Systematic manipulation of individual events in a truly random control in rats.

Truly random sequences of tone (conditioned stimuli, CSs) and shock (unconditioned stimuli, UCSs) were given to a total of 78 male Holtzman albino rats. Conditioning to the CS was measured using a conditioned suppression procedure. In Exp I eliminating chance CS-UCS pairings by systematically removing CSs that overlapped UCSs weakened conditioning monotonically as a function of the number of paired CSs removed. In Exp II systematically delaying early chance pairings, while holding constant the number of CSs, UCSs, and pairings, produced a nonsignificant weakening of conditioning. In Exp III delaying pairings again produced a nonsignificant weakening of conditioning when the early nonpaired events were CSs but significantly weakened conditioning when the early nonpairings were UCSs. Data suggest that each chance pairing in the truly random control produces an increment in conditioning unless blocked by prior UCSs alone. (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)     

The effects of backward pairings on the conditioning of the albino rabbit's jaw movement and nictitating membrane responses.

Evaluated backward conditioning of appetitive jaw movement and aversive nictitating membrane responses in rabbits, using 3 procedures: test trials, forward acquisition of the same response, and crossmotivational acquisition of the other response system. In both experiments, backward conditioning consisted of 3 conditioned stimulus/stimuli (CS) alone and 22 CS and unconditioned stimulus/stimuli (UCS) pairings. Contrasting the backward conditioning group with explicitly unpaired and no-treatment controls revealed that backward pairings produced no associative effects. The forward acquisition and crossmotivational acquisition tests suggest that excitatory backward acquisition of jaw movement was obtained. The forward acquisition test identified weak inhibitory conditioning of nictitating membrane responses, but the crossmotivational test implicated excitatory conditioning. (French abstract) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Pairings of a drug or place conditioned stimulus with lithium chloride produce conditioned sickness, not antisickness.

In different experiments, pairings of a drug (pentobarbital or morphine) or place as the conditioned stimulus (CS) with lithium-induced sickness as the unconditioned stimulus (UCS) were given to rats to produce Pavlovian conditioning. Control rats received unpaired exposures. In the test, each rat was exposed to the CS, injected with lithium, and then offered food. If such pairings produce conditioning of antisickness (i.e., a compensatory response that opposes lithium sickness), then the experimental rats should eat more than the controls. The reverse occurred. Thus, pairings of a drug or place CS with a lithium UCS resulted in conditioned sickness rather than antisickness. (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)     

Backward conditioning: Mediation by the context.

The information acquired in backward conditioning (i.e., outcome→cue) was assessed in 3 Pavlovian lick-suppression experiments with water-deprived rats as subjects. Experiment 1 confirmed previous research that few outcome→cue pairings made the cue into a conditioned excitor and additionally showed that massive posttraining extinction of the training context attenuated a backward-trained cue's excitatory value. Experiment 2 found that many outcome→cue pairings made the cue into a conditioned inhibitor and that the same context manipulation attenuated this inhibitory value. Experiment 3 confirmed the observations of Experiments 1 and 2 and demonstrated that these effects of context extinction were specific to backward-trained cues conditioned in the extinguished context. These results are interpreted in terms of cue→context and context→outcome associations. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Differential effects of the N-methyl-D-aspartate antagonist DL-2-amino-5-phosphonovalerate on acquisition of fear of auditory and contextual cues.

Intracerebroventricular (icv) administration of the N-methyl-D-aspartate (NMDA) antagonist {dl}-2-amino-5-phosphonovalerate (APV) before tone-shock pairings caused a dose-dependent suppression of acquisition of fear of contextual cues associated with shock in 3 experiments involving 110 rats. Acquisition of fear of the tone was not impaired. Exp 2 showed that the fear of the tone was associative and that this tone-shock association was less affected by APV than was a context-shock association. Rats receiving APV before context-shock pairings showed an equivalent loss of fear regardless of whether testing occurred 1 or 28 days after training. It appears that icv administration of APV blocks acquisition of context conditioning by affecting NMDA receptors in the hippocampus. Activity at these receptors at the time of acquisition seems critical for later expression of both intermediate (1 day to 2 wks) and remote (4 wks) fear memories. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Conditioning the unconditioned response: Modification of the rabbit's (Oryctolagus cuniculus) unconditioned nictitating membrane response.

Conditioning-specific reflex modification (CRM) occurs when classical conditioning modifies responding to a unconditioned stimulus/stimuli (UCS) in the absence of a conditioned stimulus (CS). Three experiments monitored rabbit (Oryctolagus cuniculus) nictitating membrane unconditioned responses to 5 intensities and 4 durations of periorbital electrical stimulation before and after CS or UCS manipulation. CRM occurred after 12 days of CS-UCS pairings but not following unpaired CS/UCS presentations or restraint. CRM survived CS-alone and CS/UCS-unpaired extinction of the conditioned response (CR) but not presentations of the UCS alone, although CRs remained intact. Thus, CRs could be weakened without eliminating CRM and CRM could be weakened without eliminating CRs. Data indicate CRM is a reliable, associative effect that is more than a generalized CR and may not be explained by habituation, stimulus generalization, contextual conditioning, or bidirectional conditioning. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Classical conditioning modifies cytochrome oxidase activity in the auditory system

