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Context renewal is identified when the conditioned response (CR) elicited by an extinguished conditioned stimulus (CS) reappears as a result of changing the contextual cues during the test. Two experiments were designed for testing contextual renewal in a conditioned taste aversion preparation. Experiment 1 assessed ABA and AAB context renewal, whereas Experiment 2 assessed ABA, AAB and ABC context renewal. The procedure of both experiments included three successive phases: acquisition, extinction and testing. The acquisition phase involved one pairing of sucrose flavor with a lithium chloride (LiCl) injection in Context A for all groups of rats. The groups were exposed to the sucrose flavor alone during the extinction phase, for the groups AAB and AAA extinction was carried out in Context A, whereas groups ABA and ABC were extinguished in Context B. Testing was done in context A for the groups ABA and AAA, whereas group AAB was tested in context B; lastly, the ABC group was tested in Context C. Results showed ABA, AAB and ABC renewal of conditioned taste aversion. The results are interpreted and discussed in relation to Bouton's (1994) retrieval of information model. (Contains 2 figures and 1 table.)

According to the composite-stimulus control model (Weiss, 1969, 1972b), an individual discriminative stimulus (S[superscript D]) is composed of that S[superscript D]'s on-state plus the off-states of all other relevant S[superscript D]s. The present experiment investigated the reversibility of composite-stimulus control. Separate groups of rats were trained to lever-press for food whenever a tone or a light S[superscript D] was present. For one group, the nonreinforced S[superscript delta] condition was tone-and-light absence. Tone-plus-light was S[superscript delta] in the other group. On a "stimulus compounding" test that recombined composite elements, maximum responding occurred to that composite consisting only of elements occasioning response increase. That was the presence of tone + light for the group trained with absence of tone + absence of light as S[superscript delta] and absence of tone + absence of light for the group trained with tone plus light S[superscript delta]. The S[superscript delta] composite was next reversed over groups in Phase 2. In Phase 2 tests, maximum responding that was comparable in magnitude to that of Phase 1 was again controlled by the composite consisting only of elements most recently occasioning response increase--whether the presence of both tone + light or the absence of both tone + light. The inhibitory conditioning history of both composite-elements currently occasioning responding did not weaken the summative effect. These results confirm and extend Weiss's composite-stimulus control model, and demonstrate that such control is fully reversible. We discuss how translating conditions of the stimulus-compounding paradigm to a composite continuum creates a functional and logical connection to intradimensional control measured through stimulus generalization, reducing the number of different behavioral phenomena requiring unique explanations. (Contains 3 tables and 1 figure.)

The control exerted by a stimulus associated with an extinction component (S-) on observing responses was determined as a function of its temporal relation with the onset of the reinforcement component (S+). Lever pressing by rats was reinforced on a mixed random-interval extinction schedule. Each press on a second lever produced stimuli associated with the component of the schedule in effect. In Experiment 1 a response-dependent clock procedure that incorporated different stimuli associated with an extinction component of a variable duration was used. When a single S- was presented throughout the extinction component, the rate of observing remained relatively constant across this component. In the response-dependent clock procedure, observing responses increased from the beginning to the end of the extinction component. This result was replicated in Experiment 2, using a similar clock procedure but keeping the number of stimuli per extinction component constant. We conclude that the S- can function as a conditioned reinforcer, a neutral stimulus or as an aversive stimulus, depending on its temporal location within the extinction component. (Contains 2 tables and 5 figures.)

To meet an increasing need to examine the neurophysiological underpinnings of behavior in rats, we developed a behavioral system for studying sensory processing, attention and discrimination learning in rats while recording firing patterns of neurons in one or more brain areas of interest. Because neuronal activity is sensitive to variations in behavior which may confound the identification of neural correlates, a specific aim of the study was to allow rats to sample sensory stimuli under conditions of strong behavioral regularity. Our behavioral system allows multimodal stimulus presentation and is coupled to modules for delivering reinforcement, simultaneous monitoring of behavior and recording of ensembles of well isolated single neurons. Using training protocols for simple and compound discrimination, we validated the behavioral system with a group of 4 rats. Within these tasks, a majority of medial prefrontal neurons showed significant firing-rate changes correlated to one or more trial events that could not be explained from significant variation in head position. Thus, ensemble recordings can be combined with discriminative learning tasks under conditions of strong behavioral regularity. (Contains 9 figures.)

Food-deprived rats in Experiment 1 responded to one of two tandem schedules that were, with equal probability, associated with a sample lever. The tandem schedules' initial links were different random-interval schedules. Their values were adjusted to approximate equality in time to completing each tandem schedule's response requirements. The tandem schedules differed in their terminal links: One reinforced short interresponse times; the other reinforced long ones. Tandem-schedule completion presented two comparison levers, one of which was associated with each tandem schedule. Pressing the lever associated with the sample-lever tandem schedule produced a food pellet. Pressing the other produced a blackout. The difference between terminal-link reinforced interresponse times varied across 10-trial blocks within a session. Conditional-discrimination accuracy increased with the size of the temporal difference between terminal-link reinforced interresponse times. In Experiment 2, one tandem schedule was replaced by a random ratio, while the comparison schedule was either a tandem schedule that only reinforced long interresponse times or a random-interval schedule. Again, conditional-discrimination accuracy increased with the temporal difference between the two schedules' reinforced interresponse times. Most rats mastered the discrimination between random ratio and random interval, showing that the interresponse times reinforced by these schedules can serve to discriminate between these schedules. (Contains 2 tables and 8 figures.)

Distinguishing safety from danger is necessary for survival, but is aberrant in individuals with post-traumatic stress disorder (PTSD). While PTSD is more prevalent in women than men, research on sex differences in safety learning is limited. Here, female rats demonstrated greater fear discrimination than males in a CS+/CS- paradigm. To determine if this sex difference transferred to fear inhibition, rats were tested for conditioned inhibition in a summation test with the CS+ and CS- presented in compound; no sex difference emerged. The results suggest sex differences in the neural mechanisms of discrimination learning but not recall of a fear inhibitor.

