Intertrial interval effects in classically conditioned fear to a discrete conditioned stimulus and to situational cues.

In Exp I fear was classically conditioned in a total of 240 female hooded and Sprague-Dawley rats with a discrete conditioned stimulus (CS) at intertrial intervals (ITIs) of 15, 45, 75, 105, 165, or 225 sec and in Exp II, with an additional 144 Ss, with or without a discrete CS at ITIs of 165, 225, or 285 sec. The amount of fear conditioned to situational cues and to the discrete CS plus situational cues was then measured by the learning of a hurdle-jumping response which allowed escape from the fear-eliciting stimuli. Results suggest that as ITI was lengthened fear conditioned to situational cues alone and to the discrete CS increased. However, following conditioning with a discrete CS, fear elicited by situational cues increased with ITI but then decreased, presumably because a discrimination had been formed between the situational cues and the compound of CS plus situational cues. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Mechanisms of second-order conditioning with a backward conditioned stimulus.

Five conditioned suppression experiments with rats examined the conditions under which backward pairings endow a first-order conditioned stimulus (CSI) with the ability to serve as a secondary reinforcer. Experiments 2-5B found evidence for excitatory second-order conditioning (SOC) if, during first-order pairings, the US-CS I interval was 0 s rather than 3 s. Levels of SOC were comparable after forward and backward pairings (Experiments 1-3), and were unaffected by extinction of CS I after SOC (Experiment 3). These results suggest that forward and backward CSIs support SOC for the same reason, and they call into question the need to invoke any special mechanism such as memory integration. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

When pentobarbital is the conditioned stimulus and amphetamine is the unconditioned stimulus, conditioning depends on the type of conditioned response.

Injections of drugs into rats were used as conditioned stimuli (CSs) and as unconditioned stimuli (UCSs). With heart rate (HR) conditioning, the pentobarbital CS produces a higher HR than under control conditions. With avfail (aversion failure) conditioning, the pentobarbital CS loses much of its capacity to induce a conditioned taste aversion. HR conditioning was obtained with forward delays of up to 30 min and backward delays of up to 270 min, where the delays are defined by the interinjection interval. Avfail was obtained with forward delays of up to 270 min but not with backward delays. Neither HR conditioning nor avfail were context specific but could be demonstrated in a test apparatus after pairings that occurred in the home cage. This indicated that the external environment was not an important part of the effective stimulus complex. When HR conditioning was obtained, its latency and duration was not related to the delay between the CS and UCS injections or whether they were forward or backward. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Conditioned stimulus determinants of conditioned response form in Pavlovian fear conditioning.

Four experiments using barpress conditioned suppression in rats found that tone evoked more freezing (immobility) than did light. Still, tone and light appeared to have similar conditioned value as assessed by suppression in Experiments 1, 2, and 3, and by blocking, second-order conditioning, and overconditioning assays in Experiments 1, 2, and 3, respectively. Experiment 4 arranged for tone to evoke less suppression than light but more freezing. Results suggest that in fear conditioning, the nature of the conditioned stimulus affects the form of conditioned responding (strong vs. weak freezing). This conclusion extends one drawn by P. C. Holland (see record 1977-12147-001) on the basis of his work in appetitive conditioning. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Heart rate conditioning with pentobarbital as a conditioned stimulus and amphetamine as an unconditioned stimulus.

Injected pentobarbital (PB) into rats 20 min after they were placed in an apparatus where heart rates were recorded. Amphetamine was injected after they were removed from the apparatus 29–30 min later. A Pavlovian conditioned response (CR) began after 3 or 4 trials in the form of a failure of conditioned Ss to show the same decline in heart rate obtained in controls after the PB injection. On later trials, the amphetamine was not injected until 50 min after the PB, and the CR was most obvious during the period 30–50 min after the PB injection. The pharmacological effects of PB were necessary for conditioning because the CR was not obtained (a) when normal saline was substituted for the PB after successful conditioning or (b) when saline was used instead of PB throughout. Because of the speed and effectiveness of the conditioning, we believe the mechanism responsible for it has homeostatic regulation as its natural role. It was puzzling that environmental cues seemed to have a role in the conditioned stimulus complex, because conditioning was not apparent when the drug–drug pairings were administered in the home cage. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Second-order conditioning and Pavlovian conditioned inhibition: Operational similarities and differences.

