Latent inhibition experiments with goldfish (Carassius auratus)

Evidence of latent inhibition was sought in a series of experiments with goldfish. In Experiment 1, goldfish were given nonreinforced preexposure to a color that subsequently predicted shock in an activity conditioning situation; their performance did not differ from that of control animals preexposed to a markedly different color. In Experiment 2, a group of goldfish given nonreinforced preexposure to a tone and an unstimulated control group were trained in an appetitive situation, with the tone serving either as a conditioned excitor or as a conditioned inhibitor. Preexposure had no significant effect in the conditioned excitation training, but it reduced the level of responding both to the positive stimulus and to the negative compound in the conditioned inhibition training. In Experiments 3 and 4, classical aversive conditioning was studied in the shuttle box. In Experiment 3, excitatory conditioning to a color was found to be impaired (relative to the performance of nonpreexposed control animals) as much by nonreinforced preexposure to the training color as by nonreinforced preexposure to a markedly different color; substantial variation in amount of preexposure was without significant effect. In the conditioned inhibition training of Experiment 4, animals with nonreinforced preexposure responded less than did unstimulated control animals both to the positive stimulus and to the negative compound. The results for goldfish can be understood on the assumption that the effect of preexposure in these animals is simply to reduce general responsiveness or level of arousal.     

Deepened extinction from compound stimulus presentation

Three experiments with rats and 2 with pigeons explored the effect of presenting 2 extinguished excitatory stimuli in compound. Four learning situations were used: Pavlovian magazine approach, Pavlovian fear conditioning, and instrumental discriminative instrumental learning in rats, as well as Pavlovian sign tracking in pigeons. All 5 experiments confirmed D. Reberg's (1972) observation that even after extinction of the individual stimuli, presenting them in compound evoked substantial responding. Moreover, nonreinforcement of that compound deepened extinction of an element more substantially than did additional presentation of that element alone. Such compound exposure reduced spontaneous recovery, reduced reinstatement, and slowed subsequent reconditioning. The primary determinant seemed to be the enhanced associative strength rather than the enhanced conditioned responding that occurred during the nonreinforced compound.     

Within-compound associations in a variety of compound conditioning procedures

Three experiments used a flavor-aversion preparation to demonstrate the occurrence of within-compound associations in various conditioning procedures. In Experiment 1 rats first received two separate two-flavor compounds, each followed by a mild US. They then received one element from one compound paired with a strong US and one element from the other compound nonreinforced. A subsequent choice test of the other two flavors from the compounds revealed that the consumption of each test flavor reflected the conditioning history of the element with which it had been paired. Rats drank less of the flavor whose associate had been paired with the strong US than of the flavor whose associate had been nonreinforced. In Experiment 2 within-compound associations were observed using a similar design in a blocking procedure in which one element from each of the two compounds was conditioned previously. Experiment 3 identified within-compound associations in a procedure where reinforced single element presentations were intermixed with nonreinforced compound presentations, as in a conditioned inhibition paradigm. These results suggest that within-compound associations occur in several important conditioning procedures which use multiple CS presentations.     

The eye blink as a time-locked response: implications for serial and second-order conditioning

A series of experiments were conducted to determine what factors control responding to the first element of a two-element serial compound in rabbit eyelid conditioning. An examination of response topography indicated that the eye-blink CR is rigidly timed to occur when the US is expected. This response-system characteristic prevents the occurrence of a CR during the first element of a serial compound or during the second-order CS in second-order conditioning. The comparison of a serial gap procedure with conventional serial and trace conditioning procedures suggested that the associative strength of the first element of a serial CS is not strongly influenced by either a second-order conditioning process or by the variable-reinforcement principle.     

