Extinction deficits in male rhesus macaques with a history of self-injurious behavior

Self-injurious behavior (SIB) occurs in both human and nonhuman primate populations. Despite the potential for harm, SIB may persist in part because of an inability to inhibit behavior that results in wounding. A lever-pressing task was used to test the prediction that monkeys with SIB would show greater persistence in lever-pressing on extinction trials than monkeys without the disorder. The subjects were 15 individually-housed adult male rhesus macaques, 10 of which (the SIB group) had a veterinary record of self-inflicted wounding. All of the monkeys were trained to lever-press for food rewards to a criterion of 400 total responses. The test procedures consisted of five daily 30-min sessions divided into six 5-min intervals. On day 1, the subjects received continuous reinforcement. On days 2-4, testing consisted of alternating reinforced/unreinforced 5-min intervals, beginning with reinforcement. Reinforced intervals were cued with a buzzer. On day 5, the subjects received no reinforcement. The number of lever-presses and behavioral responses were recorded during each session. Saliva samples were collected for cortisol measurement before and after test sessions on days 1, 2, and 5. As predicted, monkeys with SIB lever-pressed more than controls during extinction intervals on days 2-4. There was no difference on day 1 or day 5. The frequency of scratching, yawning, and abnormal behavior increased when reinforcement was intermittent (days 2-4) or absent (day 5). Cortisol levels were highest with continuous reinforcement (day 1), and may reflect differential levels of food intake rather than stress. The presence of extinction deficits suggests that SIB may persist in some monkeys because they lack the ability to regulate the intensity of their biting behavior.     

Relative numerousness judgements in crows and pigeons in the urgent comparison of stimuli earlier linked to different amounts of reinforcement]

The ability of pigeons (Colomba livia, L.) and crows (Corvus corone cornix, L.) was studied to realize urgent numerousness judgement of reinforcement consisting of discrete elements (wheat grains and meal worm larvae, respectively). In the process of preliminary training the birds mastered the information about the conformity of the feeder colour with the definite number (1-9 for pigeons and 5-12 for crows) of reinforcement units at isolated presentation of feeders. In test at presentation of pairs formed from these feeders, pigeons and crows chose the stimulus connected with a greater quantity of reinforcement. In the range of 1-8 units the precision of choice in pigeons depended on absolute and relative differences between comparing values. In crows in the range of 6-12 this dependence was not revealed. The ability to solve the given test is considered as one of manifestations of elementary reasoning.     

Assessing the risks of transgene escape through time and crop-wild hybrid persistence

Transgenes introduced into crops can escape in time, as well as space, via the seed bank. For annual plants, especially ruderals, seed bank behaviour may be the most important factor determining population persistence. Crop seeds may exhibit some dormancy and germination cueing in the soil but are expected to be less able to persist than their wild relatives, which often have considerable dormancy and longevity, as well as effective germination cueing responses. Crop-wild hybrids may have seed bank characteristics more suited to persistence, and maternal effects may favour persistence of hybrids having wild plants for their female parent. Escape of transgenes via crop-wild hybrids presents unique concerns not present for crops. Hybrids can undergo natural selection and may back-cross with wild plants. We suggest methods that can be used in conjunction with evaluation of the relative fitness of crop-wild hybrids that will determine the likelihood of back-crossing. Accurate assessment of escape in time and transgene persistence via crop-wild hybrids requires proper plant materials. We emphasize the use of null segregants as controls for transgenic crops and for generating crop-wild hybrid controls for transgenic hybrids. Since good empirical and theoretical understanding of how individual genes influence the fate of plants in different environments is lacking, evaluation of escape in time and the persistence of transgenes via crop-wild hybrids should be on a case-by-case basis.     

Effects of breaks in the interval cycle on temporal tracking in pigeons

Two experiments examined the effects of inserting a break in a cyclic interval schedule on the temporal control of keypecking responses in pigeons. In Experiment 1, pigeons were exposed to intervals that changed from 45 to 15s and returned to 45 s. A break was inserted between the last 15-s and following 45-s interval and was in effect for either 0, 60, or 120 s. Either a blackout of lights in the test chamber or turning off the response key alone signaled breaks. In Experiment 2, we examined the effects of a wider range of breaks-0, 120, and 360 s. Post-reinforcement pause (PRP) tracked changes in the interval requirement across all conditions. However, breaks in the schedule, even one lasting 360 s, did not disrupt the overall time course of responding. The only effect that a break had on temporal performance was an elevation in the rate of responding and a shorter PRP in the interval following a break. The results suggest that breaks did not affect the birds' memory for short intervals, and that the momentary increase in responding may be related to the reinforcement omission effect.     