Procedures for establishing 2nd-order excitation (CS1-unconditioned stimulus [UCS] trials followed by CS2-CS1 trials) are highly similar to those for Pavlovian conditioned inhibition (CS1-UCS trials interspersed with CS2-CS1 trials). Conditioned suppression in rats was used to identify the critical operational differences that result in 2nd-order excitation as opposed to Pavlovian inhibition. No, few, or many CS2-CS1 trials were either interspersed with or given after CS1-UCS trials. CS2 proved excitatory only after few CS2-CS1 trials, either interspersed or sequential (Exp 1). In contrast, CS2 proved inhibitory on both summation (Exp 2) and retardation (Exp 3) tests only after many CS2-CS1 trials, and then only when the excitatory status of CS1 was preserved. Apparently, the critical difference for establishing 2nd-order excitation or Pavlovian inhibition is the number of CS2-CS1 pairings. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Exposure to the unconditioned stimulus alone: Effects on retention and acquisition of conditioned suppression.

In 3 experiments with 104 male Sprague-Dawley rats, repeated exposure to an electric-shock UCS resulted in a decrement in retention of conditioned suppression evoked by a previously established excitatory CS and retarded subsequent acquisition of conditioned suppression to a novel CS paired with shock. Exp I showed that 10 sessions of exposure to shock alone were required to produce a decrement in retention of conditioned suppression, whereas retardation in the acquisition of conditioned suppression was obtained following either 5 or 10 sessions of exposure to shock alone. Exp II demonstrated that both of these effects were directly related to the intensity of the shocks. In Exp III, the decrement in retention of conditioned suppression produced by 10 sessions of exposure to shock alone was inversely related to the interval between the last exposure to shock and the test of the target CS. Findings are discussed in terms of associative and nonassociative accounts of the effects of UCS-alone procedures. (29 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Cholecystokinin conditioning in rats: Ontogenetic determinants.

Low doses (0.12–2.0 μg/kg) of cholecystokinin octapeptide (CCK), administered intraperitoneally, support the formation of conditioned odor preference in neonatal and weanling rats. Exposure to a novel odor was paired with CCK injection, and the rats' olfactory choices were assessed 24 hr later. Rats at 5, 11, and 22 days of age preferred the odor previously associated with CCK, compared with vehicle-injected littermates. In contrast, CCK failed to support olfactory conditioning in 28-day-old rats, whether they were (a) weaned and independently housed, (b) residing with the dam and suckling, or (c) fed only milk. Adult rats also did not establish an odor preference with CCK as the unconditioned stimulus. Thus, CCK's changing impact from positive to neutral probably occurs during the rats' 4th postnatal week and may be related to maturational changes occurring during the final stages of weaning. (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)     

Evidence of plasticity in the pontocerebellar conditioned stimulus pathway during classical conditioning of the eyeblink response in the rabbit

Electrical stimulation thresholds required to elicit eyeblinks with either pontine or cerebellar interpositus stimulation were measured before and after classical eyeblink conditioning with paired pontine stimulation (conditioned stimulus, CS) and corneal airpuff (unconditioned stimulus, US). Pontine stimulation thresholds dropped dramatically after training and returned to baseline levels following extinction, whereas interpositus thresholds and input-output functions remained stable across training sessions. Learning rate, magnitude of threshold change, and electrode placements were correlated. Pontine projection patterns to the cerebellum were confirmed with retrograde labeling techniques. These results add to the body of literature suggesting that the pons relays CS information to the cerebellum and provide further evidence of synaptic plasticity in the cerebellar network.     

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)     

Age-related differences in the impact of spacing, lag, and retention interval.

