Intermittent Reinforcement Schedules Research Articles

Conversation with Joseph V. Brady

Conversation with Joseph V. Brady

MOTIVATIONAL INFLUENCES ON PERFORMANCE MAINTAINED BY FOOD REINFORCEMENT

In Study 1, we examined the independent effects of reinforcer consumption during sessions and meal consumption prior to sessions on performance maintained by food reinforcement. Nine individuals with developmental disabilities participated. On alternate days, a preferred edible item was delivered during (a) seven sessions conducted before lunch (repeated-reinforcement condition) versus (b) one session each conducted before and after lunch (pre- and postmeal conditions). Results for 7 of 9 participants showed decreased response rates across sessions in the repeated-reinforcement condition; results for 3 of 9 participants showed decreased rates during postmeal relative to premeal conditions. Two participants who did not show a decrement in responding during either comparison participated in Study 2, in which reinforcer consumption during sessions, combined with meal consumption prior to sessions, also had no effect on their performance. In Study 3, we determined whether (a) choice of reinforcers, (b) increased break time between sessions, (c) varied reinforcers, or (d) intermittent reinforcement schedules mitigated the satiation effects observed for the 7 participants in Study 1. Presession choice of reinforcers resulted in maintained performance for 2 of 6 participants exposed to this condition. Varied reinforcement resulted in maintained performance for only 1 of 5 participants exposed to this condition. Neither the increased break between sessions nor the intermittent reinforcement schedule was effective in maintaining performance for the participants who were exposed to these conditions.

Acquisition of polarized-light orientation in salmonids under laboratory conditions

We examined orientation responses of juvenile salmonids to a polarized-light stimulus under laboratory conditions. Juvenile rainbow trout, Oncorhynchus mykiss, steelhead (anadromous), O.mykiss, and brook char, Salvelinus fontinalis, were trained using an operant conditioning methodology to orient relative to the axis of a linear-polarized light field. On average, rainbow trout responded correctly relative to the orientation of the light stimulus approximately 70% of the time within five training sessions. The proportion of correct responses increased further under an intermittent reinforcement schedule. We released trained and untrained fish individually in a circular tank and quantified orientation responses using a digital image-tracking system. Experimentally naı̈ve rainbow trout had no directional tendency, in contrast to trained rainbow trout, steelhead and brook char, which oriented relative to the plane of polarized light. Trained fish showed no orientation response when a diffuser was used to depolarize the light source. Rainbow trout trained to orient parallel to a polarization axis in the laboratory were tested under natural skylight before sunset. These fish oriented parallel to the bearing of maximally polarized light in the celestial hemisphere. Copyright 2003 Published by Elsevier Science Ltd on behalf of The Association for the Study of Animal Behaviour.

A history of reinforcement: The role of reinforcement schedules in behavior pharmacology.

