The temporal dynamics of a visual discrimination: The role of stimulus comparison and opponent processes.

The influence of trial spacing on simple conditioning is well established: When successive reinforced conditioned stimulus, CS+, trials are separated by a short interval (massed training), conditioned responding emerges less rapidly than when such trials are separated by longer intervals (spaced training). This study examined the influence of trial spacing on the acquisition of an appetitive visual discrimination in rats. Experiments 1 and 2 established that massed training facilitates the acquisition of such discriminations. The results of subsequent experiments demonstrated that this trial-spacing effect reflects the proximity of nonreinforced, CS-, trials to preceding (Experiment 3) and signaled (Experiment 4) presentations of the reinforcer. Experiment 5 showed that the facilitation of discrimination learning with massed training reflected an effect on learning rather than performance. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Trial spacing and trial distribution effects in Pavlovian conditioning: Contributions of a comparator mechanism.

Two experiments, involving a total of 144 rats, investigated a potential basis for trial spacing and trial distribution effects. In Exp 1, a CS (e.g., CS A) was trained with either massed (e.g., A?→?A?→?A) or spaced (e.g., A A A) trials. When trials were massed, brief exposure to the training context (a condition typical of massed training) impaired responding, whereas more extensive exposure to the context during or after training reduced this apparent massed trials deficit. In Exp 2, different CSs were trained in either a massed (e.g., A?→?A?→?A?→?B?→?B?→?B?→?C?→?C?→?C) or a distributed (e.g., A?→?B?→?C?→?A?→?B?→?C, etc.) manner. Trials massed in this sense resulted in impaired responding to the CS, and this impairment was attenuated by posttraining extinction of the context cues. Thus, trial distribution and apparent trial spacing effects were, at least in part, reversible deficits in performance rather than failures of learning. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Massed extinction trials produce better short-term but worse long-term loss of context conditioned fear responses than spaced trials.

A series of experiments studied the effects of the interval between extinction trials on the loss of context conditioned freezing responses. Rats were shocked in one context (A) but not in another (B) and subjected to extinction trials in context A. In Experiment 1, massed trials produced more rapid loss than spaced trials. A shift from spaced to massed trials maintained this loss, but the shift from massed to spaced trials restored lost responses. Experiments 2-5 examined this effect of massed trials on responding across spaced trials. They provided evidence that (a) a single trial was as effective as multiple daily massed trials, (b) learning occurred on the first of the massed trials but not on later ones, and (c) the first trial reduced the amount learned across subsequent massed trials. Finally, alternating extinction trials in A and B produced more response loss across spaced trials than blocks of trials in A and B. The results were discussed in terms of the role accorded to self-generated priming in the models developed by A. R. Wagner (1978, 1981). (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

A comparator view of Pavlovian and differential inhibition.

In 3 experiments using rats as subjects, the authors varied trial spacing to investigate the conditions under which Pavlovian and differential inhibition are observed. Experiment 1 compared Pavlovian and differential inhibition with spaced training trials. Spaced trials resulted in only the Pavlovian inhibitor passing both summation and retardation tests. Conversely, Experiment 2 compared these 2 types of inhibition with massed training trials. This training resulted in only the differential inhibitor passing both tests for conditioned inhibition. Finally, in Experiment 3 all subjects experienced Pavlovian inhibition training with massed trials. Although this training by itself did not result in behavior indicative of inhibition, subjects that also experienced posttraining extinction of the training context did pass both tests for inhibition. Overall, these results are anticipated by the extended comparator hypothesis (Denniston, Savastano, & Miller, 2001) but are problematic for most contemporary associative learning theories. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Conditioned stimulus familiarity determines effects of MK-801 on fear extinction.

Six experiments studied the role of conditioned stimulus (CS) familiarity in determining the effects of the N-methyl-d-aspartate (NMDA) receptor antagonist MK-801 on fear extinction. Systemic administration of MK-801 (0.1 mg/kg) impaired initial extinction but not reextinction learning. MK-801 impaired reextinction learning when the CS was relatively novel during reextinction training but not initial extinction learning when the CS was relatively familiar during initial extinction training. A context change failed to reinstate the sensitivity of initial fear extinction learning about a relatively familiar CS to MK-801. These experiments show that CS familiarity is an important determinant of effects of MK-801 on fear extinction learning: MK-801 impaired extinction learning about novel stimuli but spared extinction learning about familiar stimuli. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

The effect of yohimbine on the extinction of conditioned fear: A role for context.