Occasional reinforced trials during extinction can slow the rate of rapid reacquisition

Two appetitive conditioning experiments with rats examined reacquisition after conditioned responding was eliminated by either extinction or by a partial reinforcement procedure in which reinforced trials were occasionally presented among many nonreinforced trials. In Experiment 1, reacquisition to a conditional stimulus (CS) that had been conditioned and extinguished was more rapid than acquisition in a group that had received no prior conditioning. However, the addition of occasional reinforced trials to extinction slowed this rapid reacquisition effect. Experiment 2 replicated the result and showed that a procedure in which the CS and the unconditional stimulus (US) were unpaired in extinction interfered even further with reacquisition. The results suggest that rapid reacquisition is ordinarily produced when reinforced trials provide a contextual cue that can renew responding by signaling other acquisition trials (Ricker & Bouton, 1996). The effects of partial reinforcement in extinction are surprising from several theoretical perspectives and have useful clinical implications.     

Inhibitory sensory preconditioning.

In two experiments rats were preexposed to neutral stimuli. Both experiments used a between-subjects design in which a paired group was preexposed to intermixed presentations of A --> Band AX, and an unpaired control group was preexposed to intermixed presentations of A, B, and AX. After the conditioning of B, in Experiment 1, conditioned responding to X was acquired more slowly in the paired than in the unpaired group. Furthermore, in Experiment 2, X reduced conditioned responding to a separately trained excitor in a summation test but only in the paired group. Together, these results provide evidence of an inhibitory form of sensory preconditioning.     

Reminder-induced recovery from blocking as a function of the number of compound trials

Conditioned lick suppression in rats was employed to examine changes in the associative status of a blocked stimulus as a function of the number of compound stimulus conditioning trials. In each of two experiments, prior tone-footshock pairings produced similar blocking of conditioned responding to the light element of a tone-light compound when either two or six compound trials were used. In experiment 1, two exposures to the light alone in a dissimilar context during the retention interval, i.e., a reminder treatment, resulted in a restoration of responding to the light which was complete and similar for animals receiving two or six compound trials. This indicated that latent acquisition with respect to the blocked stimulus was largely complete after two compound trials. Because of a potential ceiling effect for reminder-induced recovery from blocking, Experiment 2 employed an attenuated reminder (one light exposure). This reminder reversed the blocking when six, but not when two, compound trials were used. These results suggest that, after latent acquisition to the blocked stimulus is complete, the blocked stimulus continues to be processed during additional compound stimulus conditioning trials, with a consequent latent facilitation of retrieval of associations to the blocked stimulus.     

An analysis of sequential variables in Pavlovian conditioning employing extended and limited acquisition training

Sequential theory’s memory model of learning has been successfully applied in response contingent instrumental conditioning experiments (Capaldi, 1966, Capaldi, 1967, Capaldi, 1994 and Capaldi and Miller, 2003). However, it has not been systematically tested in nonresponse contingent Pavlovian conditioning experiments. The present experiments attempted to determine if several sequential variables affect responding in Pavlovian situations as they do in instrumental ones. Of primary concern here were the effects on extinction of number of NR transitions (the number of times a nonreinforced trial is followed by a reinforced trial), N-length (the number of successive nonreinforced trials that precede a reinforced trial), and percentage of reinforcement (50 versus 100%) following either extended acquisition training (Experiment 1, 720 trials) or limited acquisition training (Experiment 3, 24 trials). In agreement with a sequential analysis, N-length increased resistance to extinction more than number of NR transitions following extensive training with the opposite occurring following limited training. In Experiment 1, greater resistance to extinction was associated with 50% than with 100% reinforcement, a partial reinforcement extinction effect (PREE). Experiment 2 examined an anomalous finding obtained in Experiment 1. A major theoretical difference between instrumental and Pavlovian conditioning has been held to be the greater ease of producing a PREE in instrumental than in Pavlovian conditioning (Kimble, 1961 and Mackintosh, 1974). However, the findings obtained here suggest that the probability of obtaining a PREE and other Pavlovian extinction effects, as in instrumental conditioning, increases along with the effectiveness of the sequential variables employed.     

