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The present study demonstrates that humans’ response to a single stimulus (S1) is determined by what follows S1's associates. The experiment used a sensory preconditioning (SPC) design where S1 was associated with both S2 and S3 on separate trials before establishing relationships between these latter stimuli with an outcome or its absence in a second phase. When S2 and S3 were associated with the same consequence, either an outcome or its absence, strong consequence-based responding to S1 was observed in a reaction time test. Participants responded quickly to indicate that the outcome was, or was not, predicted by S1. When S2 predicted the outcome and S3 did not, SPC was weaker although participants were not slower to respond to S1. Implications on the understanding of the mechanisms that allow for the response to S1 to emerge are discussed. [ABSTRACT FROM AUTHOR]

Sensory preconditioning (SPC; also known as behaviorally silent learning) consists of a combination of two neutral stimuli, none of which elicits an unconditional response. After one of them is later paired with an unconditional stimulus (US), the other neutral stimulus also yields a conditional response although it has never been paired with the US. In this study, an event-related functional magnetic resonance imaging (fMRI) paradigm was used to specify brain regions involved in SPC. The results demonstrated that SPC was associated with significant changes in activity of several regions, notably, the left amygdala, the left hippocampus, the bilateral thalamus, the bilateral medial globus pallidus, the bilateral cerebellum, the bilateral premotor cortex, and the bilateral middle frontal gyrus. This is a first effort to use fMRI to examine the effects of SPC on brain activation. Our data suggest that there is a distributed network of structures involved in SPC including both cortical and subcortical regions, therefore add to our understanding of the neural mechanisms underlying the ability to associative learning. Hum Brain Mapp 35:1297-1304, 2014. © 2013 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]

Sensory preconditioned and second-order conditioned responding are each well-documented. The former occurs in subjects (typically rats) exposed to pairings of two relatively neutral stimuli, S2 and S1, and then to pairings of S1 and a motivationally significant event [an unconditioned stimulus (US)]; the latter occurs when the order of these experiences is reversed with rats being exposed to S1-US pairings and then to S2-S1 pairings. In both cases, rats respond when tested with S2 in a manner appropriate to the affective nature of the US, e.g., approach when the US is appetitive and withdrawal when it is aversive. This paper reviews the neural substrates of sensory preconditioning and second-order conditioning. It identifies commonalities and differences in the substrates of these so-called higher-order conditioning protocols and discusses these commonalities/differences in relation to what is learned. In so doing, the review highlights ways in which these types of conditioning enhance our understanding of how the brain encodes and retrieves different types of information to generate appropriate behavior. • This review examines the neural systems involved in the two types of higher-order conditioning, sensory preconditioning and second-order conditioning. • It brings together studies that have focused on different motivational systems (appetitive and aversive) and used stimuli from a variety of sensory modalities (e.g., flavors, contexts, auditory and visual cues). In doing so, it advances our understanding of: 1) the manner in which motivational states alter processing of simple sensory information; 2) how the brain integrates sensory and motivational information; and 3) the mechanisms by which a network of sensory/motivational stimuli expands to include new information. • The review ultimately provides a set of principled statements regarding these aspects of information processing. The statements highlight the importance of higher-order conditioning protocols when trying to understand complex behaviors in simple (laboratory) environments and simple behaviors in complex (everyday) environments. [ABSTRACT FROM AUTHOR]

Highlights: [•] Orbitofrontal cortex (OFC) is critical to goal-directed behavior and learning. [•] Learning theory paradigms reveal a unique for OFC in signaling specific outcomes. [•] Learning theory a powerful tool in understanding prefrontal and neural function. [Copyright &y& Elsevier]

According to the temporal-coding hypothesis (TCH; Savastano & Miller, Behavioural Processes 44:147–162, 1998 ), acquired associations include temporal information concerning the interval between the associated elements. Moreover, the TCH posits that subjects can integrate two independently acquired associations that share a common element (e.g., S2–S1 and S1–US), which results in the creation of a third association with its own temporal relationship (S2–US). Some evidence has suggested that such temporal integration occurs at the time of testing (Molet, Miguez, Cham, & Miller, Journal of Experimental Psychology: Animal Behavior Processes 38:369–380, 2012 ). Here we report two fear-conditioning experiments with rats conducted to identify the associative structure of the integrated temporal relationship. The goal was to distinguish between two possible associative structures that could exist following an initial test on which temporal integration occurs: (1) Conditioned responding to S2 on subsequent tests could be the result of recurring successive activation of two independently learned temporal maps that remain independently stored in memory (i.e., S2–S1 plus S1–US). (2) Temporal integration at the moment of initial testing could result in the formation of a direct S2–US (or S2–response) temporal map. Integration was found to occur at test and to produce a new association that was independent of associations with the common element (S1). However, the associative status of S1 appeared to modulate whether or not the new association with S2 was US-specific (S2–US) or directly activated a fear response (S2–response). [ABSTRACT FROM AUTHOR]