Six experiments with rat subjects examined the effect of yohimbine, an alpha-2 adrenergic autoreceptor antagonist, on the extinction of conditioned fear to a tone. Experiments 1 and 2 demonstrated that systemic administration of yohimbine (1.0 mg/kg) facilitated a long-term decrease in freezing after extinction, and this depended on pairing drug administration with extinction training. However, Experiments 3 and 4 demonstrated that yohimbine did not eradicate the original fear learning: Freezing was renewed when the tone was tested outside of the extinction context. Experiments 5 and 6 found that the contextually specific attenuation of fear produced by yohimbine transferred to another extinguished conditional stimulus (CS) and not to a nonextinguished CS. The results suggest that yohimbine, when administered in the presence of a neutral context, creates a form of inhibition in that context that allows that specific context to reduce fear of an extinguished CS. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Normal conditioning inhibition and extinction of freezing and fear-potentiated startle following electrolytic lesions of medial prefrontal cortex in rats.

The authors investigated the role of medial prefrontal cortex (mPFC) in the inhibition of conditioned fear in rats using both Pavlovian extinction and conditioned inhibition paradigms. In Experiment 1, lesions of ventral mPFC did not interfere with conditioned inhibition of the fear-potentiated startle response. In Experiment 2, lesions made after acquisition of fear conditioning did not retard extinction of fear to a visual conditioned stimulus (CS) and did not impair "reinstatement" of fear after unsignaled presentations of the unconditioned stimulus. In Experiment 3, lesions made before fear conditioning did not retard extinction of fear-potentiated startle or freezing to an auditory CS. In both Experiments 2 and 3, extinction of fear to contextual cues was also unaffected by the lesions. These results indicate that ventral mPFC is not essential for the inhibition of fear under a variety of circumstances. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Classical conditioning of proboscis extension in honeybees (Apis mellifera).

Conditioned extension of the proboscis in restrained honeybees with odor as the CS and sucrose solution—delivered to the antenna (to elicit extension of the proboscis) and then to the proboscis itself—as the UCS. In a 1st series of experiments, acquisition was found to be rapid, both in massed and in spaced trials; its associative basis was established by differential conditioning and by an explicitly unpaired control procedure. Both extinction and spontaneous recovery in massed trials were demonstrated. In experiments on the nature of the UCS, eliminating the proboscis component lowered the asymptotic level of performance, whereas eliminating the antennal component was without effect; reducing the concentration of sucrose from 20% to 7% slowed acquisition but did not lower the asymptotic level of performance; and second-order conditioning was demonstrated. In experiments on the role of the UCS, an omission contingency designed to eliminate adventitious response-reinforcer contiguity was found to have no adverse effect on acquisition. In experiments designed to analyze the resistance to acquisition found after explicitly unpaired training in the 1st experiments, no significant effect was found of prior exposure either to the CS alone or to the UCS alone, although the unpaired procedure again produced resistance that was shown to be due to inhibition rather than to inattention; extinction after paired training was found to be facilitated by unpaired presentations of the UCS. (33 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Distributed practice in verbal recall tasks: A review and quantitative synthesis.

The authors performed a meta-analysis of the distributed practice effect to illuminate the effects of temporal variables that have been neglected in previous reviews. This review found 839 assessments of distributed practice in 317 experiments located in 184 articles. Effects of spacing (consecutive massed presentations vs. spaced learning episodes) and lag (less spaced vs. more spaced learning episodes) were examined, as were expanding interstudy interval (ISI) effects. Analyses suggest that ISI and retention interval operate jointly to affect final-test retention; specifically, the ISI producing maximal retention increased as retention interval increased. Areas needing future research and theoretical implications are discussed. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Temporal Specificity of Extinction in Autoshaping.

Three experiments investigated the effects of varying the conditioned stimulus (CS) duration between training and extinction. Ring doves (Streptopelia risoria) were autoshaped on a fixed CS-unconditioned stimulus (US) interval and extinguished with CS presentations that were longer, shorter, or the same as the training duration. During a subsequent test session, the training CS duration was reintroduced. Results suggest that the cessation of responding during an extinction session is controlled by generalization of excitation between the training and extinction CSs and by the number of nonreinforced CS presentations. Transfer of extinction to the training CS is controlled by the similarity between the extinction and training CSs. Extinction learning is temporally specific. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Effects of variable encoding and spaced presentations on vocabulary learning.