The effects of excitatory classical conditioning on cytochrome oxidase activity in the central auditory system were investigated using quantitative histochemistry. Rats in the conditioned group were trained with consistent pairings of a compound conditional stimulus (a tone and a light) with a mild footshock, to elicit conditioned suppression of drinking. Rats in the pseudorandom group were exposed to pseudorandom presentations of the same tone, light and shock stimuli without consistent pairings. Untrained rats in a naive group did not receive presentations of the experimental stimuli. The findings demonstrated that auditory fear conditioning modifies the metabolic neuronal responses of the auditory system, supporting the hypothesis that sensory neurons are responsive to behavioural stimulus properties acquired by learning. There was a clear distinction between thalamocortical and lower divisions of the auditory system based on the differences in metabolic activity evoked by classical conditioning, which lead to an overt learned behavioural response versus pseudorandom stimulus presentations, which lead to behavioural habituation. Increases in cytochrome oxidase activity indicated that tone processing is enhanced during associative conditioning at upper auditory structures (medial geniculate nucleus and secondary auditory cortices). In contrast, metabolic activation of lower auditory structures (cochlear nuclei and inferior colliculus) in response to the pseudorandom presentation of the experimental stimuli suggest that these areas may be activated during habituation to tone stimuli. Together these findings show that mapping the metabolic activity of cytochrome oxidase with quantitative histochemistry can be successfully used to map regional long-lasting effects of learning on brain systems.     

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)     

Redundant treatment of neutral and excitatory stimuli in autoshaping.

Five autoshaping experiments with pigeons exposed excitatory and neutral stimuli to treatments that made those stimuli uninformative with respect to the unconditioned stimulus (US). Experiment 1 found that adding sufficient USs in the absence of the conditioned stimulus (CS) to make it uninformative prevented conditioning of that CS but did not undermine conditioning of a previously trained CS. Experiment 2 found a similar data pattern when an excitatory or neutral B stimulus was embedded in an A+/BA+ paradigm with an excitatory A. Experiments 3–5 verified that finding when the salience of A and B or the conditioning level of B was varied. These results, which are largely unanticipated by current theories of conditioning, suggest that the consequences of making a stimulus redundant depend on its conditioning level prior to that treatment. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Contextual control of the extinction of conditioned fear: Tests for the associative value of the context.

When male Wistar rats received pairings of a CS with shock in one context and then extinction of the CS in another, fear of the CS was renewed when the CS was returned to and tested in the original context (Exps I and III; 40 Ss). No such renewal was obtained when the CS was tested in a 2nd context after extinction had occurred in the conditioning context (Exp IV; 24 Ss). In Exp II, shocks presented following extinction reinstated fear of the CS, but only if they were presented in the context in which the CS was tested. In each experiment, the associative properties of the contexts were independently assessed. Contextual excitation was assessed primarily with context-preference tests in which Ss chose to sit in either the target context or an adjoining side compartment. Contextual inhibition was assessed with summation tests. Although reinstatement was correlated with demonstrable contextual excitation present during testing, the renewal effect was not. There was no evidence that contextual inhibition developed during extinction. Results suggest that fear of an extinguished CS can be affected by the excitatory strength of the context but that independently demonstrable contextual excitation or inhibition is not necessary for contexts to control that fear. (41 ref) (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)     

Temporal order sensitivity of associative neural and behavioral changes in Hermissenda..

Hermissenda's neural and behavioral changes produced by light-rotation pairings were assessed as a function of the temporal relations between visual and vestibular stimulation. The results of in vitro simulations of conditioning indicated that simultaneous pairings (synchronous onsets and offsets of light and caudal hair cell stimulation) resulted in significantly greater cumulative depolarization of Type B photoreceptors than did either forward (light preceded hair cell stimulation) or backward (hair cell stimulation preceded light) pairings. Further experiments revealed that the attenuation of cumulative depolarization produced by the forward and backward pairings reflected the asynchrony of stimulus offsets that characterize these conditioning sequences, rather than their onsets. Analogous behavioral experiments revealed that intact animals trained with forward or backward pairings exhibited significantly less conditioning than those trained with simultaneous pairings. Strong parallels between the magnitude of cumulative depolarization from in vitro conditioning studies and the behavioral results for intact animals were also observed in experiments in which stimulus onset synchrony was held constant but offsets were made asynchronous, and vice versa. Thus Hermissenda exhibits a sensitivity to the temporal arrangement of light and rotation, and the results of behavioral conditioning can be predicted accurately from the outcome of in vitro conditioning of the isolated nervous system. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Representation-mediated overshadowing and potentiation of conditioned aversions.