Operant behavior maintained by schedules of intermittent reinforcement has been extensively used for the investigation of the behavioral effects of drugs that affect the central nervous system. While the primary purpose of such research has been the screening of novel compounds in the search for clinically effective drugs, much else has been discovered about drug action and behavioral processes. Current research focuses on problems of social and clinical importance, such as cocaine abuse, and involves the development of novel and modified reinforcement schedules to address problems thrown up by research involving drugs. There are also new challenges posed in studying the behavior of transgenic knock-out mice. After more than 40 years, reinforcement schedules continue to be significant research techniques in behavior pharmacology. It is important that researchers are fully trained in behavior analysis in order to contribute to this rapidly developing area of behavioral neuroscience. ********** In parallel with the development of scientific approaches to psychology, there were massive developments in our knowledge and understanding of the brain and central nervous system in the twentieth century and this progress has accelerated in recent years. Practitioners of both disciplines are aware that each discipline needs the other to provide a more complete account of human psychology and behavior. However, there is a tendency to regard brain processes as more basic in some sense, and to expect that psychological or behavioral processes will, in the end, be explained by brain processes. Whether or not one accepts this reductionist philosophical position, scientific and practical considerations show that it is necessary to investigate both behavioral processes and brain processes, and the interaction between the two. From a scientific perspective, behavioral processes are interesting in their own right, and knowledge of them and their applications can be developed without necessarily knowing about the correlated brain processes. However, scientific curiosity would lead us to investigate, where possible, the brain processes that go in parallel with behavioral or psychological phenomena. From the perspective of neuroscience the argument is even more compelling: we hardly ever investigate a process in the brain or central nervous without a parallel interest in its relationship to psychological functioning. As soon as we ask, How does this brain process affect behavior? we require an adequate account of behavioral processes to which we can relate our study of brain function. In broad terms, behavioral neuroscience is the interdisciplinary field of study that seeks to use behavioral techniques to understand brain function, and to use techniques of neuroscience to elucidate behavioral processes. Psychology and neuroscience come together in various areas of this interdisciplinary field, and to date behavior pharmacology has been one of the most productive of these areas. The last 50 years have seen the development of the modem pharmaceutical industry, which has the capacity to synthesize new chemical compounds that may be useful in treating human diseases and ameliorating psychological distress of one kind or another. A major section of the industry is concerned with agents that may alter the functioning of the human central nervous system. In the development of a new drug that affects the human central nervous system many practical, legal and scientific issues must be addressed. As well as safety issues (the drug may be toxic to many species, or especially to humans, or it may damage certain types of tissues or impair certain physiological functions, etc.), the drug must pass various positive tests as well. Even if it is chemically similar to a compound already in use for the treatment of anxiety, for example, it may not itself be effective. If it is effective, how should it be administered, and on what type of dosing regime? …

Reinforcer Rate and Stimulus Control in Discrimination Reversal Learning

Nine subjects with mental retardation were trained with a fading procedure to perform two-choice simple-discrimination tasks. Immediately after meeting a learning criterion, discrimination reversals were trained within the same sessions, also with the fading procedure. Two 10- or 16-reversal conditions were compared within subjects: High, in which all correct responses for the initial discrimination were followed by reinforcers, and Low, in which the initial discrimination was trained with an intermittent reinforcement schedule and a lower obtained reinforcer rate. In both conditions, reinforcers followed all correct responses during reversal training. Reversal learning error rates were higher in the High condition for 8 subjects. Thus, the rate of reinforcement during the initial discrimination training was related to the persistence of stimulus control, as shown by more selections of the initially positive stimulus during reversal training in the High condition. The results are consistent with behavioral momentum theory. The results indicate that (a) behavioral momentum may describe stimulus control in discrete-trial procedures, and (b) teaching procedures may sometimes benefit from tactical reductions in local reinforcement density to encourage changes or transfers of stimulus control.

Non-linear dynamics of operant behavior: a new approach via the extended return map.

Previous efforts to apply non-linear dynamic tools to the analysis of operant behavior revealed some promise for this kind of approach, but also some doubts, since the complexity of animal behavior seemed to be beyond the analyzing ability of the available tools. We here outline a series of studies based on a novel approach. We modified the so-called 'return map' and developed a new method, the 'extended return map' (ERM) to extract information from the highly irregular time series data, the inter-response time (IRT) generated by Skinner-box experiments. We applied the ERM to operant lever pressing data from rats using the four fundamental reinforcement schedules: fixed interval (FI), fixed ratio (FR), variable interval (VI) and variable ratio (VR). Our results revealed interesting patterns in all experiment groups. In particular, the FI and VI groups exhibited well-organized clusters of data points. We calculated the fractal dimension out of these patterns and compared experimental data with surrogate data sets, that were generated by randomly shuffling the sequential order of original IRTs. This comparison supported the finding that patterns in ERM reflect the dynamics of the operant behaviors under study. We then built two models to simulate the functional mechanisms of the FI schedule. Both models can produce similar distributions of IRTs and the stereotypical 'scalloped' curve characteristic of FI responding. However, they differ in one important feature in their formulation: while one model uses a continuous function to describe the probability of occurrence of an operant behavior, the other one employs an abrupt switch of behavioral state. Comparison of ERMs showed that only the latter was able to produce patterns similar to the experimental results, indicative of the operation of an abrupt switch from one behavioral state to another over the course of the inter-reinforcement period. This example demonstrated the ERM to be a useful tool for the analysis of IRT accompanying intermittent reinforcement schedules and for the study of the non-linear dynamics of operant behavior.