I examined the applicability of the encoding variability hypothesis and the spacing phenomenon to vocabulary learning in five experiments. I manipulated encoding variability by varying the number of potential retrieval routes to the word meanings, using a one-sentence context condition, a three-sentence context condition, and a no-context (definitions-only) control condition. I evaluated the spacing effect by presenting each word with or without intervening words. The results provided no evidence that the opportunity to establish multiple retrieval routes by means of contextual information is helpful to vocabulary learning, a conclusion supported unequivocally by all five experiments. By contrast, spaced presentations yielded substantially higher levels of learning than did massed presentations. I discuss the results largely in terms of educational concerns, including the utility of the learning-from-context approach to vocabulary learning. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

The effects of assessment type on verbal ratings of conditional stimulus valence and contingency judgments: Implications for the extinction of evaluative learning.

Recent research on causal learning found (a) that causal judgments reflect either the current predictive value of a conditional stimulus (CS) or an integration across the experimental contingencies used in the entire experiment and (b) that postexperimental judgments, rather than the CS's current predictive value, are likely to reflect this integration. In the current study, the authors examined whether verbal valence ratings were subject to similar integration. Assessments of stimulus valence and contingencies responded similarly to variations of reporting requirements, contingency reversal, and extinction, reflecting either current or integrated values. However, affective learning required more trials to reflect a contingency change than did contingency judgments. The integration of valence assessments across training and the fact that affective learning is slow to reflect contingency changes can provide an alternative interpretation for researchers' previous failures to find an effect of extinction training on verbal reports of CS valence. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Cocaine enhances the expression of fear-potentiated startle: Evaluation of state-dependent extinction and the shock-sensitization of acoustic startle.

Cocaine's effects on fear extinction and on the shock-sensitization of acoustic startle were examined. Following fear acquisition, rats exposed to the nonreinforced CS after cocaine administration demonstrated significant levels of fear-potentiated startle when evaluated in the drug-free state. The CS also increased startle amplitudes in subjects extinguished and tested with cocaine, indicating that mechanisms other than state-dependent learning are involved in the extinction deficit. The presentation of 10 footshocks augmented acoustic startle, and the shock enhancement was unaffected by cocaine preexposure. These data indicate that the aversive consequences of footshock relevant to the acquisition of conditional fear are not sensitized by the drug. It was suggested that cocaine reinforces fear responding to a threatening stimulus. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Functional MRI of human amygdala activity during Pavlovian fear conditioning: Stimulus processing versus response expression.

Although laboratory animal studies have shown that the amygdala plays multiple roles in conditional fear, less is known about the human amygdala. Human subjects were trained in a Pavlovian fear conditioning paradigm during functional magnetic resonance imaging (fMRI). Brain activity maps correlated with reference waveforms representing the temporal pattern of visual conditional stimuli (CSs) and subject-derived autonomic responses were compared. Subjects receiving paired CS-shock presentations showed greater amygdala activity than subjects receiving unpaired CS-shock presentations when their brain activity was correlated with a waveform generated from their behavioral responses. Stimulus-based waveforms revealed learning differences in the visual cortex, but not in the amygdala. These data support the view that the amygdala is important for the expression of learned behavioral responses during Pavlovian fear conditioning. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

The effects of neurotoxic hippocampal lesions on two effects of context after fear extinction.

Three conditioned suppression experiments with rats examined the role of the hippocampus in 2 effects of context after extinction. Reinstatement is the context-specific recovery of fear to an extinguished conditioned stimulus (CS) that occurs following independent presentations of the unconditioned stimulus (UCS), after extinction. Renewal is the recovery of fear when the CS is presented in the context in which it was conditioned, after extinction in a different context. Results indicated that neurotoxic lesions of the hippocampus, performed before conditioning, abolished reinstatement, which depends on context–UCS associations, but not renewal, which does not. This dissociation is not the result of differences in the recentness of context learning that ordinarily governs the 2 effects. The results suggest that the hippocampus is necessary for some, but not all, types of contextual learning. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Central CRF receptor antagonist α-helical CRF9-41 blocks reinstatement of extinguished fear: The role of the bed nucleus of the stria terminalis.

