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.     

ECS disruption of avoidance produced by an air blast and tone

Rats were taught to bar press for food in a Skinner box. The amount of time needed to make 10 bar presses increased significantly after a bar press was accompanied by an air blast and loud tone. This increase did not occur if the air blast and tone were followed 8 sec later by an ECS. The technique is simple and effective, and preserves the previously established effects of ECS on recent memory. Hopefully it will prove useful in situations where the investigator wishes to establish a one-trial passive avoidance without the use of painful shock.     

Lesions of Periaqueductal Gray Dissociate-Conditioned Freezing From Conditioned Suppression Behavior in Rats

It is commonly assumed that suppression of an ongoing behavior is an indirect measure of freezing behavior. We tested whether conditioned suppression and freezing are the same or distinct conditioned responses. Rats were trained to press a bar for food and then given fear-conditioning sessions in which a tone was paired with a foot shock (two pairings a day for 2 days). They then received either sham or electrolytic lesions of the periaqueductal gray (PAG). Post-training PAG lesions blocked freezing to the conditioned stimulus (CS), but had no effect on the suppression of operant behavior to the same CS. Thus, conditioned suppression and freezing, which both cause a cessation in activity, appear to be mediated by separate processes.     

Extinction instead of incubation following classical aversive conditioning in dogs.

Two dogs received a single paired classical conditioning trial, with tone CS and 12 mA shock US. Both dogs then showed a conditioned blood pressure increase in response to the nonreinforced CS, which extinguished with additional nonreinforced presentations. The CR showed spontaneous recovery four days later, but reextinguished with additional nonreinforced presentations. The results were interpreted as not supporting Eysenck's theory of "incubation" following one-trial aversive conditioning.     

A tracking procedure for determining the cat's frequency discrimination

A tracking procedure was used to investigate the time required to train cats to discriminate between a frequency-modulated tone and a steady tone. The animal was reinforced with food on a VR schedule only when the steady tone was present and the animal pressed the correct bar (one of two). After reinforcement, the steady tone usually changed to a frequency-modulated signal; by pressing the other bar, the tone could be changed to its steady state and the reinforcement then obtained as before. A major difficulty was the lack of control by the auditory stimulus on the cat's responses. This problem was solved by introducing interpress time outs which forced the animal to hesitate after every press. The use of light cues to signal the time outs and the correct bar to press accelerated the rate at which the training progressed.

With cats, this conditioning procedure apparently requires a much longer training period before the actual threshold determinations than the more commonly used avoidance conditioning procedures. However, when animals are to be tested repeatedly over a period of several months or longer, the procedure may prove the more desirable one because it reduces experimental neurosis.

     

Stimulus aspects of aversive controls: the effects of response contingent shock

A tone ending with electrical shock was periodically presented to pigeons while they pecked a key for food. Pairs of birds were run simultaneously under a yoked program which insured that both birds received the same number and temporal distribution of shocks. For one of the birds, shock was always initiated by a peck; for the other, shock was unavoidable. Both procedures led to reduced rates of pecking in the presence of the tone, and gradients of stimulus generalization were obtained. But the effects of response contingent shock extinguished more rapidly than the effects of unavoidable shock. In general, birds exposed to unavoidable shock tended to respond at intermediate rates throughout tone, whereas those exposed to response contingent shock ceased to peck for part or all of the tone period.     

Extinction instead of incubation following classical aversive conditioning in dogs

Two dogs received a single paired classical conditioning trial, with tone CS and 12 mA shock US. Both dogs then showed a conditioned blood pressure increase in response to the nonreinforced CS, which extinguished with additional nonreinforced presentations. The CR showed spontaneous recovery four days later, but reextinguished with additional nonreinforced presentations. The results were interpreted as not supporting Eysenck’s theory of “incubation” following one-trial aversive conditioning. Partial support for this study was provided by Grant 28462 from the National Heart, Lung, and Blood Institute.     

Conditioned suppression of bar-pressing behavior by stimuli associated with drugs

Ten naive male albino rats were trained to press a bar under a variable-interval 30-sec schedule with water as the reinforcer in two experiments. This behavior was disrupted by chlorpromazine in Experiment I (two rats) and by lysergic acid diethylamide (LSD) in both Experiment I (two rats) and Experiment II (six rats). The administration of the drug was paired with an originally neutral white light. After several pairings with either drug, the light also depressed behavior. When the light was no longer paired with drug, the depression effect extinguished much faster than is usually observed in conditioned suppression studies.     

