The generalization-across-dimensions model applied to conditional temporal discrimination

Can simple choice conditional-discrimination choice be accounted for by recent quantitative models of combined stimulus and reinforcer control? In Experiment 1, two sets of five blackout durations, one using shorter intervals and one using longer intervals, conditionally signaled which subsequent choice response might provide food. In seven conditions, the distribution of blackout durations across the sets was varied. An updated version of the generalization-across-dimensions model nicely described the way that choice changed across durations. In Experiment 2, just two blackout durations acted as the conditional stimuli and the durations were varied over 10 conditions. The parameters of the model obtained in Experiment 1 failed adequately to predict choice in Experiment 2, but the model again fitted the data nicely. The failure to predict the Experiment 2 data from the Experiment 1 parameters occurred because in Experiment 1 differential control by reinforcer locations progressively decreased with blackout durations, whereas in Experiment 2 this control remained constant. These experiments extend the ability of the model to describe data from procedures based on concurrent schedules in which reinforcer ratios reverse at fixed times to those from conditional-discrimination procedures. Further research is needed to understand why control by reinforcer location differed between the two experiments.

Can the prosocial benefits of episodic simulation transfer to different people and situational contexts?

Previous research has found that episodic simulation of events of helping others can effectively enhance intentions to help the same person involved and the identical situational context as the imagined scenarios. This 'prosocial simulation effect' is argued to reflect, at least in part, associative memory mechanisms whereby the simulation is reactivated when in the same situation as that imagined. However, to date, no study has examined systematically whether this 'prosocial simulation effect' can be transferred to response scenarios involving different people and/or situational contexts to the imagined scenarios, and if so, whether the degree of overlap with the imagined helping episode modulated the transfer effect. Across two experiments, we systematically varied the overlap of the simulated and response scenarios, both in terms of the persons in need and/or the situational contexts, and whether would influence the magnitude of prosocial simulation effect. Results from both experiments showed that the prosocial simulation effect can be transferred to response scenarios involving different people and situational contexts to the simulated scenarios. However, this finding was primarily driven by response scenarios that had a high degree of overlap to the simulated scenarios. The application of our findings to the practical implementation of simulation to promote prosociality in the real world is discussed.

Human social enrichment is linked with reduced mortality in artificially reared ewe lambs

Growth of lambs in an artificial rearing system is key to the economic success of sheep-milking farms. Social enrichment in the form of quiet human contact has been shown to increase growth rates of lambs after weaning, perhaps by reducing stress and enhancing immune function. One hundred fourteen artificially reared dairy ewe lambs were assigned either to a Social condition receiving quiet human company in three 20-min segments per day, or a Control condition without quiet human contact. Feeding and housing conditions were otherwise identical. Starting weights for the Social (M = 6.29 kg; SD = 1.19 kg) and Control (M = 6.38 kg; SD = 0.93 kg) lambs were the same, as were their weights after 3 weeks (Mcontrol  = 8.89 kg, SD = 2.55 kg, Msocial  = 8.63 kg, SD = 2.16 kg). Mortality rates were significantly lower (p = 0.041) in the Social condition (0%) than Control (8%). Thus, social enrichment may be important for the wellbeing of lambs and for economic viability in lamb-rearing.

Divided stimulus control depends on differential and nondifferential reinforcement: Testing a quantitative model

We investigated the effects of differential and nondifferential reinforcers on divided control by compound-stimulus dimensions. Six pigeons responded in a delayed matching-to-sample procedure in which a blue or yellow sample stimulus flashed on/off at a fast or slow rate, and subjects reported its color or alternation frequency. The dimension to report was unsignaled (Phase 1) or signaled (Phase 2). Correct responses were reinforced with a probability of .70, and the probability of reinforcers for errors varied across conditions. Comparison choice depended on reinforcer ratios for correct and incorrect responding; as the frequency of error reinforcers according to a dimension increased, control (measured by log d) by that dimension decreased and control by the other dimension increased. Davison and Nevin's (1999) model described data when the dimension to report was unsignaled, whereas model fits were poorer when it was signaled, perhaps due to carryover between conditions. We are the first to test this quantitative model of divided control with reinforcers for errors and when the dimension to report is signaled; hence, further research is needed to establish the model's generality. We question whether divided stimulus control is dimensional and suggest it may instead reflect joint control by compound stimuli and reinforcer ratios.