Blocking observed in human eyelid conditioning

Four eyelid conditioning experiments designed to be comparable to rabbit nictitating membrane (NMR) studies examined the blocking phenomenon in humans. All experiments utilized a within-subjects design, with Stage 1 of discrimination, Stage 2 of compound training, and a final test stage comparing responding to the blocked and non-blocked CSs. In two of the experiments (1 and 4) the comparison was made within subjects over all extinction trials. In Experiment 3 the test phase consisted of further reinforced training of the blocked and non-blocked CSs. These three experiments produced evidence of blocking when all extinction trials were entered into the analysis. Experiment 2, which involved a between-subjects comparison, failed to demonstrate the blocking effect. Wide variability both between and within subjects obscured the experimental effects. Post-experimental questionnaires designed to assess awareness of stimulus relations failed to identify a subjective blocking effect and showed no relationship to conditioned eyelid responding     

Importance of trials versus accumulating time across trials in partially reinforced appetitive conditioning

Four experiments with rats examined partial reinforcement in appetitive conditioning. In Experiment 1, adding nonreinforced trials to a continuous reinforcement schedule slowed acquisition, whereas deleting reinforcers did not. Trial massing suppressed performance and learning. In Experiment 2, conditioning with a short conditioned stimulus (CS) was rapid, and partial reinforcement with a short CS was as effective as continuous reinforcement with equal accumulated time in the CS. In Experiment 3, conditioning was nevertheless influenced by the probability of reinforcement. In Experiments 3 and 4, conditioning was especially disrupted when nonreinforced trials preceded reinforced trials closely in time. The results underscore the importance of temporal variables in conditioning but are more consistent with trial-based accounts than time-accumulation accounts of conditioning.     

Socially mediated fear reduction in rodents: Distraction,communication, or mere presence?

Two studies investigated why the presence of conspecifics reduces open-field fear. In Experiment 1 rats paired with another experimentally naive rat were not less fearful than alone controls. However, rats paired with a partner who had been preexposed to the open field exhibited fewer fear responses than alone controls. Since preexposed companions appeared to initiate more social interactions than nonpreexposed companions, it was argued that the preexposure effect could be the result of preexposed companions distracting actor rats from threatening stimuli. Experiment 2 tested this explanation by pairing socially sated or socially deprived rats with a socially sated or socially deprived companion. In accord with a distraction hypothesis, pairing two socially deprived rats produced the greatest fear reduction effects and also led to more social interaction than pairing socially sated rats. Alternative explanations involving conditioning and communication are discussed.     

Associative changes in elements and compounds when the other is reinforced.

Four experiments investigated the symmetry of associative changes in stimulus compounds and elements in a Pavlovian conditioned magazine-approach situation with rats. Experiment 1 used multiple groups to examine the associative changes in a conditioned element (A) as a result of its subsequent reinforcement in compound with a neutral X. These were compared with the changes in an AX compound when one of its elements (A) was subsequently reinforced alone. Although reinforcing A enhanced responding to the AX compound, compared with a control compound, reinforcing AX failed to enhance responding to A, compared with a control element. Experiments 2 and 3 made similar comparisons in a fully within-subject design, finding greater changes in a previously trained AX compound when A was subsequently conditioned than in a B element when BY was subsequently conditioned. Experiment 4 found associative decrements in A when it was reinforced in the presence of a moderately conditioned X. The results observed in each of these experiments are more consistent with an elemental model of conditioning than with a recently proposed configural model.     

Analysis of the pavlovian properties of signals for punishment

Rats learned to suppress lever pressing in the presence of a compound stimulus that signalled either response-independent shock (conditioned suppression) or response-dependent shock (punishment). Suppression to one element of this compound was blocked if the other element had previously signalled the same contingency as that holding in the presence of the compound. Two experiments, however, failed to find significant evidence of blocking if the pretrained element had signalled conditioned suppression and the compound signalled a punishment contingency. On the other hand, if one stimulus signalled punishment of one response it was able to block the acquisition of control by the second stimulus when the compound signalled punishment of a different response. Finally, although animals showed no suppression to one element of a compound signalling conditioned suppression if the other had previously signalled punishment, this apparent blocking effect concealed that what they learned was that the added element signalled the cancellation of the instrumental contingency. These results provide little support for the view that stimuli signalling punishment contingencies suppress responding in whole or in part by virtue of their Pavlovian relation to shock.     