Examined the ability of CS-evoked representations of flavored substances to modulate the conditioning of LiCl-based aversions to simultaneously presented flavors or odors. In Exps I–III, 156 thirsty Sprague-Dawley rats first received pairings of an auditory CS with a flavored-water UCS; they then received pairings of a compound stimulus with a toxin. Exp IV examined the potentiation of aversion conditioning to a novel odor using 32 Ss. In Exp I, conditioning of a flavor was partially overshadowed when it was presented in compound with a tone that had been previously paired with another flavor. Exp II replicated that result and also found that conditioning to a flavor was not overshadowed when the flavor was presented in compound with a tone that had been paired with that same flavored substance. In Exps III and IV, conditioning to an odor stimulus was potentiated when it was presented in compound with either a tone or another odor that had been previously paired with a flavor stimulus. Results suggest that evoked representations of stimuli may substitute for those events themselves in a variety of associative functions. (36 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Conserved regulatory element involved in the early onset of Hoxb6 gene expression

When administered before training to 23-day-old Long-Evans rats, scopolamine hydrobromide significantly impaired both contextual and auditory-cue fear conditioning in a dose-dependent manner. Methylscopolamine which does not cross the blood-brain barrier, however, had no effect on either form of conditioned fear. Scopolamine administered up to 3 h after training also impaired both forms of fear conditioning when administered following a single pairing of the auditory cue and shock. When rats received three pairings, however, a posttraining treatment with scopolamine only impaired contextual fear conditioning. These results suggest that central cholinergic systems are involved in the posttrial processes that establish the memory trace for the conditioning experience.     

A comparison of taste aversions induced by radiation and lithium chloride in CS-US and US-CS paradigms.

Describes 3 experiments with a total of 454 albino male Charles-River rats. Conditioned taste aversions induced by ionizing radiation and lithium chloride (LiCl) were compared with both forward (CS-UCS, conditioned stimulus-unconditioned stimulus) and backward (UCS-CS) conditioning paradigms. Taste aversions were produced when a saccharin CS preceded or followed a 100-r radiation UCS by as much as 6 hrs, but a 2%-of-body-weight, .15-mol LiCl UCS was effective only in CS-UCS pairings. It is argued that the ineffectiveness of an LiCl stimulus in UCS-CS pairings was not attributable to differences in the "strength" of the respective LiCl and radiation doses in that these doses yielded comparable aversions in forward pairings. These results are related to inadequacies of a "sickness" model of taste aversion conditioning. (22 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Stimulus contingencies that permit classical conditioning of opioid activity in the rat fetus.

In a prenatal model of classical conditioning, rat fetuses received presentations of an artificial nipple (conditioned stimulus; CS) paired with milk (unconditioned stimulus). Infusion of milk promotes activity in the kappa opioid system of the fetus, but after 2, 3, or 6 pairings with the artificial nipple, milk evoked both kappa and mu opioid activity. The nipple CS has no effect on opioid activity, but after pairing with milk evoked a mu opioid response. Conditioned mu opioid activity was evident in 60% of subjects tested after I paired conditioning trial. Significantly more fetal subjects (90%) exhibited conditioned opioid activity if preexposed to the nipple twice before conditioning. CS preexposure altered behavior during the conditioning trial, with preexposed fetuses showing more pronounced responses to milk infusion. Exposure to familiar stimuli facilitates classical conditioning of physiological responses, including opioid activity, during the first suckling episode. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Conditioned stimulus control in the rat circadian system depends on clock resetting during conditioning.

The authors examined the ability of a conditioned stimulus (CS; mild air disturbance) previously paired with an entraining light pulse to reset the circadian pacemaker in rats. Rats were entrained to a single 30-min light stimulus delivered every 25 hr or 24 hr (T cycle). Each daily light presentation was paired with the CS. After at least 20 days of stable entrainment to each of the T cycles, the rats were allowed to free run and were then presented with the CS at circadian time 15. CS-induced phase shifts in wheel-running activity rhythms were taken as evidence for conditioning. For the most part, conditioning occurred after CS-light pairings on the 25-hr but not 24-hr T cycle. The results suggest that CS control of the circadian clock phase depends on the effect that the entraining light pulse has on the clock during conditioning. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Anterior thalamic lesions and neuronal activity in the cingulate and retrosplenial cortices during discriminative avoidance behavior in rabbits.

Bilateral electrolytic lesions of the anteroventral nucleus of the thalamus given after training impaired retention performance (extinction and reacquisition) of 12 male New Zealand White rabbits in a differential avoidance conditioning task. In addition, the lesions abolished the excitatory, discriminative multiple-unit discharges that had developed in the cingulate and retrosplenial cortices to the auditory CSs during the course of behavioral acquisition, prior to the induction of the lesions. The excitatory discharges were supplanted in the Ss with lesions by CS-elicited reduction of neuronal firing to levels below the prestimulus baseline. Lesions given to 8 Ss before training did not disrupt behavioral acquisition, but they did eliminate the excitatory tone-elicited neuronal discharges that normally occur in the cortex before and during training. The CS-elicited reduction of neuronal firing did not occur at the beginning of training in the Ss given lesions before training, but it developed during the course of training. The lesions did not eliminate the excitatory and discriminative neuronal activity of the prefrontal cortex. Results demonstrate the excitatory and discriminative neuronal discharges in the cingulate and retrosplenial cortices are critically dependent on the connections of these areas with the anterior thalamic nuclei. (26 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)