Conditioned Stimulus Duration Research Articles

A comparison of two model systems of associative learning: heart rate and eyeblink conditioning in the rabbit.

ABSTRACTIn four classical conditioning experiments heart period and eyeblink responses were assessed concomitantly. The conditioned stimuli (CSs) were tones and the unconditioned stimulus (UCS) was a brief paraorbital electric shock. Using a 0.5‐s duration CS, bradycardiac conditioned responses that consisted of a 4–5 ms change from pre‐CS baseline occurred within 10–20 CS‐UCS presentations. However, eyeblink conditioned responses began to occur only after 100–150 CS–UCS presentations. A 1.0‐s CS duration resulted in bradycardiac conditioned responses of 15–30 ms change from pre‐CS baseline, which again reached asymptote within 10–20 trials. Using a 4‐s CS duration, in a differential classical conditioning paradigm, heart period discrimination between a reinforced CS+ and a nonreinforced CS− occurred within 10 trials; asymptotic performance of the heart period conditioned response occurred within 15 CS‐UCS presentations and consisted of a bradycardiac response of 40–50 ms for the 12th interbeat interval following tone onset. These data thus indicate that these two model systems of mammalian learning (based on heart period and eyeblink responses) show quite different acquisition functions. It is also significant that heart rate slowing always accompanied the eyeblink conditioned responses, even though increases in general electromyographic activity as well as eyeblink conditioned responses were simultaneously observed during CS presentation.

Effects of time of testing, stress level, and number of conditioning days on naloxone sensitivity of conditioned stress-induced analgesia in rats.

Controversy exists as to whether conditioned stress-induced analgesia (CSIA) utilizes opioid or nonopioid mechanisms. In two experiments, both an anticipatory conditioned stimulus (CS) and a shock-associated CS were used. The duration of exposure to the anticipatory CS was long, as in studies reporting opioid CSIA, whereas the duration of the shock-associated CS was short, as in studies reporting nonopioid CSIA. In addition, the effects of unconditioned stimulus (UCS) strength were investigated by using three levels of footshock (no, moderate, and high), and the development of CSIA was monitored by using different levels of training (1 to 6 days). CSIA, measured in both anticipatory and postexposure test periods, was found to be relatively stable across tail-flick trials within days and insensitive to strength of shock. As training progressed, CSIA increased with repeated CS-UCS pairings. We tested for opioid involvement using naloxone and found opioid and nonopioid mechanisms underlying CSIA; these mechanisms combined to form a stable level of analgesia. Our data suggest that stress level and amount of training interact to activate opioid and nonopioid mechanisms of CSIA. Apparent discrepancies in previous studies regarding naloxone sensitivity of CSIA may therefore be attributable to differences in stress levels, test periods and durations of exposure to shock-related cues.

CS duration and reinforcement schedule effects on conditioned enhancement and positive conditioned suppression

Two groups of rats learned a discrimination in which responses were reinforced on a variableinterval (VI) schedule during one stimulus and extinguished during another. Eight conditioned stimuli (CSs), each followed by a food unconditioned stimulus (US), were presented during each session: four during the VI component and four during the extinction component. CS duration was 2 min for Group L and 30 sec for Group S. Subjects in both groups responded at higher rates during CSs presented in the extinction component. Group L subjects also responded at higher rates during VI-component CSs, but Group S subjects responded at lower rates during VI-component CSs. These data indicate that reinforcement schedules may affect rate changes during CSs only within a limited range of CS durations.

Effects of conditioned stimulus preexposure on human electrodermal conditioning

Two experiments investigated the effects of conditioned stimulus (CS) preexposure on Pavlovian differential conditioning and extinction of the skin conductance response. In both experiments, half the subjects were exposed to 20 presentations each of the CS+ and CS-, and the other half were exposed to control stimuli. CS duration was 8 sec. The unconditioned stimulus in Experiment 1 ( N= 48) was a 1000 Hz tone of 80 dB which signalled a reaction time requirement, and in Experiment 2 ( N = 48), it was a 1 sec burst of white noise at 105 dB. The results of Experiment 1 indicated that no-preexposure groups displayed more CS+ /CS- differentiation than preexposure groups during acquisition and more resistance to extinction, at least for the first interval anticipatory response. In addition, the results of Experiment 2 indicated that no-preexposure groups displayed more differentiation than preexposure groups in terms of the second interval anticipatory response. These data constitute a demonstration of the latent inhibition effect with human subjects, and imply that there is an intrinsic relationship between the orienting response and the conditioning process.