Language and culture endow humans with access to conceptual information that far exceeds any which could be accessed by a non-human animal. Yet, it is possible that, even without language or specific experiences, non-human animals represent and infer some aspects of similarity relations between objects in the same way as humans. Here, we show that monkeys' discrimination sensitivity when identifying images of animals is predicted by established measures of semantic similarity derived from human conceptual judgments. We used metrics from computer vision and computational neuroscience to show that monkeys' and humans' performance cannot be explained by low-level visual similarity alone. The results demonstrate that at least some of the underlying structure of object representations in humans is shared with non-human primates, at an abstract level that extends beyond low-level visual similarity. Because the monkeys had no experience with the objects we tested, the results suggest that monkeys and humans share a primitive representation of object similarity that is independent of formal knowledge and cultural experience, and likely derived from common evolutionary constraints on object representation.

Using a conditional discrimination procedure, pigeons were exposed to a nonverbal analogue of qualifying autoclitics such as "definitely" and "maybe." It has been suggested that these autoclitics are similar to tacts except that they are under the control of private discriminative stimuli. Instead of the conventional assumption of privacy, which precludes direct manipulation of the controlling variable, the autoclitic was here identified as a response that is jointly determined by its function as a modifier for the consequence of the tact and by some variable that modifies stimulus control of the tact. Following this modified conceptualization, a novel conditional discrimination procedure was developed as an analogue for establishing autoclitic-like behavior in pigeons. Under some conditions, autoclitic-like behavior was established. Methodological challenges in developing an autoclitic analogue in nonhumans are discussed, followed by a consideration of the implications of such analogues for the understanding of verbal and, more broadly, human behavior.

We have previously demonstrated that voluntary exercise facilitates discrimination learning in a modified T-maze. There is evidence implicating the dorsolateral striatum (DLS) as the substrate for this task. The present experiments examined whether changes in DLS dopamine receptors might underlie the exercise-associated facilitation. Infusing a D1R antagonist into the DLS prior to discrimination learning facilitated the performance of nonexercising rats but not exercising rats. Infusing a D2R antagonist impaired the performance of exercising rats but not nonexercising rats. Exercise-associated facilitation of this task may rely on an exercise-induced decrease in D1R and increase in D2R activation in the DLS.

Temporal order discrimination in rats was analyzed using a radial maze. The task consisted of a study phase in which two to five items (arms) were presented sequentially and, after a delay, a test phase in which two of these were simultaneously presented and the rat had to choose the arm presented earlier in the study phase. Acquisition of the task was better as the number of items in the study phase increased when the total study phase length was constant (4 min). Interval length between items in the study phase did not affect the acquisition regardless of the number of items. Additionally, discrimination performance was better when two items with more intervening items in the study phase (temporal lag) were used for the test. However, this tendency was obtained only when the last item of the study phase was included in the test pair. Results suggest that the number of items presented in the sequence, but not interval length between items, is an important factor in temporal order memory, and that a larger number of intervening items, as well as containing the last item in the study phase, contributes to the occurrence of the temporal lag effect.

Monkeys will selectively and adaptively learn to avoid the most difficult trials of a perceptual discrimination learning task. Couchman, Coutinho, Beran, and Smith (2010) have recently demonstrated that this pattern of responding does not depend on animals receiving trial-by-trial feedback for their responses; it also obtains if experience of the most difficult trials occurs only under conditions of deferred feedback. Couchman et al. argued that this ruled out accounts based on low-level processes of associative learning and instead required explanation in terms of metacognitive processes of decision monitoring. Contrary to this argument, a simple associative model of reinforcement learning is shown to account for the key findings of Couchman et al.'s empirical study, along with several other findings that have previously been claimed to challenge associative models. (Contains 12 figures and 9 footnotes.)

In a temporal double bisection task, animals learn two discriminations. In the presence of Red and Green keys, responses to Red are reinforced after 1-s samples and responses to Green are reinforced after 4-s samples; in the presence of Blue and Yellow keys, responses to Blue are reinforced after 4-s samples and responses to Yellow are reinforced after 16-s samples. Subsequently, given a choice between Green and Blue, the probability of choosing Green increases with the sample duration--the context effect. In the present study we asked whether this effect could be predicted from the stimulus generalization gradients induced by the two basic discriminations. Six pigeons learned to peck Green following 4-s samples (S[superscript +]) but not following 1-s samples (S[superscript -]) and to peck Red following 4 s samples (S[superscript +]) but not following 16-s samples (S[superscript -]). Temporal generalization gradients for Green and Red were then obtained. Finally, the pigeons were given a choice between Green and Red following sample durations ranging from 1 to 16 s. Results showed that a) the two generalization gradients had the minimum at the S[superscript -] duration, an intermediate value between the S[superscript -] and the S[superscript +] durations, and the maximum at the S[superscript +] as well as more extreme durations; b) on choice trials, preference for Green over Red increased with sample duration, the context effect; and c) the two generalization gradients predicted the average context effect well. The Learning-to-Time model accounts for the major trends in the data. (Contains 3 footnotes, 3 tables, and 7 figures.)

Same-different categorization is a fundamental feat of human cognition. Although birds and nonhuman primates readily learn same-different discriminations and successfully transfer them to novel stimuli, no such demonstration exists for rats. Using a spatial discrimination learning task, we show that rats can both learn to discriminate arrays of visual stimuli containing all same from all different items and transfer this discrimination to arrays composed of novel visual items. These results are consistent with rats' engaging in same-different categorization. As such, they pave the way for investigations into the perceptual, cognitive, and neurobiological substrates of abstract categorization behavior.

Extinction and recovery of the discriminative stimulus effects of nicotine (0.3 mg/kg) was investigated with a devalued food reinforcer (rats sated). Sixteen rats were trained in a counterbalanced one manipulandum (nose-poke) drug discrimination procedure with the roles of nicotine and saline counterbalanced as S[superscript D] and S[superscript [delta]]. Discrimination training was maintained and then extinguished with the devalued reinforcer. Devaluation of the reinforcer diminished S[superscript D] response rates during discrimination training but not discriminative control. Following delays after extinction, recovery of responding occurred with the revalued but not devalued reinforcer. These data demonstrate that (a) discriminative control by nicotine is temporally stable with a devalued reinforcer following acquisition and extinction, (b) revaluation of the reinforcer promotes recovery of discriminative control, and (c) recovery of interoceptive discriminative control by nicotine following extinction is affected by changes in motivation. Theoretical implications regarding drug replacement therapy and cue-exposure therapy are discussed. (Contains 2 figures.)

