Habituation in children within a behavior suppression paradigm

Using an instrumental response suppression paradigm, Experiment 1 demonstrated habituation and long-term retention of habituation to a tone stimulus in third-grade children, but specificity of habituation was not obtained when a new tone stimulus was presented. In Experiment 2 first graders showed specificity of habituation to auditory stimuli on the response suppression measure whereas fifth graders did not. However, both age groups evidenced specificity when a skin conductance measure was employed. In addition, specificity of habituation was not affected by amount of training at either age level. Experiment 3 demonstrated specificity of habituation in fifth graders on the response suppression measure when a cross-modality stimulus change was introduced. Implications of these data for analyses of habituation and traditional learning are discussed.     

Mechanisms underlying retarded emergence of conditioned responding following inhibitory training: evidence for the comparator hypothesis

The comparator hypothesis posits that conditioned responding is determined by a comparison at the time of testing between the associative strengths of the conditioned stimulus (CS) and stimuli proximal to the CS at the time of conditioning. The hypothesis treats all associations as being excitatory and treats conditioned inhibition as the behavioral consequence of a CS that is less excitatory than its comparator stimuli. Conditioned lick suppression by rats was used to differentiate four possible sources of retarded responding to an inhibitory CS. These include habituation to the unconditioned stimulus (US), latent inhibition to the CS, blocking of the CS-US association by the conditioning context, and enhanced excitatory associations to the comparator stimuli. Prior research has demonstrated the first three phenomena. Therefore, we employed parameters expected to highlight the fourth one--the comparator process. In Experiment 1, our negative contingency training was shown to produce a conditioned inhibitor that passed inhibitory summation and retardation tests. In Experiment 2 we found transfer of retardation from an inhibitory CS to a novel stimulus when the location where retardation-test training occurred was excitatory, which is indicative of contextual blocking and/or comparator effects. In Experiment 3, extinction of the conditioning context was found to attenuate retardation regardless of whether extinction occurred before or after the CS-US pairings of the retardation test. This indicates that much of the present retardation was due to the comparator process rather than to contextual blocking. Experiment 4 demonstrated that habituation to the US did not contribute to retardation in the present case. Collectively, these studies suggest that retardation following inhibitory training can be explained without recourse to any of the traditional mechanisms of conditioned inhibition.     

Potentiation of the acoustic startle response by a conditioned stimulus paired with acoustic startle stimulus in rats

The hypothesis that the standard acoustic startle habituation paradigm contains the elements of Pavlovian fear conditioning was tested. In a potentiated startle response paradigm, a startle stimulus and a light conditioned stimulus (CS) were paired. A startle stimulus then was tested alone or following the CS. Freezing behavior was measured to index conditioned fear. The startle response was potentiated on CS trials, and rats froze more in CS than in non-CS periods. In Experiment 1, response to a previously habituated, weak startle stimulus was potentiated. In Experiment 2, response to the same stimulus used as the unconditioned stimulus (US) in training was potentiated. This CS-potentiated response retarded the course of response decrements over training sessions as compared with an explictly unpaired control group. Conditioned fear is a standard feature of this habituation paradigm, serves to potentiate the startle response, and provides an associative dimension lacking in the habituation process per se.     

A search for context-stimulus associations in latent inhibition

Six experiments investigated the effects of pre-exposure to a tone on the subsequent acquisition of conditioned suppression by rats. In Experiments 1-3 the response suppressed was drinking; in Experiments 4-6 it was food-rewarded lever pressing. Repeated exposure to the tone resulted in latent inhibition, i.e., a retardation in the acquisition of suppression. The size of the latent inhibition effect was reduced when a different context was used for conditioning from that used for pre-exposure (Experiment 2). When the context remained the same throughout, a phase of exposure to the context alone, interposed between pre-exposure and conditioning, had no influence on the size of the latent inhibition effect ultimately observed. This last result casts doubt upon Wagner's (1976, 1979) theory of the role of contextual factors in latent inhibition, and alternative accounts are considered.     

Latent Inhibition of Contextual Stimuli Reduces the US Preexposure Effect in Rats

Two experiments are reported which utilized latent inhibition of contextual stimuli prior to administering unsignaled presentations of the Us in an attempt to further assess the role of contextual stimuli in the US preexposure effect. Specifically in Experiment 1, rats received either 0-, 5-, 10-, or 15-minute exposure to the context in which the unsignaled Uss were to occur (latent inhibition). Following preexposure to the US, animals were trained in a CER paradigm with a tone CS. Measures of suppression to the tone indicated that the greatest US preexposure effect occurred in the 0 group and that no US preexposure effect was evident in the 15 group. Experiment 2 includec two important control groups which were omitted in Experiment 1 (no US preexposures) and an additional dependent variable (time to initiate licking) to measure fear to contextual stimuli. These results are discussed in terms of the role context conditioning may play in US preexposure.