Rationale: Addiction is characterized by maladaptive decision-making, in which individuals seem unable to use adverse outcomes to modify their behavior. Adverse outcomes are often infrequent, delayed, and even rare events, especially when compared to the reliable rewarding drug-associated outcomes. As a result, recognizing and using information about their occurrence put a premium on the operation of so-called model-based systems of behavioral control, which allow one to mentally simulate outcomes of different courses of action based on knowledge of the underlying associative structure of the environment. This suggests that addiction may reflect, in part, drug-induced dysfunction in these systems. Here, we tested this hypothesis. Objectives: This study aimed to test whether cocaine causes deficits in model-based behavior and learning independent of requirements for response inhibition or perception of costs or punishment. Methods: We trained rats to self-administer sucrose or cocaine for 2 weeks. Four weeks later, the rats began training on a sensory preconditioning and inferred value blocking task. Like devaluation, normal performance on this task requires representations of the underlying task structure; however, unlike devaluation, it does not require either response inhibition or adapting behavior to reflect aversive outcomes. Results: Rats trained to self-administer cocaine failed to show conditioned responding or blocking to the preconditioned cue. These deficits were not observed in sucrose-trained rats nor did they reflect any changes in responding to cues paired directly with reward. Conclusions: These results imply that cocaine disrupts the operation of neural circuits that mediate model-based behavioral control. [ABSTRACT FROM AUTHOR]

Abstract: Sensory preconditioning (SPC) is a procedure to demonstrate learning to associate between relatively neutral sensory stimuli in the absence of an external reinforcing stimulus, the underlying neural mechanisms of which have remained obscure. We address basic questions about neural processes underlying SPC, including whether neurons that mediate reward or punishment signals in reinforcement learning participate in association between neutral sensory stimuli. In crickets, we have suggested that octopaminergic (OA-ergic) or dopaminergic (DA-ergic) neurons participate in memory acquisition and retrieval in appetitive or aversive conditioning, respectively. Crickets that had been trained to associate an odor (CS2) with a visual pattern (CS1) (phase 1) and then to associate CS1 with water reward or quinine punishment (phase 2) exhibited a significantly increased or decreased preference for CS2 that had never been paired with the US, demonstrating successful SPC. Injection of an OA or DA receptor antagonist at different phases of the SPC training and testing showed that OA-ergic or DA-ergic neurons do not participate in learning of CS2–CS1 association in phase 1, but that OA-ergic neurons participate in learning in phase 2 and memory retrieval after appetitive SPC training. We also obtained evidence suggesting that association between CS2 and US, which should underlie conditioned response of crickets to CS2, is formed in phase 2, contrary to the standard theory of SPC assuming that it occurs in the final test. We propose models of SPC to account for these findings, by extending our model of classical conditioning. [Copyright &y& Elsevier]

Are humans unique in their ability to interpret exogenous events as causes? We addressed this question by observing the behavior of rats for indications of causal learning. Within an operant motor-sensory preconditioning paradigm, associative surgical techniques revealed that rats attempted to control an outcome (i.e., a potential effect) by manipulating a potential exogenous cause (i.e., an intervention). Rats were able to generate an innocuous auditory stimulus. This stimulus was then paired with an aversive stimulus. The animals subsequently avoided potential generation of the predictive cue, but not if the aversive stimulus was subsequently devalued or the predictive cue was extinguished (Exp. 1). In Experiment 2, we demonstrated that the aversive stimulus we used was in fact aversive, that it was subject to devaluation, that the cue-aversive stimulus pairings did make the cue a conditioned stimulus, and that the cue was subject to extinction. In Experiments 3 and 4, we established that the decrease in leverpressing observed in Experiment 1 was goal-directed instrumental behavior rather than purely a product of Pavlovian conditioning. To the extent that interventions suggest causal reasoning, it appears that causal reasoning can be based on associations between contiguous exogenous events. Thus, contiguity appears capable of establishing causal relationships between exogenous events. Our results challenge the widely held view that causal learning is uniquely human, and suggest that causal learning is explicable in an associative framework. [ABSTRACT FROM AUTHOR]

