Temporally specific extinction of conditioned responses in the rabbit (Oryctolagus cuniculus) nictitating membrane preparation.

Extinguishing a conditioned response (CR) has entailed separating the conditioned stimulus (CS) from the unconditioned stimulus (US). This research reveals that elimination of the rabbit (Oryctolagus cuniculus) nictitating membrane response occurred during continuous CS-US pairings. Initial training contained a mixture of 2 CS-US interstimulus intervals (ISIs): 200 ms and 1,200 ms. The CRs showed double peaks, one for each ISI. When 1 ISI was removed, its CR peak showed the hallmarks of extinction: a decline across sessions, spontaneous recovery between sessions, and rapid reacquisition when the absent ISI was reintroduced. These results support real-time models of conditioning that segment the CS into microstimuli while challenging theories that rely on contextual control, US representations, CS processing, and response inhibition. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Time-specific extinction and recovery of the rabbit's (Oryctolagus cuniculus) conditioned nictitating membrane response using mixed interstimulus intervals.

Extinguishing a conditioned response (CR) has entailed separating the conditioned stimulus (CS) from the unconditioned stimulus (US). This research reveals that elimination of the rabbit nictitating membrane response occurred during continuous CS-US pairings. Initial training contained a mixture of 2 CS-US interstimulus intervals (ISIs), 150 ms and 500 ms. The CRs showed double peaks, one for each ISI. When the 150-ms ISI was removed, its CR peak showed 2 hallmarks of extinction: a decline across sessions and spontaneous recovery between sessions. When a further stage of training was introduced with a distinctive CS using the 150-ms ISI, occasional tests of the original, extinguished CS revealed another hallmark of extinction, specifically, strong recovery of the 150-ms peak. These results support both abstract and cerebellar models of conditioning that encode the CS into a cascade of microstimuli, while challenging theories of extinction that rely on changes in CS processing, US representations, and contextual control. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

The CS–US interval and taste aversion learning: A brief look.

Temporal parameters of taste aversion learning are known to differ markedly from other learning paradigms in that acquisition occurs despite lengthy delays between exposure to conditioned (CS) and unconditioned stimulus (US). Far less consideration has been paid to very brief CS-US intervals and the possibility that this learning may also be distinguished by an ineffectiveness of close temporal contiguity between CS and US. The effectiveness of a very brief CS-US interval (10 s) was compared with that of 2 lengthier intervals (15 and 30 min). Temporal control of CS delivery (0.15% saccharin solution) into the oral cavity and US delivery (7.5 mg/kg apomorphine hydrochloride) into circulation involved infusion pumps and indwelling catheters. Using a 1-trial learning paradigm, CS-US delays of 15 and 30 min led to significant aversions whereas the 10-s CS-US interval did not, suggesting that close temporal contiguity between CS and US is neither necessary nor sufficient for conditioned taste aversion acquisition. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Comparative electrophysiologic findings between responders and nonresponders to class III antiarrhythmic drugs among patients with ventricular tachyarrhythmia

Rabbits received conditional discrimination training using contextual stimuli to set the occasion for stimulus pairings during eyelid conditioning. Specifically, animals were exposed to either the presence or the absence of an oscillating chamber light throughout the intertrial interval (50 +/- 10 s). For half the animals, this light signaled paired presentations of a discrete tone conditioned stimulus (CS) and air puff unconditioned stimulus (US) while darkness signaled presentations of only the tone CS. The remaining animals experienced the opposite contextual relationship to the conditioning stimuli. These trial types occurred pseudo-randomly across a session, with all transitions between contextual settings (i.e., light or dark) taking place immediately at the CS-US offset. Under these conditions, animals successfully utilized the contextual stimuli as conditional cues for differential responding to the shared CS. Moreover, both light and dark were equally effective as discriminative stimuli. A subset of animals received further training in which the contextual contingency was removed by restricting all conditioning to the CS-alone context. Without the contingency in place, subsequent CS presentations (paired and CS-alone) evoked equivalent conditioned responding across three sessions of training. Following the reinstatement of the contextual contingencies, discriminatory responding was immediately observed and returned to previous levels within three sessions. Finally, animals appeared to use the static representation of the conditional cue, rather than the phasic transition between cues, for discriminatory responding. These findings are discussed in terms of current neurobiological models of eyelid conditioning.     

Amount of training and cue-evoked taste-reactivity responding in reinforcer devaluation.