Generalization gradients have been investigated widely in animal conditioning experiments, but much less so in human predictive learning tasks. Here, we apply the experimental design of a recent study on conditioned fear generalization in humans (Lissek et al., 2008) to a predictive learning task, and examine the effects of a number of relevant procedural parameters drawn from the generalization literature in animal conditioning. Experiment 1 shows that prior discrimination learning and steady-state testing procedures sharpen the gradient; Experiment 2 shows that within-subjects testing of the range of generalization stimuli also sharpens the gradient. In addition, Experiment 2 shows that, in case of very flat initial generalization, an orderly gradient can reveal itself through differential rates of extinction learning. Finally, Experiment 2 also evidenced an orderly gradient of generalization-of-extinction. These results suggest that discrimination processes have an important effect on the generalization of predictive learning in humans, and highlight behavioral analogies between animal conditioning and human predictive learning. (Contains 5 figures.)

In quantity discrimination tasks, adults, infants and animals have been sometimes observed to process number only after all continuous variables, such as area or density, have been controlled for. This has been taken as evidence that processing number may be more cognitively demanding than processing continuous variables. We tested this hypothesis by training mosquitofish to discriminate two items from three in three different conditions. In one condition, continuous variables were controlled while numerical information was available; in another, the number was kept constant and information relating to continuous variables was available; in the third condition, stimuli differed for both number and continuous quantities. Fish learned to discriminate more quickly when both number and continuous information were available compared to when they could use continuous information only or number only; there was no difference in the learning rate between the two latter conditions. Our results do not support the hypothesis that processing numbers imposes a higher cognitive load than processing continuous variables. Rather, they suggest that availability of multiple information sources may facilitate discrimination learning.

In recent years, operant discrimination training procedures have been used to teach giant African pouched rats to detect tuberculosis (TB) in human sputum samples. This article summarizes how the rats are trained and used operationally, as well as their performance in studies published to date. Available data suggest that pouched rats, which can evaluate many samples quickly, are sufficiently accurate in detecting TB to merit further investigation as a diagnostic tool. (Contains 1 table.)

A neutral olfactory stimulus was employed as CS in a series of experiments with a sexually receptive female as UCS and the execution of an intromission as the UCR. Each experimental session lasted until the male ejaculated. The time the experimental subject spent in a zone adjacent to the source of the olfactory stimulus during the 10 s of CS presentation and the latency to enter that area were employed as measures of conditioning. In Experiment 1, the CS was always presented at the same location in the conditioning arena, bordering the site where the female became accessible. Conditioning resulted in a gradual increase in the time spent in the CS zone as well as in a reduction of the latency to enter it. In Experiment 2, the site of presentation of the CS was varied randomly from one side of the conditioning arena to another in order to assure that the subjects indeed associated the CS and not a particular location with access to a female. Again, the time spent in the CS zone increased and the latency to enter it was reduced by conditioning. Furthermore, after conditioning the CS was approached more than a neutral odor in a different context. In a third experiment one group of animals was randomly exposed to the CS to control for non-associative effects on performance. Another group was conditioned as in Experiment 2. The random presentation of the CS failed to enhance approach, while the paired UCS-CS had effects identical to those found in Experiment 2. In the last experiment, an olfactory CS[superscript -] was included. The CS[superscript +] and CS[superscript -] were presented in random order at random locations. Both the time spent in the CS[superscript +] and CS[superscript -] zones was increased, while the latency to enter the zone was significantly reduced only for the CS[superscript +]. These data suggest that the subjects had partially learned to discriminate between the CS[superscript +] and the CS[superscript -], at most. The procedure employed in Experiments 2 and 3 seems to be the most promising for studies of sexual conditioning in male rats. (Contains 7 figures.)

The striatum plays a major role in both motor control and learning and memory, including executive function and "behavioral flexibility." Lesion, temporary inactivation, and infusion of an N-methyl-d-aspartate (NMDA)-receptor antagonist into the dorsomedial striatum (dmSTR) impair reversal learning in adult rats. Systemic administration of MK-801 disrupts reversal learning in developing rats, as reported in an earlier work by Chadman et al., but it is not known whether NMDA-receptor function within the dmSTR plays a role in this effect. In Experiment 1, reversal learning was dose-dependently impaired following bilateral dmSTR administration of MK-801 (either 2.5 or 5.0 [micro]g) only during the reversal phase relative to saline in postnatal day (P) 26 rats. In Experiment 2, separate groups of P26 rats were trained on the same reversal learning task, but were administered bilateral dmSTR infusions during acquisition only (MK-SAL), reversal only (SAL-MK), both phases (MK-MK), or neither phase (SAL-SAL). The MK-801 effect was specific to the reversal training phase. The drug did not alter acquisition of the initial discrimination. Analysis of the pattern of errors indicates that dmSTR MK-801 treatment increased perseveration of the choice response trained in acquisition. NMDA receptors in the dmSTR play a role in reversal learning in the weanling rat.

Individuals with anxiety disorders often do not respond to safety signals and hence continue to be afraid and anxious. Consequently, it is important to develop paradigms in animals that can directly study brain systems involved in learning about, and responding to, safety signals. We previously developed a discrimination procedure in rats of the form AX+/BX-, where cues A and X presented together are paired with an aversive stimulus and cues B and X presented together predict the absence of an aversive stimulus. The present experiment adapted this procedure to the fear-potentiated startle paradigm in rhesus monkeys.

N-Methyl-D-aspartate receptors (NMDARs) mediate certain forms of synaptic plasticity and learning. We used a touchscreen system to assess NR2A subunit knockout mice (KO) for (1) pairwise visual discrimination and reversal learning and (2) acquisition and extinction of an instrumental response requiring no pairwise discrimination. NR2A KO mice exhibited significantly retarded discrimination learning. Performance on reversal was impaired in NR2A KO mice during the learning phase of the task; with no evidence of heightened perseverative responses. Acquisition and extinction of an instrumental behavior requiring no pairwise discrimination was normal in NR2A KO mice. The present findings demonstrate a significant and selective deficit in discrimination learning following loss of NR2A.