Rats (Rattus norvegicus) and pigeons (Columbia livia) are sensitive to the distance to food,but only rats request more food when distance increases

Three experiments investigated foraging by rats and pigeons. In Experiment 1, each response on a manipulandum delivered food to a cup, with the distance between the manipulandum and the cup varying across conditions. The number of responses made before traveling to collect and eat the food increased with distance for rats, but not for pigeons. In Experiment 2, two manipulanda were placed at different distances from a fixed food source; both pigeons and rats preferentially used the manipulandum closest to the food source. Experiment 3 was a systematic replication of Experiment 1 with pigeons. In different conditions, each peck on the left key increased the upcoming hopper duration by 0.5, 1.5 or 2.5 s. Completing a ratio requirement on the right key of 1, 4, 8, 16 or 32 pecks, depending on the condition, then produced the food hopper for a duration that depended on the number of prior left pecks. As the ratio requirement increased on the right key, pigeons responded more on the left key and earned more food. Overall, the results replicate previous research, underlining similarities and differences between these species. The results are discussed in terms of optimal foraging, reinforcer sensitivity and delay discounting.     

Influence of budget and reinforcement location on risk-sensitive preference

Little is known about the effect that procedural variables have on risk-sensitive preference. This study assessed the effect of procedural variables on pigeons' choice between a fixed and variable amount of reinforcement (amount risk) and, in a separate condition, between a fixed and variable delay until reinforcement (delay risk). Experiment 1 investigated the impact of water reinforcement and risk dimension when pigeons were in a restrictive budget, where access to water was less than that necessary to maintain current body weight, and a condition where the pigeons had ample access to water. Pigeons exhibited a greater tendency to prefer the variable alternative for delay risk than for amount risk in both restrictive and ample budgets. Varying water budget had no effect on risk preference. Experiment 2 investigated the influence of water reinforcer location while in a restrictive budget, in which reinforcers were delivered to a single location, two distinct locations, or a randomly selected location. With amount risk, pigeons were risk averse when reinforcers were delivered in separate or random locations and were indifferent to risk when delivered to a single location. With delay risk, pigeons were generally risk prone with no effect from reinforcement location. The finding that pigeons were risk averse when reinforcers were delivered to separate locations and were indifferent to risk when delivered to a single location offers a methodological explanation to the inconsistent findings in the literature with amount risk.     

Resistance to change varies inversely with reinforcement context

We report two experiments which test whether resistance to prefeeding and satiation for a variable-interval (VI) schedule that delivers a constant rate of reinforcement varies inversely with the reinforcement rate for an alternative schedule. In Experiment 1, eight pigeons responded in a multiple schedule in which the red key was always associated with a VI 90-s schedule and the green key with either a richer (VI 18s) or leaner (VI 540s) schedule in different conditions. After baseline training in each condition, prefeeding test sessions were conducted in which 10g, 20g, 30g, 40g, and 50g food were provided one-hour prior to test. Additional baseline training was given between each test session. In Experiment 2, two groups of pigeons responded in a multiple schedule similar to Experiment 1. After baseline training, pigeons were exposed to a 5-h satiation test session in which the VI 90-s schedule was available continuously. Test sessions were conducted when pigeons were maintained at 85%, 95%, and 85% of their body weights in an ABA design. Results of both experiments showed that responding in the VI 90-s schedule that alternated with a leaner schedule during baseline was more resistant to prefeeding and satiation. These data rule out alternative explanations for results of previous studies, and confirm that resistance to change varies inversely with reinforcement context.     