An experiment is reported that examines age-related differences in the lag effect and its relation to retention interval. A total of 30 young and 30 older adults received both once-presented pairs and twice-presented pairs that were tested in a continuous cued-recall paradigm either after a short retention interval (2 pairs intervening between the last presentation of a pair and its test) or a long retention interval (20 pairs intervening between the last presentation of a pair and its test). In addition, the twice-presented pairs were separated by either 0, 1, 4, 8, or 20 intervening pairs. The results replicated the interaction between retention interval and lag that has been reported by Glenberg (1976, Journal of Verbal Learning and Verbal Behavior, 15, 1–26). Furthermore, although the older adults performed considerably lower than the younger adults in overall recall performance, their data were remarkably similar to the younger adults in the patterning of means. A mathematical modeling procedure was used to fit the data to Estes' stimulus fluctuation model. The results of this modeling procedure suggest that, compared with younger adults, older adults (a) encode less contextual information at a given point in time and (b) have a slower rate of contextual fluctuation across time. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Hippocampal age differences reoccur after modification of stimulus configurations in rabbit jaw movement conditioning

A bacteriocin-like activity produced by Enterococcus faecium BC25, isolated from the the rumen of a cow, was partially purified and characterized. The active substance was prepared by ammonium sulfate and chloroform/methanol precipitation of culture supernatant. The bacteriocin was a protein of molecular mass 17.5 kDa. Activity was inactivated by trypsin and proteinase K. The bacteriocin BC25 inhibited growth of amylolytic ruminal strains of Streptococcus bovis, including S. bovis AO 24/85. Results showed that bacteriocine substance BC25 has a bacteriostatic effect when present in concentrations exceeding 125 AU/ml. Agar overlays and batch culture growth experiments proved that E. faecium BC25 was producing bacteriocin that inhibited the growth of S. bovis.     

Odor-guided fear conditioning in rats: 2. Lesions of the anterior perirhinal cortex disrupt fear conditioned to the explicit conditioned stimulus but not to the training context.

Previous studies examining the neural substrates of fear conditioning have indicated unequivocally that the acquisition and expression of conditioned fear depends critically on the integrity of the amygdala. The extent to which the rhinal cortical areas contribute to fear conditioned to either the explicit conditioned stimulus (CS) or to the training context is less clear, however. The effects of pretraining lesions of the anterior perirhinal (PRH) cortex on fear conditioned to an explicit odor CS and to the context in which CS–unconditioned stimulus pairing took place was examined in rats. Rats with PRH cortex lesions demonstrated a robust attenuation of fear conditioned to the explicit CS, but no attenuation of fear conditioned to the training context. These data suggest that the PRH cortex is an important component of the neural system supporting the association between olfactory cues and footshock and add to a growing body of evidence implicating the rhinal cortical regions in associative learning. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Morphine-conditioned changes in locomotor activity: Role of the conditioned stimulus.

When a multisensory environment was reliably paired with morphine (2 mg/kg) in rats, that environment, in a drug-free test, evoked a hyperactive conditioned response (CR). When an olfactory cue (banana odor) was the only stimulus element reliably paired with morphine, it also elicited a hyperactive CR. However, a gustatory cue (saccharin solution) evoked a hypoactive CR. This taste-elicited decrease in activity was dose dependent; morphine at 2 and 4 mg/kg conditioned hypoactivity, whereas a higher dose (8 mg/kg) did not. A robust conditioned saccharin aversion occurred only at the highest dose of morphine, suggesting disassociation between the hypoactive CR and taste aversion. A taste cue present during context conditioning also prevented either acquisition or expression of the hyperactive CR to the context. The modality of the conditioned stimulus is a critical determinant of the form of the CR in a morphine locomotor conditioning paradigm. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Pro-Leu-Gly-NH? serves as a conditioned stimulus in the acquisition of conditioned tolerance.