The present experiments assessed the necessity of central CRF in reinstatement of extinguished fear. Using the fear-potentiated startle procedure, rats were given light-shock pairings (fear conditioning) followed by light-alone extinction training. Rats were then given unsignaled shocks to reinstate fear to the light conditioned stimulus (CS). Intracerebroventricular administration of the CRF antagonist α-Helical CRF9-41 prior to reinstatement training dose-dependently prevented reinstatement. Further, α-Helical CRF9-41 administration prior to reinstatement training or the test for reinstatement of fear to the extinguished CS prevented reinstatement at both treatment times, suggesting that CRF activity is critical for this type of return of fear to an extinguished CS. The abolition of reinstatement by drug administration was not due to state-dependent learning, as rats treated with the drug prior to both reinstatement training or testing also failed α-Helical CRF9-41 in the bed nucleus of the stria terminalis suggested that this area is a site at which central CRF is involved in this form of relapse. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Differential effects of forward or simultaneous conditioned stimulus-unconditioned stimulus intervals on the defensive behavior system of the Norway rat (Rattus norvegicus).

To test several predictions derived from a behavior-systems approach, the authors assessed Pavlovian fear conditioning in rats after 30 trials of forward, simultaneous, or unpaired training. Direct evidence of conditioned fear was collected through observation of flight and freezing reactions during presentations of the conditioned stimulus (CS) alone. The authors also tested the CS's potential to reinforce an instrumental escape response in an escape-from-fear paradigm. On the one hand, rats that received forward training showed conditioned freezing, but no conditioned flight was observed. On the other hand, rats that received simultaneous training showed conditioned flight, but no conditioned freezing was observed. Rats that received either forward or simultaneous pairings showed instrumental learning of the escape-from-fear response. Implications for several theories of Pavlovian conditioning are discussed. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Rehearsal strategies can enlarge or diminish the spacing effect: Pure versus mixed lists and encoding strategy.

Using 5 experiments, the authors explored the dependency of spacing effects on rehearsal patterns. Encouraging rehearsal borrowing produced opposing effects on mixed lists (containing both spaced and massed repetitions) and pure lists (containing only one or the other), magnifying spacing effects on mixed lists but diminishing spacing effects on pure lists. Rehearsing with borrowing produced large spacing effects on mixed lists but not on pure lists for both free recall (Experiment 1) and recognition (Experiment 2). In contrast, rehearsing only the currently visible item produced spacing effects on both mixed lists and pure lists in free recall (Experiment 3) and recognition (Experiment 4). Experiment 5 demonstrated these effects using a fully within-subjects design. Rehearse-aloud protocols showed that rehearsal borrowing redistributed study from massed to spaced items on mixed lists, especially during massed presentations. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Trial spacing is a determinant of cue interaction.

Four conditioned lick suppression experiments with rats examined the effect of trial spacing on cue interaction. Experiments 1 and 2 found overshadowing to be eliminated with massed compound stimulus- outcome pairings and the usual trial spacing effect to be reversed with compound acquisition trials. Experiment 3 found that whether acquisition compound- outcome pairings were massed or spaced determined the effect of posttraining extinction treatment. Extinction of the overshadowing cue reduced responding following massed training and increased responding following spaced training. Extinction of the context decreased responding following massed training. Experiment 4 found the conditioning and devaluation results to be associative and stimulus specific. These results are in accord with the extended comparator hypothesis (J. C. Denniston, H. I. Savastano, & R. R. Miller, 2001). (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Conditioning with masked stimuli affects the timecourse of skin conductance responses.

In Pavlovian fear conditioning, an aversive unconditional stimulus (UCS) is repeatedly paired with a neutral conditional stimulus (CS). As a consequence, the subject begins to show conditional responses (CRs) to the CS that indicate expectation and fear. There are currently two general models competing to explain the role of subjective awareness in fear conditioning. Proponents of the single-process model assert that a single propositional learning process mediates CR expression and UCS expectancy. Proponents of a dual-process model assert that these behavioral responses are expressions of two independent learning processes. We used backward masking to block perception of our visual CSs and measured the effect of this training on subsequent unmasked performance. In two separate experiments we show a dissociation between CR expression and UCS expectancy following differential delay conditioning with masked CSs. In Experiment I, we show that masked training facilitates CR expression when the same CSs are presented during a subsequent unmasked reacquisition task. In Experiment II we show that masked training retards learning when the CS+ is presented as part of a compound CS during a subsequent unmasked blocking task. Our results suggest that multiple memory systems operate in a parallel, independent manner to encode emotional memories. (PsycINFO Database Record (c) 2010 APA, all rights reserved)