Prior research has found that when subjects independently acquire 2 associations with a common element (e.g., S1-S2 and S2-US), each with its own temporal relationship, they behave as if the 2 unique cues (i.e., S1 and US) have a known temporal relationship despite their never having been paired. This is interpreted as indicative of temporal integration of the memories acquired during Phase 1 and Phase 2 of training based on the element common to both experiences (i.e., S2). There are 2 times at which such integration of independent temporal relationships could plausibly occur: at the time of acquisition of the second relationship or at the time of testing. Three lick-suppression experiments with rats were performed to determine when integration occurs. This question of the moment of temporal integration was assessed by extinguishing the mediating element (S2) between learning of the second temporal relationship and testing of SI. Experiment 1 (using sensory preconditioning) and Experiment 2 (using second-order conditioning) found that this manipulation interfered with behavioral control by SI, suggesting that temporal integration occurred at the time of testing. Experiment 3 used spontaneous recovery, a hallmark phenomenon of extinction, to confirm that the S2-alone presentations in Experiments 1 and 2 attenuated integration as a result of extinction of S2. Implications for the temporal coding hypothesis (e.g., Savastano & Miller, 1998) are discussed. [ABSTRACT FROM AUTHOR]

Abstract: This experiment adapted a sensory preconditioning (SPC) procedure using human participants to determine if conditioning (Cond) to one flavor (the conditioned flavor) will enhance liking for another flavor (the SPC flavor) associated with it prior to training. Participants in one of three groups (N =40 per group) consumed and rated plain or sweetened cherry and grape kool-aids in four phases. In baseline and SPC phase, ratings for a plain cherry, grape, and cherry-grape mixture were similar. In training, one flavor was sweetened (SPC+Cond and Cond Only groups) or unsweetened (SPC Only group) and ratings increased only for the flavor that was sweetened. In test, Group SPC+Cond rated the conditioned flavor and the SPC flavor as more liked and tasting sweeter. Group Cond Only rated only the conditioned flavor as more liked and tasting sweeter. Group SPC Only showed no change in ratings from baseline to test. These are the first data to show SPC learning using a flavor preference paradigm with human participants. [Copyright &y& Elsevier]

Abstract: Lesion studies have suggested that the prefrontal cortex is involved in memory for contextual details surrounding the prior observation of objects or events, but it is unknown whether it is crucial for encoding details about the location at which cues are experienced, or for recall of that information. We used intracranial infusions of the GABAA receptor agonist muscimol in rodents to directly assess the role of the medial prefrontal cortex (mPFC) during incidental encoding and retrieval of information about the location of a cue during a spatial sensory preconditioning procedure. Rats experienced a single, discrete, sensory cue as they explored an open platform, and then were tested after a 24h delay on recollection of the prior location of the cue. Activity in the mPFC was suppressed with muscimol during either encoding or retrieval of the information, with a control group receiving saline infusions before both phases. We found that mPFC suppression during the encoding phase blocked the formation of incidental memory about the cues but mPFC suppression during retrieval had no effect. Moreover, animals with suppressed frontal cortical activity in the encoding phase expressed smaller cue-directed orienting responses, indicating they attended less to the cue. These results suggest that the frontal cortex may be required to sustain attention to incidental cues in order to later recollect the location in which they have been previously experienced, but that once the location information is encoded the frontal cortex is not required for retrieval of that information. [Copyright &y& Elsevier]

Abstract: We previously found that young infants spontaneously associate stimuli that they merely see together. Using a sensory preconditioning paradigm with 6- and 9-month-olds, we asked how long such associations remain latent before being forgotten and what exposure conditions affect their persistence. Groups were preexposed to two puppets for 1h/day for 2 days, 1h on 1 day, or 1h on 1 day in two sessions; 1–27 days later, target actions were modeled on one puppet, and infants were tested with the other puppet 1 day later. The longest delay after which infants imitated the actions on the other puppet defined how long they remembered the association. The data revealed that the preexposure regimen determined retention. Regardless of exposure time, both ages remembered the association longer after two sessions, and younger infants remembered longer than older infants – for 4 weeks – after two 30-min sessions on 1 day. [Copyright &y& Elsevier]

