The effects of ITI fillers in autoshaping

The present experiments used pigeons in an autoshaping procedure to examine the effects of a nonreinforced, nontarget stimulus presented during the intertrial interval on responding to a target CS. Experiments 1 and 2 found that a filler stimulus presented during a substantial portion of the ITI retarded responding to the target CS relative to a group not exposed to the filler. This group difference in performance was subsequently abolished by continued training and omitting the filler in the former group (Experiment 1) or adding a filler for the latter group (Experiment 2). Experiment 3 found that when the “filler” occupied most of the interreinforcement interval, CS-US pairings embedded within the “filler” stimulus yielded superior autoshaping relative to a group that received CS-US pairings embedded in static apparatus cues in the absence of the filler. The results are discussed with reference to the ways that a nontarget stimulus during the ITI can influence contextual modulation of responding to a discrete CS and the necessity for comparator theories to incorporate a “local context” view of cycle time to explain the present findings.     

Duration of signals for intertrial reinforcement and nonreinforcement in random control procedures

In the random control procedure, responding to a conditioned stimulus (target CS) is prevented when the probability of unsignaled, unconditioned stimuli (USs) in the intertrial interval (ITI) is equal to the probability of the US in the presence of the target CS. Three experiments used an autoshaping procedure with White Carneaux pigeons to examine the effects of the temporal duration of signals for the ITI USs (cover CSs) and for concomitant periods of nonreinforcement. In Experiment 1, a short duration cover, but not a long duration cover, resulted in responding to the target CS. In Experiment 2, an explicit CS- cue during periods of nonreinforcement did not affect target acquisition. In Experiment 3, a long CS-, but not a short cover CS, was a sufficient condition for the acquisition of responding to the target CS. These results imply that the acquisition of responding to a target CS requires a discriminable period of nonreinforcement that is long relative to the target CS duration.     

Intertrial interval as a contextual stimulus: further analysis of a novel asymmetry in temporal discrimination learning

Four experiments investigated discrimination learning when the duration of the intertrial interval (ITI) signaled whether or not the next conditional stimulus (CS) would be paired with food pellets. Rats received presentations of a 10-s CS separated half the time by long ITIs and half the time by short ITIs. When the long ITI signaled that the CS would be reinforced and the short interval signaled that it would not be (Long+/Short-), rats learned the discrimination readily. However, when the short ITI signaled that the CS would be reinforced and the long interval signaled that it would not (Short+/Long-), discrimination learning was much slower. Experiment 1 compared Long+/Short- and Short+/Long- discrimination learning with 16-min/4-min or 4-min/1-min ITI combinations. Experiment 2 found no evidence that Short+/Long- learning is inferior because the temporal cue corresponding to the short interval is ambiguous. Experiment 3 found no evidence that Short+/Long- learning is poor because the end of a long ITI signals a substantial reduction in delay to the next reinforcer. Long+/Short- learning may be faster than Short+/Long-because elapsing time involves exposure to a sequence of hypothetical stimulus elements (e.g., A then B), and feature-positive discriminations (AB+/A-) are learned quicker than feature-negative discriminations (A+/AB-). Consistent with this view, Experiment 4 found a robust feature-positive effect when sequentially presented CSs played the role of elements A and B.     

Overshadowing and stimulus duration

In 3 experiments, the authors investigated the effect of stimulus duration on overshadowing. Experiments 1 and 2 examined responding to a target conditioned stimulus (CS1) when it was conditioned in compound with a coterminating overshadowing stimulus (CS2) that was longer, shorter, or of the same duration (the long, short, and matched groups, respectively). Equal overshadowing of conditioning to CS1 was obtained in all 3 conditions relative to a control group conditioned to the light alone. There were, however, differences in responding to CS2 as a function of its absolute duration. Experiment 3 examined the contribution of the food-food interval/CS onset-food interval ratio to these findings. In Experiments 1 and 2, the ratio differed for the overshadowing CS but not for the target CS. In Experiment 3, this arrangement was reversed, but the pattern of results remained the same. The implications of these findings for trial-based and real-time models of conditioning are discussed.     

