Variable-interval Schedules Of Reinforcement Research Articles

Hyperbolae.

Hyperbolic relations between independent and dependent variables are ubiquitous in the experimental analysis of behavior, mentioned in over 150 articles in the Journal of the Experimental Analysis of Behavior. There are two principal forms of hyperbolae: The first describes the relation between response rate and reinforcement rate on variable-interval schedules of reinforcement; it rises asymptotically toward a maximum. The second describes the relation between the current equivalent value of an incentive and its delay or (im)probability; it falls from a maximum toward an asymptote of 0. Where do these come from? What do their parameters mean? How are they related? This article answers the first two questions and addresses the last.

Topographical aspects of brief-stimulus presentations: A re-examination of the problem of conditioned reinforcement

Three pigeons were exposed to second-order schedules in which responding under a fixed-interval (FI) component schedule was reinforced according to a variable-interval (VI) schedule of food reinforcement. Completion of each component resulted in either (1) brief presentation of a stimulus present during reinforcement (paired brief stimulus), (2) brief presentation of a stimulus not present during reinforcement (nonpaired brief stimulus), or (3) no stimulus presentation (tandem schedule). Under the two nonpaired brief stimulus conditions, either a change in keylight color or onset of houselight illumination was used as the brief stimulus. Similar patterns of keypecking occurred under tandem and nonpaired keylight brief-stimulus presentations, whereas nonpaired houselight brief-stimulus presentations generated positively accelerated within-component keypeck patterning for two pigeons. When the same keylight brief stimulus was paired with food, positively accelerated patterns of keypecking were obtained for all pigeons. Differences in the effects of nonpaired brief-stimulus presentations on second-order schedule performance suggest that component schedule patterning under nonpaired brief-stimulus procedures is a function of the particular type of stimulus used (i.e., houselight versus keylight). These results suggest that (1) brief houselight illumination may function as a sensory reinforcer, and (2) a briefly presented food-paired stimulus can function as an effective conditioned reinforcer.

Longer operant lever-press duration requirements induce fewer but longer response bouts in rats.

Operant behavior is organized in bouts that are particularly visible under variable-interval (VI) schedules of reinforcement. Previous research showed that increasing the work required to produce a response decreases the rate at which bouts are emitted and increases the minimum interresponse time (IRT). In the current study, the minimum effective IRT was directly manipulated by changing the minimum duration of effective lever presses reinforced on a VI 40-s schedule. Contrary to assumptions of previous models, response durations were variable. Response durations were typically 0.5 s greater than the minimum duration threshold; durations that exceeded this threshold were approximately log-normally distributed. As the required duration threshold increased, rats emitted fewer but longer bouts. This effect may reflect an effort-induced reduction in motivation and a duration-induced facilitation of a response-outcome association.

Repeated resurgence with and without a context change

Resurgence is the reoccurrence of a target response when reinforcement for a more recently reinforced alternative response is eliminated or reduced. The present study arranged two successive three-phase procedures to assess whether resurgence decreases with repeated assessments. Moreover, we arranged a contextual change from the first to second assessment for some groups. Phase 1 reinforced a target response on a touchscreen computer with typically developing adults as participants according to either variable-ratio or variable-interval schedules of reinforcement. Phase 2 extinguished target responding and reinforced alternative responding. Phase 3 tested for resurgence by extinguishing alternative responding. Resurgence reliably occurred in all tests and decreased from the first to second exposure to the procedures but there were no effects of context change. Therefore, repeated exposures to resurgence tests reduced those effects but contextual changes had no effect.

Contingency, contiguity, and causality in conditioning: Applying information theory and Weber's Law to the assignment of credit problem.