Oscillations following periodic reinforcement

Three experiments examined behavior in extinction following periodic reinforcement. During the first phase of Experiment 1, four groups of pigeons were exposed to fixed interval (FI 16 s or FI 48 s) or variable interval (VI 16 s or VI 48 s) reinforcement schedules. Next, during the second phase, each session started with reinforcement trials and ended with an extinction segment. Experiment 2 was similar except that the extinction segment was considerably longer. Experiment 3 replaced the FI schedules with a peak procedure, with FI trials interspersed with non-food peak interval (PI) trials that were four times longer. One group of pigeons was exposed to FI 20 s PI 80 s trials, and another to FI 40 s PI 160 s trials. Results showed that, during the extinction segment, most pigeons trained with FI schedules, but not with VI schedules, displayed pause-peck oscillations with a period close to, but slightly greater than the FI parameter. These oscillations did not start immediately after the onset of extinction. Comparing the oscillations from Experiments 1 and 2 suggested that the alternation of reconditioning and re-extinction increases the reliability and earlier onset of the oscillations. In Experiment 3 the pigeons exhibited well-defined pause-peck cycles since the onset of extinction. These cycles had periods close to twice the value of the FI and lasted for long intervals of time. We discuss some hypotheses concerning the processes underlying behavioral oscillations following periodic reinforcement.     

Processing of hierarchic stimulus structures has advantages in humans and animals

Carmesin and Schwegler (1994) have determined theoretically that a linear hierarchical stimulus structure can be encoded by a parallel network of minimal complexity. The experiments reported here compare the efficiency with which humans and pigeons process sets of stimulus pairs embodying different inequality structures. Groups of subjects of each species were taught to discriminate all 10 pairwise combinations of 5 stimuli with an operant conditioning method. For one group, the reward/punishment allocations within the pairs agreed with a linear hierarchy. For a second and third group, the reinforcement allocations of one or three, respectively, of the stimulus pairs deviated from such ordering. The time it took the subjects to learn the tasks as well as the final choice latencies and/or error rates increased with the number of deviating inequalities. The results agree with the assumption that both humans and pigeons encode stimulus inequality structures with parallel processing neural networks rather than with a sequentially processing algorithm.     

Shifts in the psychophysical function in rats

The primary goal was to compare results from a free-operant procedure with pigeons [Machado, A., Guilhardi, P., 2000. Shifts in the psychometric function and their implications for models of timing. J. Exp. Anal. Behav. 74, 25-54, Experiment 2] with new results obtained with rats. The secondary goal was to compare the results of both experiments with dependent variables that were not used in the original publication. As in the original study with pigeons, rats were trained on a two-alternative free-operant psychophysical procedure in which left lever press responses were reinforced during the first and second quarters of a 60-s trial, and right lever press responses were reinforced during the third and fourth quarters of the trial. The quarters were reinforced according to four independent variable interval (VI) schedules of reinforcement. The VI duration was manipulated in each quarter, and shifts in the psychophysical functions that relate response rate with time since trial onset were measured. The results obtained with rats were consistent with those previously obtained with pigeons. In addition, results not originally reported were also consistent between rats and pigeons, and provided insights into the perception, memory, and decision processes in Scalar Expectancy Theory and Learning-to-Time Theory.     

Viral escape from the neutralizing antibody response: the lymphocytic choriomeningitis virus model

In addition to CD8+ cytotoxic T lymphocyte (CTL) responses, neutralizing antibodies contribute substantially to the long-term immune control of noncytopathic viruses, as demonstrated during infection with the lymphocytic choriomeningitis virus (LCMV). The high virus load during the initial phase of an infection and the ability of this RNA virus to spontaneously acquire mutations are important prerequisites for escaping an ongoing immune response. In this context, LCMV escape from the humoral response by single point mutations in neutralizing envelope protein determinants may occur, particularly during conditions of CTL deficiency, leading to virus persistence.     

Quantitative analyses of observing and attending

We review recent experiments examining whether simple models of the allocation and persistence of operant behavior are applicable to attending. In one series of experiments, observing responses of pigeons were used as an analog of attending. Maintenance of observing is often attributed to the conditioned reinforcing effects of a food-correlated stimulus (i.e., S+), so these experiments also may inform our understanding of conditioned reinforcement. Rates and allocations of observing were governed by rates of food or S+ delivery in a manner consistent with the matching law. Resistance to change of observing was well described by behavioral momentum theory only when rates of primary reinforcement in the context were considered. Rate and value of S+ deliveries did not affect resistance to change. Thus, persistence of attending to stimuli appears to be governed by primary reinforcement rates in the training context rather than conditioned reinforcing effects of the stimuli. An additional implication of these findings is that conditioned "reinforcers" may affect response rates through some mechanism other than response-strengthening. In a second series of experiments, we examined the applicability of the matching law to the allocation of attending to the elements of compound stimuli in a divided-attention task. The generalized matching law described performance well, and sensitivity to relative reinforcement varied with sample duration. The bias and sensitivity terms of the generalized matching law may provide measures of stimulus-driven and goal-driven control of divided attention. Further application of theories of operant behavior to performance on attention tasks may provide insights into what is referred to variously as endogenous, top-down, or goal-directed control of attention.     