Revaluation of overselected stimuli: Emergence of control by underselected stimuli depends on degree of overselectivity

Stimulus overselectivity describes strong control by one stimulus element at the expense of other equally relevant elements. Research suggests that control by underselected stimuli emerges following extinction of the overselected stimulus ("revaluation") and the emergence is larger when overselectivity is greater. We compared such revaluation effects with a control compound or condition in two experiments. Human participants chose between compound S+ and S- stimuli. Then, to assess control by compound-stimulus elements, participants chose between individual elements in a testing phase without feedback. The S+ element chosen most often (the overselected element) underwent revaluation, during which choice of that element was extinguished and choice of a novel element reinforced. Thereafter, participants completed a retesting phase. Revaluation reduced choice of the overselected element. Choice of the underselected element decreased for participants with low overselectivity but increased for participants with high overselectivity. This was not the case for a control compound that did not undergo revaluation (Experiments 1 and 2) or in a control condition in which the overselected element continued to be reinforced during revaluation (Experiment 2). These findings suggest that overselectivity levels may modulate revaluation effects, and they also highlight the importance of the contingency change in postrevaluation changes in stimulus control.

Is superstitious responding a matter of detectability? A replication of Killeen (1978)

Organisms may sometimes behave as if a contingency exists between behavior and consequences, even if this is not actually the case. Killeen (1978) suggested that such superstition occurs because of factors that bias subjects to behave "superstitiously" rather than because of failures of discrimination. We systematically replicated Killeen's experiment and compared contingency discrimination between different consequences. Six pigeons responded in a matching-to-sample procedure in which a response-independent or response-dependent stimulus change, food delivery, or blackout occurred. The pigeons reported whether the consequence was response dependent or response independent by choosing between two side keys. Discrimination was strongest after stimulus changes, weaker after blackouts, and weakest after food deliveries. These differences persisted even after additional training, suggesting asymmetries that may reflect differences in the disruptive effects of different consequences on remembering and/or behavioral mnemonics. Importantly, the pigeons were not biased to report response-dependent consequences unless that response was consistent with locational biases; that is, they behaved "superstitiously" when there was a reason to be biased to do so. These findings corroborate Killeen's and demonstrate that behavior may deviate from contingencies not necessarily because subjects cannot discriminate those contingencies but because they are biased to behave otherwise.

Quantitative Indices of Student Social Media Engagement in Tertiary Education: A Systematic Review and a Taxonomy

Recent studies have evaluated the use of social media as learning aids in tertiary education. Emerging research in this area has focused primarily on non-quantitative approaches to student social media engagement. However, quantitative engagement outcomes may be extracted from student posts, comments, likes, and views. The goal of the present review was to provide a research-informed taxonomy of quantitative and behavior-based metrics of student social media engagement. We selected 75 empirical studies comprising a pooled sample of 11,605 tertiary education students. Included studies used social media for educational purposes and reported student social media engagement outcomes (source databases: PsycInfo and ERIC). We used independent raters and stringent interrater agreement and data extraction processes to mitigate bias during the screening of references. Over half of the studies (52%, n = 39) utilized ad hoc interviews and surveys to estimate student social media engagement, whereas thirty-three studies (44%) used some form of quantitative analysis of engagement. Based on this literature, we present a selection of count-based, time-based, and text-analysis metrics. The proposed taxonomy of engagement metrics resulting provides the methodological basis for the analysis of social media behavior in educational settings, particularly, for human operant and behavioral education studies. Implications for future research are discussed.

Supplementary Information

The online version contains supplementary material available at 10.1007/s10864-023-09516-6.

Not bird-brained: Chickens use prior experience to solve novel timing problems

Despite differences between bird and human brain anatomy, birds have recently demonstrated capacities thought to be uniquely human, including planning and problem-solving. Many avian demonstrations of 'complex' behaviors rely on species-specific behavior (e.g., caching, tool use), or use birds who have evolved largely in similarly undomesticated circumstances (e.g., pigeons). In the present experiment, we asked how a species domesticated thousands of years ago, chickens (Gallus gallus domesticus), used past experience to navigate novel problems in the double-bisection task. The double-bisection task which has been used extensively with pigeons, allowing a comparison of signatures of chicken and pigeon performance on the same task. Our findings revealed chickens, like pigeons, show flexible learning that is sensitive to the broader context in which events occur. Further, as with pigeons, our chickens' patterns of performance could be divided into two distinct categories which may reflect differences in the specific behaviors in which organisms engage during a timing task. Our findings demonstrate remarkable similarity in how chickens and pigeons use past experience to navigate novel problems. Further, these findings add to a growing body of knowledge suggesting the simplest forms of learning common across species-operant and respondent conditioning-are more flexible than is typically assumed.