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.

A role for conditioned stimulus duration in toxiphobia conditioning.

A role for conditioned stimulus duration in toxiphobia conditioning.

A role for conditioned stimulus duration in toxiphobia conditioning.

The effect of conditioned stimulus (CS) exposure duration on toxiphobia conditioning was investigated. When a relatively long (20--30 min) CS exposure was administered on the conditioning trial, rats subsequently displayed a reduced conditioned aversion in comparison with when CS exposure was relatively brief (5--10 min). This result was obtained in an odor-aversion learning task with neonatal rats as subjects (Experiments 1 and 2) and in a flavor-aversion learning task with adult rats (Experiment 3). This phenomenon was further investigated in the odor-aversion task by presenting neonates with two discrete 10-min exposures to the odor CS and by varying both the temporal proximity of the two CS presentations to each other and to the occurrence of the unconditioned stimulus (US). Two exposures to the CS reduced conditioning in comparison with a single exposure but only when the second CS presentation preceded US occurrence by at least 5 min (Experiments 4 and 5). Both nonassociative and associative interpretations of the data were discussed.

Temporal properties of responding during stimuli that precede response-independent food

Pigeons' keypecks were reinforced with grain on the average of once per minute by schedules that maintained low response rates and by schedules that maintained high response rates. During these schedules, a fixed-duration conditioned stimulus (CS) ranging from 7.5 to 120 sec in duration across conditions terminated with response-independent food. Response rates during the CS were inversely related to CS duration. The rates and the temporal patterns of responding during the shortest CS were similar whether the ongoing schedule maintained high response rates or low response rates. As CS duration increased, the rate and pattern of responding during the CS converged on the rate and pattern of responding maintained by the baseline schedule. These data indicate that changes in responding during stimuli that signal response-independent reinforcement are not homogeneous throughout the CS; that response measures, such as “suppression ratios”, which presume homogeneity may mislead us; and that conditioned suppression and conditioned enhancement may be better talked about in terms of species-specific approach and avoidance than in terms of emotional states.

Taste aversions conditioned by the aversiveness of insulin and formalin: Role of CS specificity.

Experimenters in the past have reported that when insulin is used as the unconditioned stimulus (US), rats will learn an aversion to a sodium chloride but not a sucrose solution, whereas with formalin as the US, they will learn an aversion to a sucrose but not a saline solution. The present experiments failed to confirm these findings. Aversions to sucrose were conditioned with insulin and aversions to sodium chloride were conditioned with formalin. The use of a more concentrated sucrose solution in the present study may have been responsible for the successful sucrose-aversion conditioning with insulin. Although the source of the discrepancy in findings concerning aversion conditioning with formalin remains unclear, experiments ruled out numerous possibilities. These experiments also showed that sodium chloride aversion conditioning with formalin is a highly robust phenomenon that occurs with a variety of conditioned stimulus durations and formalin doses, with distributed and massed training, in male and female rats, and even if saline is not the only novel solution presented during conditioning. Furthermore, the aversion can be detected with both single-stimulus and choice test procedures.

Taste aversions conditioned by the aversiveness of insulin and formalin: role of CS specificity.

Experimenters in the past have reported that when insulin is used as the unconditioned stimulus (US), rats will learn an aversion to a sodium chloride but not a sucrose solution, whereas with formalin as the US, they will learn an aversion to a sucrose but not a saline solution. The present experiments failed to confirm these findings. Aversions to sucrose were conditioned with insulin and aversions to sodium chloride were conditioned with formalin. The use of a more concentrated sucrose solution in the present study may have been responsible for the successful sucrose-aversion conditioning with insulin. Although the source of the discrepancy in findings concerning aversion conditioning with formalin remains unclear, experiments ruled out numerous possibilities. These experiments also showed that sodium chloride aversion conditioning with formalin is a highly robust phenomenon that occurs with a variety of conditioned stimulus durations and formalin doses, with distributed and massed training, in male and female rats, and even if saline is not the only novel solution presented during conditioning. Furthermore, the aversion can be detected with both single-stimulus and choice test procedures.

Residual fear of the conditioned stimulus as a function of response prevention after avoidance or classical defensive conditioning in the rat.