Abstract: Previous animal studies have defined the ability to remember the details of what, where, and when of an event as an episodic-like memory to be used to model episodic memory in humans. Numerous findings indicate that the hippocampal–frontal cortical circuitry plays a major part in its neural mechanism. Researchers have intensively studied roles of diverse hippocampus sub-regions using animal models. By contrast, the impact of prefrontal cortex lesions on episodic-like memory in animals is still unknown. Here we show that Wistar rats with bilateral medial prefrontal cortex lesions failed to use the temporal–contextual information to retrieve memory of a fear-conditioning event, indicating impairments in their episodic-like memory. Subsequent experiments excluded alternative interpretations that the manipulation impaired the fear-conditioning per se, or interfered with the sensory preconditioning process. We concluded that damages in this area might impair temporal information processing, or interfere with integrating temporal and contextual elements of fear-conditioning events to form a conjunctive entity. These findings can help understand how the medial prefrontal cortex contributes to episodic-like memory. [ABSTRACT FROM AUTHOR]

Abstract: The characteristics of memory in infants and adults seem vastly different. The neuromaturational model attributes these differences to an ontogenetic change in the basic memory process, namely, to the hierarchical maturation of two distinct memory systems. The early-maturing (implicit) system is functional during the first third of infancy and supports the gradual learning of perceptual and motor skills; the late-maturing (explicit) system supports representations of contextually specific events, relationships, and associations. An alternative model holds that the basic memory process does not change, but what infants and adults select to encode for learning does. This ontogenetic change in selective attention has been mistaken for an ontogenetic shift in the basic memory process. Over the last 25 years, evidence from transfer studies with developing rats and human infants has revealed that the first third of infancy is actually a period of exuberant learning that ends, not coincidentally, at the same age that the late-maturing memory system presumably emerges. This article reviews data from recent studies of sensory preconditioning, potentiation, associative chains, and transitive inference with human infants that support this conclusion—data for which the neuromaturational model cannot account. Fast mapping is a general learning mechanism that accounts for this evidence. [Copyright &y& Elsevier]

Abstract: We developed a single-trial cue-location memory task in which rats experienced an auditory cue while exploring an environment. They then recalled and avoided the sound origination point after the cue was paired with shock in a separate context. Subjects with medial prefrontal cortical (mPFC) lesions made no such avoidance response, but both lesioned and control subjects avoided the cue itself when presented at test. A follow up assessment revealed no spatial learning impairment in either group. These findings suggest that the rodent mPFC is required for incidental learning or recollection of the location at which a discrete cue occurred, but is not required for cue recognition or for allocentric spatial memory. [Copyright &y& Elsevier]

Abstract: In avoidance learning studies, a warning signal (Sd) is directly paired with an aversive unconditioned stimulus (US) unless a particular response is performed. Research by Augustson and Dougher (1997) demonstrated that not only warning signals but also stimuli that are indirectly paired with the aversive event are capable of affecting the avoidance response. In the present study, we extended these results by incorporating a sensory preconditioning paradigm in an avoidance learning procedure. Sensory preconditioning training influenced the selection of avoidance responses in the presence of warning signals that were never paired directly with the aversive events. Moreover, results suggest that the selection of the avoidance responses was based on an integration of knowledge about the relation between the Sd and the US (S-S knowledge) and knowledge about the relation between the avoidance response and the US (R-S knowledge). As the expectancy-based theory of Lovibond (2006) is the only theory of avoidance that incorporates both knowledge structures, the results provide unique support for this theory. [Copyright &y& Elsevier]

How the memory of adults evolves from the memory abilities of infants is a central problem in cognitive development. The popular solution holds that the multiple memory systems of adults mature at different rates during infancy. The early-maturing system (implicit or nondeclarative memory) functions automatically from birth, whereas the late-maturing system (explicit or declarative memory) functions intentionally, with awareness, from late in the first year. Data are presented from research on deferred imitation, sensory preconditioning, potentiation, and context for which this solution cannot account and present an alternative model that eschews the need for multiple memory systems. The ecological model of infant memory development (N. E. Spear, 1984) holds that members of all species are perfectly adapted to their niche at each point in ontogeny and exhibit effective, evolutionarily selected solutions to whatever challenges each new niche poses. Because adults and infants occupy different niches, what they perceive, learn, and remember about the same event differs, but their raw capacity to learn and remember does not. [ABSTRACT FROM AUTHOR]