Contingency is a critical concept for theories of associative learning and the assignment of credit problem in reinforcement learning. Measuring and manipulating it has, however, been problematic. The information-theoretic definition of contingency-normalized mutual information-makes it a readily computed property of the relation between reinforcing events, the stimuli that predict them and the responses that produce them. When necessary, the dynamic range of the required temporal representation divided by the Weber fraction gives a psychologically realistic plug-in estimates of the entropies. There is no measurable prospective contingency between a peck and reinforcement when pigeons peck on a variable interval schedule of reinforcement. There is, however, a perfect retrospective contingency between reinforcement and the immediately preceding peck. Degrading the retrospective contingency by gratis reinforcement reveals a critical value (.25), below which performance declines rapidly. Contingency is time scale invariant, whereas the perception of proximate causality depends-we assume-on there being a short, fixed psychologically negligible critical interval between cause and effect. Increasing the interval between a response and reinforcement that it triggers degrades the retrograde contingency, leading to a decline in performance that restores it to at or above its critical value. Thus, there is no critical interval in the retrospective effect of reinforcement. We conclude with a short review of the broad explanatory scope of information-theoretic contingencies when regarded as causal variables in conditioning. We suggest that the computation of contingencies may supplant the computation of the sum of all future rewards in models of reinforcement learning. (PsycINFO Database Record (c) 2019 APA, all rights reserved).

Reversing Breathing Paralysis through Optimization of Intermittent Hypoxia Treatment after Cervical Spinal Cord Injury

Spinal cord injury (SCI) most commonly occurs at the cervical level and can interrupt descending neural pathways, causing paralysis of the diaphragm, as well as profound breathing motor difficulties which threaten survival and greatly decrease quality of life. Intermittent hypoxia (IH) treatment is often utilized in preclinical models to attenuate breathing motor deficits resulting from cervical SCI by inducing a prolonged increase in respiratory motor output known as long term facilitation (LTF), a form of breathing motor plasticity. IH typically consists of the repeated, alternating exposure of a subject to consistent and equal 5‐minute periods of hypoxia and normoxia and thus can be fittingly termed fixed interval intermittent hypoxia (FIH). FIH exhibits similarity to the psychological construct of operant conditioning in which the increased incidence and persistence of a desired, spontaneous behavior is trained through reinforcement. As such, each interval of hypoxia can be construed as the period during which the subject responds with heightened respiratory drive and is subsequently reinforced by an interval of normoxia. Provided that IH is a form of operant conditioning, it can be optimized through application of seminal psychological findings which established variable interval schedules of reinforcement as more effective than fixed interval schedules for learning. Therefore, using the duration of the hypoxic interval as our independent variable, we hypothesize that varied interval intermittent hypoxia (VIH) treatment will induce a greater, more prolonged increase in respiratory motor output than FIH after injury. We utilized the C2 hemisection injury model in rats and treated with VIH or FIH for 5 days at 1‐week post‐injury. Following treatment, we conducted diaphragm electromyograph recordings to assess breathing motor recovery within each animal by comparing baseline activity to maximal output induced by nasal occlusion, occurring immediately after cessation of IH treatment. Preliminary results show that 1‐week post‐injury VIH treated animals exhibited recovery on average equaling 33.87±7.89% as compared to the 19.28±6.02% recovery in FIH treated animals (differences nonsignificant by unpaired t‐test, p>0.05 with uncertainty shown as standard error of the mean). These data suggest that VIH may induce more increased and prolonged recovery than FIH in post‐injury models. Ongoing work includes evaluation of VIH treatment in animals at 8 weeks after injury and will expand outcome measurement timepoints to include 1 week after cessation of treatment to interrogate the persistence of induced recovery. Additionally, further exploration will focus on the molecular markers present within the phrenic motor nucleus of the cervical spinal cord and on the development of IH paradigms based on breathing frequency.Support or Funding InformationNINDS R01NS101105 (Warren J. Alilain)This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Predator videos and electric shock function as punishers for zebrafish (Danio rerio).

Zebrafish (Danio rerio) are a promising animal model for studying the effects of gene-environment interactions on behavior. Two experiments were conducted to assess punishment effects of presenting predator videos (Indian leaf fish; Nandus nandus) and electric shock on operant approach responses in zebrafish. In Experiment 1, the predator video and shock stimuli were presented upon a response maintained by a single variable-interval schedule of food reinforcement in different groups of fish. In Experiment 2, the predator video and shock stimuli were presented upon one of two response alternatives maintain by concurrently available variable-interval schedules of food reinforcement in different groups of fish. Responding decreased when the predator video and shock stimuli were presented relative to their absence in both experiments. Moreover, responding on an unpunished alternative did not reliably decrease in Experiment 2. These results indicate that the decrease in responding resulted from the punishment contingency rather than from elicited species-specific defense responses or conditioned avoidance. Thus, the predator video and electric shock functioned as punishers of operant behavior for zebrafish. Identifying punishers for this species could lead to research on how gene-environment interactions influence individual differences in sensitivity to punishment.