Three tests of 'anticipatory responding' as an account for induction produced by upcoming food-pellet reinforcement

Previous research has demonstrated that rats' rates of lever pressing for low-concentration liquid-sucrose reinforcers are increased when food-pellet, rather than sucrose, reinforcement will be upcoming in the same session (i.e. induction). The present experiments were designed to determine whether this induction was the product of 'anticipatory responses' for the upcoming food pellets being added to the responses being made for the currently available sucrose reinforcement. Experiment 1 tested this idea by summing sucrose-reinforced responding and 'anticipatory responding' from different conditions and comparing the sum to responding from a third condition in which subjects responded for sucrose when food-pellet reinforcement was upcoming. The comparison yielded similar response rates. Experiment 2 employed a blackout, of different durations in different conditions, to delay the upcoming food-pellet reinforcement. Consistent with the anticipatory-responding account, the delay decreased the size of the induction. However, results from the blackouts were not entirely consistent with the anticipatory-responding explanation. Experiment 3 provided, in some conditions, sucrose and food-pellet reinforcement in the first and second halves of the session, respectively, for responding on separate levers. These conditions separated 'anticipatory responses' for the food pellets from responses for the sucrose reinforcers. However, induction in responding for sucrose was still present. Together, these experiments demonstrate that, although anticipatory responses may contribute to induction in some instances, they are not solely responsible for the effect.     

Pigeons do not perceptually complete partly occluded photos of food: an ecological approach to the "pigeon problem"

Humans routinely complete partly occluded objects to recognize the whole objects. However, a number of studies using geometrical figures and even conspecific images have shown that pigeons fail to do so. In the present study, we tested whether pigeons complete partially occluded objects in a situation simulating a natural feeding context. In Experiment 1, we trained pigeons to peck at any photograph of food and not to peck at any containing a non-food object. At test, we presented both photos of food partly occluded by pigeon's feather and photos simply truncated at the same part. We predicted that if the pigeons perceptually completed the occluded portion, then they would discriminate the photos of occluded food better than the truncated photos. The result was that the pigeons pecked at the truncated photos earlier than the occluded photos. Placing the occluder next to all of the stimuli in Experiment 2 or substituting indented lozenge for the feather in Experiment 3 did not affect the results. Thus, even in a simulated ecologically significant situation, pigeons continued to not show evidence of perceptual completion.     

Response-reinforcer relations and resistance to change

Behavioral momentum theory suggests that the relation between a response and a reinforcer (i.e., response-reinforcer relation) governs response rates and the relation between a stimulus and a reinforcer (i.e., stimulus-reinforcer relation) governs resistance to change. The present experiments compared the effects degrading response-reinforcer relations with response-independent or delayed reinforcers on resistance to change in conditions with equal stimulus-reinforcer relations. In Experiment 1, pigeons responded on equal variable-interval schedules of immediate reinforcement in three components of a multiple schedule. Additional response-independent reinforcers were available in one component and additional delayed reinforcers were available in another component. The results showed that resistance to disruption was greater in the components with added reinforcers than without them (i.e., better stimulus-reinforcer relations), but did not differ for the components with added response-independent and delayed reinforcement. In Experiment 2, a component presenting immediate reinforcement alternated with either a component that arranged equal rates of reinforcement with a proportion of those reinforcers being response independent or a component with a proportion of the reinforcers being delayed. Results showed that resistance to disruption tended to be either similar across components or slightly lower when response-reinforcer relations were degraded with either response-independent or delayed reinforcers. These findings suggest that degrading response-reinforcer relations can impact resistance to change, but that impact does not depend on the specific method and is small relative to the effects of the stimulus-reinforcer relation.     