To test the assumptions of two models of timing, Scalar Expectancy Theory (SET) and Learning to Time (LeT), nine pigeons were exposed to two temporal discriminations, each signaled by a different cue. On half of the trials, pigeons learned to choose a red key after a 1.5-s horizontal bar and a green key after a 6-s horizontal bar; on the other half of the trials, they learned to choose a blue key after a 6-s vertical bar and a yellow key after a 24-s vertical bar. During subsequent test trials, they were exposed to the horizontal or vertical bar, for durations ranging from 1.5 to 24s, and given a choice between novel key combinations: red vs. yellow, or green vs. blue. Results showed a strong effect of sample duration--as the test signal duration increased, preference for green over blue increased and preference for red over yellow decreased. The effect of sample cue was obtained only on the green-blue test trials. These effects are discussed in light of SET and LeT.

Learning styles in capuchin monkeys were assessed with a computerized reversal-learning task called the mediational paradigm. First, monkeys were trained to respond with 90% accuracy on a two-choice discrimination (A+B-). Then the authors examined differences in performance on three different types of reversal trials (A-B+, A-C+, B+C-), each of which offered differing predictions for performance, depending on whether the monkeys were using associative cues or rule-based strategies. Performance indicated that the monkeys mainly learned to avoid the B stimulus during training, as the A-C+ condition produced the best performance levels. Therefore, negative stimuli showed greater control over responding after reversal and reflected a more associative rather than rule-based form of learning. (Contains 1 table and 1 figure.)

A robust finding in humans and animals is that intermixed exposure to 2 similar stimuli (AX/BX) results in better discriminability of those stimuli on test than does exposure to 2 equally similar stimuli in 2 separate blocks (CX_DX)--the intermixed-blocked effect. This intermixed-blocked effect may be an example of the superiority of spaced over massed practice; in the intermixed, but not the blocked exposure regime, each presentation of a given stimulus (e.g., AX) is separated from the next by the presentation of its partner (BX). Two experiments with human participants replicated the intermixed-blocked effect and showed that the effect was not due to the spacing of exposure trials. A mechanism for the intermixed-blocked effect is proposed, which combines theories from associative learning and memory.

Two groups of rats were trained in a symbolic delayed matching-to-sample task with a 0-s delay to discriminate sample stimuli that consisted of sequences of tone bursts. For one group, sequences varied in number with total sequence duration controlled. For the other group, total sequence duration, sum of the tone durations, and sum of the gap durations were all controlled. The former group of rats acquired the discrimination, but the latter group of rats were not able to learn the discrimination with the additional temporal cues controlled. Retention functions for the group that acquired the discrimination exhibited a choose-many bias at the 1-s delay and a choose-few bias at the 8-s delay regardless of the similarity in the intertrial interval and delay interval illumination condition. The asymmetrical retention functions appeared to be due to instructional ambiguity which resulted from the similarity of the delay interval to temporal features of the tone burst sequence, such as the gap between tone bursts and the total duration of tone. In Experiment 2, diagnostic tests provided evidence that rats discriminated the sequences of tone bursts by timing and summing the duration of individual tone bursts rather than using an event switch to count tones. Like pigeons, rats use multiple temporal features rather than number to discriminate sequences of successive events.

In Experiment 1, rats were trained on a discrimination in which one occasion setter, A, signaled that one cue (conditioned stimulus, CS), x, would be followed by one outcome, p (unconditioned stimulus, US), and a second CS, y, by a different outcome, q (x [right arrow] p and y [right arrow] q); a second occasion setter, B signalled the reverse CS-outcome relations (x [right arrow] q and y [right arrow] p). In a subsequent stage, the animals were divided into two groups, and trained as before, except that both A and B were presented in compound with a novel occasion setter, C. For Group S (same) the CS-outcome relations following A and B were identical to those in the pretraining stage, whereas in Group D (different) they were reversed. In a subsequent test, stimulus C was shown to be a more effective occasion setter in Group D than in Group S. In Experiment 2, rats were trained on a negative occasion-setting discrimination in which CS x signaled outcome p, and y outcome q; when x and y were signaled by the occasion setter A then no outcome followed (x [right arrow] p, y [right arrow] q, A:x-, and A:y-). In a subsequent stage, A was now trained as a positive occasion setter, signaling reinforcement of x and y. In Group S, x and y signaled the same outcomes as in the prior training stage (x-, y-, A:x [right arrow] p, and A:y [right arrow] q), whereas in Group D they signaled the opposite outcomes (x, y, A:x [right arrow] q, and A:y [right arrow] p); more efficient test performance was seen in the latter group. These results suggest that the each occasion setter conveyed information about the specific combination of CS and US paired in its presence (i.e., x [right arrow] p and y [right arrow] q, or x [right arrow] "no" p and y [right arrow] "no" q). These results are consistent with the suggestion that occasion setters operate, at least in part, on a specific CS-US association.

Increases in cyclic adenosine monophosphate (cAMP) are proposed to initiate learning in a wide variety of species. Here, we measure changes in cAMP in the olfactory bulb prior to, during, and following a classically conditioned odor preference trial in rat pups. Measurements were taken up to the point of maximal CREB phosphorylation in olfactory bulb mitral cells. Using both drug and natural unconditioned stimuli we found effective learning was associated with an increase in cAMP at the end of the conditioning trial, followed by a decrease 5 min later. This early timing of a transient cAMP increase occurred only when the odor was paired with an effective drug or natural unconditioned stimulus (US). The data support the hypothesis that the rate of adenylate cyclase activation is enhanced by pairing calcium and G-protein activation and that the timing of transient cAMP signaling is critical to the initiation of classical conditioning.