Choosing a future from a murky past: A generalization-based model of behavior

Remembering the past appears critical in allowing organisms to detect order in an environment, and hence to behave in accordance with likely future events. Yet the shortcomings of remembering and perceiving typically mean that the remembered past differs from the actual past, and hence that behavior does not perfectly track the structure of the environment. Here, we outline how the process of generalization might be used to understand differences between what an organism does, and the structure of the past and potential structure of the environment. We explore how different sources of generalization - both from within the same stimulus situation, and from different stimulus situations - might be modeled quantitatively, and how predictions made by this modeling approach are supported by research. Finally, we discuss how generalization from multiple stimulus situations, longer-term experience, and from stimulus situations in the past that are not identical to the stimulus situation in the present, might contribute to our understanding of how an organism's experience translates into behavior.

Effects of brief post-sample cues signaling presence or absence of reinforcers in delayed matching-to-sample

When short-term memory is assessed in the delayed matching-to-sample (DMTS) procedure, performance is better when cues signal larger reinforcer magnitudes or higher reinforcer probabilities for correct responding. Previous studies demonstrating signaled-magnitude or signaled-probability effects presented cues for a prolonged period during the sample stimulus and/or retention interval. The present study asked whether a signaled-probability effect would occur with brief post-sample cues that signaled the presence or absence of reinforcement. Five pigeons responded in a DMTS task in which sample stimuli were sometimes followed by a 0.5-s cue signaling that reinforcers would either be available or not available in the current trial, and the retention interval varied from 0.5s to 20s. A reliable signaled-probability effect was found when reinforcers were arranged independently and for all correct responses, whereas a smaller, less systematic effect was found when reinforcers were arranged dependently and probabilistically. These findings highlight the importance of reinforcement contingencies and contingency discriminability in remembering, and add to the evidence showing that cues signaling differential reinforcement in DMTS may affect processes during the retention interval and comparison phase, rather than attention to the sample stimulus.

Machine learning with a snapshot of data: Spiking neural network 'predicts' reinforcement histories of pigeons' choice behavior

An accumulated body of choice research has demonstrated that choice behavior can be understood within the context of its history of reinforcement by measuring response patterns. Traditionally, work on predicting choice behaviors has been based on the relationship between the history of reinforcement—the reinforcer arrangement used in training conditions—and choice behavior. We suggest an alternative method that treats the reinforcement history as unknown and focuses only on operant choices to accurately predict (more precisely, retrodict) reinforcement histories. We trained machine learning models known as artificial spiking neural networks (SNNs) on previously published pigeon datasets to detect patterns in choices with specific reinforcement histories—seven arranged concurrent variable‐interval schedules in effect for nine reinforcers. Notably, SNN extracted information from a small ‘window’ of observational data to predict reinforcer arrangements. The models' generalization ability was then tested with new choices of the same pigeons to predict the type of schedule used in training. We examined whether the amount of the data provided affected the prediction accuracy and our results demonstrated that choices made by the pigeons immediately after the delivery of reinforcers provided sufficient information for the model to determine the reinforcement history. These results support the idea that SNNs can process small sets of behavioral data for pattern detection, when the reinforcement history is unknown. This novel approach can influence our decisions to determine appropriate interventions; it can be a valuable addition to our toolbox, for both therapy design and research.

Modeling choice across time: Effects of response-reinforcer discriminability

This experiment asks whether timing is affected by animals' discrimination of response-reinforcer contingencies, and if so, how this effect can be understood. Six pigeons were trained on a procedure in which concurrent-schedule reinforcer ratios between left and right keys changed at 30 s after the last reinforcer. One stimulus signaled a reinforcer-ratio reversal from 9:1 to 1:9 on that key, and the other stimulus signaled the inverse reversal, with the key on which these stimuli occurred randomized. Across conditions, the physical difference between the stimuli signaling the two responses was varied and the directional changes in the reinforcer ratio signaled by each stimulus were reversed. Choice changed appropriately across time when the two stimuli were discriminable, and points of subjective equality fell with decreasing stimulus difference. A model which assumed that reinforcers obtained in time bins were redistributed across other time bins according to ogivally changing standard deviations, and between response locations according to an ogivally changing redistribution measure, accounted well for the data. This model was shown to be preferable to one in which across-time redistributions were scalar, and across-location redistribution was constant. These results show the critical importance of stimulus-response-reinforcer discriminability to measures of timing.