Studies employing response prevention (RP) are reviewed. Considering assessment difficulties and conflicting findings, it is questionable whether RP actually reduces fear to a conditioned stimulus (CS). This study measured fear after RP via a conditioned emotional response (CER) paradigm. Hypotheses were that fear of an auditory CS (conditioned in an avoidance paradigm) is reduced during RP, and that fear conditioning would occur to aspects of the conditioning environment per se. Also evaluated was the effectiveness of RP when fear had been learned under two different conditions: (a) avoidance or (b) classical defensive conditioning. Seven groups of 10 experimentally naive female rats were run. Animals were initially trained to bar press for food pellets on a variable interval (VI) 2 schedule. Three groups were then avoidance trained in a two-way shuttle box to a criterion of 10 successive avoidances. Immediately following acquisition, one group received RP (blocking) in the shuttle box (Condition A-B). This consisted of placing a door between the two sides of the box and presenting the 85 dB (A) white noise CS for 15 20-sec periods with a variable 1-min interstimulus interval. One group did not receive RP (nonblocked) and was instead immediately returned to its home cage (Condition A-NBHC). The third group was treated as was A-B except the CS was not presented (Condition A-NBSB). Two other groups were trained in a classical defensive paradigm. These animals were matched to A-B animals in terms of number, order, and duration of CSs and USs. Following conditioning, one group received the same treatment as A-B (Condition CD-B), and the other received the same treatment as A-NBHC (Condition CD-NBHC). Two groups served as controls. A backward control (Condition BC-NBHC) was matched to A-NBHC in terms of number, order, and duration of CSs and USs. A sensitization control (Condition SC-NBHC) was matched to A-NBHC in terms of number, order, and duration of CS presentations. Immediately following conditioning trials, control animals received the same treatment as A-NBHC animals. After differential treatments all animals were immediately returned to the lever box in which they had learned to bar press, a VI 2 schedule was reinstated, and the CER was measured. A-B showed significant suppression initially but significantly less than A-NBHC, suggesting that although RP was effective in reducing fear to the CS, some fear remained. Controls showed essentially no suppression and did not differ. A-B did not differ from A-NBSB, suggesting that conditioning of fear did occur to the environment and that this fear was subsequently reduced in A-NBSB. A-B suppressed significantly more than CD-B, suggesting that RP was more effective when fear was learned in a classical as compared to an avoidance paradigm. Theoretical implications and generalizations to implosive therapy are discussed.

EFFECTS OF A PRE‐REWARD STIMULUS ON A TEMPORAL DISCRIMINATION

Pigeons responded in a situation in which a response on key B could be followed by reinforcement if a response on key A preceded it by 10 sec. A‐to‐B responses with intervals of greater than 10 sec. duration were reinforced on a random ratio schedule. The effects of a conditioned stimulus terminating in response‐independent reinforcement were assessed against this baseline. Two levels of response‐independent reinforcement (6 or 10 sec. hopper access) and two conditioned stimulus durations (1.5 min. and 3 min.) were used. Except for the combination of the 3 min. conditioned stimulus with the 6 sec. response‐independent reinforcement, all other combinations produced a suppression of A‐to‐B responding during the conditioned stimulus. Concomitantly, the number of correct (i.e. > 10 sec.) responses, and in some cases the percentage of total responses that were correct, were reduced. All of these effects were seen most clearly using the conditioned stimulus of shorter duration.

Conditioned stimulus duration and delay of reward as variables in a lever pulling situation

Ninety kindergarten children served as Ss in a lever-pulling experiment. They were randomly assigned to five experimental groups. A group (LIMI) receiving immediate CS offset and immediate marble delivery did not differ significantly on a starting speed measure from a group (LI) receiving only immediate CS offset. In contrast, Group LIMI was significantly superior to Group LI on a movement speed measure. These results were interpreted as being consistent with the contention that starting speed differences mainly reflect the effect of “dompeting responses”, while movement speed differences principally reflect effects of motivational variables. In another phase of the experiment, a factorial design (which included Group LIMI but excluded Group LI) utilizing two values of delay of marble reward and two values of delay of CS (“Black Light”) offset yielded results which indicate that delay of marble delivery affected starting speeds and that delay of CS offset affected movement speeds. Other results obtained indicate that children's performance on the first trial block was influeneed by watching E perform the same task during an initial demonstration trial, that the trial block to trial block starting speed correlations were positive but smaller in magnitude than corresponding positive movement speed correlations, and that the correlation between starting and movement time for 5-year, 7-month-old children was positive.