The effects of qualitative and quantitative variation in the US on individual components of Pavlovian appetitive conditioned behavior in rats

The form of rats’ Pavlovian conditioned responses to visual and auditory conditioned stimuli (CSs) paired with a variety of unconditioned stimuli (USs) was examined in three experiments using direct behavioral observation techniques. In Experiment 1, the form of conditioned behavior occurring most frequently during later portions of the CS-US interval depended only on which of several appetitive USs was used, but the form of behavior occurring most frequently during early portions of the CS-US interval depended only on the nature of the CS. US-dependent behaviors resembled the response to the US, and CS-dependent behaviors resembled the original orienting response (OR) to the CS. In Experiment 2, the use of larger magnitude appetitive USs resulted in higher frequencies of US-dependent behaviors, but lower frequencies of CS-dependent behaviors in the presence of auditory and visual CSs. In Experiment 3, US-dependent conditioned behavior to auditory and visual CSs paired with shock was more frequent when high-intensity shocks were used, but CS-dependent behavior was more frequent when low-intensity shocks were used. These results suggested that Pavlovian conditioned responding may involve two independent types of behavior—one appropriate to the US and another based on the original OR to the CS.     

Inflation of comparator stimuli following CS training

Water-deprived rats served in seven conditioned lick suppression experiments designed to assess the effects on responding to a target CS of a series of unsignaled USs given in the training context following completion of CS training. Such treatment has been hypothesized to increase (inflate) the associative strength of the background cues from training (putatively, the CS’s comparator stimuli), thereby reducing responding to excitatory CSs and increasing the inhibitory potential of inhibitory CSs. Although posttraining extinction (deflation) of the CS’s comparator stimuli usually decreases inhibitory potential and increases excitatory potential of the target CS, posttraining inflation of the comparator stimuli had no effect on either excitatory responding to the target CS or summation test performance indicative of conditioned inhibition. This outcome was consistently obtained across a number of training, inflation, and test conditions selected to maximize sensitivity to any possible effects of comparator inflation. Implications of these null results for the comparator hypothesis of conditionedresponse generationare discussed.     

Stimulus-reinforcer interactions in Pavlovian conditioning of pigeons: Implications for selective associations

In a Pavlovian procedure, groups of pigeons were presented with a compound auditory-visual stimulus that terminated with either response-independent electric shock or food. In a subsequent test, the tone CS was dominant in aversive conditioning, reliably eliciting conditioned head raising and prancing. The red light CS was dominant in appetitive conditioning, reliably eliciting pecking. This result was replicated in a second experiment, in which trials were widely spaced. Pour additional groups of pigeons received pairings of the separate element CSs with the USs. Red light, but not tone, was an effective CS in appetitive conditioning, whereas tone, but not red light, was effective in aversive conditioning. There was no discriminative responding in zero-contingency control groups. Several theoretical accounts of these data are discussed.     

On the generality of conditioned bradycardia in rabbits: Assessment of CS and US modality

To determine whether the magnitude of heart rate (HR) slowing induced by classical conditioning contingencies is comparable under a broad range of stimulus conditions, experiments were conducted in which rabbits were exposed to tones, increases in illumination, or vibratory stimuli as conditioned stimuli (CSs) and in which paraorbital electric shocks, corneal airpuffs, or intraoral pulses of water served as unconditioned stimuli (USs). The results indicated that conditioned bradycardia was elicited by all three CSs. Moreover, when a corneal airpuff served as the US, small but reliable CS-evoked HR decelerations also occurred. Finally, CS-evoked HR slowing also occurred in response to a tone CS employed in an appetitive task, in which water was the US. These findings suggest that HR slowing is a general phenomenon that occurs when rabbits are exposed to signals that systematically predict aversive or appetitive consequences according to a Pavlovian conditioning paradigm.     

A contingency analysis of taste aversion conditioning

The acquisition of conditioned taste aversion was assessed relative to five control procedures. That is, forward conditioning using multiple-trial, brief-duration taste CSs and weak USs over a 30-min CS-US delay was compared to backward, CS alone, US alone, sham CS and sham US, and random control procedures. The outcome supported an associative conditioning interpretation of the learned aversion. While there were no differences between the various control procedures, all were different from the forward conditioning group. The argument was made that some of the distinctive associative and nonassociative phenomena attributed to taste aversion conditioning (but not seen in the present study) may in part be due to the duration and intensity of both the CS and US events.     

