Effects of the correlation between a positive and negative US on conditioned suppression in rats

For three groups of rats, an auditory CS, presented while the animals were responding on a variable-interval schedule for food reinforcement, was terminated on half of the trials with a noncontingent footshock. For two groups, half of the trials were also followed after 5 sec by the delivery of free food. In the positively correlated condition (PC) the free food was presented on shocked trials and in the negatively correlated condition (NC) on the nonshocked trials, while the remaining group (S) never received free food. In a fourth group the shock was omitted and free food delivered on half of the trials. Although all shocked groups showed a significant suppression, the magnitude was greater for Group PC than for Groups NC and S, which did not differ. Suppression did not result from the pairing of the CS with food alone. These results do not support the counterconditioning hypothesis that the pairing of a normally noxious stimulus with food reduces its aversiveness.     

CS and US duration effects in one-trial simultaneous fear conditioning as assessed by conditioned suppression of licking in rats

In three experiments, rats received a single presentation of an auditory conditioned stimulus (CS) beginning simultaneously with an electric grid-shock unconditioned stimulus (US). Later, the CS was presented while the rats licked a drinking tube for water, and CS-elicited suppression of licking was taken as an index of the excitation conditioned to the CS. It was found that conditioning increased as a joint function of the duration of CS-US overlap and US duration. The evidence suggested that weak conditioning due to a brief CS-US overlap could be increased by extending the US beyond CS termination. Extending CS duration beyond US termination, however, did not strengthen conditioning; indeed, extending the CS 60 sec beyond US termination weakened conditioning significantly. It is suggested that these results shed light on a discrepancy in the recent literature on simultaneous conditioning.     

The effects of activity conditioned in random CS/US training on performance in autoshaping

Random presentations of keylights and food retarded acquisition and suppressed asymptotic rates of keypecking in autoshaping. Sequences of 10 sessions of random training alternated with 10 sessions of autoshaping resulted in poor performance (in terms of both the acquisition and asymptotic indices) relative to a group that received sequences of CS-only (rather than random) training alternating with autoshaping. When the birds that previously were trained with the random sequence were exposed to CS-alone extinction, retardation of acquisition was alleviated but the asymptotic suppression was not (Experiment 1). Pigeons with a history of autoshaping without prior random training showed no asymptotic suppression when exposed to the random procedure. These birds, when switched between two-session sequences of random training alternating with two-session sequences of autoshaping, were able to (1) surpass pigeons that received CS-only rather than random treatment in terms of asymptotic levels of responding in autoshaping, and (2) showed improvement in extinction performance with repeated random/autoshaping sequences (Experiment 2). Detailed observations of pigeons in random training showed that stereotypic activity behaviors were acquired, and these generally persisted in autoshaping; the degree of change in these behaviors was related to asymptotic rates of keypecking in autoshaping (Experiment 3). The same kinds of behaviors were observed when pigeons initially were autoshaped, and these persisted in subsequent random and fixed-interval 10-sec training. We suggest that the suppression effect is reflected in activity, conditioned in random training, which persists in autoshaping (especially if the activity is compatible with the kinds of behaviors elicited by the autoshaping contingency itself), and that the effect may be a deficit due to performance factors rather than to associative learning.     

The effects of compound stimulus preexposure of two elements differing in salience on the acquisition of conditioned suppression

Conditioned attention theory (CAT) of latent inhibition (LI) states that parallel learning processes occur during reinforced and nonreinforced stimulus presentation. The present experiments investigated the effects of nonreinforced preexposure of either a compound CS or elements of that compound which differed in salience. Three predictions were advanced: (1) Both the compound and its elements will show an increase in LI as a function of the number of preexposures; (2) the two elements will show different levels of LI, with more LI accruing to the more salient element; (3) overshadowing will occur during compound preexposure. Two experiments, using rats as subjects and a conditioned suppression test, are reported. In Experiment 1, groups received 0, 20, 40, or 80 nonreinforced preexposures to a compound whose elements differed in salience. The results of the subsequent test confirmed predictions 1 and 2. Experiment 2, in which groups were preexposed to either the elements or the compound, provided evidence for an overshadowing effect, confirming prediction 3 from CAT.     