We explored the circumstances in which rats engage either declarative memory (and the hippocampus) or habit memory (and the dorsal striatum). Rats with damage to the hippocampus or dorsal striatum were given three different two-choice discrimination tasks (odor, object, and pattern). These tasks differed in the number of trials required for learning ([similar to]10, 60, and 220 trials). Dorsal striatum lesions impaired discrimination performance to a greater extent than hippocampal lesions. Strikingly, performance on the task learned most rapidly (the odor discrimination) was severely impaired by dorsal striatum lesions and entirely spared by hippocampal lesions. These findings suggest that discrimination learning in the rat is primarily supported by the dorsal striatum (and habit memory) and that rats engage a habit-based memory system even for a task that takes only a few trials to acquire. Considered together with related studies of humans and nonhuman primates, the findings suggest that different species will approach the same task in different ways.

Small deviations from bilateral symmetry (fluctuating asymmetries) are cues to fitness differences in some animals. Therefore, researchers have considered whether animals use these small asymmetries as visual cues to determine appropriate behavioral responses (e.g., mate preferences). However, there have been few systematic studies of animals' abilities to visually discriminate such minor asymmetries. If the asymmetries cannot be discriminated, fluctuating asymmetry can not be a visual cue. Here, we report an investigation of European starlings' ("Sturnus vulgaris") abilities to discriminate small size asymmetries. We trained starlings, through operant conditioning in a free-flight aviary, to discriminate achromatic, symmetric paired stimuli from size-matched asymmetric stimuli. By starting the learning process with a large asymmetry and progressing through sequential trials of decreasing asymmetry, we elucidated a behavioral limit to asymmetry discrimination. We found that starlings are capable of discriminating a 10% size asymmetry. There was weaker evidence for discrimination of 5% asymmetry but no evidence for signal discrimination at 2.5% size asymmetry. This level of asymmetry discrimination suggests that many size asymmetry cues in nature can be discriminated by birds. At each level of asymmetry discrimination, we also tested whether starlings could generalize their learned symmetry preference to unreinforced novel images. Consistent with previous findings, we found that starlings could generalize their symmetry preferences. (Contains 1 table and 4 figures.)

While many studies have investigated duration discrimination in human adults and in nonhuman animals, few have investigated this ability in infants. Here, we report findings that 6-month-old infants are able to discriminate brief durations, and, as with other animal species, their discrimination function is characterized by Weber's Law: proportionate difference rather than absolute difference between stimuli determined successful discrimination. Importantly, paralleling results found with nonhuman animals, the Weber function that we found for infants' discrimination of time is the same as that found for their discrimination of number. Infants discriminated durations of an audiovisual event differing by a 1:2 ratio, but not those differing by a 2:3 ratio, over a range of durations. This suggests that (a) in human as in nonhuman animals, the same mental mechanism may underlie the ability to measure duration as to represent number, and (b) we may share this mental mechanism with other animal species.

Nonhuman animals, like humans, appear sensitive to the structure of the elements of sequences, perhaps even when the structure relates nonadjacent elements. In the present study, we examined the contribution of chunking, rule learning, and item memory when rats learned serial patterns composed of two interleaved subpatterns. In one group, the first interleaved subpattern was a formally simple sequence, whereas in two other groups the first subpattern was formally more complex, containing 2 or 4 violation elements, respectively. In all groups, the second interleaved subpattern encountered was formally simple. Evidence from the study suggests that rats chunked their interleaved patterns into component subpatterns, that is, they cognitively sorted pattern elements to form chunks based on nonadjacent structural relationships that can be characterized as rules. They also learned interitem associations via traditional discrimination learning to use adjacent elements as compound or configural cues for later events in the sequence. Thus, the evidence suggests that rats used chunking, rule learning, and interitem association learning concurrently to master these complex patterns.

Using a context discrimination procedure and rats as the subjects, the formation of context-dependent aversions to novel and familiar fluids was investigated. Experiment 1 revealed that context dependency could be established to a novel fluid (saccharin) after three cycles of context discrimination training and that the acquired context dependency was revealed also to a second familiar fluid (water) presented in the following test. Experiment 2 showed that the formation of the context-dependent aversion and its transfer to a second fluid was not affected by whether fluid presented during discrimination was novel (saccharin) or familiar (water). Experiment 3 demonstrated that when the same water was given both in the two training contexts and in the home cages of the subjects during discrimination, the context-dependent aversion formed was specific to it. These findings can be explained in terms of a simple summation effect of fluid-nausea and context-nausea associations.

Six male Sprague-Dawley rats were used to examine the long-term retention of an olfactory discrimination learning set. Rats were trained on 30 odor-unique, 5-trial discrimination problems, transferred to an olfactory discrimination reversal task, and then given a 6-week break in training. The rats demonstrated use of a learning set by performing significantly better than expected by chance on Trial 2 on both the olfactory discrimination problems and the olfactory discrimination reversal problems. Following the 6-week break in training, retention of learning set was tested using 20 new odor-unique discrimination problems. The rats again performed significantly better than expected by chance on Trial 2 in the retention test indicating they had retained the learning set. (Contains 3 figures.)

To date, only 1 study has evaluated the impact of a Pavlovian drug conditional stimulus (CS) on operant responding. A within-subject operant 1-lever go/no-go (across sessions) design was used to evaluate the impact of Pavlovian contingencies on the discriminative stimulus effects of nicotine (0.4 mg/kg) and ethanol (800 mg/kg) in male Sprague Dawley rats. Drugs were administered 10 min before each acquisition and test session. One drug predicted sessions of food reinforcement and the other drug predicted sessions of nonreinforcement; stimulus roles were counterbalanced. In Experiment 1 (n = 7), operant lever pressing was initially maintained on a VI-30 s schedule of food reinforcement. This phase was followed by 20 (CS+ vs. CS-) Pavlovian drug discrimination training sessions without the levers present. Two extinction tests revealed significantly more operant lever pressing under the CS+ drug conditions compared to the CS- drug conditions, suggesting evidence for Pavlovian-instrumental transfer. Operant training significantly strengthened stimulus control. In Experiment 2 (n = 7), the drugs functioned as operant drug discriminative stimuli first. Next, the predictive roles of the drug [S[superscript D]s and [S[superscript[delta]]]s were reversed under Pavlovian CS- and CS+ contingencies, respectively. The original stimulus control was significantly undermined but was not reversed. These studies suggest that Pavlovian drug-reinforcer contingencies embedded within the operant 3-term contingency may play a partial role in mediating the discriminative stimulus effects of drugs. (Contains 2 tables and 1 figure.)