Extending a misallocation model to children's choice behavior

Nonhuman animal models show that reinforcers control behavior through what they signal about the likelihood of future events, but such control is generally imperfect. Imperfect control by the relation between past and likely future events may result from imperfect detection of those events as they occur, which result in imperfect detection of the relation between events. Such an approach would suggest the involvement of more complex psychological processes like memory in simple operant learning. We extended a research paradigm previously examined with nonhuman animals to test the ability of a quantitative model that assumes imperfect control by the relation between events arises because of (a) occasional misallocation of reinforcers to the wrong response, causing imperfect control by the relation between events; and (b) a tendency to explore or exploit which is independent of the relation between events. Children played a game in which one of two different responses could produce a reinforcer. The likelihood of a reinforcer for the same response that produced the last one varied across three conditions (.1, .5, .9). As with nonhuman animal models, children's choices followed these probabilities closely but not perfectly, suggesting strong control by what one reinforcer signals about subsequent reinforcers. Choice was well described by the quantitative model. This same model also provides a good description of nonhuman animal-model data, suggesting fundamentally similar mechanisms of control across species. These findings suggest reinforcers control behavior to the extent the relation between reinforcers can be detected-that is, simple operant learning may be more complex than is typically assumed. (PsycInfo Database Record (c) 2021 APA, all rights reserved).

Pigeons prefer to invest early for future reinforcers

Five pigeons were trained in a series of conditions in which food was delivered after 25 responses, but only when a different (Investing) response had been made before the 25 responses had been completed. If an Investing response was not made, the 25 responses ended in blackout. In various conditions, effective Investing responses either had to be made before the first of the 25 responses, or anywhere within the 25 responses; and effective Investing responses either resulted in a stimulus change or did not. Pigeons Invested even when the consequences were temporally and spatially distant, but Investing was most likely when it produced an immediate stimulus change. When given the choice, pigeons preferred to make Investing responses at the beginning of a trial. These findings again demonstrate that behavior may be maintained by events that are separated in time and space from the present.

On the "Strength" of Behavior

The place of the concept of response strength in a natural science of behavior has been the subject of much debate. This article reconsiders the concept of response strength for reasons linked to the foundations of a natural science of behavior. The notion of response strength is implicit in many radical behaviorists' work. Palmer (2009) makes it explicit by applying the response strength concept to three levels: (1) overt behavior, (2) covert behavior, and (3) latent or potential behavior. We argue that the concept of response strength is superfluous in general, and an explication of the notion of giving causal status to nonobservable events like latent behavior or response strength is harmful to a scientific endeavor. Interpreting EEG recordings as indicators of changes in response strength runs the risk of reducing behavior to underlying mechanisms, regardless of whether such suggestions are accompanied by behavioral observations. Many radical behaviorists understand behavior as a discrete unit, inviting conceptual mistakes reflected in the notion of response strength. A molar view is suggested as an alternative that accounts for the temporally extended nature of behavior and avoids the perils of a response-strength based approach.

Stimulus control depends on the subjective value of the outcome

Stimuli that provide information about likely future reinforcers tend to shift behavior, provided a reliable relation between the stimulus and the reinforcer can be discriminated. Stimuli that are apparently more reliable exert greater control over behavior. We asked how the subjective value (measured in terms of preference) of reinforcers associated with stimuli influences stimulus control. Five pigeons worked on a concurrent chains procedure in which half of all trials ended in a smaller reinforcer sooner, and the other half in a larger reinforcer later. In Signaled trials, the color and flash duration on the keys in the initial link signaled the outcome of the trial. In Conflicting probe trials, the color and the flash duration signaled conflicting information about the outcome of the trial. Choice in Signaled trials shifted toward the signaled outcome, but was never exclusive. In Conflicting probe trials, control was divided idiosyncratically between the 2 stimulus dimensions, but still favored the outcome with the higher subjective value. Thus, stimulus control depends not only on the perceived reliability of stimuli, but also on the subjective value of the outcome.

Generalizing from the Past, Choosing the Future

Behavior in the present depends critically on experience in similar environments in the past. Such past experience may be important in controlling behavior not because it determines the strength of a behavior, but because it allows the structure of the current environment to be detected and used. We explore a prospective-control approach to understanding simple behavior. Under this approach, order in the environment allows even simple organisms to use their personal past to respond according to the likely future. The predicted future controls behavior, and past experience forms the building blocks of the predicted future. We explore how generalization affects the use of past experience to predict and respond to the future. First, we consider how generalization across various dimensions of an event determines the degree to which the structure of the environment exerts control over behavior. Next, we explore generalization from the past to the present as the method of deciding when, where, and what to do. This prospective-control approach is measurable and testable; it builds predictions from events that have already occurred, and assumes no agency. Under this prospective-control approach, generalization is fundamental to understanding both adaptive and maladaptive behavior.