Manipulation of CS-US conditional probability and of the US-US trace interval on conditioning to the CS and to a background stimulus in a CER situation

Two experiments examined the relationship between conditioning to the CS and background using a novel CER paradigm, in which a long background stimulus played the role of more conventional contextual cues. Experiment 1 manipulated the probability of US occurrence given the CS (p(US/CS)). Conditioning to the background was not a monotonically decreasing function of p(US/CS) at all shock intensities, and conditioning to the CS was remarkably insensitive to the value of p(US/CS) when assessed off the baseline. Experiment 2 manipulated the trace interval between the CS and US. Although conditioning to the CS decreased as the trace interval increased, conditioning to the background was dependent upon whether it served as the interstimulus interval (ISI; interval between the CS and US) or intertrial interval (ITI; interval between CS-US pairs) stimulus. Conditioning to the ISI background decreased as the trace interval increased, but conditioning to the ITI background at first increased, but then decreased as the trace interval was further increased. These results are discussed with respect to the adequacy of contemporary models of conditioning.     

The role of contextual associations in producing the partial reinforcement acquisition deficit

Three conditioned suppression experiments with rats as subjects assessed the contributions of the conditioned stimulus (CS)-context and context-unconditioned stimulus (US) associations to the degraded stimulus control by the CS that is observed following partial reinforcement relative to continuous reinforcement training. In Experiment 1, posttraining associative deflation (i.e., extinction) of the training context after partial reinforcement restored responding to a level comparable to the one produced by continuous reinforcement. In Experiment 2, posttraining associative inflation of the context (achieved by administering unsignaled outcome presentations in the context) enhanced the detrimental effect of partial reinforcement. Experiment 3 found that the training context must be an effective competitor to produce the partial reinforcement acquisition deficit. When the context was down-modulated, the target regained behavioral control thereby demonstrating higher-order retrospective revaluation. The results are discussed in terms of retrospective revaluation, and are used to contrast the predictions of a performance-focused model with those of an acquisition-focused model.     

Effect of signaling intertrial unconditioned stimuli in autoshaping

Context-unconditioned-stimulus (US) associations have been suggested as the mediator of the response decrement that occurs when extra USs are added to the intertrial intervals (ITIs) of an otherwise standard Pavlovian conditioning situation. The present autoshaping experiments were concerned with the effect of signaling those extra USs, since such signaling might be expected to lessen their ability to condition the context. Experiments 1 and 2 showed that signaling the ITI USs did reduce their detrimental effects on responding to the conditioned stimulus (CS). To determine whether that reduction was due to an impact of signaling on the target-CS-US association or on performance to the target-CS, Experiment 3 examined responding to differentially trained CSs in a common context, as well as responding to identically trained CSs in differentially trained contexts. Whether the CS was tested in a context of relatively high or low associative strength, more responding occurred to the CS trained with signaled, as compared with unsignaled, ITI USs; further, there was more responding to that CS in the more highly valued context. The pattern of results suggests that contextual value does interact with CS-US learning and may also affect performance to the CS.     

Conditioned stimulus informativeness governs conditioned stimulus-unconditioned stimulus associability

In a conditioning protocol, the onset of the conditioned stimulus ([CS]) provides information about when to expect reinforcement (unconditioned stimulus [US]). There are two sources of information from the CS in a delay conditioning paradigm in which the CS-US interval is fixed. The first depends on the informativeness, the degree to which CS onset reduces the average expected time to onset of the next US. The second depends only on how precisely a subject can represent a fixed-duration interval (the temporal Weber fraction). In three experiments with mice, we tested the differential impact of these two sources of information on rate of acquisition of conditioned responding (CS-US associability). In Experiment 1, we showed that associability (the inverse of trials to acquisition) increased in proportion to informativeness. In Experiment 2, we showed that fixing the duration of the US-US interval or the CS-US interval or both had no effect on associability. In Experiment 3, we equated the increase in information produced by varying the C/T ratio with the increase produced by fixing the duration of the CS-US interval. Associability increased with increased informativeness, but, as in Experiment 2, fixing the CS-US duration had no effect on associability. These results are consistent with the view that CS-US associability depends on the increased rate of reward signaled by CS onset. The results also provide further evidence that conditioned responding is temporally controlled when it emerges.     