Inhibitory conditioning resulting from a reduction in the magnitude of reinforcement

Five experiments, all employing conditioned suppression in rats, studied inhibitory conditioning to a stimulus signalling a reduction in shock intensity. Experimental subjects were conditioned to a tone signalling a 1·0 mA shock and to a tone-light compound signalling a 0·4 mA shock. On a summation test in which it alleviated the suppression maintained by a third stimulus also associated with the 1·0 mA shock, the light was established as a conditioned inhibitor. Retardation tests gave ambiguous results: the light was relatively slow to condition when paired, either alone or in conjunction with another stimulus, with the 0·4 mA shock, but the difference from a novel stimulus control group was not significant. Two final experiments found no evidence at all of inhibition on a summation test in which the light was presented in conjunction with a stimulus that had itself been associated with the 0·4 mA shock. The results of these experiments have implications for the question of what animals learn during the course of inhibitory conditioning.     

Appetitive-aversive interactions: Superconditioning of fear by an appetitive cs

Four groups of rats received conditioned suppression training in which a tone and light compound was reinforced with shock. If the light had been previously paired with free food, enhanced fear conditioning accrued to the tone during compound training relative to control groups pre-exposed to the light alone, the light semi-randomly associated with food, or the light unpaired with food. The second experiment replicated the difference in aversive conditioning for the groups receiving the light either paired or unpaired with food. The results are discussed in terms of the functional similarity of a conditioned excitor for food and a conditioned inhibitor for shock.     

Control of avoidance by a response-produced stimulus

Three pigeons were trained to press a foot pedal to postpone electric shock. The safety signal was a tone of 1000 Hz, which sounded for 5 sec following each press; silence therefore served as a warning signal. After the performance approached asymptote, test sessions were interspersed among the training sessions. Following a warm up, the shock was omitted (extinction), and no tone or a tone of 250, 500, 1000, 2000, or 4000 Hz was presented following each press for the remainder of the session. Gradients of generalization plotted in terms of the number of times the tone was produced on each test peaked at 1000 Hz, indicating that the performance was controlled by the frequency. Since this dimension is orthogonal to absence of tone, the gradient cannot be attributed to differences in the magnitude of change in the warning signal. It was concluded that response-produced stimuli reinforce avoidance behavior.     

Effects of fixed-time shocks and brief stimuli on food-maintained behavior of rats

When a fixed-time schedule of shocks was presented to rats lever pressing for food on a random-interval schedule, a pattern of behavior developed with a high rate of pressing after shock declining to near zero before the next shock was delivered. Once this pattern had stabilized, one-quarter of the shocks were replaced with brief auditory stimuli (tones) in a random sequence. Tone maintained behavior similar to shock, although tone was never paired with shock. Both tone and shocks elicited responding when presented at various times as probe stimuli, and responding was usually totally suppressed if neither stimulus occurred at the beginning of the fixed-time interval. When other stimuli were paired with tone and shock, only those paired with tone gained discriminative control and elicited responding. These findings suggest that stimuli that signal a shock-free, or safe, period will maintain the pattern of behavior generated by shock on a fixed-time schedule. There is a parallel between this phenomenon and the control of behavior on second-order schedules of positive reinforcement with nonpaired brief stimuli.     

Some aspects of self aversive stimulation in the hooded rat

Three hooded rats were trained to bar press for variable-ratio liquid reinforcement after which an electric shock was delivered following the response. Initially, the shock was presented on a FR 100 basis but the frequency was gradually increased until all responses were punished. Finally, a partial extinction procedure was conducted to determine if the shock resulted in increased bar pressing. No durable suppression of responding occurred, although one subject's rate was reduced during continuous shock. The overall trend for the three animals was one of increased responding. Changes in the pattern of responding were also observed suggesting that the suppressive effects of the punishment were largely restricted to the first response following reinforcement. Increased responding as a function of shock reintroduction during extinction was also observed.     

Associations in Pavlovian conditioned inhibition

Two experiments with rat subjects used a variety of transfer tests to examine the associations learned when Pavlovian inhibition is established by an A+, AX− paradigm. Experiment 1 found in a conditioned suppression situation that inhibition conditioned to X with one exciter (A) readily transferred to another exciter (B) which had been paired with the same shock US. Transfer occurred even when the response to A had been extinguished prior to testing with B. However, X did not inhibit a general activity response produced by a B which had been subsequently paired with a food US. Experiment 2 employed a Pavlovian conditioning situation in which A and B, when separately paired with the same food US, evoked dissimilar responses. Nevertheless, an inhibitor trained in an A+, AX− paradigm successfully inhibited the different response evoked by B. However, such an X did not inhibit the behaviors acquired by A or B when they were subsequently paired with a shock US. The transfer of Pavlovian inhibition across conditioned stimuli and responses but not across unconditioned stimuli is consistent with the notion that a conditioned inhibitor acts to prevent activation of a US representation which would normally be activated by conditioned exciters.     