A137 COLONOSCOPY RELATED ADVERSE EVENTS IN A POPULATION-BASED COLON SCREENING PROGRAM

Background

The British Columbia Colon Screening Program (BCCSP) is a population-based program enrolling 50–74 year old individuals for biennial FIT (OC-Sensor, cut-off 10 mcg/g) with follow-up colonoscopy for positive FIT. The neoplasia detection rate is 50–55% and over 75% of colonoscopies have a specimen taken. Previously reported colonoscopy adverse event rates for FIT based screening programs vary widely: 0.03–6.2% and 0–2.7% for bleeding and perforation, respectively. Mortality as a result of colonoscopy is rare but has been reported in 0.0004%-0.0074% of colonoscopies. The rate of colonoscopy related adverse events in BCCSP participants is unknown.

Aims

To determine the rate of colonoscopy related serious adverse events within the BCCSP.

Methods

This is a retrospective cohort study of all participants undergoing colonoscopy in BCCSP from November 15, 2013 to December 31, 2017. BCCSP contacts screening participants by phone 14 days post colonoscopy to determine unplanned medical visits the day prior (during bowel preparation) or following the colonoscopy. Unplanned events underwent chart review if the event was a perforation, cardiovascular or respiratory event, or resulted in death, hospitalization, or significant intervention including repeat colonoscopy, interventional radiology, surgery, blood transfusion, cardioversion, casting of a fracture or suturing of a laceration. Chart review was conducted by a Colonoscopy Lead and reviewed by BCCSP Quality Committee. Unplanned events were defined as serious adverse events (SAE) if they resulted in death, hospitalization or significant intervention and further classified as probably, possibly, or unlikely related to the colonoscopy.

Results

A total of 108,004 colonoscopies were performed. Unplanned events were reported in 1753 participants, of which 586 met criteria for review. Of these, 578 were confirmed unplanned events and 409 were SAEs of which 367 (89.7%) were probably, 22 (5.4%) possibly and 20 (4.9%) unlikely associated with colonoscopy. 36/10,000 colonoscopies were associated with a SAE that was probably or possibly related: perforation in 5/10,000, bleeding 22/10,000. Three deaths occurred in the 14 days following colonoscopy that were probably (2 perforations) or possibly related to the colonoscopy (0.3/10,000).

Conclusions

The BCCSP has a colonoscopy SAE rate in keeping with previous publications, particularly in the context of a very high proportion of procedures associated with polypectomy, a known risk factor for perforation and bleeding. This study will help inform screening participants about the risks of colonoscopy in the BC program. Future studies are required to confirm these rates using hospital admission data.

Funding Agencies

None

Being there on time: Reinforcer effects on timing and locating

In research on timing, reinforcers often are assumed to influence discrimination of elapsed time. We asked whether changes in choice used to measure timing arise because of joint control by elapsed time and reinforcers, rather than from the direct modification of control by elapsed time by reinforcers. Pigeons worked on a concurrent-choice task in which 1 response was 9 times more likely to produce a reinforcer, reversing between locations when 19 s had elapsed since the marker event. Across conditions, we manipulated the percentage of reinforcers arranged before the probability reversal from 5 to 95%. These changes in reinforcer percentages altered control by location-based elements of the contingency, but not by time-based elements. Choice was well described by a model that assumes that control by the contingency is weakened by generalization across the time and location of reinforcers, and that these generalizations become more likely at later times since a marker. These findings add to a growing body of research that suggests that reinforcers share the same function as other environmental events in determining how the environment controls behavior.

Timing compound stimuli: Relative reinforcer probabilities divide stimulus control in the multiple peak procedure

Previous research suggests that when multiple stimuli signal the location of future reinforcers, the extent of control by each stimulus depends on the relative reinforcer probability associated with that stimulus. In this experiment, we asked whether relative reinforcer probabilities also divide control between stimuli that signal the time to future reinforcers. Six pigeons responded in a multiple peak procedure in which 2 dimensions of a compound stimulus-a color (red or green) and a frequency of on/off alternation (fast or slow)-signaled a fixed-interval 2- or 8-s schedule. Across conditions, we varied the probability of reinforcer deliveries associated with each dimension from .1 to .9. Relative reinforcer probabilities determined the degree of control by each stimulus dimension; as the probability of reinforcer deliveries associated with one dimension increased, a peak in response rates at the time signaled by that dimension became apparent while a peak at the time signaled by the other dimension diminished. However, these effects of relative reinforcers were smaller than in previous research, probably because elapsed time also competed for control over behavior. These findings extend the relation between relative reinforcers and divided stimulus control from spatial to temporal discriminations. (PsycInfo Database Record (c) 2020 APA, all rights reserved).