The effects of relative intensity of cue and marker on marked trace conditioning in pigeons

Pigeons were trained on trace conditioning procedures (autoshaping) in which a keylight conditioned stimulus (CS) was presented for 5 sec and followed after a 10-sec temporal gap by food. In Experiment 1, acquisition of conditioned responding to the CS was facilitated when it was immediately followed by a brief stimulus, but only when the brief stimulus was of greater intensity than the trace CS. In Experiment 2, this facilitation effect was found when a more intense brief stimulus followed the trace CS immediately, but not when it was delayed. In both experiments, responding to the target CS was compared within-subjects to that to a control CS, which eliminated factors such as second-order conditioning as an explanation for the facilitation effects. Together, the results of Experiments 1 and 2 suggest that a brief stimulus may mark immediately preceding events in memory and thus facilitate their association with delayed reinforcement only if the brief stimulus is relatively more intense, and thus more salient, than the target events. In Experiment 3, we found substantial generalization of this facilitation effect from the marked CS to the control CS and conclude that the results of Experiments 1 and 2 may underestimate the magnitude of these marking effects.     

The extinction of an aversion: Role of the interval between non-reinforced presentations of the averted stimulus

A toxiphobia conditioning paradigm was used to examine the relation between the intertrial interval (ITI) and the extinction of an aversion. The design employed was based upon that developed by Davis (1970) to study the relation between the ITI and the habituation of a response. After conditioning, the conditioned stimulus (CS) was presented on two occasions at times T1 and T2, and this interval (the ITI) was varied across groups. However, the interval from the CS exposure at T2 to the extinction test was common for all groups. On the test, ITIs of 6 to 24 hr were found to have promoted more extinction than ITIs of 0.5 hr with odour (Experiments 1a and b) and flavour (Experiment 1c) CSs. Further, this facilitation of extinction by the long ITIs compared to the short ones was not due to differences in the intervals between the initial CS exposure at T1 and the test (Experiment 2). The final experiment examined the relation between the ITI and extinction when different CSs were presented at T1 and T2. A short interval between the presentation of CS1 at T1 and the presentation of CS2 at T2 was found to have facilitated the extinction of the aversion to CS2 compared either to a long interval between these presentations or to the presentation of CS2 in the absence of a prior CS1 presentation. The results were discussed in terms of the model developed by Wagner (1981).     

Automaintenance,contrast, and contingencies: Effects of local vs. overall and prior vs. impending reinforcement context

The pigeon's keypeck was investigated in a variety of multiple schedules of response-independent or -dependent reinforcement. Experiments 1 and 2 found that keypecking developed to a reinforcement-associated cue that signaled an increase in local rate of reinforcement, but signaled either no change or a decrease in the overall rate of reinforcement, defined as that prevailing in the cue's absence. In Experiment 3 a greater rate of responding to a target stimulus was observed when it was preceded by a second signal—associated with the same, or a lower, rate of reinforcement—and followed by extinction than when this temporal sequence was reversed. In Experiment 4 responding to a target cue increased when either a temporally prior or a subsequent reinforcement-associated cue was changed to signal extinction. Experiment 5 examined the conditioned reinforcing effectiveness of target cues in two types of situations varying the local context of reinforcement. Stimuli associated with selected target components of a response-dependent multiple schedule of reinforcement could appear as the terminal-link consequences in a two-link chain schedule. Enhanced responding during a target cue which accompanied the introduction of an extinction period following this cue was paralleled by an increase in the conditioned reinforcement effects of this cue. No such increase was found for target cues in which an enhancement of responding had been produced by the interposition of an extinction period prior to the cue.     

Contextual control of appetitive conditioning: Influence of a contextual stimulus generated by a partial reinforcement procedure

Three experiments with rat subjects examined the effects of a context switch after conditioning treatments in which the conditioned stimulus (CS) was either paired with food on every presentation (continuous reinforcement) or on some of its presentations (partial reinforcement). In each experiment, a target CS was given one of these treatments in Context A, and another CS was given a treatment during sessions that were intermixed in Context B. Final tests of the target CS in Context A and Context B often revealed no loss of responding with the switch to B. However, a loss was observed when partial reinforcement had been associated with Context A and continuous reinforcement had been associated with Context B. Those conditions caused equal decrements in responding to partially reinforced and continuously reinforced targets. The results suggest that under the present conditions partial reinforcement can generate a contextual stimulus that becomes associated with the physical context and controls responding to the CS.     