Nurr1 expression is up-regulated in the brain following associative learning experiences, but its relevance to cognitive processes remains unclear. In these studies, rats initially received bilateral hippocampal infusions of control or antisense oligodeoxynucleotides (ODNs) 1 hour prior to training in a holeboard spatial discrimination task. Such pre-training infusions of "nurr1" antisense ODNs caused a moderate effect in learning the task and also impaired LTM tested 7 days later. In a second experiment, ODN infusions were given immediately after the animals had received two sessions of training, during which all animals showed normal learning. Although antisense treated rats were significantly impaired during the post-infusion stages of acquisition of the task, no group differences were observed during the LTM test given 7 days later. These animals were subjected 3 days later to reversal training in the same maze in the absence of any additional treatments. Remarkably, rats previously treated with antisense ODNs displayed perseveration: The animals were fixated with the previously learned pattern of baited holes, causing them to be significantly impaired in the extinction of acquired spatial preferences and future learning. The authors postulate that Nurr1 function in the hippocampus is important for normal cognitive processes.

We have previously reported that sound sequence discrimination learning requires cholinergic inputs to the auditory cortex (AC) in rats. In that study, reward was used for motivating discrimination behavior in rats. Therefore, dopaminergic inputs mediating reward signals may have an important role in the learning. We tested the possibility in the present study. Rats were trained to discriminate sequences of two sound components, and licking behavior in response to one of the two sequences was rewarded with water. To identify the dopaminergic inputs responsible for the learning, dopaminergic afferents to the AC were lesioned with local injection of 6-hydroxydopamine (6-OHDA). The injection attenuated sound sequence discrimination learning, while it had no effect on discrimination between the sound components of the sequence stimuli. Local injection of 6-OHDA into the nucleus accumbens attenuated sound discrimination learning. However, not only discrimination learning of sound sequence but also that of the sound components were impaired. SCH23390 (0.2 mg/kg, i.p.), a D1 receptor antagonist, had no effect on sound sequence discrimination learning, while it attenuated the licking behavior to unfamiliar stimuli. Haloperidol (0.5 mg/kg, i.p.), a D2 family antagonist, attenuated sound sequence discrimination learning, while it had no clear suppressive effect on discrimination of two different sound components and licking. These results suggest that D2 family receptors activated by dopaminergic inputs to the AC are required for sound sequence discrimination learning.

Recognition memory, involving the ability to discriminate between a novel and familiar object, depends on the integrity of the perirhinal cortex (PRH). Glutamate, the main excitatory neurotransmitter in the cortex, is essential for many types of memory processes. Of the subtypes of glutamate receptor, metabotropic receptors (mGluRs) have received less study than NMDA receptors; thus, the reported experiments examined the role of mGluRs in familiarity discrimination in the rat PRH. Experiments 1 and 2 assessed the effects of systemic administration of MPEP, a group I mGluR (specifically mGluR5) antagonist, and/or LY341495, a group II mGluR antagonist, on a spontaneous object novelty preference task. Simultaneous antagonism of both group I and II mGluRs impaired familiarity discrimination following a 24-h but not a 15-min delay, while antagonism of either mGluR subtype alone had no effect at either delay. The impairment was in acquisition, as in Experiment 3 coadministration of MPEP and LY341495 did not affect recognition memory performance when administered either after the sample phase or prior to test. The impairment in long-term recognition memory was mediated by mGluRs in the PRH, as localized intracortical antagonism of group I and II mGluRs also produced a deficit (Experiment 4). No evidence was found for an involvement of group III mGluRs in the acquisition of long-term familiarity discrimination (Experiment 5). These findings establish that glutamatergic neurotransmission in the PRH via group I and II mGluRs is crucial for the acquisition, but not for the consolidation or retrieval of long-term object recognition memory. (Contains 3 tables and 3 figures.)

This experiment assesses spatial and nonspatial relational memory in freely moving 9-mo-old and adult (11-13-yr-old) macaque monkeys ("Macaca mulatta"). We tested the use of proximal landmarks, two different objects placed at the center of an open-field arena, as conditional cues allowing monkeys to predict the location of food rewards hidden in one of two sets of three distinct locations. Monkeys were tested in two different conditions: (1) when local visual cues marked the two sets of potentially baited locations, so that monkeys could use both local and spatial information to discriminate these locations from never-baited locations; and (2) when no local visual cues marked the two sets of potentially baited locations, so that monkeys had to rely on a spatial relational representation of the environment to discriminate these locations. No 9-mo-old or adult monkey associated the presence of the proximal landmarks, at the center of the arena, with the presence of food in one set of three distinct locations. All monkeys, however, discriminated the potentially baited locations in the presence of local visual cues, thus providing evidence of visual discrimination learning. More importantly, all 9-mo-old monkeys tested discriminated the potentially baited locations in absence of the local visual cues, thus exhibiting evidence of spatial relational learning. These findings indicate that spatial memory processes characterized by a relational representation of the environment are present as early as 9 mo of age in macaque monkeys. (Contains 1 table and 2 figures.)

Learning to discriminate between spatial locations defined by two adjacent arms of a radial maze in the conditioned cue preference paradigm requires two kinds of information: latent spatial learning when the rats explore the maze with no food available, and learning about food availability in two spatial locations when the rats are then confined in one arm with food and the other with no food. Previous research showed that a functional dorsal hippocampus is not required for latent learning. The present experiments show that it is required for learning about food availability, and during retrieval of both types of information. (Contains 3 figures.)