Behavioral intoxication following voluntary oral ingestion of tetramethylenedisulfotetramine: Dose-dependent onset, severity, survival, and recovery

Tetramethylenedisulfotetramine (tetramine, or TETS) is a highly toxic rodenticide that has been responsible for over 14,000 accidental and intentional poisonings worldwide. Although the vast majority of TETS poisonings involved tainted food or drink, the laboratory in vivo studies of TETS intoxication used intraperitoneal injection or gavage for TETS exposure. Seeking to develop and characterize a more realistic model of TETS intoxication in the present study, rats were trained to rapidly and voluntarily consume a poisoned food morsel. Initially, the overt toxic effects of TETS consumption across a large range of doses were characterized, then a focused range of doses was selected for more intensive behavioral evaluation (in operant test chambers providing a variable-interval schedule of food reinforcement). The onset of intoxication following voluntary oral consumption of TETS was rapid, and clear dose-dependent response-rate suppression was observed across multiple performance measures within the operant-chamber environment. At most doses, recovery of operant performance did not occur within 30h. Food consumption and body weight changes were also dose dependent and corroborated the behavioral measures of intoxication. This voluntary oral-poisoning method with concomitant operant-behavioral assessment shows promise for future studies of TETS (and other toxic chemicals of interest) and may be extremely valuable in characterizing treatment outcomes.

EFEITOS DA EXIGÊNCIA DE DESEMPENHOS ENTRELAÇADOS SOBRE LINHA DE BASE EM ESQUEMA SIMPLES DE REFORÇO

Foram investigados os efeitos da exigência de desempenhos entrelaçados, espaçamento de respostas de três participantes, sobre o responder de tríades em esquemas de reforço em intervalo variável. A tarefa consistiu em um jogo, no qual a única resposta possível, clicar o botão do mouse, era livre para ocorrer e podia produzir consequências individuais (em VI 3 s ou VI 12 s) e consequências contingentes à exigência de que os participantes clicassem espaçadamente em relação uns aos outros. Doze estudantes universitários formaram quatro tríades. Duas condições foram programadas em um delineamento de reversão ABAB: Condição A, apenas contingência operante; Condição B, contingências operante e cultural em vigor. Os resultados mostraram que, independente do parâmetro do esquema de reforçamento individual, a sobreposição da exigência de desempenho entrelaçado selecionou um padrão de coordenação entre as respostas dos participantes. A suspensão e reintrodução da exigência do entrelaçamento mostrou que mudanças sistemáticas no padrão de ocorrência dos desempenhos entrelaçados foram função da variável manipulada. A exigência de espaçamento entre as respostas dos participantes para produção de uma consequência arranjada experimentalmente pode ser caracterizada como uma metacontingência. O procedimento, nesse sentido, demonstra a atuação de níveis diferentes de variação e seleção em ação simultaneamente para o comportamento dos participantes e para desempenhos entrelaçados. Palavras-chave: Seleção cultural, contingências comportamentais entrelaçadas, esquemas de reforço, operante livre.

乱比率(RR)及び変間隔(VI)食餌強化スケジュールの下でのハトのタイムアウト行動

乱比率(RR)及び変間隔(VI)食餌強化スケジュールの下でのハトのタイムアウト行動

Using Variable Interval Reinforcement Schedules to Support Students in the Classroom: An Introduction With Illustrative Examples

When behaviors are reinforced with a variable interval reinforcement schedule, reinforcement is available only after an unknown period of time. These types of reinforcement schedules are most useful for reinforcing slow and steady responding and for differentially reinforcing behaviors that are incompatible with some problematic behaviors. This review helps define variable interval reinforcement schedules, uses the example of a strategy to manage thumb-sucking behavior to illustrate the implementation of these schedules, and describes potential applications in school and clinical settings.