In two experiments, rats received minimal (16) pairings of one auditory conditioned stimulus (CS) cue with a sucrose reinforcer, and extensive (112) pairings of another auditory CS with that reinforcer. After sucrose was devalued by pairing it with lithium chloride in some rats (Devalue groups) but not others (Maintain groups), taste reactivity (TR) and other responses to unflavored water were assessed in the presence of the auditory CSs alone. The minimally trained CS controlled substantially more evaluative TR responses than the extensively trained CS. Those TR responses were hedonic (positive) in the Maintain groups, but aversive (negative) in the Devalue groups. By contrast, food cup entry and other responses thought not to reflect evaluative taste processing were controlled more by the extensively trained cue. These responses were reduced by sucrose devaluation comparably, regardless of the amount of training. The results suggest rapid changes in the content of learning as conditioning proceeds. Early in training, CSs may be capable of activating preevaluative processing of an absent food reinforcer that includes information about its palatability, but that capability is lost as training proceeds. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Is the number of trials a primary determinant of conditioned responding?

Acquisition of conditioned responding is thought to be determined by the number of pairings of a conditioned stimulus (CS) and an unconditioned stimulus (US). However, it is possible that acquisition is primarily determined not by the number of trials but rather by quantities that often correlate with the number of trials, such as cumulative intertrial interval (ITI) and the number of sessions. Four experiments examined whether the number of trials has an effect on acquisition of conditioned responding, once cumulative ITI and number of sessions are equated. Results of the experiments with rats and mice favor the hypothesis that over an eightfold range, variation in number of CS-US pairings has little effect. It is suggested that learning curves might more accurately be plotted across cumulative ITI or number of sessions, and not across number of trials. Results pose a challenge to trial-centered accounts of conditioning, as demonstrated by simulations of the Rescorla-Wagner model, a simplified version of Wagner's standard operating procedure model (SOP), and Stout & Miller's sometimes competing retrieval model (SOCR). A time-centered account, rate estimation theory (RET), predicts the main finding but has trouble with other aspects of the learning process more easily accommodated by trial-centered models. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Motivational Control of Second-Order Conditioning.

Two experiments examined the motivational specificity of the associations that support 2nd-order conditioning. In the 1st phase of each experiment rats were exposed to 2 visual conditioned stimuli (CSs) paired with either a saline or food pellet unconditioned stimulus (US) prior to exposure to 2nd-order conditioning using 2 auditory CSs, 1 paired with each visual CS. Rats' motivational state was then shifted prior to a test such that if and only if specific motivational features of the 1st-order training US played a role in the 2nd-order associative structure would responding to the 2nd-order cues shift appropriately with the state change. Even when the US was irrelevant to the training motivational state, shifts in state revealed that it was encoded within the associative structure supporting 2nd-order responding. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Associative effects of US preexposure: Modulation of conditioned responding by an excitatory training context.

Conducted 2 experiments with 120 naive Sprague-Dawley rats to examine factors that contribute to retarded emergence of conditioned responding to a conditioned stimulus/stimuli (CS) trained in a context in which unsignaled unconditioned stimulus/stimuli (UCS) had previously been administered. In both experiments, water-deprived Ss were used in a conditioned lick suppression task to measure the conditioned response (CR) elicitation potential of the CS and the training context. From Exp I, it was determined that nonreinforced exposure to the excitatory context after UCS preexposure and prior to CS–UCS pairings in that context eliminated the CR deficit observed on a subsequent test of the CS. From Exp II, it was determined that the recovery induced by contextual deflation after CS training was specific to deflation of the context in which the CS was trained as opposed to another excitatory context. (28 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Temporal specificity of fear conditioning: Effects of different conditioned stimulus--unconditioned stimulus intervals on the fear-potentiated startle effect.

Separate groups of rats were given 30 pairings of a light (conditioned stimulus, CS) and a 500-ms shock (unconditioned stimulus, US) at CS–US intervals of 0, 25, 50, 100, 200, 800, 3,200, 12,800, or 51,200 ms. Other groups had lights and shocks inconsistently paired. The startle reflex was elicited 2–4 days later with a noise burst alone or 25–51,200 ms after light onset. After CS–US pairings over a range of intervals (25–51,200 ms), startle was potentiated in testing, as rapidly as 50 ms after light onset. Magnitude of potentiation and resistance to extinction were generally greater with longer CS–US intervals. In several groups, potentiation was maximal at a test interval that matched the CS–US interval used in training. This temporal specificity sharpened with increasing numbers of training trials but even occurred with a single training trial in which a 51,200-ms CS–US interval was used. Data indicate that even during simple fear conditioning (FC), animals rapidly learn a temporal CS–US relationship. This has implications for understanding the neural mechanisms of FC. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Conditioning-specific modification of the rabbit's unconditioned nictitating membrane response.