A model of conditional discrimination performance (Davison & Nevin, 1999) is combined with the notion that unmeasured attending to the sample and comparison stimuli, in the steady state and during disruption, depends on reinforcement in the same way as predicted for overt free-operant responding by behavioral momentum theory (Nevin & Grace, 2000). The rate of observing behavior, a measurable accompaniment of attending, is well described by an equation for steady-state responding derived from momentum theory, and the resistance to change of observing conforms to predictions of momentum theory, supporting a key assumption of the model. When probabilities of attending are less than 1.0, the model accounts for some aspects of conditional-discrimination performance that posed problems for the Davison-Nevin model: (a) the effects of differential reinforcement on the allocation of responses to the comparison stimuli and on accuracy in several matching-to-sample and signal-detection tasks where the differences between the stimuli or responses were varied across conditions, (b) the effects of overall reinforcer rate on the asymptotic level and resistance to change of both response rate and accuracy of matching to sample in multiple schedules, and (c) the effects of fixed-ratio reinforcement on accuracy. Some tests and extensions of the model are suggested, and the role of unmeasured events in behavior theory is considered. (Contains 1 table, 11 figures and 2 footnotes.)

This experiment examined the effects of reinforcement probability on resistance to change of remembering and response rate. Pigeons responded on a two- component multiple schedule in which completion of a variable-interval 20-s schedule produced delayed matching-to-sample trials in both components. Each session included four delays (0.1 s, 2 s, 4 s, and 8 s) between sample termination and presentation of comparison stimuli in both components. The two components differed in the probability of reinforcement arranged for correct matches (i.e., rich, p = 0.9; lean, p = 0.1). Response rates during the variable-interval portion of the procedure were higher in the rich component during baseline and more resistant to the disruptive effects of intercomponent food and extinction. Forgetting functions were constructed by examining matching accuracy as a function of delay duration. Baseline accuracy was higher in the rich component than in the lean component as measured by differences in the y-intercept of the forgetting functions (i.e., initial discrimination), rather than from differences in the slope of the forgetting function (i.e., rate of forgetting). Intercomponent food increased the rate of forgetting relatively more in the lean component than in the rich component, but initial discrimination was not systematically affected. Extinction reduced initial discrimination relatively more in the lean component than in the rich component, but did not systematically affect rate of forgetting. These results are consistent with our previous data suggesting that, as for response rate, accuracy and resistance to change of discriminating are positively related to rate of reinforcement. These data also suggest that the disruptability of remembering depends on the conditions of reinforcement, but the way in which remembering is disrupted depends on the nature of the disruptor. (Contains 1 table and 6 figures.)

Fos protein immunodetection was used to investigate the neuronal activation elicited in some olfactory-related areas after either learning of an olfactory discrimination task or its reactivation 10 d later. Trained rats (T) progressively acquired the association between one odor of a pair and water-reward in a four-arm maze. Two groups of pseudotrained rats were used: PO rats were not water restricted and were submitted to the olfactory stimuli in the maze without any reinforcement, whereas PW rats were water-deprived and systematically received water in the maze without any odorous stimulation. When the discrimination task was well mastered, a significantly lower Fos immunoreactivity was observed in T rats compared to PW and PO rats in most of the analyzed brain areas, which could reflect the post-acquisition consolidation process. Following memory reactivation, differences in Fos immunoreactivity between trained and some pseudotrained rats were found in the anterior part of piriform cortex, CA3, and orbitofrontal cortex. We also observed that Fos labeling was significantly higher in trained rats after memory reactivation than after acquisition of the olfactory task in most of the brain areas examined. Our results support the assumption of a differential involvement of neuronal networks after either learning or reactivation of an olfactory discrimination task. (Contains 1 table and 5 figures.)

The neural mechanisms of fear suppression most commonly are studied through the use of extinction, a behavioral procedure in which a feared stimulus (i.e., one previously paired with shock) is nonreinforced repeatedly, leading to a reduction or elimination of the fear response. Although extinction is perhaps the most convenient index of fear inhibition, a great deal of behavioral work suggests that postextinction training conditioned stimuli are both excitatory and inhibitory, making it difficult to determine whether a neural manipulation affects inhibition, excitation, or some combination thereof. For this reason we sought to develop a behavioral procedure that would render a stimulus primarily inhibitory while at the same time avoiding some of the issues raised by the traditional conditioned inhibition paradigm, namely second-order conditioning, external inhibition, and configural learning. Using the fear-potentiated startle paradigm, we adapted an AX+, BX- training procedure in which stimuli A and X were presented simultaneously and paired with shock, and stimuli B and X were presented simultaneously in the absence of shock. In testing, high levels of fear-potentiated startle were seen in the presence of A and AX and much lower levels were seen in the presence of B and AB, as would be predicted if stimulus B were a conditioned inhibitor. We believe this method is a viable alternative to the traditional conditioned inhibition training procedure and will be useful for studying the neural mechanisms of fear inhibition. (Contains 7 figures and 1 table.)

The present study was aimed at simultaneously determining on the same subject, the effects of stress on retrieval of flexible (contextual or temporal) or stable (spatial) information. Three behavioral paradigms carried out in a four-hole board were designed as follows: (1) Simple Discrimination (SD), in which mice learned a single discrimination; (2) Contextual and Serial Discriminations (CSD), in which mice learned two successive discriminations on two different internal contexts; (3) Spatial Serial Discriminations (SSD), in which mice learned two successive discriminations on an identical internal context. The stressor (three inescapable electric footshocks) was delivered 5 min before retention, occurring 5 min or 24 h after acquisition. Results showed that this stressor increased plasmatic corticosterone levels and fear reactivity in an elevated-plus-maze, as compared with nonstressed mice. The stressor reversed the normal pattern of retrieval observed in nonstressed controls in the CSD task, this effect being context dependent, as it was not observed in the SSD task. Overall, our study shows that stress affected the retrieval of flexible and old information, but spared the retrieval of stable or recent ones. Therefore, these behavioral paradigms allow us to study simultaneously, on the same animal, the effects of stress on distinct forms of memory retrieval. (Contains 1 table and 7 figures.)