Stepwise reductions in US density and intensity: Maintenance and extinction of conditioned suppression

Information was sought on how the strength of fear, acquired and extinguished in a classical conditioning paradigm, might be affected by certain of the circumstances that normally are associated with the course of avoidance learning. Following preconditioning exposures to the conditioned stimulus, one group of rats (Group PRF) was given continuous conditioned stimulus-unconditioned stimulus (CS-US) acquisition trials followed by stepwise reductions in US probability (given the CS) over three phases to about 11% for the final phase. Another group, Group US(lo), was given continuous CS-US pairings throughout, but, following acquisition, received stepwise reductions in US intensity (to permit evaluation of a progressively changing feature of the US over a wide range without having to employ excessive shock) over the same phases. A third group, Group US(hi), received unchanging (from acquisition) CS-US pairings over these phases, and an explicitly unpaired control group (Group RU) was included. Although outcomes differed somewhat depending upon whether suppression ratios or absolute measures of responding were considered, the major findings were that suppression to the CS was complete by the end of acquisition and persisted thereafter throughout the three phases for all but control subjects. In contrast to Group US(lo), which showed relatively little resistance to extinction of suppression to subsequent CS-only exposures, Groups US(hi) and PRF displayed marked resistance over two separate, extended sets of extinction sessions. Suppression to context cues was pronounced only for Groups US(hi) and RU, and varied as a function of US parameters and not whether the shocks were or were not signaled. Theoretical reconciliation of these findings was most difficult for Groun PRF.     

Context sensitivity of conditioned suppression following preexposure to the conditioned stimulus

Conditioned suppression in rats is often unaffected when the context (or set of background stimuli) is changed following conditioning. This suggests that responding to the conditioned stimulus (CS) can be relatively insensitive to the context in which the CS is presented. In two experiments, we examined whether sensitivity to contextual stimuli is affected by preexposure to the CS. In Experiment 1, when the CS was novel at the outset of conditioning, conditioned suppression was not affected when the context was changed following conditioning. However, when the CS had been preexposed, responding was weaker when extinction occurred outside of the conditioning context. In Experiment 2, responding was again sensitive to the test context, regardless of whether preexposure occurred in the conditioning context or in an alternate context. These results suggest that the extent to which responding is sensitive to context can depend on the conditioning history of the CS.     

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

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

Differential conditioned suppression in the Konorski-Lawicka paradigm

In conditioned suppression discriminations, the Konorski-Lawicka paradigm involves A+ trials, where conditioned stimulus A is followed by shock, and sh → A? trials, where A is preceded by shock Rats easily mastered this A+/sh → A? discrimination, as indicated by suppression of food-reinforced barpressing on A+ trials and acceleration of barpressing on sh → A? trials. A history of A+ conditioning resulted in nearly perfect discrimination performance on the very first day of A+/sh → A ? training, but a history of sh→ A? conditioning retarded development of the discrimination. The basis for the development of the discrimination was discussed in terms of an inferred stimulus (sh′) arising from the aftereffects of shock.     

The response-shock interval and conditioned suppression of avoidance in rats

Rats were trained to avoid unsignaled shocks with response-shock intervals of 30, 60, or 120 sec. When CSs of 60 sec duration paired with unavoidable shocks were then superimposed upon the avoidance baseline, responding decreased during the CS. Reductions in responding resulted in extra shocks which were potentially avoidable in all response-shock interval conditions, with the greatest increase in shocks in the response-shock 30-sec condition. Decreases in responding were greater when the CS was paired with a 2.0-mA unavoidable shock than with a 1.0-mA shock.     

The effects of signal intensity upon conditioned suppression: Effects upon responding during signals and intersignal intervals

Two rat experiments were run to study the effects of a wide range of signal (CS) intensities on the suppression of licking. In Experiment 1, a light CS was varied over five levels, including zero intensity. Conditioned suppression was found to vary directly with CS intensity, but basal lick rates were not different among groups. In Experiment 2, an attempt was made to disturb the basal rate of licking, while a tone CS was varied over four levels, including zero intensity. Here the suppression of the CS rates was found to be directly related and basal rates inversely related to CS intensity. The results as a whole were consistent with the Perkins-Logan hypothesis regarding the effects of CS intensity upon conditioning, but not with the Rescorla-Wagner model of classical conditioning.     