A behavioral approach to understanding and treating pathological gambling.

This report provides a behavioral account of gambling and its treatment. It describes the similarities between gambling and other behaviors maintained by intermittent schedules of reinforcement, the relationship between response cost and gambling behaviors, and how magnitudes of reinforcers affect gambling behaviors. In addition, the relationship between immediacy of reinforcement and behavior is described. Using these behavioral phenomena, behavioral and cognitive-behavioral treatments of pathological gambling are described. Finally, we present the rationale and framework for our cognitive-behavioral treatment, and we provide a behavioral interpretation of 12-step groups.

Oral drug self-administration: an overview of laboratory animal studies

Many abused drugs can be established as orally delivered reinforcers for rhesus monkeys and other animals. Benzodiazepines, barbiturates, opioids, psychomotor stimulants, dissociative anesthetics, and ethanol can come to serve as reinforcers when taken by mouth. The principal problems in establishing drugs as reinforcers by the oral route of administration are (1) aversive taste, (2) delay in onset of central nervous system effects, and (3) consumption of low volumes of drug solution. Strategies have been devised to successfully overcome these problems, and orally delivered drugs can be established as effective reinforcers. Reinforcing actions are demonstrated by consumption of greater volumes of drug solution than volumes of the water vehicle, and supporting evidence for reinforcing effects consists of the maintenance of behavior under intermittent schedules of reinforcement and the generation of orderly dose-response functions. This article presents an overview of studies of behavior reinforced by oral drug reinforcement. Factors that control oral drug intake include dose, schedule of reinforcement, food restriction, and alternative reinforcers. Many drugs, administered by the experimenter, can alter oral drug reinforcement. Relative reinforcing effects can be assessed by choice procedures and by persistence of behavior across increases in schedule size. In general, reinforcing effects increase directly with dose. Rhesus monkeys prefer combinations of reinforcing drugs to the component drugs. The taste of drug solutions may act as a conditioned reinforcer and a discriminative stimulus. Consequences of drug intake include tolerance and physiological dependence. Findings with orally self-administered drugs are similar to many findings with other positive reinforcers, including intravenously self-administered drugs.

Negative reinforcement and substance abuse: Using a behavioral conceptualization to enhance treatment.