Solving Pavlov's puzzle: Attentional, associative, and flexible configural mechanisms in classical conditioning

This article introduces a new "real-time" model of classical conditioning that combines attentional, associative, and "flexible" configural mechanisms. In the model, attention to both conditioned (CS) and configural (CN) stimuli are modulated by the novelty detected in the environment. Novelty increases with the unpredicted presence or absence of any CS, unconditioned stimulus (US), or context. Attention regulates the magnitude of the associations CSs and CNs form with other CSs and the US. We incorporate a flexible configural mechanism in which attention to the CN stimuli increases only after the model has unsuccessfully attempted learn input-output combinations with CS-US associations. That is, CSs become associated with the US and other CSs on fewer trials than they do CNs. Because the CSs activate the CNs through unmodifiable connections, a CS can become directly and indirectly (through the CN) associated with the US or other CSs. In order to simulate timing processes, we simply assume that a CS is formed by a temporal spectrum of short-duration CSs that are activated by the nominal CS trace. The model accurately describes 94?% of the basic properties of classical conditioning, using fixed model parameters and simulation values in all simulations.     

Temporal integration in second-order conditioning and sensory preconditioning

Lick suppression experiments with rats revealed that the magnitude of both second-order conditioning (Experiment 1) and sensory preconditioning (Experiment 2) was superior when that conditioning was based on backward (US→CS) relative to forward (CS→US) first-order pairings of a CS and US. The superiority of backward relative to forward first-order conditioning on suppression to the higher order cues can be understood by assuming that the magnitude of higher order conditioning was determined by a memory representation of the higher order cues that provided information about the expected temporal location of the US. The results suggest that temporal information such as order between paired CSs and USs was encoded, preserved, and integrated with memory for the higher order stimuli. The relevance of these findings to memory integration in Pavlovian learning, the temporal coding hypothesis (Barnet, Arnold, & Miller, 1991; Matzel, Held, & Miller, 1988), backward excitatory conditioning, and the associative structure that underlies second-order Pavlovian fear conditioning are discussed.     

The effects of signaling the US in backward conditioning: A shift from excitatory to inhibitory learning

The associative effects of “backward” US-CS pairings were compared when the pairing occasions were either consistently preceded by a well-trained CS+ or were unannounced. The investigation employed a conditioned emotional response procedure with rats, under conditions in which all subjects received the same schedule of shock USs, some signaled and others not, and the back-ward CS was arranged to follow either the former or the latter, in separate groups. The major finding was that although the backward CS became excitatory when it followed unsignaled USs, it became inhibitory when it followed signaled USs. This outcome, which is in line with prior findings of Wagner and Terry (1975), is in accordance with a “sometimes-opponent-process” model proposed by Wagner (1981). It is contrary to data reported by Fowler, Kleiman, and Lysle (1984) that may have resulted from a confounding of the different circumstances of backward conditioning with differences in the predictability of the US in the experimental context.     

The order of presentation of conditioned stimuli during extinction

Three experiments examined the effects of prior extinction with nontarget CSs on a target CS’s rate of extinction and amount of associative loss. After fear was conditioned to several CSs, rats received extinction training with the target CS either before or after extinction with nontarget CSs. In general, neither rate of extinction nor amount of associative change, as measured by reinstatement and relearning, were significantly affected by order of extinction. The present results do not support Rescorla’s interpretation of extinction.     

The role of signaled periods of nonreinforcement in responding on a random schedule in autoshaping

Little responding develops to a conditioned stimulus (CS) that is placed in a random relation to an unconditioned stimulus (US). However, if the USs not preceded by that CS are themselves signaled by another stimulus, then the CS does come to elicit responding. This result has been attributed (e.g., by Durlach, 1983) to the signal’s blocking of conditioning to background cues that otherwise would prevent conditioning of the CS. However, Goddard and Jenkins (1987) have suggested the alternative that signaling the USs promotes responding due to the adventitious creation of periods of signaled nonreinforcement. Two experiments were conducted to assess this alternative, involving an autoshaping preparation in pigeons. In Experiment 1, little responding to a keylight CS presented in a random relation to a food US occurred, despite the explicit presentation of a discrete noise signaling periods of no food in the intertrial interval (ITI). Experiment 2 was designed to replicate the procedure of Goddard and Jenkins, in which an auditory stimulus extended throughout the ITI of a random schedule, terminating only prior to extra USs and during the CS. Contrary to their findings, little responding developed to the target CS. However, responding did develop when the sound-free period occurred only prior to the extra USs. These results offer little support for the hypothesis that signaled periods of nonreinforcement promote responding on random schedules. However, they are consistent with the view that signaling of ITI USs acts by preventing conditioning of potentially competitive background cues.     

