Backward and forward blocking in human electrodermal conditioning: Blocking requires an assumption of outcome additivity

Blocking was observed in two human Pavlovian conditioning studies in which colour cues signalled shock. Both forward (Experiment 1) and backward (Experiment 2) blocking was demonstrated, but only when prior verbal and written instructions suggested that if two signals of shock (A+ and B+) were presented together, a double shock would result (AB++). In this case, participants could assume that the outcome magnitude was additive. Participants given non-additivity instructions (A+ and B+ combined would result in the same outcome, a single shock) failed to show blocking. Modifications required for associative models of learning, and normative statistical accounts of causal induction, to account for the impact of additivity instructions on the blocking effect, are discussed. It is argued that the blocking shown in the present experiments resulted from the operation, not of an error-correction learning rule, nor of a simple contingency detection mechanism, but of a more complex inferential process based on propositional knowledge. Consistent with the present data, blocking is a logical outcome of an A+/AB+ design only if participants can assume that outcomes will be additive.     

Salience modulation in serial preexposure: implications for perceptual learning

In three experiments rats were given serial preexposure to two flavor stimuli. In Experiment 1, some animals were given exposure to AX followed by the presentation of BX, a forward schedule; the others were given backward preexposure (BX→AX). Conditioning and test trials with the A element showed that salience or effectiveness of A was better protected in the forward than in the backward condition. Experiments 2 and 3 assessed the relevance of this salience modulation mechanism for perceptual learning. In these experiments, generalization of a conditioned aversion from AX to BX was reduced in the forward (but not in the backward) condition only after prolonged exposure, indicating that the establishment of an inhibitory link from B to A is required for successful discrimination. However, generalization to a novel compound stimulus, NX, was reduced in the forward group both after short and long preexposure, suggesting the existence of salience modulation processes that work in parallel with associative inhibition. These results seem to support the existence of a salience modulation mechanism that seems to be beyond the scope of current theories of perceptual learning.     

Backward blocking in honeybees

Three experiments with foraging honeybees were designed to study the effect of experience with A on responding to B after AB+ training. In the first experiment, responding to B was the same whether the AB+ training was preceded or followed by A+ training. In the second experiment, responding to B after AB+ training was less in animals that also had A+ training than in control animals that were equally often reinforced in the absence of A; whether the A+ training preceded, was concurrent with, or followed the AB+ training made no difference. In the third experiment, responding to B after AB+ training was less when the AB+ training was followed by A+C- training than when it was followed by C+/A- training. These results, like those of some recent vertebrate experiments, take us beyond the traditional explanation of blocking in terms of impaired conditioning of B on AB+ trials and support the suggestion that the mechanism, still poorly understood, may nevertheless be a relatively simple one.     

Solving XOR

Three experiments examined the way in which exclusive-or (XOR) problems are solved by rats. All rats first received food-rewarded positive and negative patterning problems with two stimulus sets: either A+, B+, AB- and C-, D-, CD+, or A-, B-, AB + and C+, D +, and CD-. Subsequently, rats received revaluation trials in which A was paired with shock and C was not, prior to generalization test trials with B, D, AB, and CD (Experiments 1 & 2); or received A-->shock trials prior to tests with B and CD (Experiment 3). There was greater generalized fear to B than to either D (Experiments 1 & 2) or AB (Experiment 2) and CD (Experiments 2 & 3). These results are inconsistent with configural, connectionist models, but are consistent with an alternative connectionist model that can represent the logical structure of XOR problems.     