Reinforcement history is a particularly potent predictor of future addictive behaviors. For the most part, researchers have studied the operant in order to understand and control positive reinforcement that maintains substance use behavior. However, negative reinforcement can be just as powerful in increasing the likelihood of substance use in the future, and some data suggest that negative (and not positive) reinforcement predicts increased difficulties associated with substance abuse over a person's lifetime. Negative reinforcement may be occurring if clients are avoiding withdrawal symptoms, escaping reality, self-medicating, or avoiding loss of friends. In such cases, therapists may wish to use strategies such as response prevention, generating alternative activities, environmental interventions within the family and community, and emotion regulation and distress tolerance skills. ********** Examining the function of substance use behavior is the foundation of credible psychosocial research and clinical practice concerning the prevention and treatment of addictive behaviors. Reinforcement history is a particularly potent predictor of future addictive behaviors. For the most part, researchers have studied the operant in order to understand and control positive reinforcement that maintains substance use behavior. The most obvious example of positive reinforcement concerns physical euphoria associated with drug use. The rewarding effects can be immediate and quite powerful, making it likely that the behavior will be repeated in the future. Furthermore, if the euphoria is not always predictable and occurs intermittently, then reduction or cessation of the substance use becomes even more difficult since intermittent positive reinforcement schedules tend to maintain persistent behavior in the absence of consistent access. However, negative reinforcement also can powerfully increase the likelihood of substance use in the future. Some research findings suggest that negative (and not positive) reinforcement may predict increased difficulties associated with substance abuse over a person's lifetime (e.g., Carey & Correia, 1997). People who abuse substances often make statements such as using makes me feel normal, which provide clues to the function of the behavior. The language implies that without the substance use the person feels abnormal, and that substance use takes away the aversive abnormality (hence, negative reinforcement). NEGATIVE REINFORCEMENT OF SUBSTANCE USE Negative reinforcement is understood to increase the likelihood that a behavior will be repeated in the future. A negative reinforcer strengthens behavior that reduces an aversive outcome (e.g., Skinner, 1974). Negative reinforcement of substance use means that an aversive outcome is reduced by the use of the substance, making it more likely that substance use behavior will occur again in the future. Substance use is often negatively reinforced on an intermittent schedule, making extinction of the behavior more difficult. Indeed, extinction of substance use does appear to be a difficult proposition, perhaps owing to the power of the intermittent reinforcement schedule. One function of problematic drug use may be to reduce aversive withdrawal symptoms. A coffee-drinking reader who has experienced the pain of a caffeine headache after a prolonged period without coffee may understand the function of drinking coffee to reduce the headache. For other more life threatening forms of addictive behaviors, one common question asked of people to determine the extent of a substance use problem is something like, do you use substances to reduce a hangover or stop the shakes? A positive response to such a question often suggests a greater constellation of consequences associated with problematic substance use (Buchsbaum, Buchanan, Centor, Schnoll, & Lawton, 1991; Dyson, et al., 1998). In the case of reduction of aversive withdrawal symptoms, negative reinforcement via substance use can be extraordinarily powerful. …

Self-administration of remifentanil, an ultra-short acting opioid, under continuous and progressive-ratio schedules of reinforcement in rats.

Remifentanil is a mu-opioid agonist with an exceptionally short duration of action. Evaluating remifentanil's effects within the self-administration model of drug abuse may provide insight into the relationship between a drug's duration of action and its effectiveness as a reinforcer. This study was conducted to establish a dose-effect function for intravenous remifentanil self-administration in rats and to assess the drug's ability to maintain responding under intermittent schedules of reinforcement. Inter-infusion intervals were recorded under two continuous-reinforcement schedules of remifentanil self-administration. In the fixed-dose schedule, the unit dose (0.25-32 micrograms/kg) was held constant within sessions but varied across sessions. In the variable-dose schedule, four different doses were self-administered in random order within each session. For comparison, heroin (6.25-125 micrograms/kg) was studied with the variable-dose schedule. Remifentanil and heroin were also compared under a progressive-ratio schedule of reinforcement in which the response requirements increased exponentially with each successive infusion until responding ceased within each session. Under the continuous-reinforcement schedules, inter-infusion intervals for both drugs increased monotonically as a function of dose, with the remifentanil curve being considerably flatter. Under the progressive-ratio schedule, breaking points varied as an inverted-U shaped function, and the highest breaking points maintained by remifentanil and heroin were similar. At the doses that maintained the highest breaking points under the progressive-ratio schedule, post-infusion pauses under the continuous-reinforcement schedule were about three times shorter with remifentanil than with heroin. Although rates of self-administration are clearly influenced by a drug's duration of action, the ability to maintain responding under intermittent schedules of reinforcement may be independent of duration of action.

Psychophysiological assessment of compulsive gamblers' arousal to gambling cues: a pilot study.