Examined representation of pictorial exemplars of humans by 3- and 4-month olds. Results demonstrated an asymmetry regarding the exclusivity of categorical representations formed for humans and non-human animals. Categorical representations for humans included exemplar information, whereas categorical representation for non-human animals was based on summary information. Asymmetry did not extend to more basic process of discriminating human versus non-human animals. (Author/KB)

We studied object-location binding in pigeons using a sequence learning procedure. A sequence of four objects was presented, one at a time at one of four locations on a touchscreen. A single peck at the object ended the trial, and food reinforcement was delivered intermittently. In Experiment 1, a between-subjects design was used to present objects, locations, or both in a regular sequence or randomly. Response time costs on nonreinforced probe tests on which object order, location order, or both were disrupted revealed sequence learning effects. Pigeons encoded location order when it was consistent, but not object order when it alone was consistent. When both were consistent, pigeons encoded both, and showed evidence of object-location binding. In Experiment 2, two groups of pigeons received training on sequences where the same object always appeared at the same location. For some pigeons a consistent sequence was used while for others sequence order was randomized. Only when sequence order was consistent was object-location binding found. These experiments are the first demonstrations of strong and lasting feature binding in pigeons and are consistent with a functional account of learning.

Reversal learning paradigms are widely used assays of behavioral flexibility with their probabilistic versions being more amenable to studying integration of reward outcomes over time. Prior research suggests differences between initial and reversal learning, including higher learning rates, a greater need for inhibitory control, and more perseveration after reversals. However, it is not well-understood what aspects of stimulus-based reversal learning are unique to reversals, and whether and how observed differences depend on reward probability. Here, we used a visual probabilistic discrimination and reversal learning paradigm where male and female rats selected between a pair of stimuli associated with different reward probabilities. We compared accuracy, rewards collected, omissions, latencies, win-stay/lose-shift strategies, and indices of perseveration across two different reward probability schedules. We found that discrimination and reversal learning are behaviorally more unique than similar: Fit of choice behavior using reinforcement learning models revealed a lower sensitivity to the difference in subjective reward values (greater exploration) and higher learning rates for the reversal phase. We also found latencies to choose the better option were greater in females than males, but only for the reversal phase. Further, animals employed more win-stay strategies during early discrimination and increased perseveration during early reversal learning. Interestingly, a consistent reward probability group difference emerged with a richer environment associated with longer reward collection latencies than a leaner environment. Future studies should systematically compare the neural correlates of fine-grained behavioral measures to reveal possible dissociations in how the circuitry is recruited in each phase. (PsycInfo Database Record (c) 2021 APA, all rights reserved).

Serotonergic hallucinogens such as lysergic acid diethylamide (LSD) induce head twitches in rodents via 5-HT2A receptor activation. The goal of the present investigation was to determine whether a correlation exists between the potency of hallucinogens in the mouse head-twitch response (HTR) paradigm and their reported potencies in other species, specifically rats and humans. Dose-response experiments were conducted with phenylalkylamine and tryptamine hallucinogens in C57BL/6J mice, enlarging the available pool of HTR potency data to 41 total compounds. For agents where human data are available (n = 36), a strong positive correlation (r = 0.9448) was found between HTR potencies in mice and reported hallucinogenic potencies in humans. HTR potencies were also found to be correlated with published drug discrimination ED50 values for substitution in rats trained with either LSD (r = 0.9484, n = 16) or 2,5-dimethoxy-4-methylamphetamine (r = 0.9564, n = 21). All three of these behavioral effects (HTR in mice, hallucinogen discriminative stimulus effects in rats, and psychedelic effects in humans) have been linked to 5-HT2A receptor activation. We present evidence that hallucinogens induce these three effects with remarkably consistent potencies. In addition to having high construct validity, the HTR assay also appears to show significant predictive validity, confirming its translational relevance for predicting subjective potency of hallucinogens in humans. These findings support the use of the HTR paradigm as a preclinical model of hallucinogen psychopharmacology and in structure-activity relationship studies of hallucinogens. Future investigations with a larger number of test agents will evaluate whether the HTR assay can be used to predict the hallucinogenic potency of 5-HT2A agonists in humans. "This article is part of the special issue entitled 'Serotonin Research: Crossing Scales and Boundaries'.

Accurate knowledge of species colour discrimination is fundamental to explain colour based behaviours and the evolution of colour patterns. We tested how the receptor noise limited model, widely used in behavioural ecology, matched actual colour discrimination thresholds obtained using behavioural tests. Guppies (Poecilia reticulata) were first trained to push a target coloured disk placed among eight grey disks of various luminances on a grey plate. Guppies were then tested to find target disks, which varied in colour contrast from the plate. The target disks followed a gradient going from high contrast to inconspicuous against the grey background. We plotted the percentage of correct choices of each colour in the gradient against the model prediction and determined the discrimination thresholds using the inflection point of the fitted sigmoid curve. We performed the experiment on six colour gradients: red, orange, yellow, green, blue and purple. Four colour gradients: red, orange, green and blue, showed a discrimination threshold that matched the model predictions. However, deviations of the model for the yellow and purple gradients suggest that ecological relevance of some colours could affect decision-making in behavioural tests and that we can no longer assume that the rules for colour discrimination are independent of colours.

Occasion setting refers to the ability of 1 stimulus, an occasion setter, to modulate the efficacy of the association between another, conditioned stimulus (CS) and an unconditioned stimulus (US) or reinforcer. Occasion setters and simple CSs are readily distinguished. For example, occasion setters are relatively immune to extinction and counterconditioning, and their combination and transfer functions differ substantially from those of simple CSs. Similarly, the acquisition of occasion setting is favored when stimuli are separated by longer intervals, by empty trace intervals, and are of different modalities, whereas the opposite conditions typically favor the acquisition of simple associations. Furthermore, the simple conditioning and occasion setting properties of a single stimulus can be independent, for example, that stimulus may simultaneously predict the occurrence of a reinforcer and indicate that another stimulus will not be reinforced. Many behavioral phenomena that are intractable to simple associative analysis are better understood within an occasion setting framework. Besides capturing the distinction between direct and modulatory control common to many arenas in neuroscience, occasion setting provides a model for the hierarchical organization of memory for events and event relations, and for contextual control more broadly. Although early lesion studies further differentiated between occasion setting and simple conditioning functions, little is known about the neurobiology of occasion setting. Modern techniques for precise manipulation and monitoring of neuronal activity in multiple brain regions are ideally suited for disentangling contributions of simple conditioning and occasion setting in associative learning. (PsycINFO Database Record (c) 2019 APA, all rights reserved).