Effects of contingency violations on the extinction of a conditioned fear inhibitor and a conditioned fear excitor

Rats were used in a conditioned-suppression paradigm to assess the effects of contingency variations on responding to a conditioned inhibitor (CS-) and a conditioned excitor (CS+). In Experiment 1, various unconditioned stimulus (US) frequencies were equated across the presence and absence of a CS- in the context of either background cues (continuous-trial procedure) or an explicit neutral event (discrete-trial procedure). With both procedures, a CS-alone treatment enhanced inhibition, whereas treatments involving 50% or 100% reinforcement for the CS- eliminated inhibition without conditioning excitation to that CS. The latter outcome also occurred in Experiment 2, with discrete-trial training equating considerably reduced US frequencies for the presence and absence of the CS-. In further evidence that inhibition was eliminated without conditioning excitation to the CS-, Experiment 3 showed that a novel CS did not acquire excitation when 25%, 50%, or 100% reinforcement was equated across the presence and absence of that CS in the context of a discrete-trial event. Using the procedures of Experiment 1, Experiment 4 showed that a CS+ was extinguished by a CS-alone treatment but was substantially maintained by treatments involving 50% or 100% uncorrelated reinforcement. These effects for a CS+ and a CS- implicate CS-US contiguity, rather than contingency, as the factor determining the extinction of a CS.     

Prevention of the degraded-contingency effect by signalling training trials

Presentation of unsignalled unconditioned stimuli (USs) interspersed among Pavlovian excitatory conditioning trials weakens conditioned responding to a target conditioned stimulus (CS; Rescorla, 1968). However, signalling these intertrial USs with another cue (a cover stimulus) has been shown to alleviate this degraded-contingency effect (e.g. Durlach, 1982, 1983). In contrast to signalling the inter-trial USs, the present experiments examined the effect on the degraded-contingency effect of signalling the target CS-US pairings. Experiment 1, using parameters selected to avoid overshadowing, found that consistently presenting a cover stimulus immediately prior to the target CS-US pairings during degraded-contingency training alleviated the degraded-contingency effect. Experiment 2 examined the underlying mechanism responsible for this cover-stimulus effect through posttraining associative inflation of the cover stimulus or the context, and found that inflation of the cover stimulus attenuated responding to the target CS (i.e. empirical retrospective revaluation). The results are discussed in terms of various acquisition- and expression-focused models of acquired responding.     

The role of context in intertrial interval effects in autoshaping

Two experiments evaluated the role of differential conditioning of the context in mediating the effect of intertrial interval (ITI) in autoshaping. In Experiment 1 pigeons were given acquisition with two keylights, each presented in a particular context. A given keylight/context combination had associated with it either a short (10-sec) or a long (2-min) ITI. Acquisition was more rapid with the long ITI. Tests with those keylights in a common third context indicated that the longer ITI had resulted in greater conditioning. On the other hand, pigeons trained on keylights with mixed ITIs in a third context evoked more responding when they were tested in the short ITI context compared with the long ITI context. That suggests that a context with a history of a short ITI enhances performance. In Experiment 2, two keylights were initially conditioned with mixed ITIs and then extinguished in different contexts under different ITI lengths. Extinction was more rapid for the keylight presented with a short ITI. That difference persisted when the keylights were tested with mixed ITIs in a common third context, suggesting a difference in associative strength of the keylights. The results are interpreted in terms of differential context conditioning resulting in differences in learning about the keylight.     

Responding to a conditioned stimulus depends on the current associative status of other cues present during training of that specific stimulus.

The comparator hypothesis is a response rule stating that responding to a Pavlovian conditioned stimulus (CS) reflects the associative strength of the CS relative to that of other cues (comparator stimuli) that were present during CS training. Thus, modulation of the associative strength of a CS's comparator stimulus should alter responding to that CS. These studies examined the stimulus specificity of this effect using within-subjects designs. Rats were trained on 2 CSs, each with a unique comparator stimulus, to determine the degree to which posttraining extinction of the comparator stimulus for one CS influences responding to the other CS. Using negative contingency (Experiments 1 and 2), overshadowing (Experiment 3), and local context (Experiment 4) preparations, stimulus specificity was observed. In each case, posttraining extinction of the comparator stimulus for one CS had greater impact on responding to that CS than on responding to the alternate CS.     