Psychophysiological assessments measuring heart rate, systolic and diastolic blood pressure, and skin resistance level were conducted on 7 male compulsive gamblers and on 7 age and gender matched controls while both groups performed mental arithmetic and listened to individualized tapes of the gamblers' preferred form of gambling and an individualized fear tape. Heart rate responses of the gamblers to the 2 gambling audiotapes were significantly greater than those found for the controls whereas the groups did not differ on mental arithmetic or the fear provoking scene, confirming some degree of cue-specific arousal in gamblers. The other physical responses did not yield such strong results. If physiological arousal provides the motivational basis for gambling and is maintained on an intermittent schedule of reinforcement, the findings may have implications for the treatment of compulsive gambling.

Discrimination and self-administration of nicotine by inbred strains of mice

These studies aim to characterize the discriminative stimulus effects of nicotine in two inbred strains of mice that differ in many pharmacological responses, and to investigate the feasibility of IV self-administration studies with nicotine in one of the strains. For discrimination studies, three groups of C57BL/6 and one group of DBA/2 mice were trained in a two-lever operant conditioning paradigm with a tandem VI-30" FR-10 schedule of food reinforcement. After 40 training sessions, accuracy reached 57.5, 77.5 and 90.0% in C57BL/6 mice trained with (-)-nicotine (SC) in doses of 0.4, 0.8 and 1.6 mg/kg, respectively (n = 8). DBA/2 mice trained with 0.8 mg/kg nicotine attained similar (73.3 %) accuracy (n = 9). Results from extinction tests showed that all groups of mice yielded orderly dose-response curves for nicotine (0.03-1.6 mg/kg), but stimulus control remained notably weaker for the mice trained with 0.4 mg/kg nicotine than for any other group. Overall rates of responding in the undrugged state were lower for DBA/2 than for C57BL/6 mice; DBA/2 mice were also slightly less sensitive than C57BL/6 mice to the response rate-reducing effect of nicotine. The nicotine antagonist mecamylamine (1.5 mg/kg SC) blocked the discriminative stimulus effect of the training dose of nicotine in all groups. The results of the IV self-administration study suggest that nicotine (0.1 mg/kg) can serve as a positive reinforcer in drug-naive C57BL/6J mice (n = 13). Behaviour maintained by 0.1 mg/kg nicotine injections was significantly greater than behaviour maintained by vehicle injections, and it was maintained under an intermittent schedule of reinforcement (FR4). The methods described provide possible approaches for genetic analyses of strain differences in sensitivity to the discriminative and reinforcing stimulus properties of nicotine.

Inherited Hypohedonia Leads to Learned Helplessness: A Conjecture Updated

The reformulated learned helplessness theory posits depressogenic explanatory style (DES) as a vulnerability factor in depression. Early-life adversity has been suggested as the source of DES, but this is contradicted by empirically established features of the learned helplessness phenomenon itself and by the evidence for human resilience in the face of adversity. This article extends a conjecture first advanced by P. E. Meehl (1975) and argues that an inherited defect in hedonic capacity would be sufficient to produce DES by causing intermittent schedules of reinforcement to be experienced as extinction schedules, resulting in a pervasive and unremitting sense of helplessness. This proposed hedonic defect also provides a means for integrating the original and reformulated versions of learned helplessness theory and for integrating learned helplessness with emerging research and theory on the biology of vulnerability to depression.

Inherited hypohedonia leads to learned helplessness: A conjecture updated.

The reformulated learned helplessness theory posits depressogenic explanatory style (DES) as a vulnerability factor in depression. Early-life adversity has been suggested as the source of DES, but this is contradicted by empirically established features of the learned helplessness phenomenon itself and by the evidence for human resilience in the face of adversity. This article extends a conjecture first advanced by P. E. Meehl (1975) and argues that an inherited defect in hedonic capacity would be sufficient to produce DES by causing intermittent schedules of reinforcement to be experienced as extinction schedules, resulting in a pervasive and unremitting sense of helplessness. This proposed hedonic defect also provides a means for integrating the original and reformulated versions of learned helplessness theory and for integrating learned helplessness with emerging research and theory on the biology of vulnerability to depression.