Response-bout analysis of interresponse times in variable-ratio and variable-interval schedules

The purpose of this study was to analyze the response pattern difference between variable-ratio (VR) and variable-interval (VI) schedules of reinforcement by modeling interresponse time distributions of rats’ lever presses. All eight rats showed higher response rates under VR 30 than under inter-reinforcement intervals yoked VI. The 30 models consisting of single Exponential (with and without the lower limit on interresponse times), Weibull, Normal, Log-Normal or Gamma distributions, all possible two component combinations of those, and 3 and 4 component models consisting of Weibull, Normal, Log-Normal, or Gamma distribution combinations were compared. The 4 component Log-Normal model was the best in terms of the Akaike information criterion and visual inspection of fitting outcome. Parameter estimates for the L4 model showed that the VR-VI response rate difference is due to a difference in short interresponse times or within bout responses. This results suggests that the VR-VI response rate difference is not an indication of a difference in the overall tendency to respond but it is rather a difference in terms of what types of response patterns are engendered between the two schedules.

The distribution of response bout lengths and its sensitivity to differential reinforcement.

Response bouts are clusters of responses that occur in rapid succession and are punctuated by pauses during which the response does not occur. Under variable interval schedules of reinforcement, the number of responses in each bout (the bout length) varies among bouts. This experiment was aimed at determining whether the relative rate of reinforcement influenced the relative frequency of bouts of different lengths. Lever pressing in rats was reinforced under a tandem variable time (VT) 150-s fixed ratio (FR) X, where X could be 1 or 5 and varied randomly after each reinforcer. Two conditions were included: majority FR1 (mFR1) and majority FR5 (mFR5). In mFR1, 75% of reinforcers had a tandem FR requirement of 1 and 25% had a tandem FR requirement of 5; this distribution was reversed in mFR5. The dynamic bi-exponential refractory model of response bouts was fitted to the interresponse times (IRTs) in each condition. Model parameter estimates and IRTs were then used to simulate probable distributions of bout lengths. These distributions comprised a mixture of short geometrically-distributed bout lengths and long negative-binomially-distributed bout lengths. Long bouts were significantly longer in the mFR5 condition than in the mFR1 condition. In conjunction with previous data, the present study suggests that the prevalence of long bouts increases with the proportion of reinforcers with FR5 requirement. These results suggest that bouts of different lengths are sensitive to the rate at which they are reinforced.

Deconstructing 5-HT6 receptor effects on striatal circuit function

Medium spiny neurons (MSNs) constitute 95% of neurons in the dorsal striatum subdivided into direct (striatonigral) and indirect (striatopallidal) pathways. Whereas D1 and D2 receptors and several neuropeptides, including dynorphin and enkephalin, are differentially expressed in these neurons, 5-hydroxytryptamine 6 receptors (5-HT6) are expressed in both pathways. Previous results demonstrate that concurrent 5-HT6 receptor overexpression in MSNs of both pathways in the dorsomedial striatum (DMS) interferes with instrumental learning and that 5-HT6 overexpression in the dorsolateral striatum (DLS) relieves rats from inflexible habitual behaviors. We hypothesized that 5-HT6 receptor-mediated co-activation of both pathways interferes with the differential activation/inhibition of direct/indirect pathways by dopamine. To test this idea, we cloned novel viral vectors to selectively overexpress 5-HT6 receptors in direct or indirect pathway MSNs to deconstruct their role in modulating instrumental learning and habitual responding. We found that increasing 5-HT6 receptor expression in either direct or indirect pathway MSNs of the posterior DMS selectively enhanced or impaired initial acquisition of a discrete instrumental learning task respectively, though all rats were ultimately able to learn the task. In a separate set of experiments, 5-HT6 receptor overexpression in indirect pathway MSNs of the DLS facilitated behavioral flexibility in rats overtrained on a repetitive pressing task using a variable interval schedule of reinforcement, during an omission contingency training session and subsequent probe testing. Together these findings further the notion that 5-HT6 signaling causes balanced activation of opposing MSN pathways by serotonin in sub-regions of the dorsal striatum allowing for more reflective modalities of behavior.

Regional variation in phasic dopamine release during alcohol and sucrose self-administration in rats.