Elemental and non-elemental olfactory learning in Drosophila

Brain complexity varies across many orders of magnitude between animals, and it is often assumed that complexity underpins cognition. It is thus important to explore the cognitive capacity of widely used model organisms such as Drosophila. We systematically investigated the fly’s ability to learn discriminations involving compound olfactory stimuli associated with shock. Flies could distinguish binary mixtures (AB+ CD-), including overlapping mixtures (AB+ BC-). They could learn positive patterning (AB+ A- B-) but could not learn negative patterning (A+ B+ AB-) or solve a biconditional discrimination task (AB+ CD+ AC- BD-). Learning about the elements of a compound (AB+) was not affected by prior conditioning of one of the elements (A+ AB+): flies do not exhibit blocking in this task. We compare these results with the predictions from simulation of several well-known theoretical models of learning, and find none are fully consistent with the overall pattern of observed behaviour.     

Blocking of conditioned suppression: role of the first compound trial.

Four experiments, all employing the conditioned suppression of licking in thirsty rats, examined the extent to which reinforcement of one component, A, of a compound conditioned stimulus, AB, would "block" conditioning to the other element, B, on the first compound trial. Suppression to B was unaffected by prior reinforcement of A; that is, no evidence of blocking was obtained. If additional reinforced trials were given to the AB compound, further conditioning to B was blocked by prior reinforcement of A. Thus blocking appeared to take at least one trial to develop. These results suggest that blocking is not due to any competition between stimuli for association with reinforcement, but is a consequence of the animals' ignoring the added element once they have learned that it is redundant.     

Forward blocking in human learning sometimes reflects the failure to encode a cue–outcome relationship

In two “allergist” causal judgement experiments, participants were trained with a blocking design (A?+?|AB+). The procedure allowed different food cues to be paired with different fictitious allergic reactions. On test, participants were asked to rate the causal efficacy of the target cues and to recall the particular allergic reaction (outcome) that had followed each cue during training. Forward blocking was observed on the causal judgement measure and on the outcome recall measure in both Experiment 1 and Experiment 2. A backward blocking contingency was also trained in Experiment 2 (AB?+?|A+). Backward blocking was not observed either on the causal judgement or on the outcome recall measure. The evidence from the recall measure suggests that forward blocking in this task results from a failure to encode the B–outcome relationship during training. Associative and nonassociative mechanisms of forward blocking are discussed.     

Overshadowing and blocking procedures in latent inhibition

Three experiments used rats as subjects and a flavour-aversion procedure to examine the effect of compound flavour (AB) pre-exposure on the extent of latent inhibition to an element (A) of that compound. In Experiment 1 a group of rats exposed to AB exhibited less latent inhibition to stimulus A than a group for which A was presented in isolation or than a group that received A followed by a second stimulus, B, during the pre-exposure phase. Experiment 2 and Experiment 3 showed that the effectiveness of B during compound pre-exposure depended on its novelty. Thus a group that received exposure to stimulus B prior to AB pre-exposure showed more latent inhibition than a group that did not receive prior experience of the B stimulus. Two explanations for these effects were considered: that compound pre-exposure might reduce effective processing of the elements of that compound during the pre-exposure phase; alternatively, that exposure to an element as a part of a compound may modify the way in which that element is perceived.     

An extended comparator hypothesis account of superconditioning

Three conditioned taste aversion experiments with rats investigated superconditioning. In each experiment, alternate exposures of 2 flavor compounds with a common element (i.e., AB/AS) were administered to establish an inhibitory relationship between the 2 unique elements, B and S, and prior to testing, S was paired with lithium chloride (LiCl). In Experiment 1, pairings of a neutral cue (X) with S in compound with B after the AB/AS exposures resulted in superconditioning between X and S. Extinction of the common element (A) just before the S-LiCl pairing attenuated both the inhibitory relationship between B and S (Experiment 2) and superconditioning between X and S (Experiment 3). These observations suggest that superconditioning consists of enhanced performance rather than enhanced associative acquisition.     