     

Influence of long-term sensitization on long-term habituation of the acoustic startle response in rats: central gray lesions, preexposure, and extinction

The relation between long-term decrements of the acoustic startle response in rats and the development of freezing behavior during habituation training was examined. Freezing behavior developed over the initial trials of habituation training, and the rate of long-term response decrements was found to be inversely related to the development of freezing. Manipulations (neurological or behavioral) that either reduced the level of freezing or retarded its development promoted startle response decrements. In Experiment 1, rats receiving electrolytic lesions of the ventrolateral periaqueductal gray demonstrated both accelerated long-term startle response decrements and retarded development of freezing behavior. In Experiment 2, preexposure to the startle apparatus (i.e., latent inhibition) accelerated long-term startle decrements and inhibited development of freezing. In Experiment 3, exposure to the startle apparatus following initial habituation training (i.e., extinction) reduced both freezing behavior and startle response amplitudes. The results are discussed in terms of the influence of Pavlovian fear conditioning on long-term habituation of the acoustic startle response.     

Associative effects of US preexposure: modulation of conditioned responding by an excitatory training context

In two experiments we examined factors that contribute to retarded emergence of conditioned responding to a conditioned stimulus (CS) trained in a context in which unsignaled unconditioned stimuli (USs) had previously been administered. In both experiments water-deprived rats were used in a conditioned lick suppression task to measure the conditioned response elicitation potential of the CS and the training context. From Experiment 1 we determined that nonreinforced exposure to the excitatory context after US preexposure and prior to CS-US pairings in that context eliminated the conditioned response deficit observed on a subsequent test of the CS. The recovery from the US preexposure deficit was nearly as great in animals that received nonreinforced exposure to the excitatory training context after the CS-US pairings but prior to the ultimate test of the CS. From Experiment 2 we determined that the recovery induced by contextual deflation after CS training was specific to deflation of the context in which the CS was trained as opposed to another excitatory context. In total, these experiments suggest that context-US associations partially mask the expression of a learned CS-US association. These results are discussed in terms of recent models of conditioned response generation.     

Recovery of an overshadowed association achieved by extinction of the overshadowing stimulus

Three experiments are reported in which conditioned lick suppression by water-deprived rats was used as an index of associative strength. In Experiment 1, overshadowing of a light by a tone was observed when the light-tone compound stimulus was paired with foot shock. After initial compound pairings, the tone-shock association was extinguished in one group of subjects. Subsequently, these animals demonstrated significantly higher levels of suppression to the light relative to a control group in which the tone had not been extinguished. Experiment 2 replicated this effect while failing to find evidence to support the possibilities that extinction presentations of the overshadowing tone act as retrieval cues for the light-shock association, or that, via second-order conditioning, the light-shock association is actually formed during extinction of the tone. Experiment 3 determined that the recovery from overshadowing observed in Experiments 1 and 2 was specific to the extinction of the overshadowing stimulus rather than the extinction of any excitatory cue. Collectively, these results suggest that the debilitated response to an overshadowed stimulus does not represent an acquisition failure, but rather the failure of an acquired association to be manifest in behavior.     

Contextual potentiation of acquired behavior after devaluing direct context-US associations

In two experiments, water-deprived rats were exposed to tone-footshock pairings in a particular conditioning context. In Experiment 1, the training parameters were selected on the basis of parametric pilot data so as not to produce direct associations between the conditioning context and footshock. Despite independent evidence of no appreciable direct association between the conditioning context and footshock, animals tested in the conditioning context exhibited more lick suppression to the tone than other animals tested with the tone outside of the conditioning context. Thus, the conditioning context potentiated suppression to the tone in the absence of appreciable fear of the conditioning context. In Experiment 2, training parameters were used that permitted the formation of direct associations between the conditioning context and footshock; however, for half the animals, these associations to the context were then extinguished. With the tone absent, a comparison of extinguished animals tested either in the conditioning context or outside the conditioning context detected no differences in suppression, demonstrating the effectiveness of the extinction manipulation. However, other animals also subjected to extinction of direct context-footshock associations displayed greater suppression to the tone within the conditioning context than comparable animals tested outside the context. Collectively the data indicate that the superior retention-test performance seen within the training context can arise both from the commonly assumed direct associations between the training context and the unconditioned stimulus and from the potentiation by the training context of the associations between the nominal conditioned stimulus and unconditioned stimulus.     

