Effect of CS-US pairings on shock-elicited aggression as a function of US intensity

In two experiments with paired rats, the effect of superimposing CS-US pairings on a baseline of shock-elicited aggression was studied. Baseline shocks (3.0 mA, 0.125-sec duration) occurred at a rate of 20 shocks per min throughout each session. In Experiment I, each independent group of two pairs of subjects received (in addition to baseline shocks) US shocks of 1.0, 3.0, or 5.0 mA and 5-sec duration, each shock signalled by a 1-min CS. At all three US intensities, aggression increased during the CS. In Experiment II, pairs of subjects received each unconditioned stimulus intensity in a within-subjects design. This procedure revealed a direct relationship between rate of responding and unconditioned shock intensity.     

The effect of shock intensity upon responding under a multiple-avoidance schedule

The effect of two shock intensities (1.00 and 2.00 mA) were studied in the acquisition, maintenance, and extinction of unsignalled avoidance by albino rats. Single and multiple avoidance schedules were employed, with shock intensity being the principal condition that differed between schedule components. The higher shock intensity was generally more effective in producing avoidance. Higher response rates and lower shock rates were observed under high-intensity shock when performance stabilized. When the multiple schedule was introduced, the six rats trained under a single shock intensity all showed poorer performance under the new shock intensity, whether it was higher or lower than the training intensity. Performance under the original shock intensity did not change substantially with the introduction of a different shock intensity in the other multiple schedule component. Performance under the new shock intensity showed gradual improvement with continued exposure to it. All of the rats showed persistent “warm-up”, receiving approximately 40% of the total session shocks in the first one-sixth of the session. The degree of warm-up was unrelated to avoidance shock intensity.     

Density and delay of punishment of free-operant avoidance

In two experiments, the free-operant shock-avoidance behavior of rats was punished by electric shock. Two aspects of the schedule of response-produced shock were varied: the frequency of punishment over time (punishment density) and the temporal interval between the punished response and the punishment (punishment delay). The general finding was that response-produced shock suppressed avoidance responding under most of the density-delay combinations studied, and suppression increased as a function of increases in density and decreases in delay. Rate increases of small magnitude also were observed, usually as an initial reaction to the lesser densities and longer delays. Response suppression, while decreasing the number of punishment shocks received, also increased the number of avoidance shocks, so that the total number of shocks received usually was greater than the minimal number possible. The results were discussed from the standpoint of similarities between the effects of punishing positively and negatively reinforced behavior. The finding that subjects did not minimize the total number of shocks suggested that when avoidance behavior is punished, responding is controlled more by the local consequences of responding than by overall shock frequencies during the course of the session.     

Variable-interval escape from stimuli accompanied by shocks

Individual performances of three rats were examined under a procedure in which steady rates of bar pressing were maintained by conditioned aversive stimulation. Originally neutral visual and auditory stimuli were accompanied by widely and irregularly spaced pulses of shock; they were terminated on a variable-interval schedule by pressing a bar. The contingencies between behavior and shock were also duplicated in a control procedure in which no visual or auditory stimuli were provided. Pressing observed under the control procedure was attributed to differences in the aversiveness of pressing and nonpressing behavior engendered by differences in the incidence of shock following the two classes of behavior. Increased rates with visual and auditory stimuli were attributed to termination of conditioned aversive stimulation. Control rates declined more rapidly than did experimental rates as the mean interval between successive shocks was lengthened; both rates tended to decline when less than 60 sec was allowed as time out from shocks following the successful response. In the control procedure, discrimination between the continuation and discontinuation of the shock series, as measured by relative rates, depended on the relative length of the interval between shocks and the time-out period. Regular warm-up accelerations in rate were noted following an initial delay in responding at the beginning of each session. The length of time required for the warm-up depended on the length of the mean interval between shocks, indicating that exposure to a certain amount of shock was required to establish a supporting state for the observed performance.     

Avoidance based on shock intensity reduction with no change in shock probability

Rats were trained on a free-operant avoidance procedure in which shock intensity was controlled by interresponse time. Shocks were random at a density of about 10 shocks per minute. Shock probability was response independent. As long as interresponse times remained less than the limit in effect, any shocks received were at the lower of two intensities (0.75 mA). Whenever interresponse times exceeded the limit, any shocks received were at the higher intensity (1.6 mA). The initial limit of 15 seconds was decreased in 3-second steps to either 6 or 3 seconds. All animals lever pressed to avoid higher intensity shock. As the interresponse time limit was reduced, the response rate during the lower intensity shock and the proportion of brief interresponse times increased. Substantial warmup effects were evident, particularly at the shorter interresponse-time limits. Shock intensity reduction without change in shock probability was effective in the acquisition and maintenance of avoidance responding, as well as in differentiation of interresponse times. This research suggests limitations on the generality of a safety signal interpretation of avoidance conditioning.     