Noradrenaline loss and the disruption of between-CS stimulus generalisation effects in aversion learning

In stimulus blocking the previous conditioning to one stimulus, A, followed by conditioning to the compound stimulus, AB, results in less conditioning to the other stimulus, B. Three experiments were performed to study the effect of the prior pairing of a stimulus A (noisy bottle) with lithium chloride followed by stimulus AB (saccharin plus noisy bottle) and lithium chloride pairings upon the strength of the aversion to the target stimuls B (saccharin) in NA-depleted and control rats. The results obtained were not in keeping with a "blocking" explanation since enhancement, rather than blocking, of the saccharin aversion was obtained, and confirm several recent demonstrations of the "anti-blocking" effect from investigations using the conditioned suppression procedure. DSP4 treatment attenuated the aversion-enhancement ("anti-blocking") effects in all three experiments. The possibility that DSP4-induced noradrenaline depletions may cause some disruption of between-CS associations leading to a lesser higher order conditioning or stimulus generalisation is discussed and the present findings do add further evidence to the conclusion that noradrenaline is intimately involved in attentional processes.     

The influence of context and UCS intensity on the UCS preexposure effect in a flavor aversion paradigm

Three studies were conducted to evaluate the influence of environmental context and UCS preexposure intensity on the acquisition of a flavor aversion. In the first study, rats were exposed to two UCS-alone presentations at one of three dose levels of LiCl. The rats were given the two UCS preexposures in one of two contexts (familiar or novel). Following the UCS-alone treatment, all of the subjects received two pairings of sucrose and LiCl (1.25 meq) in the familiar context. Animals receiving UCS preexposures in the same context as used during conditioning consumed significantly more sucrose than did saline control animals. In contrast, the influence of prior UCS preexposure was not evident when UCS preexposure was experienced in a context different from that experienced during conditioning. In Experiment 2, all animal received UCS preexposure and conditioning in the novel room. Animals received either two or four preexposures at 0, 1.25, or 3.75 meq dose of LiCl. Two UCS preexposures at 3.75 meq dose produced a stronger UCS preexposure effect than did the 1.25 meq dose. Animals in Experiment 3 received two exposures to either 0-, 1.25-, or 3.75-meq LiCl in one of two contexts, followed by conditioning with the 1.25-meq LiCl dose in the same context as preexposure. Greater UCS interference occurred in animals preexposed to LiCl in the novel rather than familiar environment, with no specific dose effects in either context. These observations are discussed in terms of context blocking and generalization decrement models of the UCS preexposure effect.     

Rats are sensitive to ambiguity

In the present study, we investigated response decisions made under conditions of incomplete information in rats. In Experiment 1, rats were trained on either a positive patterning (PP; A-, B-, AB+) or a negative patterning (NP; A+, B+, AB-) instrumental lever-press discrimination. Subjects that had learned an NP discrimination responded less to Cue A when Cue B was covered at test. The cover did not, however, affect test responses to Cue A in the PP condition. In Experiment 2, rats received concurrent training on both PP and NP discriminations. After concurrent training, responses to Cue A were different with B covered versus uncovered for both NP and PP discriminations. We discuss possible accounts for why exposure to a nonlinearly soluble discrimination (NP) may have affected sensitivity to cue ambiguity produced by the cover. These results have interesting implications for representational processes engaged in problem solving.     

Context conditional flavor preferences in the rat based on fructose and maltodextrin reinforcers

In three experiments, rats were exposed to a flavor preference procedure in which flavor A was paired with the reinforcer and flavor B presented alone in Context 1, while in Context 2 flavor A was presented alone and flavor B with the reinforcer. With fructose as the reinforcer both two- and one-bottle training procedures produced a context-dependent preference (Experiments 1 and 2). With maltodextrin as the reinforcer two-bottle training produced a context-dependent preference (Experiment 1). Following one-bottle training with maltodextrin reinforcement rats demonstrated a context-dependent preference when the conditioned stimulus (CS)- was presented with a dilute solution of the reinforcer during training (Experiment 3B) but not when the CS- was presented alone (Experiments 2 and 3A). The pattern of results with maltodextrin reinforcement suggests that there was competition between the cue flavors and the taste of the maltodextrin as predictors of the postingestive consequences of the maltodextrin reinforcer. The fact that rats were able to display context-dependent flavor preferences is consistent with the idea that learned flavor preferences rely on the sort of cue-consequence associations that underpin other forms of conditioning which produce accurate performance on biconditional tasks. The differences between fructose- and maltodextrin-based preferences are discussed in terms of configural and elemental learning processes.     