No sex difference in contextual control over the expression of latent inhibition and extinction in Pavlovian fear conditioning in rats

Three experiments with Wistar rats searched for a sex difference in contextual control over the expression of latent inhibition and extinction. Experiment 1 used a latent inhibition procedure; Experiments 2 and 3 employed an extinction preparation. All experiments used a shock as the unconditioned stimulus, a tone as the conditioned stimulus, and suppression of food magazine visits as the measure of conditioned responding to the tone. Each experiment revealed a reliable context effect on conditioned responding to the tone; after conditioning in a separate context, conditioned responding in the former latent inhibition or extinction context was attenuated relative to conditioned responding in a control context. There was no sex difference in the magnitude of this effect. These results are discussed in the framework of sex differences in the hippocampus and of the putative role of this structure in various instances of contextual learning.     

The effect of cuing after backward conditioning trials

Rats were given backward pairings of a tone (CS) and shock (US) and were then tested for response to the CS in a lick suppression test. Animals given a cuing or reminder treatment prior to the test exhibited enhanced suppression in the presence of the tone relative to a variety of control conditions. The cue-induced suppression enhancement did not appear to result from sensitization or short-term motivational changes nor did it seem to depend on second-order conditioning of the test context. The effect appeared more robust as the number of backward conditioning trials preceding the cue increased. The results were discussed in terms of current explanations of backward conditioning effects.     

Contextual dependencies in predictive learning

Two experiments assessed contextual dependencies in a predictive-learning task. Subjects learned to associate each of four pictorial stimuli with the occurrence or non-occurrence of a specific outcome. Each of these stimuli, the intentional stimuli, was presented against one of two different visual (Experiment 1) or auditory (Experiment 2) context stimuli. These context stimuli were incidental: subjects were not explicitly instructed to pay any attention to them and each of them in isolation was not predictive of the outcome. During acquisition and testing, subjects expressed the expected relationship between intentional stimulus and outcome by an appropriate key press. At test, intentional stimuli were presented either with the same contextual stimulus as also present during acquisition (same trials), or with the other one (switched trials). The response latency was slower on switched trials than on same trials in each experiment, a result extending previous findings on the effect of environmental contextual stimuli on task performance. Results are discussed in the framework of contextual occasion setting and habituation to contextual stimuli.     

Contextual dependencies in predictive learning

Two experiments assessed contextual dependencies in a predictive-learning task. Subjects learned to associate each of four pictorial stimuli with the occurrence or non-occurrence of a specific outcome. Each of these stimuli, the intentional stimuli, was presented against one of two different visual (Experiment 1) or auditory (Experiment 2) context stimuli. These context stimuli were incidental: subjects were not explicitly instructed to pay any attention to them and each of them in isolation was not predictive of the outcome. During acquisition and testing, subjects expressed the expected relationship between intentional stimulus and outcome by an appropriate key press. At test, intentional stimuli were presented either with the same contextual stimulus as also present during acquisition (same trials), or with the other one (switched trials). The response latency was slower on switched trials than on same trials in each experiment, a result extending previous findings on the effect of environmental contextual stimuli on task performance. Results are discussed in the framework of contextual occasion setting and habituation to contextual stimuli.     

Reminder-induced recovery of associations to an overshadowed stimulus

The possibility of reversing the deficit produced by overshadowing through the use of memory reactivation was investigated. Using lick suppression as a measure of associative strength, water-deprived rats were conditioned in a Pavlovian paradigm which produced reliable overshadowing of a flashing light by a tone. It was found, however, that exposure to the overshadowed stimulus outside of the conditioning context during the retention interval (reminder treatment) caused an increase in lick suppression during testing in animals that had undergone overshadowing, relative to nonreminded overshadowed animals. Subjects that received the reminder treatment but were conditioned without overshadowing showed no increase in lick suppression. Additional control groups ensured that the increase in suppression observed in the overshadowed subjects following reminder treatment was not due to nonspecific fear. The results suggest that the performance deficit produced by overshadowing is due at least in part to a reversible failure to efficiently retrieve associations to the overshadowed stimulus at the time of testing, rather than a failure to form those associations during conditioning.     

The comparator hypothesis of conditioned response generation: manifest conditioned excitation and inhibition as a function of relative excitatory strengths of CS and conditioning context at the time of testing

In the present research water-deprived rats were used in a conditioned lick suppression paradigm to test and further develop Rescorla's (1968) contingency theory, which posits that excitatory associations are formed when a conditioned stimulus (CS) signals an increase in unconditioned stimulus (US) likelihood and that inhibitory associations develop when the CS signals a decrease in US likelihood. In Experiment 1 we found that responding to a CS varied inversely with the associative status of the context in which the CS was trained and that this response was unaltered when testing occurred in a distinctively dissimilar context with a different conditioning history, provided associative summation with the test context was minimized. These results suggest that manifest excitatory and inhibitory conditioned responding is modulated by the associative value of the training context rather than that of the test context. In Experiment 2 it was demonstrated that postconditioning decreases in the associative value of the CS training context reduced the effective inhibitory value of the CS even when testing occurred outside of the training context. Moreover, this contextual deflation effect was specific to the CS training context as opposed to any other excitatory context. Collectively, these studies support the comparator hypothesis, which states that conditioned responding is determined by a comparison of the associative strengths of the CS and its training context that occurs at the time of testing rather than at the time of conditioning. This implies that all associations are excitatory and that responding indicative of conditioned inhibition reflects a CS-US association that is below (or near) the associative strength of its comparator stimulus. It is suggested that response rules which go beyond a monotonic relation between associative value and response strength can partially relieve learning theories of their explanatory burdens, thereby allowing for simpler models of acquisition.     