Response Stereotypy in Humans Maintained by Response‐Contingent Events

The present study examined stereotyped behaviors developed during human performances that were generated by response‐dependent intermittent schedules of reinforcement. Thirty university students were assigned to either fixed‐interval 30‐s or fixed‐ratio 30‐s schedules in which points or monetary reinforcers were produced only by presses on the number keys of a 41‐key computer keyboard. Behavior patterns developed by all subjects were classified into four categories: optimal, random, unique, and general stereotypes. The general stereotypes category was further subdivided into five idiosyncratic types: connection, order, shift, repeat, and restriction. Analysis of the data demonstrated the role of contiguity: Whatever behavior happened to precede reinforcers was repeated even though reinforcers did not depend on that behavior. These findings support the argument that much of idiosyncratic and stereotyped human behavior is produced and maintained by contingencies of reinforcement, rather than schedule‐induced or adjunctive behavior.

Anatomical Substrates of Behavioral Impairment Induced by Developmental Lead Exposure in Monkeys: Inferences from Brain Lesions

There is a substantial literature exploring the behavioral consequences of developmental lead exposure in the monkey; deficits have been observed on a number of tasks assessing learning and memory including spatial delayed alternation, discrimination reversal, matching to sample, and concurrent discrimination. Differences in performance between control and lead-exposed monkeys have also been observed on intermittent schedules of reinforcement. Comparison of the effects of lead with the extensive literature on the consequences of lesions in discrete areas of brain on the same tasks may provide insight into the possible sites of brain damage responsible for lead-induced behavioral impairment. Available data strongly suggest that prefrontal cortical areas are damaged by lead, based on the pattern of performance deficits across specific tasks. In addition, a constellation of global deficits including perseveration, increased distractibility, inability to change response strategy, and inability to inhibit inappropriate responding are hallmarks of both prefrontal damage and developmental lead exposure. Evidence also implicates basal forebrain structures in behavior impairment produced by lead based on the pattern of deficits across numerous tasks, although the evidence is much weaker than for prefrontal cortex. In contrast, the pattern of behavioral impairment produced by limbic system lesions is different in many respects from that produced by lead; in addition, the scant neuropathological data available suggest that limbic structures are not a target of lead even at high blood lead levels in the monkey. Comparison of the pattern of damage following lead exposure with the effects of lesions, presented here, provides direction for further morphological or neurochemical exploration of lead-induced brain damage in the monkey.

The relationship between gambling and video-game playing behavior in children and adolescents

It is suggested that commercial video-games (e.g.Nintendo &Sega) and gambling activities have similar attractive features and intermittent reinforcement schedules. This research seeks to examine the nature of this relationship amongst children. One hundred and four children aged 9 to 14, from grades 4, 6, and 8, participated. A questionnaire exploring issues related to video-game playing and gambling behavior in children and adolescents was completed and a computerized blackjack game was individually administered. High frequency video-game players were compared to low frequency video-game players with respect to their gambling performance on the blackjack gambling task as well as on information gathered from the questionnaire. Of particular concern is the risk-taking strategies used by avid video-game players, whether or not children perceive gambling and video-games as involving similar amounts of skill or whether they realize that gambling is primarily a game of chance. The findings, in general, suggest that high frequency video-game players gamble more than low frequency video-game players, report that gambling makes them feel more important, and take greater risks on the blackjack gambling task although no overall differences in success were found. Males exhibited greater risk-taking tendencies on the blackjack task than females. The clinical implications of the findings are addressed.