Conditioned inhibition from incomplete reductions in the probability of reinforcement

Five experiments, all using appetitive, discrete trial operant conditioning, studied the properties of a stimulus that signalled a reduction in the probability of reward. Discriminations were trained in which reinforcement was available on 100% of trials signalled by a tone, but only on some occasions when the tone was presented simultaneously with a light. The properties of the light were assessed in summation tests with a clicker. The first two experiments established that if the tone-light compound signalled reinforcement on only 25%, 33% or 50% of trials the light acted as a discriminative inhibitor, suppressing responding maintained by the clicker. In these experiments reinforcement had been available on 86.7% of clicker trials during initial training. Experiments 3, 4 and 5 examined further the properties of the light after animals had been trained on a discrimination where reinforcement was available on 50% of trials signalled by the tone-light compound. The light was evaluated in a summation test with a clicker, which had signalled only a 25% or 15% probability of a reinforcement. In this instance, the light did not suppress the responding maintained by the clicker. Instead, the light acted as an excitatory discriminative stimulus to enhance that responding. This dependence of the properties of the light on the reinforcement history of the clicker poses problems for most contemporary models of inhibitory performance.     

Behavioral decrement following stimulus preexposure: effects of number of preexposures, presence of a second stimulus, and interstimulus interval in children and adults.

This article presents an analysis of the similarities between the stimulus familiarization effect and latent inhibition. Just as latent inhibition has been readily demonstrated in infrahumans, and only with considerable difficulty in adult humans, the experiments reported here obtained a significant interaction between the stimulus familiarity effect and age. Children exhibited increased reaction time to preexposed stimuli as opposed to nonpreexposed stimuli, but adults did not show this effect. In addition, a number of variables were shown to affect the amount of decrement. The decrement increased as a function of the number of preexposures and decreased with the addition of a second stimulus placed in a conditioning relationship to the to-be-tested stimulus during preexposure. However, an increased in the interstimulus interval was only marginally effective in increasing the inhibitory effects of preexposure. The data were discussed in relation to a conditioned attention explanation for these phenomena of behavioral decrement.     

Unequal associative changes when excitors and neutral stimuli are conditioned in compound

In four experiments a compound of a previously conditioned excitor and a neutral stimulus was either reinforced with food or nonreinforced. Two experiments used a magazine approach procedure in rats, and two used an autoshaping procedure in pigeons. All experiments employed a novel compound test procedure, which permitted evaluation of the size of the associative change that took place for the excitatory and neutral stimuli. Reinforcement of the compound resulted in greater increments in the associative strength of the neutral stimulus than of the excitor. Nonreinforcement of the compound resulted in greater decrements in the associative strength of the excitor than of the neutral stimulus. These results agree with earlier experiments with compounds of excitors and inhibitors and provide an additional challenge to contemporary error-correction models of conditioning.     

Cholinergic modulation of trace conditioning trained in serial compound: A developmental analysis