Prepulse inhibition of the startle eyeblink as an indicator of temporal summation

The inhibition of the human startle eyeblink response was assessed in three experiments in which the duration of the prepulse was manipulated. In all cases, inhibition of startle was more pronounced as prepulse duration increased from 6 to 50 msec. Inhibition of startle amplitude for single prepulses was not significantly different from that for paired prepulses (Experiment 1), but inhibition was more pronounced as prepulse intensity increased (Experiment 3). Varying the interval between prepulse offset and startle-stimulus onset had no significant effect on inhibition (Experiment 2). These data demonstrate the sensitivity of startle inhibition to prepulse duration, and suggest that this response system can be used to evaluate early temporal summation in the auditory system.     

Contextual dependencies in a stimulus equivalence paradigm

Two experiments with human subjects assessed contextual dependencies in a stimulus equivalence paradigm. Subjects learned to form two sets of stimuli in a matching-to-sample training procedure. Each set was presented against one of two different background colours, the contextual cues. At test, the influence of a context change--that is, presenting each set against the other context--was measured on baseline, symmetry, and equivalence trials. These three trial types reflect a difference in task complexity. It was predicted that the magnitude of context-dependent effects would be a positive function of task complexity. In Experiment 1, the context change was realized by switching the stimulus set at test while keeping the background colour constant. In Experiment 2, the stimulus set remained constant, and the background colour was switched. In both experiments, a change in context only resulted in an increase in response latency on equivalence trials; no effect was seen on symmetry and baseline trials. Results were discussed in the framework of switch costs, habituation to contextual stimuli, and a model based on Shea and Wright (1995) that explains the differential influence of a context switch on easy versus difficult tasks.     

The effects of context change on long-term habituation of the orienting response in humans

Four experiments examined the effects of context change (Experiments 1-3) and context extinction (Experiment 4) on long-term habituation of the skin conductance response. In all experiments, subjects received 15 presentations of a target stimulus in each of two sessions. In Experiment 1 (N=60) there was a 15-min interval between training and test sessions. This interval was extended to 24 hours in Experiments 2 (N=60) and 3 (N=60). The experimental treatment in each of these studies involved a change in context between the two training sessions. None of the experiments provided evidence of context dependency in measures of long-term habituation. In Experiment 4 (N=60) experimental subjects received a period of context extinction during which they remained in the laboratory environment between two series of habituation trials. Again, however, there was no evidence that long-term habituation was contextually mediated. Thus, the results fail uniformly to support theories that argue that long-term habituation is context-dependent.     

Fear-potentiated startle conditioning to explicit and contextual cues in Gulf War veterans with posttraumatic stress disorder

Aversive conditioning to explicit and contextual cues was examined in Gulf War veterans with and without posttraumatic stress disorder (PTSD) by use of the startle reflex methodology. Veterans participated in a differential aversive conditioning experiment consisting of 2 sessions separated by 4 or 5 days. Each session comprised two startle habituation periods, a preconditioning phase, a conditioning phase, and a postconditioning extinction test. In contrast to the non-PTSD group, the PTSD group showed a lack of differential startle response in the presence of a conditioned stimulus with or without an unconditioned stimulus in Session 1 and an increase in the baseline startle response during Session 2. The PTSD group also exhibited normal differential conditioning following reconditioning in Session 2. These data suggest that individuals with PTSD tend to generalize fear across stimuli and are sensitized by stress.     

Blocking between occasion setters and contextual stimuli

Two appetitive conditioning experiments with rats used a blocking procedure to compare the mechanisms through which contexts and positive occasion setters control responding to conditioned stimuli (CSs) in discrimination learning. In Experiment 1, a light (L) initially set the occasion for food reinforcement of a tone CS (T). Then a compound of L and a novel context signaled reinforcement of T. Previous learning about L blocked contextual control of responding to T. Blocking was not due to simple excitation conditioned to L during discrimination training; comparable excitation to L in a control group did not result in blocking. Conversely, in Experiment 2, initial learning about the context blocked the acquisition of occasion setting to L. Excitation conditioned to the context could not account for the blocking. In general, the results suggest that contexts and occasion setters may control responding to CSs through similar mechanisms.