Response-cost punishment via token loss with pigeons

Two experiments were conducted to investigate punishment via response-contingent removal of conditioned token reinforcers (response cost) with pigeons. In Experiment 1, key pecking was maintained on a two-component multiple second-order schedule of token delivery, with light emitting diodes (LEDs) serving as token reinforcers. In both components, responding produced tokens according to a random-interval 20-s schedule and exchange periods according to a variable-ratio schedule. During exchange periods, each token was exchangeable for 2.5-s access to grain. In one component, responses were conjointly punished according to fixed-ratio schedules of token removal. Response rates in this punishment component decreased to low levels while response rates in the alternate (no-punishment) component were unaffected. Responding was eliminated when it produced neither tokens nor exchange periods (Extinction), but was maintained at moderate levels when it produced tokens in the signaled absence of food reinforcement, suggesting that tokens served as effective conditioned reinforcers. In Experiment 2, the effect of the response-cost punishment contingency was separated from changes in the density of food reinforcement. This was accomplished by yoking either the number of food deliveries per component (Yoked Food) or the temporal placement of all stimulus events (tokens, exchanges, food deliveries) (Yoked Complete), from the punishment to the no-punishment component. Response rates decreased in both components, but decreased more rapidly and were generally maintained at lower levels in the punishment component than in the yoked component. In showing that the response-cost contingency had a suppressive effect on responding in addition to that produced by reductions in reinforcement density, the present results suggest that response-cost punishment shares important features with other forms of punishment.     

Tradeoffs among delay, rate, and amount of reinforcement

In Experiment 1, an adjusting-delay procedure was used to measure pigeons' choices between a single delayed reinforcer and a range of different variable-time schedules. Indifference points showed an inverse relation between rate of reinforcement and delay that was well described by a hyperbolic equation. An adjusting-amount procedure was used in Experiment 2, in which pigeons chose between an adjusting amount of food delivered after a 0.5-s delay and 3 s of food delivered after a range of different delays, and the effects of delay were similar to those found in Experiment 1. The results from both experiments indicated that, for pigeons, the strength of a reinforcer decreased rapidly with increasing delay. Estimates of a decay rate parameter in the hyperbolic equation were similar to those found in other studies with pigeons, but the rates of temporal discounting were three or four times faster than those found in studies with rats, suggesting a possible species difference.     

Bacterial motility confers fitness advantage in the presence of phages

Dispersal provides the opportunity to escape harm and colonize new patches, enabling populations to expand and persist. However, the benefits of dispersal associated with escaping harm will be dependent on the structure of the environment and the likelihood of escape. Here, we empirically investigate how the spatial distribution of a parasite influences the evolution of host dispersal. Bacteriophages are a strong and common threat for bacteria in natural environments and offer a good system with which to explore parasite‐mediated selection on host dispersal. We used two transposon mutants of the opportunistic bacteria, Pseudomonas aeruginosa, which varied in their motility (a disperser and a nondisperser), and the lytic bacteriophage ФKZ. The phage was distributed either in the central point of colony inoculation only, thus offering an escape route for the dispersing bacteria; or, present throughout the agar, where benefits of dispersal might be lost. Surprisingly, we found dispersal to be equally advantageous under both phage conditions relative to when phages were absent. A general explanation is that dispersal decreased the spatial structuring of host population, reducing opportunities for parasite transmission, but other more idiosyncratic mechanisms may also have contributed. This study highlights the crucial role the parasites can play on the evolution of dispersal and, more specifically, that bacteriophages, which are ubiquitous, are likely to select for bacterial motility.     

Reaction times identify a Pavlovian component in a two-choice discrimination

Six pigeons discriminated on discrete trials between two colors. In Experiment 1, two luminous spots were both either blue or green and the reinforced responses were “peck left” for blue and “peck right” for green. In Experiment 2, the hue of a center spot controlled subsequent choice pecks to left or right. In both experiments response bias was manipulated in two ways. During stimulus frequency (“SF”) sessions correct responses brought food on 40% of trials; in “imbalanced” blocks of sessions one hue appeared on 80% of trials and the other on 20%. During reinforcement probability (“RNF”) sessions the hues appeared equally often, but in imbalanced blocks the hues signaled different reinforcement probabilities, either 64% or 16%. In “balanced” control blocks the hues appeared equally often and were both reinforced at 40%. The experiments gave similar results. When bias was computed from choice percentages the imbalanced conditions yielded substantial response bias, and the amount of bias was about the same under RNF and SF treatments. However, reaction times (RTs) gave a different outcome. RNF imbalance slowed responses directed at the less reinforced stimulus, but SF imbalance had little RT effect (Experiment 1) or no effect (Experiment 2). These results suggest that choice was controlled by an instrumental stimulus-response-reinforcement association, whereas RTs were controlled by a Pavlovian stimulus-reinforcement association.