Abstract: Exposure to banana scented salty water produced a preference for that smell in rats tested under a sodium appetite (experiment 1), and exposure to almond scented sweet water produced avoidance of that smell when rats were tested after developing an aversion to the sweet taste (experiment 2). The consolidation of this within-event learning was disrupted when exposure to the solutions were followed by social isolation. These results duplicate the disruptive effect of social isolation on context learning and raise the possibility that within-event learning like context learning may involve the hippocampal formation. [Copyright &y& Elsevier]

One recent perspective (Blundell et al., 2001; 2003; Killcross and Blundell, 2002; Balleine et al. 2003) on the function of the basolateral region of the amygdala (BLA) suggests that it plays an important role in the representation of the sensory features of motivationally significant events. This predicts that lesions of the BLA will not produce a decrement in performance in conditioning procedures based on the formation of associations between the sensory aspects of neutral events but will interfere with conditioning based on associations between neutral cues and motivationally significant events. This prediction is supported by the evidence that BLA lesions were without effect on a sensory preconditioning procedure (experiment 1A) that used neutral cues but that BLA lesions did significantly impair representation-mediated conditioning (experiment 1B)when the target cues were motivationally significant at the time of training. These results demonstrate that animals with lesions of the BLA can represent the sensory aspects of neutral events but not the sensory aspects of motivationally significant events. [ABSTRACT FROM AUTHOR]

The ability of rats with ibotenate lesions of the entorhinal cortex to form memories for events was assessed by using a gustatory within-event learning procedure. Rats first received exposure to 2 events, AX and BY, each composed of a pair of flavors. Following this exposure period, Flavor X alone was paired with the delivery of lithium chloride. Lesioned and control rats showed a greater aversion to A than to B and to AX than to BX. These results challenge theories that suppose that the entorhinal cortex plays a general role in forming representations of patterns of stimulation. [ABSTRACT FROM AUTHOR]

The authors used a touch screen-based visual-search task to investigate spatial integration in pigeons. First, pigeons were presented with a consistent spatial relationship between compound visual landmarks (LMs) A-X and B-Y, separately. Next, pigeons learned to find a hidden goal on the monitor in the presence of LMs A and B. The goal bore a consistent spatial relationship to LM A, but not to LM B. On nonreinforced probe tests, the peak and distribution of responses to LMX suggest that pigeons computed a novel X-goal spatial relationship on the basis of X-A and A-goal spatial vectors. Responses to LM Y, however, revealed no evidence of spatial integration. These results replicate and extend those of A. P. Blaisdell and R. G. Cook (2005) using an open-field task. [ABSTRACT FROM AUTHOR]

Rats with neurotoxic lesions centered in the anterior thalamic nuclei were trained in two versions of a nonspatial, sensory preconditioning procedure. In both versions, two stimulus compounds (AX and BY) were first presented and then X, but not Y, was paired with an aversive unconditioned stimulus. This procedure resulted in greater conditioned responding to A than B. Anterior thalamic lesions had no apparent effect on these two examples of sensory preconditioning, nor did they affect fear conditioning or conditioned taste aversion. In contrast, the same lesions led to a severe deficit on a test of spatial memory. These results help to refine our understanding of the contribution of the anterior thalamic nuclei to spatial memory. [Copyright &y& Elsevier]

Equivalence of flavour cues, each of which had been paired with a common antecedent, was demonstrated with rats in a three-stage design. In the first stage, a group of thirsty rats were given each of two target flavour cues after a common antecedent flavour (X→A and X→B), while a second group of rats were given A and B after differential antecedent flavours (X→A and Y→B). Another group of rats was allowed to drink A and B after familiar tap water. In the second stage, aversion to A was established by a lithium chloride injection after drinking A. The acquired equivalence effect was verified in the third stage by strong aversion to B in the group trained with the common antecedent compared with the remaining two groups. The representation-mediation, rather than response-mediation, hypothesis seems to fit the backward acquired equivalence effect obtained here. [Copyright &y& Elsevier]