Transgenic superoxide dismutase mice

Consequent to the insertion of the human Cu/Zn-superoxide dismutase (SOD) transgene, SOD transgenic mice (SOD-Tg) show higher concentrations of the primary receptor thought to be involved in opioid reinforcement (µ). These increases are observed in areas specifically associated with the primary neurotransmitter thought to be involved in addiction (dopamine), and in neuroanatomical regions thought to mediate substance abuse (mesolimbic). In the present study we have tested the idea that these increases in µ-receptors are associated with parallel changes in µ-mediated behaviors. Baseline and morphine-induced locomotor activity were significantly altered in the SOD-Tg mice. A qualitative change in the nature of acute and chronic morphine-induced locomotor activity was demonstrated by a significant change in the slope of the dose-effect curve. SOD-Tg mice were significantly more sensitive to the locomotor stimulant effects of morphine. Intravenous morphine-reinforced behavior was also altered in the SOD-Tg mice. SOD-Tg mice showed significant dose-related changes in operant lever-press responding maintained by morphine injection and significantly greater amounts of behavior were maintained by the drug than by vehicle under an intermittent schedule of reinforcement. In addition, SOD-Tg mice increased operant responding for the drug when the amount of behavior required to maintain drug intake increased 10-fold under a PR schedule of reinforcement. In contrast, in wild-type mice morphine injections failed to maintain greater amounts of behavior than vehicle, there were no dose-related changes in behavior, and when response requirements increased under the PR schedule, morphine intake decreased significantly. Thus, SOD transgene insertion significantly enhanced the efficacy of morphine as a reinforcer.

Fixed interval/fixed ratio performance in adult monkeys exposed in utero to methylmercury.

Previous studies in monkeys and rodents have shown the fixed interval/fixed ratio (FI/FR) schedule to be a sensitive indicator of neurotoxicity. In the present study, monkeys (Macaca fascicularis) were exposed in utero to methylmercury (MeHg). Maternal doses of MeHg of 50, 70, or 90 micrograms/kg b.wt./day resulted in infant blood mercury levels at birth ranging from 1.04 to 2.45 ppm. Monkeys were tested on a multiple FI/FR schedule of reinforcement at 8-10 years of age. Four FI/FR cycles were run per session. Pause time and run rate were calculated for FI and FR components, as well as FI quarter-life and local FI response rates. MeHg treatment and sex effects were investigated by fitting a linear orthogonal polynomial regression to each monkey's profile across sessions and performing two-way ANOVAs on the resulting linear and intercept terms. There were no treatment-related effects on either the FI or FR component for pause time or run rate. Analysis of the quarter-life revealed a significant treatment by sex effect as well as a main effect for sex. Post hoc t-tests revealed a significant difference in quarter-life of treated male and female monkeys and a marginal difference between treated and control males. The FI run rate of the male monkeys was significantly greater than that of the female monkeys whereas the FR run rate of the males was marginally greater. These results indicate that there may be a differential effect of MeHg on male and female monkeys, which could be interpreted as an effect on temporal discrimination. Overall, adult monkeys exposed to in utero MeHg exhibited a very limited sex-related effects on the FI/FR intermittent schedule of reinforcement.

Responding maintained by intermittent reinforcement: implications for the use of extinction with problem behavior in clinical settings.

Results of basic research have demonstrated that behavior maintained on an intermittent schedule of reinforcement (INT) will be extinguished more slowly than behavior maintained on a continuous schedule (CRF). Although these findings suggest that problem behaviors may be difficult to treat with extinction if they have been maintained on INT rather than on CRF schedules, few applied studies have examined this phenomenon with human behavior in clinical settings. The purpose of this study was to determine whether problem behavior maintained on CRF schedules would be extinguished more rapidly than behavior maintained on INT schedules. Three individuals diagnosed with profound mental retardation participated after results of pretreatment functional analyses had identified the sources of reinforcement that were maintaining their self-injury, aggression, or disruption. Subjects were exposed to extinction following baseline conditions with CRF or INT schedules alternated within reversal or multielement designs. Results suggested that problem behavior may not be more difficult to treat with extinction if they have been maintained on INT rather than CRF schedules. However, switching from an INT to a CRF schedule prior to extinction may lower the baseline response rate as well as the total number of responses exhibited during extinction.