Procedure and reliability of conditioned respiratory suppression

Changes in respiration amplitude, respiration rate, and heart rate in response to a conditioned signal for shock were measured concurrently in kittens and adult cats. The data were analyzed with respect to qualitative and quantitative variability across trials and subjects; correlation among measures; skewness; and kurtosis. Suppression of respiration amplitude was the most reliable response across trials and subjects, with increases in respiration rate second and heart rate by far the least reliable. Correlations between each pair of measures were moderate. Respiration-amplitude responses were negatively skewed, but this deviation from normality was moderate and consistent across subjects. The measurement of conditioned respiratory suppression is a viable addition or alternative to the conditioned emotional response procedure in studies of classically conditioned fear.     

Recovery of conditioned suppression after backward pairings

In four conditioned suppression experiments with rats (Rattus norvegicus), backward pairings of a shock unconditioned stimulus (US) and a tone conditioned stimulus (CS) eliminated an already established conditioned response (CR), but there was recovery of the CR if the shock was later withheld. In Experiment 1, there was recovery after backward pairings, regardless of whether the period after the US was normally shock free or not. In Experiment 2, the occurrence of recovery depended on the CS’s being presented closely after the US in response elimination. Levels of recovery were positively correlated with the resistance of the response to elimination during backward pairings (Experiments 3 and 4). Taken together, these data support the notion that recovery after backward pairings is a form of renewal (see, e.g., Bouton, 1991) and is not due toprotection from extinction.     

Transfer of conditioned suppression and conditioned acceleration from instrumental to consummatory baselines

Following training on a variable-interval food reinforcement schedule, rats were exposed to Pavlovian procedures which produced reliable conditioned suppression and conditioned acceleration of the leverpressing (instrumental) baseline. When free food was simultaneously made available in the test cage, all subjects spent the majority of each session “freeloading,” that is, eating food from a dish rather than leverpressing for it. When superimposed upon the freeloading baseline, the conditioned suppression and conditioned acceleration procedures affected the rate of pellet consumption identically in magnitude and direction to their previous effects on leverpressing. These results suggest a motivational mechanism for conditioned suppression and acceleration, rather than one which depends upon spurious punishment of specific response sequences.     

Retardation of rabbit nictitating membrane conditioning following US preexposures depends on the distribution and number of US presentations

Retarded conditioned response (CR) acquisition produced by unconditioned stimulus (US) preexposures has been attributed either to interference resulting from contextual conditioning or to habituation of the US. Both perspectives assume that the amount of retardation is directly related to the number of US preexposures. This assumption was examined in two experiments. In Experiment 1, separate groups of rabbits received 200 paraorbital shock US preexposures either in one session or spread equally over 10 daily sessions. Subsequently, all subjects received 150 CS-US pairings. Acquisition of nictitating membrane CRs was retarded relative to a naive control group only in the group that received the preexposures over 10 sessions. Thus, the number of US preexposure sessions, rather than the number of US preexposures, determined whether or not retarded acquisition was observed. In Experiment 2, four groups of rabbits received 1, 5, 20, or 40 shock US preexposures in each of 10 sessions. Over the subsequent 150 CS-US pairings, similar levels of retarded CR acquisition were observed in groups that received 20 and 40 US preexposures per session, a weak retardation effect was observed with 5 preexposures per session, and no retardation was observed with 1 preexposure per session. Thus, Experiment 2 suggested that retarded CR development was not greatly influenced by increasing the number of US preexposures above some minimum threshold number of exposures per session. Implications for current theories were discussed.     

Omission training and positive conditioned suppression in the rat

Suppression of operant responding during a conditioned stimulus (CS) was studied in two procedures. In both procedures, operant leverpressing was maintained by a variable-interval 1-min food-delivery schedule, and insertion of a second lever served as the CS. In the first procedure, autoshaping, food followed each CS presentation irrespective of a subject’s behavior during the CS. In the second procedure, omission training, contact with the CS canceled the delivery of food scheduled for the end of that CS. In the first experiment, subjects were exposed to omission training followed by autoshaping; these procedures were reversed in the second experiment. In each experiment, the omission contingency resulted in fewer CS contacts and less suppression of operant responding during the CS than did autoshaping. These differences were more notable in subjects receiving the sequence autoshaping→omission training (Experiment 2). Direct observations in Experiment 2 revealed that, for subjects that were contacting the CS frequently when the omission contingency was introduced, reductions in signal contacts were accompanied by redistributions of behavior. The form of these redistributions depended upon behavior allocation at the time the omission contingency was imposed.     