Timing or counting? Control by contingency reversals at fixed times or numbers of responses

Behavior may come under the control of time or number when such cues predict food availability, but time tends to exert stronger control than does number. We asked how the control of behavior is divided between time and number in a procedure where the likely location of a reinforcer reversed systematically at a particular point in a trial. In Phase 1, we asked whether pigeons would choose to time or count when both time and number of responses could be used to discriminate the occurrence of the reversal. Pigeons preferred to time, even when numbers of events were more reliable predictors of reinforcer availability. In the Phase 2, we asked whether control could be shifted from time to number when number was highly discriminable and time was a highly unreliable predictor of the reversal. In this phase, pigeons continued to favor time over number. These findings suggest divided stimulus control between time and numbers depends both on recent experience with relative reinforcer rates and on factors relating to the animal's longer term history, such as training. This research adds to a growing body of work demonstrating strong control by elapsed time when elapsed time is not the most reliable predictor of reinforcer availability. (PsycINFO Database Record (c) 2019 APA, all rights reserved).

Environment tracking and signal following in a reinforcer-ratio reversal procedure

Several studies suggest that the degree of control by reinforcer ratios (environment tracking) and by exteroceptive stimuli that signal future reinforcer availability (signal following) depends on environmental certainty: As reinforcers become more likely at one location, environmental contingencies exert stronger control and exteroceptive stimuli exert weaker control. This research has not yet been extended to environments in which reinforcer availability changes across time, even though such changes are present in most natural environments. Thus, in the present experiment, we examined environment tracking and signal following in a concurrent schedule in which the reinforcer ratio reversed to its reciprocal 30 s after a reinforcer delivery and keylight-color stimuli signaled the likely or definite time or location of the next reinforcer. Across conditions, we manipulated environmental certainty by varying the probability of reinforcer deliveries on the locally richer key. This made the location of future reinforcers at a particular time more or less certain, but did not change the overall reinforcer ratio. Changes in local environmental certainty had little to no effect on environment tracking and signal following; in all conditions, keylight-color stimuli strongly controlled choice and reinforcer ratios exerted weak control. The present findings suggest that the extent of environment tracking and signal following is primarily determined by global, not local, environmental certainty.

Generalization of response patterns in a multiple peak procedure

Stimulus generalization is typically assessed by analyzing overall response rates. Studies of generalization of response-rate patterns across time are less common, despite the ubiquitous nature of time and the strong temporal control over behavior in the natural world. Thus, we investigated generalization of response-rate patterns across time using a multiple peak procedure in pigeons. The frequency (fast or slow) at which the color of a keylight changed signaled a fixed-interval (FI) 5-s or 20-s schedule, counterbalanced across subjects. In peak trials, the frequency of keylight-color changes was varied. For the fast and slow training stimuli, response rates in peak trials were controlled by the arranged FI schedule value; they increased as the arranged reinforcer time approached, and decreased thereafter. Response-rate patterns to all test stimuli were similar to response-rate patterns to the slow training stimulus for all subjects. Thus, overall, strong generalization from the slow training stimulus to all test stimuli was evident, whereas there was little to no generalization from the fast training stimulus. These findings extend past research examining generalization of temporally controlled response-rate patterns, and provide a useful starting point for future investigations of generalization of fixed-interval responding. A thorough understanding of generalization processes requires analysis of dependent variables other than overall response rates, especially when responding is likely to be temporally controlled.

Does a negative discriminative stimulus function as a punishing consequence?

The study and use of punishment in behavioral treatments has been constrained by ethical concerns. However, there remains a need to reduce harmful behavior unable to be reduced by differential-reinforcement procedures. We investigated whether response-contingent presentation of a negative discriminative stimulus previously correlated with an absence of reinforcers would punish behavior maintained by positive reinforcers. Across four conditions, pigeons were trained to discriminate between a positive discriminative stimulus (S+) signaling the presence of food, and a negative discriminative stimulus (S-) signaling the absence of food. Once learned, every five responses on average to the S+ produced S- for a duration of 1.5 s. S+ response rate decreased for a majority of pigeons when responses produced S-, compared to when they did not, or when a neutral control stimulus was presented. In Condition 5, choice between two concurrently presented S+ alternatives shifted away from the alternative producing S-, despite a 1:1 reinforcer ratio. Therefore, presenting contingent S- stimuli punishes operant behavior maintained on simple schedules and in choice situations. Development of negative discriminative stimuli as punishers of operant behavior could provide an effective approach to behavioral treatments for problem behavior and subverting suboptimal choices involved in addictions.