Forward blocking in human learning sometimes reflects the failure to encode a cue-outcome relationship

In two “allergist” causal judgement experiments, participants were trained with a blocking design (A + |AB+). The procedure allowed different food cues to be paired with different fictitious allergic reactions. On test, participants were asked to rate the causal efficacy of the target cues and to recall the particular allergic reaction (outcome) that had followed each cue during training. Forward blocking was observed on the causal judgement measure and on the outcome recall measure in both Experiment 1 and Experiment 2. A backward blocking contingency was also trained in Experiment 2 (AB + |A+). Backward blocking was not observed either on the causal judgement or on the outcome recall measure. The evidence from the recall measure suggests that forward blocking in this task results from a failure to encode the B-outcome relationship during training. Associative and nonassociative mechanisms of forward blocking are discussed.     

Evidence that blocking is due to associative deficit: Blocking history affects the degree of subsequent associative competition

Rats were trained on an appetitive discriminated operant procedure according to a blocking design, in which training with a compound stimulus (A + B) was given with or without prior training with A alone. Stimulus B was then presented in compound with a new stimulus (B + C compound). The ability of Stimulus B to block acquisition of control by Stimulus C was decreased by B having itself been blocked by A during the prior training. The results imply that the mechanism of blocking during the initial blocking procedure was a reduced association between Stimulus B and the reinforcer, and not a performance deficit such as that postulated by comparator models of learning.     

Effects of prior exposure on conditioned taste aversion in the rat: androgen- and estrogen-dependent events

The effects of preexposure and gonadal hormone manipulation on the extinction of a conditioned taste aversion were investigated. In Experiment 1, male rats were given one prior exposure to sucrose at some selected time (Days 4, 2, or 1) before a second exposure (Day 0) to sucrose and an LiCl injection. Controls received no prior exposure to sucrose but experienced only the sucrose + LiCl condition (Day 0). under the single exposure condition (Day 0) castrated animals extinguished the aversion faster than either testosterone-treated castrated rats or sham-operated rats. Sham-operated and castrated rats that had received preexposure treatment (Days 4, 2, or 1) were not distinguishable on a within-groups comparison of behavior but differed from their respective controls by exhibiting faster rates of extinction. However, in the testosterone-treated group only the Day 4 preexposure group exhibited a faster extinction rate. In Experiment 2, estradiol, dihydrotestosterone, and testosterone were studied by using only a Day 1 preexposure condition. The testosterone-treated group maintained the aversion for the longest period of time, followed by dihydrotestosterone-treated, sham, castrated, and estradiol-treated groups. It appears that estradiol augments the castration effect whereas dihydrotestosterone attenuates the effect. In Experiment 3, estradiol was administered alone or in combination with two different doses of dihydrotestosterone, and a Day 1 preexposure condition was used. The findings indicate that the outcome of behavior is dependent on the ratio of estradiol to dihydrotestosterone, with variations in this ratio resulting in fast (estrogen effect) to slow (androgen effect) rates of extinction.     

Predictive learning in nutrient-based flavor conditioning

This paper presents two experiments on nutrient-based flavor conditioning with rats as subjects and sucrose as the unconditioned stimulus (US). Experiment 1 was aimed at establishing an optimal control for conditioning, comparing simultaneous and serial presentations of a flavor and the US. The results showed that simultaneous, but not serial training, produced conditioning. Experiment 2 was designed to obtain evidence of summation as an index of both conditioned inhibition and predictive learning. Group Simultaneous received Pavlovian conditioned inhibition training during which flavor A was simultaneously paired with sucrose on excitatory trials (A+), and forming an unreinforced compound with flavor B on inhibitory trials (AB-). An independent excitor for the summation test was also trained by simultaneous pairings with sucrose (C+). In the control group (Blocked), the AB- trials were presented forming a block at the beginning of training to avoid a negative contingency-relationship with sucrose, and flavor A received serial rather than simultaneous pairing with sucrose (A --> +). On the summation test, only in group Simultaneous was consumption of the CB compound lower than that of flavor C alone, suggesting that, during training, flavor A activated an expectancy of the US occurrence.     