Robust classical conditioning modifies responding to the unconditioned stimulus (US) in the absence of the conditioned stimulus (CS), a phenomenon the researchers called conditioning-specific reflex modification. Unconditioned responses (URs) to periorbital stimulation varying in intensity and duration were assessed before and after 1, 3, or 6 days of paired, explicitly unpaired, or no presentations of tone and electrical stimulation. After 3 days of pairings, conditioned responding (CRs) reached 94%, and there was an increase in latency to the peak of URs. The peak latency increase was replicated in a 2nd experiment where rabbits reached asymptotic conditioning during 6 days of pairings. There was also a conditioning-specific increase in the amplitude of URs. There were no UR changes as a function of low level of CRs following 1 day of pairings. Data suggest that there are learning-specific changes in pathways mediating the US/UR, as well as in those mediating the CS/CR. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

A Role for CS-US Contingency in Pavlovian Conditioning.

Two experiments evaluated the role of conditioned stimulus-unconditioned stimulus (CS-US) contingency in appetitive Pavlovian conditioning in rats. In both experiments, some groups received a positively contingent CS signaling an increased likelihood of the US relative to the absence of the CS. These groups were compared with control treatments in which the likelihood of the US was the same in the presence and absence of the CS. A trial marker served as a trial context. Experiment 1 found contingency sensitivity. There was a reciprocal relationship between responding to the CS and the trial marker. Experiment 2 showed that this result was not stimulus or response specific. These results are consistent with associative explanations and the idea that rats are sensitive to CS-US contingency. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Mechanisms underlying retarded emergence of conditioned responding following inhibitory training: Evidence for the comparator hypothesis.

The comparator hypothesis posits that conditioned responding is determined by a comparison at the time of testing between the associative strengths of the conditioned stimulus/stimuli (CS) and stimuli proximal to the CS at the time of conditioning. The hypothesis treats all associations as being excitatory and treats conditioned inhibition as the behavioral consequence of a CS that is less excitatory than its comparator stimuli. Conditioned lick suppression by rats was used to differentiate 4 possible sources of retarded responding to an inhibitory CS. These include habituation to the unconditioned stimulus/stimuli (UCS), latent inhibition to the CS, blocking of the CS-UCS association by the conditioning context, and enhanced excitatory associations to the comparator stimuli. Prior research has demonstrated the 1st 3 phenomena. Therefore, we employed parameters expected to highlight the 4th one—the comparator process. In Exp I, our negative contingency training produced a conditioned inhibitor that passed inhibitory summation and retardation tests. In Exp II we found transfer of retardation from an inhibitory CS to a novel stimulus when the location where retardation-test training occurred was excitatory. In Exp III, extinction of the conditioning context attenuated retardation regardless of whether extinction occurred before or after the CS-UCS pairings of the retardation test. Exp IV demonstrated that habituation to the UCS did not contribute to retardation in the present case. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Acquisition of conditioned associations in Hermissenda: Additive effects of contiguity and the forward interstimulus interval.

Examined conditioned suppression of photokinesis (CSPK) by the marine mollusc in 3 experiments. In each experiment, groups of Ss received light (conditioned stimulus, CS) paired with high-speed orbital rotation (unconditioned stimulus, UCS), light and rotation explicitly unpaired, or no exposure to these stimuli. 24 hrs after training, all Ss were tested for CSPK in the presence of the light. 50 CS–UCS pairings resulted in a marginal CSPK, whereas 100 and 150 pairings produced strong CSPK. In Exp 2, delay between CS onset and UCS onset was varied between 1 and 10 s. The 10-s interstimulus interval (ISI) did not support conditioning, whereas 1-s and 2-s ISIs were effective. In Exp 3, CS–UCS pairings in which the CS preceded the onset of the UCS and ended with the offset of the UCS evoked stronger CSPK than either a CS that preceded the UCS and ended with its onset or a CS that was paired in simultaneous compound with the UCS. CS–UCS contiguity and the forward ISI act additively to establish the CS–UCS association. No differences were observed between groups that were untreated and that received the CS and UCS unpaired. Similarities are noted in the temporal characteristics of associative learning in these Ss and vertebrate species. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Timed excitatory conditioning under zero and negative contingencies.