Animals seem to learn and use internal models; they learn to anticipate predictable events, and their behavior in the sensory preconditioning paradigm reflects formation of novel associative chains. To investigate possible neural correlates, the temporal difference model (TD model) was extended to an internal model approach. The proposed model learns reward prediction error signals that resemble dopamine neuron activity. In contrast to the original TD model, the reward prediction error signals of the proposed model are influenced by the formation of novel associative chains in the sensory preconditioning experiment. This is consistent with experimental findings, as striatal dopamine concentration is influenced by the formation of novel associative chains in this paradigm. Comparison of the model architecture with biological neural networks suggests that chains of neurons with tonic anticipatory activity may underlie the formation of novel associative chains. These findings suggest that dopamine neuron activity may reflect the processing of an internal model. [ABSTRACT FROM AUTHOR]

Of all frontocortical subregions, the anterior cingulate cortex (ACC) has perhaps the most overlapping theories of function. 1–3 Recording studies in rats, humans, and other primates have reported diverse neural responses that support many theories, 4–12 yet nearly all these studies have in common tasks in which one event reliably predicts another. This leaves open the possibility that ACC represents associative pairing of events, independent of their overt biological significance. Sensory preconditioning 13 provides an opportunity to test this. In the first phase, preconditioning, value-neutral sensory stimuli are paired (A→B). To test whether this was learned, subjects are given standard conditioning during which one of the previously neutral sensory cues is paired with a biologically meaningful outcome (B→outcome). During the final probe test, the neutral cue which was never paired with a biologically meaningful outcome is presented alone (A→) and will elicit a conditional response, suggesting that subjects had learned the associative structure during preconditioning and use that knowledge to infer presentation of the biologically relevant outcome (A→B→outcome). Inference-based responding demonstrates a fundamental property of model-based reasoning 14,15 and requires learning of the associations between neutral stimuli before rewards are introduced. 16–19 ACC neurons developed firing patterns that reflected the learning of sensory associations during preconditioning, even though no rewards were present. The strength of these correlates predicted rats' ability to later mobilize and use that associative information during the probe test. These results demonstrate that clear biological significance is not necessary to produce correlates of learning in ACC. • ACC ensembles signal associations between value-neutral sensory cues • Value-neutral associative correlates predict value-based inference behavior • ACC neurons acquire responses to reward predictive cues • ACC ensembles infer value Hart et al. use single unit recording and behavior to show that rat anterior cingulate ensembles signal the learning of associations between value-neutral sensory cues, as well as inference-based memory retrieval [ABSTRACT FROM AUTHOR]

Previous studies have shown that pipradrol enhances the acquisition of responding with conditioned reinforcement. The present experiments replicated this finding and assessed the possible role of nonspecific stimulus change, feeding in the test environment and procedure consisted of three phases: Operant rates of pressing two levers, one of which produced a 3-s tone, were measured in the pre-exposure phase; the tone was paired with food in the four conditioning sessions; and conditioned reinforcement was demonstrated in the test phase by a relative increase in pressing the tone lever. A group ( N=8) receiving food but no tone during the conditioning phase also increased responding on the tone lever in the test phase and this effect was enhanced in a group ( N=8) receiving pipradrol prior to the test. A group ( N=8) receiving neither tones nor pellets during conditioning and pipradrol prior to test failed to show a change in lever bias, ruling out the possibility that pipradrol produced a nonspecific enhancement of responding for stimulus change. It was hypothesized that the conditioned reinforcement effect seen in the food-alone groups resulted from sensory preconditioning. According to this hypothesis, the tone was conditioned to environmental stimuli during the pre-exposure phase; subsequent presentation of pellets resulted in a learned association between environmental stimuli and food, leading to increased responding for the tone in the test phase. The results of two control experiments supported this view. One group ( N=8) received no tone in the pre-exposure phase, pellets alone in the conditioning phase and then pipradrol prior to test. The second group ( N=8) received pellets alone in the conditioning phase, but under altered environmental stimulus conditions, and then was given pipradrol prior to test. Neither group showed evidence of conditioned reinforcement. It was concluded that pipradrol enhanced acquisition of responding with conditioned reinforcement even if the conditioned reinforcing stimulus was established with sensory preconditioning procedures. [ABSTRACT FROM AUTHOR]