Effects of expectancies of different reward magnitudes in transfer from noncontingent pairings to instrumental performance

In two experiments, rats received noncontingent pairings of two stimuli with food reward, one paired with small reward and the other with large reward, and received bar press training with large reward or with small reward. When the noncontingent stimuli (NS) were presented for test during subsequent rewarded bar pressing and during early extinction of bar pressing, responding for each group was faster in the presence of the NS which was paired with the same reward magnitude that group received in bar press training than to the NS which had been paired with a different reward magnitude. As extinction progressed, all groups responded more slowly in the presence of the NS which had been paired with the large reward than in the presence of the NS which had been paired with small reward. These results were interpreted as indicating that responding in the presence of an NS depends on: (i) whether the reward expectancy elicited by the NS has been conditioned to the instrumental response, and (ii) the relationship between the reward expected in the presence of the NS and that received in test.     

A study of cardiac conditioning in man

Conditioning procedures for EEG, ECG, and PGR responses were evaluated in 23 male and female human Ss. A 5 sec. 310 cps tone served as the CS+; a 130 cps 5 sec. tone served as the CS?. The US was a 10 msec. 6 ma shock to the finger. After orienting responses had extinguished, conditioning trials were initiated and continued until the S showed conditional responses on 5 consecutive occasions. PGR and ECG results were in accord with the findings of other authors working on animals. The significance of the responses studied was discussed in terms of data recorded by other investigators.     

Chaining and secondary reinforcement based on escape from shock

Three white rats were trained to press a bar while being shocked. This produced a white noise. After 30 sec they were allowed to terminate both the shock and the noise by nosing a pigeon key. Comparison of the rates of pressing before and after the onset of the noise indicated that the noise itself was the immediate reinforcing agent for pressing. Furthermore, control tests showed that pressing was maintained only if it produced the noise: either omission of the noise or elimination of the dependency of the noise on the occurrence of the response led to a gradual abolition of pressing. When automatic termination of the shock was substituted for the key nosing requirement, however, only the key nosing extinguished. This indicated that the effectiveness of the noise as a reinforcer did not depend on its status as a discriminative stimulus for some other form of operant behavior.     

Acceleration and suppression of rats' responding to avoid foot shock and tail shock

Signalled response-independent shocks were superimposed on rats' wheel-turn responding to avoid shock administered to their feet through a grid floor or to their tails through fixed electrodes. In Experiment I, a tone paired with response-independent foot shock increased responding in three of four rats; a tone paired with tail shock increased responding in only one of four rats and suppressed responding in two rats. In Experiment II, a tone presented randomly with respect to response-independent shock had no reliable effect on responding to avoid foot shock or tail shock. In Experiment III, tail shock and foot shock were compared in a within-subject design while the temporal pattern of responding during conditioned stimuli was recorded. Responding during the conditioned stimulus preceding foot shock was characterized by initial suppression of responding at tone onset, followed by increased responding just before response-independent shock. Responding was suppressed throughout the conditioned stimulus preceding tail shock. Foot shock elicited bursts of responding, but tail shock did not.     

The cardiac orienting response and its relationship to the cardiac conditional response in dogs

Orienting tones of two durations (11 seconds and 42 seconds) were presented to two groups of dogs, and the heart rate-orienting responses (HR-OR) were measured. With the brief tone (11 seconds) the initial HR-OR was usually a bradycardia. With the longer tone (42 seconds) the HR-OR was a biphasic response, an initial bradycardia followed by a tachycardia. The bradycardia component extinguished more rapidly than the tachycardia component of this response. The bradycardia also extinguished more rapidly to the long tone than to the brief tone. With the brief tone the degree of HR-OR was not highly correlated with the degree of HR conditional response given by the same dog after the tone had been reinforced by shock.     

Chronic stress impairs recall of extinction of conditioned fear

Chronic restraint stress produces retraction of apical dendrites of pyramidal neurons in medial prefrontal cortex. To begin to examine the functional significance of this dendritic reorganization, we assessed the effects of chronic restraint stress on a prefrontally mediated behavior, extinction of conditioned fear. After bar press training to obtain a baseline of activity against which to measure freezing, rats were either unstressed or stressed via placement in a plastic restrainer (3 h/day for 1 week). After an additional day of bar press training, rats underwent fear conditioning and extinction. Rats received five habituation trials to a 30-s tone (4.5 kHz, 80 db) followed by seven pairings of tone and footshock (500 ms, 0.5 mA). One hour later, rats received tone-alone extinction trials to criterion. The next day, rats received 15 additional extinction trials. Percent freezing was assessed during all phases of training. Stress did not significantly affect unconditioned responding to tone, acquisition of conditioned fear, or initial extinction, but significantly increased freezing on extinction day 2. Thus, consistent with the regressive dendritic changes seen in medial prefrontal cortex, one week of restraint stress specifically impaired recall of extinction, a pattern of deficits typical of animals with impaired medial prefrontal function.