Melioration revisited: a systematic replication of Vaughan (1981)

Organisms that behave so as to forfeit a relatively higher overall rate of reinforcement in favor of a relatively lower rate are said to engage in suboptimal choice. Suboptimal choice has been linked with maladaptive behavior in humans. Melioration theory offers one explanatory framework for suboptimal choice. Melioration theory suggests behavior is controlled by differences in local reinforcer rates between alternatives. Vaughan (1981) arranged two experimental conditions in which maximizing the overall rate of reinforcement required behavior that was compatible, or incompatible, with melioration. Vaughan found pigeons allocated more time to a locally richer alternative even when doing so resulted in suboptimal choice. However, Vaughan did not show whether these effects could systematically reverse and did not provide within-session data to show that choice across short time spans remains under the control of differences in local reinforcer rates. The present study used pigeons to replicate and extend Vaughan's findings. We investigated shifts in overall- and within-session choice across repeated conditions, according to arranged local contingencies. Behavior systematically followed changes in local contingencies for most pigeons. Within-session data suggests that, providing differences in local reinforcer rates are discriminated, pigeons will allocate more time to a locally richer alternative, even if this leads to suboptimal choice. These findings facilitate the more confident use of similar procedures that investigate how melioration contributes to suboptimal choice.

A model for discriminating reinforcers in time and space

Both the response-reinforcer and stimulus-reinforcer relation are important in discrimination learning; differential responding requires a minimum of two discriminably-different stimuli and two discriminably-different associated contingencies of reinforcement. When elapsed time is a discriminative stimulus for the likely availability of a reinforcer, choice over time may be modeled by an extension of the Davison and Nevin (1999) model that assumes that local choice strictly matches the effective local reinforcer ratio. The effective local reinforcer ratio may differ from the obtained local reinforcer ratio for two reasons: Because the animal inaccurately estimates times associated with obtained reinforcers, and thus incorrectly discriminates the stimulus-reinforcer relation across time; and because of error in discriminating the response-reinforcer relation. In choice-based timing tasks, the two responses are usually highly discriminable, and so the larger contributor to differences between the effective and obtained reinforcer ratio is error in discriminating the stimulus-reinforcer relation. Such error may be modeled either by redistributing the numbers of reinforcers obtained at each time across surrounding times, or by redistributing the ratio of reinforcers obtained at each time in the same way. We assessed the extent to which these two approaches to modeling discrimination of the stimulus-reinforcer relation could account for choice in a range of temporal-discrimination procedures. The version of the model that redistributed numbers of reinforcers accounted for more variance in the data. Further, this version provides an explanation for shifts in the point of subjective equality that occur as a result of changes in the local reinforcer rate. The inclusion of a parameter reflecting error in discriminating the response-reinforcer relation enhanced the ability of each version of the model to describe data. The ability of this class of model to account for a range of data suggests that timing, like other conditional discriminations, is choice under the joint discriminative control of elapsed time and differential reinforcement. Understanding the role of differential reinforcement is therefore critical to understanding control by elapsed time.

On the joint control of preference by time and reinforcer-ratio variation

Five pigeons were trained in a procedure in which, with a specified probability, food was either available on a fixed-interval schedule on the left key, or on a variable-interval schedule on the right key. In Phase 1, we arranged, with a probability of 0.5, either a left-key fixed-interval schedule or a right-key variable-interval 30s, and varied the value of the fixed-interval schedule from 5s to 50s across 5 conditions. In Phase 2, we arranged either a left-key fixed-interval 20-s schedule or a right-key variable-interval 30-s schedule, and varied the probability of the fixed-interval schedule from 0.05 to 1.0 across 8 conditions. Phase 3 always arranged a fixed-interval schedule on the left key, and its value was varied over the same range as in Phase 1. In Phase 1, overall preference was generally toward the variable-interval schedule, preference following reinforcers was initially toward the variable-interval schedule, and maximum preference for the fixed-interval schedule generally occurred close to the arranged fixed-interval time, becoming relatively constant thereafter. In Phase 2, overall left-key preference followed the probability of the fixed-interval schedule, and maximum fixed-interval choice again occurred close to the fixed-interval time, except when the fixed-interval probability was 0.1 or less. The pattern of choice following reinforcers was similar to that in Phase 1, but the peak fixed-interval choice became more peaked with higher probabilities of the fixed interval. Phase 3 produced typical fixed-interval schedule responding. The results are discussed in terms of reinforcement effects, timing in the context of alternative reinforcers, and generalized matching. These results can be described by a quantitative model in which reinforcer rates obtained at times since the last reinforcer are distributed across time according to a Gaussian distribution with constant coefficient of variation before the fixed-interval schedule time, changing to extended choice controlled by extended reinforcer ratios beyond the fixed-interval time. The same model provides a good description of response rates on single fixed-interval schedules.