Rats (rattus norvegicus) anticipated the arrival of a food pellet unconditioned stimulus (US) even when the conditioned stimulus (CS) signaled no overall change or a substantial decrease in the overall rate of US occurrence. Pellet USs were scheduled probabilistically in the intertrial interval at either an equivalent rate (Experiment 1) or a four times higher rate (Experiments 2 and 3) than in the CS, which included one fixed-time target US. Conditioning has been said to involve learning "whether" (contingency) the CS signals a change in the US, and if so, "when" (contiguity) the US is scheduled to arrive. Our results suggest that "when" trumps "whether," challenging the received view that a positive CS-US contingency is necessary for successful conditioning. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Odor-guided fear conditioning in rats: 1. Acquisition, retention, and latent inhibition.

Three experiments examined the acquisition, retention, and latent inhibition of odor-guided fear conditioning in rats. The results of Experiment 1 indicate that forward conditioned stimulus (CS)–unconditioned stimulus (US) pairings resulted in robust freezing responses to subsequent presentation of the CS alone. In Experiment 2, rats in one group (PRE) received unreinforced preexposures to the odorant CS, and those in a second group (NON) were not preexposed to the odorant. All rats then received forward CS–US pairings. PRE rats exhibited a marked attenuation of freezing to subsequent exposure to the CS relative to NON rats. All rats were then retested at one of the following posttraining delays: 17, 24, or 31 days. Freezing behavior of the NON rats declined significantly across these delays, whereas rats in the PRE group froze no more at any delay than they had 24 hr after training. Experiment 3 examined the contextual specificity of latent inhibition. Only those rats that were preexposed and were trained in the same context exhibited latent inhibition. These results indicate that odor-guided fear conditioning is a robust and useful paradigm suitable for future studies of the neural bases of associative learning. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Classical conditioning with electrical stimulation of cerebellum as both conditioned and unconditioned stimulus.

Stimulating electrodes were implanted in rabbit cerebellum, providing an electrical conditioned stimulus (CS) activating cortical parallel fibers and thence Purkinje and other cells, and an electrical unconditioned stimulus (US) activating underlying white matter and eliciting unconditioned responses. Paired CS-US presentations led to the development of conditioned responses, which showed extinction following CS-alone trials and reacquisition with significant savings on reinstatement of paired trials. Increased local excitability as a result of paired training (but not following unpaired stimulus presentations) was observed in cerebellar cortex, as manifested in substantial decreases in CS threshold for response elicitation in all subjects. This preparation offers a model for the study of plastic neuronal interactions within cerebellar networks critically involved in associative learning. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Single Neurons in the Dentate Gyrus and CA1 of the Hippocampus Exhibit Inverse Patterns of Encoding During Trace Fear Conditioning.

Trace fear conditioning is a hippocampus-dependent learning task that requires the association of an auditory conditioned stimulus (CS) and a shock unconditioned stimulus (US) that are separated by a 20-s trace interval. Single-neuron activity was recorded simultaneously from the dentate gyrus (DG) and CA1 of rats during unpaired pseudoconditioning and subsequent trace fear conditioning. Single neurons in DG showed a progressive increase in learning-related activity to the CS and US across trace fear conditioning. Single neurons in CA1 showed an early increase in responding to the CS, which developed into a decrease in firing later in trace conditioning. Correlation analyses showed that DG and CA1 units exhibit inverse patterns of responding to the CS during trace fear conditioning. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

The role of contingency in classical conditioning.

The assumption that classical conditioning depends on a contingent relation between the conditioned stimulus (CS) and the unconditioned stimulus (US), which was proposed some decades ago as an alternative to the traditional contiguity assumption, still is widely accepted as an empirical generalization, if no longer as a theoretical postulate. The first support for the contingency assumption was provided by experiments in which occasional CS–US pairings produced no response to the CS in random training—i.e., training in which the probability of the US was the same in the presence and absence of the CS. Those early experiments, the results of which too often are taken at face value, are reconsidered along with various later experiments that show conditioning, both of the CS and its context, in random training. The evidence suggests that CS–US contingency is neither necessary nor sufficient for conditioning and that the concept has long outlived any usefulness it may once have had in the analysis of conditioning. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Opioid Receptors Regulate the Extinction of Pavlovian Fear Conditioning.

Rats received a single pairing of an auditory conditioned stimulus (CS) with a footshock unconditioned stimulus (US). The fear (freezing) that had accrued to the CS was then extinguished. Injection of naloxone prior to this extinction significantly impaired the development of extinction. This impairment was mediated by opioid receptors in the brain and was not observed when naloxone was injected after extinction training. Finally, an injection of naloxone on test failed to reinstate extinguished responding that had already accrued to the CS. These experiments show that opioid receptors regulate the development, but not the expression, of fear extinction and are discussed with reference to the roles of opioid receptors in US processing, memory, and appetitive motivation. (PsycINFO Database Record (c) 2010 APA, all rights reserved)