Secondary extinction in Pavlovian fear conditioning

Pavlov (1927/1960) reported that following the conditioning of several stimuli, extinction of one conditioned stimulus (CS) attenuated responding to others that had not undergone direct extinction. However, this secondary extinction effect has not been widely replicated in the contemporary literature. In three conditioned suppression experiments with rats, we further explored the phenomenon. In Experiment 1, we asked whether secondary extinction is more likely to occur with target CSs that have themselves undergone some prior extinction. A robust secondary extinction effect was obtained with a nonextinguished target CS. Experiment 2 showed that extinction of one CS was sufficient to reduce renewal of a second CS when it was tested in a neutral (nonextinction) context. In Experiment 3, secondary extinction was observed in groups that initially received intermixed conditioning trials with the target and nontarget CSs, but not in groups that received conditioning of the two CSs in separate sessions. The results are consistent with the hypothesis that CSs must be associated with a common temporal context during conditioning for secondary extinction to occur.     

Evaluating conditioning of related and unrelated stimuli using a compound test

Three experiments used a compound test procedure to evaluate whether superior conditioning results from the pairing of stimuli that are related to each other. In each case, a stimulus compound was tested after its component conditioned stimuli (CSs) had been conditioned by the same unconditioned stimuli (USs) arranged such that either related or unrelated CSs and USs were paired. Experiment 1 explored auditory and gustatory stimuli conditioned by LiCl or shock, using rats. Experiments 2 and 3 used second-order conditioning in pigeons to pair stimuli that were similar by virtue either of qualitative features or of shared physical location. In each case, the compound test provided clear evidence that pairing related stimuli produces superior associative learning.     

Effect of signaling extra unconditioned stimuli on autoshaping

Acquisition to a target conditioned stimulus (CS) is prevented when extra, unsignaled unconditioned stimuli (USs) are presented with sufficient frequency to remove contingency between target CS and US. Acquisition occurs, however, when the extra USs are signaled by another CS. According to the Rescorla-Wagner theory, signaling reduces contextual conditioning, which otherwise prevents acquisition. Results of Experiment 1 led to the rejection of a rival explanation derived from scalar expectancy theory by showing that acquisition does not occur when only half of the extra USs are signaled. The results of Experiment 2 were, however, contrary to the Rescorla-Wagner theory because they showed equivalent acquisition when the stimulus used to signal the extra USs was also present concurrently with the target CS. Signaling may exert its effect by converting the intertriai interval to CS?.     

CS modality,context conditioning,and conditioned freezing

Two studies used a one-trial-a-day aversive conditioning procedure with rats as subjects to investigate the effects of a noise versus a light CS on conditioned freezing. Experiment 1 demonstrated that less conditioned freezing was elicited by the light, although the two CSs led to similar levels of freezing to the contextual cues of the conditioning chamber. Experiment 2 replicated these outcomes and showed that the manipulation of CS intensity produced results similar to those of modality, with the more intense CSs eliciting less freezing. The second experiment also determined that freezing to contextual cues resulted from context conditioning. According to the Rescorla-Wagner model, CSs that condition poorly should generate little competition with context conditioning. Since neither the modality nor intensity factor reliably influenced context conditioning, as measured by context-evoked freezing, the studies provide no support for the view that the effects on CS-evoked freezing represent differences in the strength of conditioning to the various stimuli. This finding raises the possibility that all of the CSs conditioned well but varied in their abilities to elicit freezing because they differed in terms of the form of defensive behavior under their control.     

Excitatory and inhibitory consequences of explicitly unpaired and truly random conditioning procedures on heart rate in rats

A comparison was made of heart-rate (HR) responses of restrained rats to CSs that were part of an explicitly unpaired or a truly random control procedure. Subsequent to these procedures, an assessment was made of the relative capacities of these CSs to affect an established HR CR in a combined-cue paradigm and to impede the development of a HR CR in a reversal-conditioning situation. The principal findings were (1) that the explicitly unpaired and truly random CSs generated HR responses of opposite direction, i.e., HR acceleration vs. HR deceleration, respectively, and (2) that conditioning of a decelerative HR CR to the CS that had earlier been employed in the explicitly unpaired procedure was retarded compared to what was obtained to the truly random CS. The two CSs did not have reliably different effects in the combined-cue test. It was suggested that the truly random CS may have produced both associative and nonassociative influences on HR. It was hypothesized further than the explicitly unpaired CS may have acquired the capacity to function as a conditioned inhibiting stimulus.     

Transfer across CS-US intervals and sensory modalities in classical conditioning of the rabbit

Two experiments investigated transfer of the rabbit’s conditioned nictitating membrane response (NMR) from shorter to longer CS-US intervals in conjunction with a change in CS modality, for example, light to tone. In Experiment 1, three experimental groups received initial training with a 400-msec CS-US interval, which produced substantial CR acquisition, and three control groups received initial training with a 2,800-msec CS-US interval, which produced minimal CR acquisition. Subsequently, the experimental and control groups received training with an 800-, 1,800-, or 2,800-msec CS-US trace interval. At the same time, the modality of the CS was changed from tone to light (or vice versa). Experiment 2 contained three groups that received initial exposure to a 400-msec CS-US interval, a 2,800-msec CS-US interval, or just the experimental chambers. Subsequently, all three groups received training with an 800-msec CS-US interval in a different CS modality. The results of both experiments revealed substantial positive transfer across CS modalities from the 400-msec CS-US interval to the 800-msec CS-US interval. There was also significant transfer to the 1,800-msec but not the 2,800-msec CS-US interval. The transfer did not appear immediately on test presentations of the second CS. Rather, the transfer appeared as an enhancement in the rate of CR acquisition after reinforced training with the second CS had commenced. The results are discussed with respect to mechanisms of transfer and facilitation of trace conditioning.     