In four experiments the effects of serial compound conditioning on responding to a trace-conditioned CS were evaluated using a fear conditioning paradigm. The subjects were 18- and 25-day-old Sprague-Dawley rats, previously shown to exhibit little or no trace fear conditioning. Here, animals as young as 18 days of age were shown to be capable of trace conditioning between a visual CS1 and a shock US, provided the trace interval was filled with a non-target CS2 during serial conditioning trials (CS1-->CS2-->US). To explore cholinergic mechanisms involved in trace and serial conditioning, additional experiments assessed conditioned responding following pre-training administration of the muscarinic receptor antagonist scopolamine. Scopolamine produced a dose-dependent reduction in responding to the trace CS1, regardless of whether subjects were trained with standard trace (CS1-->trace interval-->US) or serial (CS1-->CS2-->US) trials. Responding to CS2 was unaffected by scopolamine. These data suggest that central cholinergic systems are functional in the young animals, but are not normally sufficiently activated by standard trace conditioning procedures. The results suggest that serial compound conditioning can promote trace conditioning in young rats, as it does in adults, perhaps by enhancing cholinergic activity during training. Implications for the late ontogenetic emergence of trace conditioning as it relates to maturation of neural pathways and their role in the potentiating effects of a gap filler are discussed.     

A partial reinforcement extinction effect despite equal rates of reinforcement during Pavlovian conditioning

In 4 experiments rats received appetitive Pavlovian conditioning followed by extinction. Food accompanied every trial with the conditioned stimulus (CS) for the continuously reinforced groups and only half of the trials for the partially reinforced groups. In contrast to previous experiments that have compared the effects of partial and continuous reinforcement, the rate at which food was delivered during the CS was the same for both groups. The strength of the conditioned response during extinction weakened more rapidly in the continuously than in the partially reinforced groups. The results demonstrate that the partial reinforcement extinction effect is a consequence of the nonreinforced trials with the CS, rather than the rate at which the unconditioned stimulus is delivered during the CS.     

Preconditioning exposure to the unconditined stimulus affects the acquisition of a conditioned emotional response

Four experiments examined the UCS preexposure phenomenon using conditioned suppression of food-reinforced responding as a measure of excitatory conditioning, and electric shock as a UCS. In Experiment 1, groups of rats were preexposed to unsignaled 0.8-mA electric shocks for 0, 1, 3, 5, or 10 days, and then conditioned with a 0.8-mA electric shock. Preexposure to electric shock 1 day prior to conditioning enhanced the acquisition of a CER, whereas preexposure to electric shock for 3, 5, or 10 days prior to conditioning attenuated the acquisition of a CER as a direct function of the number of days of preexposure. In Experiments 2 and 2A, groups of rats were preexposed to unsignaled electric shocks of 0.3, 0.5, 0.8, or 1.3 mA for 10 days, and then conditioned with a 0.8-mA electric shocl. All groups preexposed to electric shock acquired the CER at a slower rate than a group not preexposed to electric shock. The greatest attenuation of CER conditioning occurred when the same intensity electric shock was used during both the preexposure and conditioning phases. In Experiment 3, groups of rats were preexposed to signaled electric shocks of either 0.5, 0.8, or 1.3 mA, and then conditioned with a 0.8-mA electric shock. All groups preexposed to electric shock acquired the CER at a slower rate than a group not preexposed to electric shock. As in Experiments 2 and 2A, the greatest attenuation of CER conditioning occurred when the same intensity electric shock was used during both the preexposure and conditioning phases. In Experiment 4, groups of rats were preexposed to series of 0.5, 0.8, or 1.3-mA electric shocks which they could escape by performing a chain-pull response. Rats in each of these groups had yoked partners which received the same number, intensity, and temporal pattern of electric shocks, but could not perform a response to escape shock. All groups were then conditioned with a 0.8-mA electric shock. Rats preexposed to escapable electric shocks showed equal or greater attenuation of CER conditioning than rats which could not escape shock during the preexposure phase. These results are discussed in terms of nonassociative and associative explanations of the UCS preexposure phenomenon.     

Associative explanations of discriminative inhibition effects

In two experiments rats were trained to discriminate between trials signalled by a tone during which responding was reinforced, and trials signalled by the tone in compound with a light. During tone-light trials reinforcers were still delivered, but there was no contingency between responding and reinforcement. Subsequent transfer tests showed that the light had acquired the ability to suppress operant responding. Various explanations of this phenomenon are discussed. It is concluded that the data are inconsistent with traditional accounts of discriminative control.