Flavor preferences produced by backward pairing with wheel running

In 3 experiments rats given 8 sessions of preexposure to wheel running acquired a preference for a flavor that was given immediately after each of 4 subsequent sessions of wheel running. Such flavor preference was less likely when rats were given the same conditioning procedure but without preexposure to wheels (Experiment 1) or when access to flavor was delayed by 30 min following a wheel session (Experiment 2). When rats were given a flavor before each wheel session, the resulting conditioned aversion was greater in rats that had no prior exposure to wheel running (Experiment 3). These results show that whether an aversion or preference for a flavor is produced by wheel running depends on an interaction between prior wheel experience and the sequence of events.     

Associative interference with taste aversions after contextual discrimination learning

In two experiments, rats were first given discriminative training with two distinctive contexts, such that a flavor was paired with lithium chloride (LiCl) in one context, alternating with presentations of the flavor alone in another context. Contextual control of the fluid ingestion was observed in that rats reduced the fluid intake in the LiCl-paired context but drank the solution in the context never paired with lithium. Having learned this discrimination, the rats were now given a second flavor in their home cage before being injected with LiCl and transferred to the previously lithium-paired context. In Experiment 1, the acquisition of an aversion to the novel flavor was blocked when this flavor and the contextual cues are conditioned as a compound. In Experiment 2, the blocking effect and the conditional control over fluid consumption were abolished when the associative strength of the LiCl-paired context had been extinguished by exposing the animals to water in the contexts after discriminative training. These results are interpreted as evidence that context dependency of conditioned taste aversions is mediated by a summative effect of the context–LiCl and flavor–LiCl associations.     

Signalling and incentive processes in instrumental reinforcer devaluation

We have previously reported that conditioning an aversion to the reinforcer using an isotonic lithium chloride (LiCl) solution following instrumental training reduces performance in a subsequent extinction test only if animals are re-exposed to the reinforcer prior to the test. Rescorla (1992), in contrast, reported an immediate devaluation effect using a hypertonic LiCl solution that did not depend upon re-exposure. In two experiments we examined the effect of using a hypertonic LiCl solution to condition the aversion to the reinforcer on subsequent instrumental performance in extinction, with and without re-exposure. In Experiment 1 thirsty rats were trained to press a lever for a sucrose solution before being injected with 0.6 M LiCl either immediately or after a delay. Half of the immediate and delay groups were then re-exposed to the sucrose in the absence of the lever, with the remainder being exposed to water. Contrary to the previously reported effects of isotonic LiCl, a hypertonic solution induced a reinforcer devaluation effect in all the immediately poisoned animals, which did not depend upon re-exposure to the reinforcer. In Experiment 2 the possibility that this devaluation effect was induced by the discomfort associated with the hypertonicity of the solution was assessed by replicating Experiment 1 but, in addition, using two immediately poisoned groups given the LiCl injection under anaesthesia. In the absence of anaesthesia, the devaluation effect observed without re-exposure to the reinforcer in Experiment 1 was replicated. When the injection was given under anaesthesia, however, a reinforcer devaluation effect was observed only in animals that were re-exposed to the reinforcer prior to the extinction test. These results were interpreted as evidence that a reinforcer devaluation effect induced by pairing the reinforcer with illness depends upon a process of incentive learning, whereas a devaluation effect mediated by learning a signalling relationship between the reinforcer and somatic discomfort does not.