce the effectiveness of a reinforcer. In addition, habituation may lead to a better understanding of several puzzling phenomena in the conditioning literature (e.g., extinction, behavioral contrast), and it may provide a more precise and accurate description of the dynamics of many different types of behavior.

Common processes may contribute to extinction and habituation.

Psychologists routinely attribute the characteristics of conditioned behavior to complicated cognitive processes. For example, many of the characteristics of behavior undergoing extinction have been attributed to retrieval from memory. The authors argue that these characteristics may result from the simpler process of habituation. In particular, conditioned responding may decrease during extinction partially because habituation occurs to the stimuli that control responding when those stimuli are presented repeatedly or for a prolonged time (e.g., the experimental context, the conditioned stimulus in classical conditioning). This idea is parsimonious, has face validity, and evokes only processes that are well established by other evidence. In addition, behavior undergoing extinction shows 12 of the fundamental properties of behavior undergoing habituation. However, this model probably cannot provide a complete theory of extinction. It provides no obvious explanation for some of the other characteristics of extinguished behavior.

Participation by women in developmental, social, cognitive, and general psychology: A context for interpreting trends in behavior analysis.

We examined participation by women in journals devoted to social, developmental, cognitive, and general psychology. Authorship and first authorship by women increased from 1978 to 1997 for most journals. Participation by women on the editorial staff did not keep pace with their increased authorship for social and developmental psychology. Based on these trends, women's participation decreased with increases in the selectivity of the position for social and developmental psychology (a glass ceiling). The development of a glass ceiling suggests that the contributions of men and women are not always treated equally (gender inequity). Because a similar glass ceiling was reported for journals in behavior analysis (McSweeney, Donahoe, & Swindell, 2000; McSweeney & Swindell, 1998), the causes of this inequity appear to be relatively widespread. The failure to find a glass ceiling for general and cognitive psychology suggests that the inequity might be reduced by subtle pressure for diversity in editorial positions and by adopting actions that encourage women to pursue research positions.