Lever attacking and pressing as a function of conditioning and extinguishing a lever-press avoidance response in rats

Six experimental rats were conditioned to press one of two available levers to avoid shock. The levers registered bites as well as presses. For four of these rats, shock was contingent on lever bites when a specified time period had elapsed after the previous shock. An extinction period, in which only periodic noncontingent shocks were presented, followed avoidance training. Six yoked-control rats received the same sequence of shocks as did the corresponding experimental rats in both the conditioning and extinction phases. All six experimental rats repeatedly bit the avoidance lever. Four bit it more than the nonavoidance lever during conditioning, and five bit it more during extinction. Five of the six experimental rats consistently bit the levers many more times during each session than did their respective control rats, suggesting that avoidance conditioning facilitated lever biting. Rates of lever biting and pressing by all of the experimental rats and by some of the control rats were highest immediately following shock throughout both phases. During later portions of the intervals following shock, characteristic effects of conditioning and extinction were observed. This finding suggests that extinction of avoidance behavior by unavoidable shock presentations can be demonstrated more readily when shock-elicited responding is extricated from the data.     

The effects of escape conditioning and shock intensity on responding during inescapable shock

Eight albino rats, conditioned to press a lever to escape shock, continued to lever press during short inescapable shocks presented subsequently. The rate of this behavior was found to be higher for higher shock intensities regardless of the order in which shock values were presented. Relative to the immediately preceding escape rate, responding during inescapable shock was higher following conditioning at higher fixed-ratio escape requirements. Four subjects not conditioned to escape shock pressed the lever very infrequently during inescapable shock and showed little change with changes in shock intensity. The escape conditioning effects suggest that responding during inescapable shock is superstitious escape behavior. The effects of shock intensity on this behavior appear to be similar to reported effects of shock intensity on escape behavior.     

Preconditioning exposure to the unconditined stimulus affects the acquisition of a conditioned emotional response

Four experiments examined the UCS preexposure phenomenon using conditioned suppression of food-reinforced responding as a measure of excitatory conditioning, and electric shock as a UCS. In Experiment 1, groups of rats were preexposed to unsignaled 0.8-mA electric shocks for 0, 1, 3, 5, or 10 days, and then conditioned with a 0.8-mA electric shock. Preexposure to electric shock 1 day prior to conditioning enhanced the acquisition of a CER, whereas preexposure to electric shock for 3, 5, or 10 days prior to conditioning attenuated the acquisition of a CER as a direct function of the number of days of preexposure. In Experiments 2 and 2A, groups of rats were preexposed to unsignaled electric shocks of 0.3, 0.5, 0.8, or 1.3 mA for 10 days, and then conditioned with a 0.8-mA electric shocl. All groups preexposed to electric shock acquired the CER at a slower rate than a group not preexposed to electric shock. The greatest attenuation of CER conditioning occurred when the same intensity electric shock was used during both the preexposure and conditioning phases. In Experiment 3, groups of rats were preexposed to signaled electric shocks of either 0.5, 0.8, or 1.3 mA, and then conditioned with a 0.8-mA electric shock. All groups preexposed to electric shock acquired the CER at a slower rate than a group not preexposed to electric shock. As in Experiments 2 and 2A, the greatest attenuation of CER conditioning occurred when the same intensity electric shock was used during both the preexposure and conditioning phases. In Experiment 4, groups of rats were preexposed to series of 0.5, 0.8, or 1.3-mA electric shocks which they could escape by performing a chain-pull response. Rats in each of these groups had yoked partners which received the same number, intensity, and temporal pattern of electric shocks, but could not perform a response to escape shock. All groups were then conditioned with a 0.8-mA electric shock. Rats preexposed to escapable electric shocks showed equal or greater attenuation of CER conditioning than rats which could not escape shock during the preexposure phase. These results are discussed in terms of nonassociative and associative explanations of the UCS preexposure phenomenon.     

Effects of post-trial administration of naloxone and beta-endorphin on shock-induced fighting in rats

To study the involvement of endogenous opioid peptides in the development of shock-induced fighting, naloxone (2 mg/kg) or beta-endorphin (10 micrograms/kg) was administered subcutaneously immediately after the session and during nine consecutive daily sessions to rats repeatedly exposed to electric shocks. beta-Endorphin blocked the development of shock-induced fighting while naloxone facilitated it but only when shock-induced fighting occurred at a low rate. The effects of beta-endorphin were time dependent since when beta-endorphin was injected 90 min after the shock session instead of immediately after, its impairing effect disappeared. In addition, naloxone blocked the impairment produced by beta-endorphin. Differential postsession treatment of each member of pairs of rats with naloxone and beta-endorphin resulted in a higher probability of rats treated with naloxone to be dominant over rats treated with beta-endorphin in the test situation. These results are discussed in relation with the possible involvement of endogenous opioids in the modulation of the physiological consequences of defensive behavioral responses to shock.     

Actual versus potential shock in making shock situations function as negative reinforcers

The relative importance of potential and actual shocks in making shock situations function as negative reinforcers was studied. Shocks were scheduled to occur at the same rate during two stimuli. During one, squirrel monkeys could avoid the shocks; during the other, they were unavoidable. For the two stimuli the potential rate of shocks was the same, but the actual rate was lower during avoidance because of avoidance responding. Fixed-ratio responding was maintained by the change from unavoidable shock to avoidance, indicating that the change was reinforcing when it resulted in a reduction in actual shock rate with no reduction in potential shock rate. Further increases in the rate of potential shock during avoidance had little effect upon the fixed-ratio responding until the rate was increased to the point that the actual shock rate during avoidance was comparable with that during unavoidable shock. At that point, the fixed-ratio response rate decreased nearly to zero. These findings show that actual shocks are more important than potential shocks in determining whether or not a shock situation will function as a negative reinforcer; this explains why the change from unavoidable shock to avoidable shock is reinforcing.     