Posttrial effects of presenting vs. omitting expected shock USs in the conditioned suppression procedure: Concurrent measurement of barpress suppression and freezing

Barpress suppression in a 1-min interval following CS trials was investigated using 16 rats in a conditioned suppression procedure with a two-stage design. For one group, each CS co-terminated with a brief shock US in Stage 1; then, in Stage 2, only half the CSs ended with a shock, which in turn was followed 1 min later by a second shock. For a second group, the two stages were reversed. When CSs were followed by single shocks in Stage 1, posttrial suppression weakened across trials; but when, in Stage 2, double shocks followed half the CSs, posttrial suppression grew stronger. When half the trials were followed by double shocks in Stage 1, posttrial suppression was maintained at initial levels but weakened in Stage 2 when single shocks followed each trial. In both stages, posttrial suppression was stronger on nonreinforced than on reinforced trials. Two factors were hypothesized to control posttrial suppression. First, posttrial suppression weakens with training under the single-shock procedure because post-shock temporal stimuli come to inhibit fear unless themselves paired with shock. Second, posttrial suppression is stronger on nonreinforced trials than on reinforced trials because freezing behaviors initiated during the CS are not disrupted by a US and so persist into the posttrial interval.     

The role of number and spacing of CS-US pairings in conditioned suppression

In four studies, with rats as Ss. acquisition and. where appropriate, extinction trials were presented against a baseline of ongoing licking. At shock intensities of 0.1. 0.5. 1.0, or 2.0 mA, acquisition performance was a function of number of CS-US pairings: spacing of trials (one or two per day) did not affect acquisition performance. Resistance to extinction could be predicted from terminal acquisition performance and reached a maximum after three CS-US pairings.     

Dissociation of the blocking of conditioned eyeblink and conditioned fear following a shift in US locus

Two experiments with rabbits investigated the concordance of two measures of conditioning, eyeblink and potentiated startle, during a blocking sequence with paraorbital shock reinforcement. In both, a shift in the locus of shock from one eye to the other between the conditioning of an element and a compound of that element and a new cue had differential effects on the two measures of conditioning to the new cue. When the shock was unchanged, diminished conditioning in relation to controls (i.e., blocking) was observed on both measures. When the shock was changed, little conditioning was observed in startle, but control-equivalent amounts were observed in eyeblink (i.e., blocking occurred on the former but not the latter). The results are interpreted as showing a dissociation of the associative learning involving the emotive features of Pavlovian reinforcers and that involving the remaining sensory-perceptual features, and more compatible with a diminished US-processing, than with a CSprocessing, view of blocking.     

Directed approach responses and positive conditioned suppression in the rat

Three experiments evaluated an alternative to accounts of positive conditioned suppression that stress central (i.e., motivational or emotional) states. This “competing-response” interpretation was tested by analyzing directed movements that develop in rats during a visual or an auditory stimulus (CS) that signals an appetitive reinforcer (US) in a situation where the subject is also emitting an instrumental response for food. In each experiment, positive conditioned suppression (i.e., a reduction in the rate of such instrumental responding during CS presentations) was accompanied by responses directed toward the CS source and/or the US-delivery site. In Experiment 1, a diffuse (auditory) CS signaled a US delivered at some specific place in the chamber and rats approached the US-delivery site during CS. In Experiments 2 and 3, the spatial proximity of a localized visual CS and US-delivery site determined whether CS-directed or US-directed behavior predominated during the CS. The results suggest that the topographies of conditioned responses on any positive conditioned suppression procedure depend upon the spatial arrangements of features that elicit and support these behaviors. They further suggest that the identification of these features and their spatial arrangements is necessary for the analysis of appetitive classical-instrumental interactions.