While dopamine input to the dorsal striatum is well-known to be critical for action selection, including alcohol-motivated behaviors, it is unknown whether changes in phasic dopamine accompany these behaviors. Long-term alcohol abuse is believed to promote alterations in the neurocircuitry of reward learning in both ventral and dorsal striatum, potentially through increasing dopamine release. Using fast-scan cyclic voltammetry, we measured phasic dopamine release in the dorsal and ventral striatum during alcoholic and nonalcoholic reward-seeking behavior and reward-related cues in rats trained on a variable-interval schedule of reinforcement. We observed robust phasic dopamine release in the dorsolateral striatum after reinforced lever presses and inconsistent dopamine release in the dorsomedial striatum. Contrary to our expectations, alcohol did not enhance dopamine release in rats drinking alcoholic rewards. Cue-induced dopamine release was also observed in the nucleus accumbens core of rats drinking the reward solutions. These data demonstrate that alcoholic and nonalcoholic reward self-administration on a variable-interval schedule of reinforcement in rats is accompanied by phasic dopamine release time-locked to reinforcement in the dorsolateral striatum and the nucleus accumbens, but not the dorsomedial striatum.

E-mail Anonymous: A Physician's Addiction

DOI: 10.1200/JCO.2014.58.9705 The next time you are sitting in Grand Rounds, standing in the cafeteria line, rounding with your team, or giving a talk, take a look at your colleagues. Chances are, they are staring at a screen, and many are probably checking e-mail. E-mail has changed how physicians communicate. Whether related to patient care, academic pursuits, or administrative tasks, our interactions are now frequently conducted electronically. This form of communication has a new pressure attached: who would not prefer to collaborate with someone who responds within minutes? The patient-based aspect of e-mail reinforces the importance of a prompt reply—patient care is paramount, and therefore, checking e-mail frequently feels like something that good doctors should do. Whether the shift from live dialogue to e-mail has led to improved communication, enhanced productivity, or—most importantly—improved patient outcomes is unknown. The true extent and impact of daily e-mail use by physicians has also not been formally quantified. However, my own e-mail usage can be formally qualified. I am an e-mailaholic. It is hard for me to know exactly when the addiction began, but by the time I became an attending physician, I would check my e-mail at nearly every moment. The e-mail browser would be open in my office or clinic throughout the day, and when not at a computer, my e-mail would be just a few seconds away, accessible via the phone in my righthand pocket. Before going to bed and first thing on waking, while waiting for my coffee to brew or walking through the parking lot, I was checking e-mail. It was an action devoid of thought; it just became second nature for my thumb to push, slide right, enter password, and push once more—I can open e-mail with my eyes closed. Most of us can. A retrospectively self-administered CAGE questionnaire (the name of which is derived from questions used to screen for alcohol addiction; these questions involve the words cut down, annoyed, guilty, and eye-opener) would have easily produced three “yes” responses: I often thought about the need to cut down my e-mail usage and spend more time on the task at hand or with the people around me; my children were at times annoyed with me for not paying attention to our conversations; and I felt uneasy without a daily eye-opener: a check of my e-mail soon after waking up. Constant communication may have its benefits. Queries are answered more quickly, both by colleagues and patients (who may feel less intimidated getting in touch with their physician via e-mail), and perhaps projects and tasks are advanced a little faster. But e-mail also decreases spoken and face-to-face interactions, arguably eroding relationships. Questions answered by e-mail may result in misunderstandings that require many follow-up e-mails. This ongoing e-mailing leads to interruptions and multitasking: patient care, writing, reading, thinking, and generally being present in any given moment become fragmented. Many knowledge workers frequently multitask, but some believe that the term multitasking is a misnomer and that one is actually rapidly switching from task to task, which is a hindrance to creativity and intense focus. What is the cost of this constant switching of thought and attention? Although no prospective and unequivocal studies exist, e-mail has been associated with stress and loss of control, and in an academic environment, users spent approximately 23% of their time on e-mail, checking it up to 36 times per hour. In older adults, multitasking led to significant working memory disruption. When e-mail was shut off for 13 knowledge workers, they tended to multitask less and exhibited longer task focus; stress was also lower without e-mail. Interestingly, some companies are now asking their employees to shut off e-mail for certain lengths of time during the day to increase productivity. The gravitational pull of e-mail is strong and draws on the variable-interval reinforcement schedule concept—reward is unpredictable and e-mail becomes addictive. Furthermore, a recent study in default-mode processing (inward-directed thought) revealed that people not only do not seem to enjoy sitting quietly and thinking with no distractions, but that 43% (67% of men and 25% of women) selfadministered electric shocks rather than simply sit for 15 minutes without anything to do (all of them previously said they would pay money to avoid such JOURNAL OF CLINICAL ONCOLOGY A R T O F O N C O L O G Y VOLUME 33 NUMBER 3 JANUARY 2

Saccadic reaction time distributions follow the matching law in a concurrent variable interval reinforcement schedule

Saccadic reaction time distributions follow the matching law in a concurrent variable interval reinforcement schedule

Contextual control of operant behavior: evidence for hierarchical associations in instrumental learning.