The effect of Pro-Leu-Gly-NH? (MIF) on the acquisition of tolerance to morphine-induced antinociception and its efficacy as a cue predictive of morphine administration was examined. Daily administration of MIF prior to morphine injection did not attenuate the acquisition of tolerance to the antinociceptive properties of morphine, as measured by the latency to hindpaw lick in a hot-plate test of analgesia. When the animals were tested 72 hrs later without MIF pretreatment, they appeared to lose tolerance, as indicated by longer latencies to paw lick. These data suggest that in some situations MIF may interfere with the acquisition of tolerance by acting as a cue that reliably predicts the antinociceptive properties of morphine. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Contextual and cued fear conditioning in C57BL/6J and DBA/2J mice: Context discrimination and the effects of retention interval.

Context discrimination and time course studies of contextual fear conditioning revealed strain differences between C57BL/6J (B6) and DBA/2J (D2) mice. Both strains discriminated contexts, but D2 mice exhibited less freezing in a shock-paired context. The strains did not differ immediately, or at 2 and 3 hr after contextual fear conditioning training. D2 mice showed less freezing at 15 min, 30 min, and 24 hr after training. B6 mice exhibited exaggerated generalized freezing and poor discrimination between the context and altered context 7-30 days after training. The acoustic startle response in B6 mice was also enhanced at 14 days after training. D2 mice did not show this pattern of generalized freezing. B6, but not D2, mice retained contextual memories for at least 60 days. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Sexual approach conditioning: Unconditioned stimulus factors.

Unconditioned stimulus (US) factors were investigated in a Pavlovian sexual conditioning paradigm with male Japanese quail (Coturnix coturnix japonica). Copulation with a female quail was more effective than exposure to a female without copulation, but the latter also produced conditioned responding (Exp 1). The greater effectiveness of copulatory opportunity as a US was probably not due to nonassociative effects of copulation (Exp 2). Visual cues of the female, as presented on a taxidermic model, were not effective unless the Ss had prior sexual experience that increased their response to the model (Exps 3a and 3b). Successful conditioning with noncopulatory female exposure is significant because it allows for sexual learning to occur in a broader range of circumstances than does conditioning with copulation and because it permits conducting multiple trials per day because males do not become satiated as rapidly. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Eyeblink conditioning in the rabbit (Oryctolagus cuniculus) with stimulation of the mystacial vibrissae as a conditioned stimulus.

Eyeblink conditioning is a well-understood paradigm for the study of learning and memory and has been successfully employed with the use of auditory and visual conditioned stimuli (CSs). In this study, vibrotactile stimulation of the mystacial vibrissae was examined as an alternative CS in the rabbit ( Oryctolagus cuniculus). The technique is described and acquisition of eyeblink conditioning (EBC) with stimulation of a single row of vibrissae in a delay paradigm is reported. Extinction of EBC with presentation of the CS alone is demonstrated, as well as reacquisition with stimulation of a single whisker. Finally, control experiments ensure that the CS has no auditory components. Ipsilateral presentation of the CS and airpuff is a more effective combination for training than contralateral presentations. Vibrotactile stimulation of the vibrissae as a CS will enable further examination of the neural correlates of learning in a well-characterized sensory system. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Competition between novelty and cocaine conditioned reward is sensitive to drug dose and retention interval.

The conditioned rewarding effects of novelty compete with those of cocaine for control over choice behavior using a place conditioning task. The purpose of the present study was to use multiple doses of cocaine to determine the extent of this competition and to determine whether novelty’s impact on cocaine reward was maintained over an abstinence period. In Experiment 1, rats were conditioned with cocaine (7.5, 20, or 30 mg/kg ip) to prefer one side of an unbiased place conditioning apparatus relative to the other. In a subsequent phase, all rats received alternating daily confinements to the previously cocaine paired and unpaired sides of the apparatus. During this phase, half the rats had access to a novel object on their initially unpaired side; the remaining rats did not receive objects. The ability of novelty to compete with cocaine in a drug free and cocaine challenge test was sensitive to cocaine dose. In Experiment 2, a place preference was established with 10 mg/kg cocaine and testing occurred after 1, 14, or 28 day retention intervals. Findings indicate that choice behaviors mediated by cocaine conditioning are reduced with the passing of time. Taken together, competition between cocaine and novelty conditioned rewards are sensitive to drug dose and retention interval. (PsycINFO Database Record (c) 2010 APA, all rights reserved)