Outcome-specific conditioned inhibition in Pavlovian backward conditioning

In the present experiments, the outcome specificity of learning was explored in an appetitive Pavlovian backward conditioning procedure with rats. The rats initially were administered Pavlovian backward training with two qualitatively different unconditioned stimulus conditioned-stimulus (US-CS) pairs of stimuli (e.g., pellet --> noise or sucrose --> light), and then the effects of this training were assessed in Pavlovian-to-instrumental transfer (Experiment 1) and retardation-of-learning (Experiment 2) tests. In the transfer test, it was shown that during the last 10-sec interval, the CSs selectively reduced the rate of the instrumental responses with which they shared a US, relative to the instrumental responses with which they did not share a US. The opposite result was obtained when the USs (in the absence of the CSs) were presented noncontingently. In the retardation test, conditioned magazine approach, responding to the CSs was acquired more slowly when the stimulus-outcome combinations in the backward and the forward conditioning phases were the same, as compared with when they were reversed. These results are collectively in accord with the view that Pavlovian backward conditioning can result in the formation of outcome-specific inhibitory associations. Alternative views of backward conditioning are also examined.     

General transfer across sensory modalities survives reductions in the original conditioned reflex in the rabbit

Two experiments demonstrated that transfer of training between CSs from different sensory modalities survived substantial reductions in responding to the first CS. In both experiments, animals received three stages of training. Stage 1 entailed CS-US training with a CS from one modality (e.g., light), and Stage 3 entailed CS-US training with a CS from another modality (e.g., tone). The experiments differed in treatment during Stage 2. In Experiment 1, animals either remained in their home cages or received unreinforced exposures to the first CS, which extinguished the original CR. In Experiment 2, the animals received either continued CS-US training or exposure to the CS and US but at a long interval (2,800 msec), which eliminated the original CR. As the baseline for detection of transfer effects, each experimental group had a control group that received Stage 1 training with a 2,800-msec CS-US interval, which produced minimal CR acquisition. The results of both experiments revealed substantial positive transfer across CS modalities regardless of the treatment during Stage 2. The transfer did not appear immediately on test presentations of the second CS in Stage 3. Rather, the transfer appeared as an enhancement in the rate of CR acquisition after reinforced training with the second CS had commenced. The results are discussed with respect to stimulus generalization, neutralization of background stimuli, and learning processes superordinate to specific associations.     

Conditioned suppression of a positively reinforced shuttle response

Rats were trained to run up and down an alleyway for sucrose reinforcement on a variable interval schedule. Differential aversive classical conditioning with auditory CSs was then conducted in a separate apparatus (“off the baseline”) prior to those CSs being presented while the subjects were responding for sucrose in the alleyway. Once the effects of the CSs had extinguished, shock was reintroduced following one CS but not the other (“on the baseline” differential aversive classical conditioning). Both “off the baseline” and “on the baseline” conditioning resulted in conditioned suppression to the CS followed by shock, but little effect of the CS followed by no shock was found. In the “on the baseline” phase, total suppression of baseline responding occurred at moderate US intensities, and this appeared to result from the subject avoiding the location at which he was last shocked. At lower values, both baseline response rate and relative suppression ratio were functions of US intensity. The results are discussed in relation to the effects found in similar experiments using avoidance baselines.     

Transfer of contextual control across similarly trained conditioned stimuli

Two autoshaping experiments with pigeons examined contextual control of responding to conditioned stimuli (CSs) when separate phases of reinforcement and nonreinforcement occurred in different contexts. In Experiment 1, a CS was first conditioned in Context A prior to nonreinforcement in Context B. In Experiment 2, a conditional discrimination reversal procedure was employed in which one CS was first reinforced in A prior to nonreinforcement in B, and another CS was first nonreinforced in A and then reinforced in B. In both experiments, the extent of contextual control was assessed by testing the CSs in A and B. The CS specificity of control was assessed by examining control of CSs that had been treated like the original CSs, but in a different pair of contexts. The results show that contexts control responding to CSs through a CS-specific mechanism, as well as through a mechanism that is independent of the identity of the CS. However, the extent to which control is mediated by a CS-independent mechanism depends on the training history of the CS.