Recent research has suggested that operant responses can be weakened when they are tested in new contexts. The present experiment was therefore designed to test whether animals can learn a context-(R-O) relation. Rats were given training sessions in context A, in which one response (R1; lever pressing or chain pulling) produced one outcome (O1) and another response (R2; chain pulling or lever pressing) produced another outcome (O2) on variable interval reinforcement schedules. These sessions were intermixed with training in context B, where R1 now produced O2 and R2 produced O1. Given the arrangement, it was possible for the animal to learn two distinct R-O associations in each specific context. To test for them, rats were then given aversion conditioning with O2 by pairing its presentation with lithium-chloride-induced illness. Following the aversion conditioning, the rats were given an extinction test with both R1 and R2 available in each context. During testing, rats showed a selective suppression in each context of the response that had been paired with the reinforcer subsequently associated with illness. Rats could not have performed this way without knowledge of the R-O associations in effect in each specific context, lending support to the hypothesis that rats learn context-(R-O) associations. However, despite a complete aversion to O2, responding was not completely suppressed, leaving the possibility open that rats form context-R associations in addition to context-(R-O) associations.

Choice for response alternatives differing in reinforcement frequency in dopamine D2 receptor mutant and Swiss-Webster mice

A previous study showed that dopamine (DA) D2 receptors (D2Rs) are involved in the reinforcing effectiveness of food, but the specific involvement of DA D2Rs in choice among food reinforcers remains unclear. The current study used genetic and pharmacological approaches to assess the role of D2Rs in choice among food-reinforcement frequencies using the generalized matching law (GML), which specifies that logged response and time allocation ratios vary linearly with logged reinforcer ratios. Congenic D2R knockout (KO) and wild-type (WT) mice were exposed to concurrent variable-interval schedules of reinforcement with scheduled relative-reinforcement rates from 4:1 to 1:4. Effects of the D2R antagonist (-)-eticlopride (0.1-1.0 mg/kg) were assessed in Swiss-Webster mice. Response and time allocation ratios were related to obtained reinforcement ratios as predicted by the GML. GML fits accounted for ≥ 92 % of the variance in allocation ratios and did not differ in D2R KO and WT mice. Similarly, there were no significant effects of (-)-eticlopride dose on GML fits, despite effects on overall response rates. The current results demonstrate that neither deletion nor acute blockade of D2Rs affects choice among response alternatives varying in food-reinforcement frequencies. Because previously published results suggest a role of D2Rs in choice between response alternatives differing in reinforcer magnitude and delay or magnitude and probability, the current findings suggest that D2Rs play a role in choice only among certain parameters of reinforcement. Furthermore, these findings suggest parameters of reinforcement may only be fungible in a complex manner.

Prelimbic and Infralimbic Neurons Signal Distinct Aspects of Appetitive Instrumental Behavior

It is thought that discrete subregions of the medial prefrontal cortex (mPFC) regulate different aspects of appetitive behavior, however, physiological support for this hypothesis has been lacking. In the present study, we used multichannel single-unit recording to compare the response of neurons in the prelimbic (PL) and infralimbic (IL) subregions of the mPFC, in rats pressing a lever to obtain sucrose pellets on a variable interval schedule of reinforcement (VI-60). Approximately 25% of neurons in both structures exhibited prominent excitatory responses during rewarded, but not unrewarded, lever presses. The time courses of reward responses in PL and IL, however, were markedly different. Most PL neurons exhibited fast and transient responses at the delivery of sucrose pellets, whereas most IL neurons exhibited delayed and prolonged responses associated with the collection of earned sucrose pellets. We further examined the functional significance of reward responses in IL and PL with local pharmacological inactivation. IL inactivation significantly delayed the collection of earned sucrose pellets, whereas PL inactivation produced no discernible effects. These findings support the hypothesis that PL and IL signal distinct aspects of appetitive behavior, and suggest that IL signaling facilitates reward collection.