Reinforcement: food signals the time and location of future food

It has long been understood that food deliveries may act as signals of future food location, and not only as strengtheners of prefood responding as the law of effect suggests. Recent research has taken this idea further--the main effect of food deliveries, or other "reinforcers", may be signaling rather than strengthening. The present experiment investigated the ability of food deliveries to signal food contingencies across time after food. In Phase 1, the next food delivery was always equally likely to be arranged for a left- or a right-key response. Conditions were arranged such that the next food delivery was likely to occur either sooner on the left (or right) key, or sooner on the just-productive (or not-just-productive) key. In Phase 2, similar contingencies were arranged, but the last-food location was signaled by a red keylight. Preference, measured in 2-s bins across interfood intervals, was jointly controlled by the likely time and location of the next food delivery. In Phase 1, when any food delivery signaled a likely sooner next food delivery on a particular key, postfood preference was strongly toward that key, and moved toward the other key across the interreinforcer interval. In other conditions in which food delivery on the two keys signaled different subsequent contingencies, postfood preference was less extreme, and quickly moved toward indifference. In Phase 2, in all three conditions, initial preference was strongly toward the likely-sooner food key, and moved to the other key across the interfood interval. In both phases, at a more extended level of analysis, sequences of same-key food deliveries caused a small increase in preference for the just-productive key, suggesting the presence of a "reinforcement effect", albeit one that was very small.

Germline truncating mutations in both MSH2 and BRCA2 in a single kindred

There has been interest in the literature in the possible existence of a gene that predisposes to both breast cancer (BC) and colorectal cancer (CRC). We describe the detailed characterisation of one kindred, MON1080, with 10 cases of BC or CRC invasive cancer among 26 first-, second- or third-degree relatives. Linkage analysis suggested that a mutation was present in BRCA2. DNA sequencing from III: 22 (diagnosed with lobular BC) identified a BRCA2 exon 3 542G>T (L105X) mutation. Her sister (III: 25) had BC and endometrial cancer and carries the same mutation. Following immunohistochemical and microsatellite instability studies, mutation analysis by protein truncation test, cDNA sequencing and quantitative real-time PCR revealed a deletion of MSH2 exon 8 in III: 25, confirming her as a double heterozygote for truncating mutations in both BRCA2 and MSH2. The exon 8 deletion was identified as a 14.9 kb deletion occurring between two Alu sequences. The breakpoint lies within a sequence of 45 bp that is identical in both Alu sequences. In this large BC/CRC kindred, MON1080, disease-causing truncating mutations are present in both MSH2 and BRCA2. There appeared to be no increased susceptibility to the development of colorectal tumours in BRCA2 mutation carriers or to the development of breast tumours in MSH2 mutation carriers. Additionally, two double heterozygotes did not appear to have a different phenotype than would be expected from the presence of a mutation in each gene alone.

Payment by salary or fee-for-service. Effect on health care resource use in the last year of life

OBJECTIVE

To investigate the effect of physician payment method on use of health care resources.

DESIGN

Retrospective analysis of patient health care data collected for 3 years (1994 to 1996) from the Vital Statistics Department of the British Columbia Ministry of Health. Billing numbers identified physician payment method.

SETTING

Salaried and fee-for-service primary care practices in the Capital Region District of Victoria, BC.

PARTICIPANTS

A total of 582 patients in their last year of life: 106 were attended by salaried family physicians at a community health clinic; 476 were attended by fee-for-service practitioners. Groups were comparable in age, sex, and geographical location.

MAIN OUTCOME MEASURES

Number and cost of specialist and diagnostic services and medications, number of days in hospital (acute and extended care), and main causes of death.

RESULTS

None of the dependent measures showed any statistically significant differences based on comparisons between many variables for patients in the two groups. Costs of pharmaceutical, specialist, and diagnostic services were not significantly different for the two groups. There were three main causes of death, according to codes on death certificates: heart disease, malignant neoplasms, and cerebrovascular disease.

CONCLUSION

Whether physicians were paid by salary or fee-for-service had no empirical effect on health care resource use.