Appetitive pavlovian-instrumental interactions: Effects of inter-stimulus interval and baseline reinforcement conditions

Experiment I manipulated two variables which appear to influence whether a signal for food enhances or suppresses food-rewarded instrumental performance: interstimulus interval (ISI) during classical conditioning and instrumental reinforcement schedule during testing. In two groups a 10-s conditioned stimulus (CS) and food were paired (10-s and 20- to 100-s ISI), while in a third group they were unpaired. During signalled reinforcement of lever-pressing (S+), the CS suppressed responding in both paired groups. During signalled extinction (S-), responding in the 10-s ISI group was suppressed during the CS and enhanced for 60 s after CS offset; responding in the 20- to 100-s ISI group was enhanced both during the CS and for 120 s after CS offset. Experiment II examined whether the long ISI enhancement effect would occur when the baseline response rate was lowered by satiation rather than extinction. A 20- to 100-s CS and food were paired in one group and unpaired in another. After near-satiation on a CRF schedule, CS presentations caused a reduction in responding in both groups, with no significant difference between the two groups. The results of the two experiments were interpreted in terms of an interaction between the expectancy of food generated by stimuli conditioned at short and long ISIs and the expectancy of food availability controlled by the instrumental schedule.     

Stimulus and temporal cues in classical conditioning

In 2 experiments, separate groups of rats were given stimulus conditioning, temporal conditioning, untreated control and (in Experiment 2) learned irrelevance control procedures, followed by a compound with both stimulus and temporal cues. Stimulus conditioning consisted of a random 15-s duration conditioned stimulus (CS) followed by food; temporal conditioning consisted of food-food intervals of fixed 90 s (Experiment 1) or fixed 75 + random 15 s (Experiment 2). The stimulus group abruptly increased responding after CS onset, and the temporal group gradually increased responding over the food-food interval. When the food-food interval was fixed 90 s, the temporal cue exerted stronger control in the compound, whereas when the food-food interval was fixed 75 + random 15 s, the stimulus cue exerted stronger control. The strength of conditioning, temporal gradients of responding, and cue competition effects appear to reflect simultaneous timing of multiple intervals.     

Interaction between preexposure and overshadowing: Further analysis of the extended comparator hypothesis

Three experiments with rats used conditioned suppression of barpress to test predictions of the extended comparator hypothesis, which assumes that the effectiveness of (first-order) comparator stimuli in modulating responding to a target conditioned stimulus (CS) is itself modulated by other (second-order) comparator stimuli. Experiment 1 demonstrated that both pretraining exposure to the target CS alone (i.e., CS-preexposure effect, also known as latent inhibition) and pretraining exposure to a compound of the target CS and nontarget CS (i.e., compound-CS-preexposure effect) counteract overshadowing, and that posttraining deflation (i.e., extinction) of the overshadowing stimulus attenuates responding to the target CS when overshadowing is preceded by a CS-preexposure treatment (i.e., yields a CS-preexposure effect), but not when overshadowing is preceded by a compound-CS-preexposure treatment. Experiment 2 examined the consequences of posttraining associative inflation of the overshadowing stimulus or the preexposure companion stimulus following conjoint compound-CS-preexposure and overshadowing treatment. Experiment 3 examined the consequences of posttraining inflation of the overshadowing stimulus or the context following conjoint CS-alone preexposure and overshadowing treatment. The results support the expression-focused comparator view in contrast to recent acquisition-focused models of retrospective revaluation.     

Temporal learning in random control procedures

Experiments 1 and 2 delivered conditioned stimuli (CSs) at random times and unconditioned stimuli (USs) at either fixed (Experiment 1) or random (Experiment 2) intervals. In Experiment 3, CS duration was manipulated, and US deliveries occurred at random during the background. In all 3 experiments, the mean rate of responding (head entries into the food cup) in the background was determined by the mean US-US interval, and the mean rate during the CS was a linear combination of responding controlled by the mean US-US and mean CS onset-US intervals; the pattern of responding in time was determined by the interval distribution form (fixed or random). An event-based timing account, Packet theory, provided an explanation of the results.