Schedules using noxious stimuli. III. Responding maintained with response-produced electric shocks

Responding was maintained in two squirrel monkeys under several variations of a 10-min fixed-interval schedule of electric shock presentation. The monkeys were first trained under a 2-min variable-interval schedule of food presentation, and then under a concurrent schedule of food presentation and shock presentation. In one monkey, when shocks (12.6 ma) followed each response during the last minute of an 11-min cycle ending with a timeout period, responding was increased during the first 10 min and suppressed during the last minute of each cycle. When the shock schedule was eliminated, both the enhancement and suppression disappeared, and a steady rate of responding was maintained under the variable-interval schedule. When the food schedule was eliminated, the shock schedule maintained a characteristic fixed-interval pattern of responding during the first 10 min, but suppressed responding during the last minute of each cycle. The fixed-interval pattern of responding was maintained when the timeout period was eliminated and when only one shock could occur at the end of the cycle. In the second monkey, responding under the concurrent food and shock schedule was suppressed when responses produced shocks after 3-min. Under an 11-min cycle, responding continued to be maintained at increasing shock intensities. When the food schedule was eliminated, a fixed-interval pattern of responding was maintained under a 10-min schedule of shock presentation (12.6 ma). Whether response-produced electric shocks suppressed responding or maintained responding depended on the schedule of shock presentation.     

Immunization of opioid analgesia: Effects of prior escapable shock on subsequent shock-induced and morphine-induced antinociception

In Experiment 1, it was shown that experience with escapable foot shock 4 hr prior to a session of 80 inescapable tail shocks prevented the occurrence of an analgesic response normally observed immediately following the tail shock. It has been suggested by J. W. Grau, R. L. Hyson, S. F. Maier, J. Madden, and J. D. Barchas (Science, 1981, 213, 1409–1411) that the analgesia that occurs following this number of inescapable tail shocks is mediated by endogenous opioid systems. To further explore the influence of escapable shock on opiate-mediated analgesia, Experiment 2 examined the effects of prior escapable shock on the long-term analgesia reaction that occurs upon brief exposure to shock 20 hr after morphine administration. Rats were given escapable shock, inescapable shock, or no shock 4 hr prior to a morphine injection. Twenty hours following the injection, all subjects received 5 brief foot shocks and were then immediately given tail-flick analgesia tests. Subjects which received inescapable shock or no shock prior to the morphine injection displayed a significant analgesic response. However, subjects which received escapable shock prior to morphine were not analgesic following brief exposure to shock. Thus, escapable shock seems to directly influence the activation of opioid analgesia systems.     

Punishment shock intensity and basal skin resistance

The relationship between punishment shock intensity and basal skin resistance (BSR) was investigated in two sessions with human females selected for their ability to maintain a fairly substantial operant rate under a wide range of shock intensities. In both sessions each button-pressing response was reinforced with a counter tally. Subjects were paid one cent for each 20 counts. In session 1, punishment followed each response during alternate 4-min periods; in session 2 punishment was programmed in all 4-min periods. Shock intensities were presented randomly among the 4-min shock periods, with the restriction that the first three presentations occurred in ascending order. Operant responding showed some suppression at higher shock intensities in session 1, with substantial recovery in most subjects during session 2. Respondent behavior was characterized by greater activity at successively higher intensities, with recovery at all shock levels, especially the lowest levels, apparent during the second session.     

Periodic shock with added clock

Rats were shocked every 6 min while responding was maintained on a variable-interval schedule of reinforcement. With some rats, shocks were interspersed with a sequence of three different stimulus conditions (S3→S2→S1), or clock cues, each lasting 2 min. For other rats, a single stimulus condition prevailed between shocks at the beginning of the experiment and clock cues were introduced later. Response rate decreased from S3 to S1. Response rate in S3, S2, and S1 was inversely related to shock intensity. When clock cues were added, response rate increased in all 2-min intershock periods. During clock cues, an index of curvature, indicating the degree of negative acceleration of response rate, was greatest for S1 and least for S3, and was directly related to shock intensity. The response-facilitating effect of shock and its relation to a possible discriminative function of shock and to behavioral contrast is discussed.     

Persistent shock-elicited responding engendered by a negative-reinforcement procedure

A procedure in which responses reduced intermittently presented electric shocks to one quarter of their originally scheduled intensity, effectively engendered and maintained lever pressing in hooded rats. This contingency also markedly increased the response rates of rats initially trained under an unsignaled avoidance procedure. The responding of all animals extinguished rapidly when shock was withdrawn. Subsequently, it was discovered that high response rates could be maintained solely through presentation of shocks that were not affected by responses. Variations in the interval between shocks and changes in shock intensity over a wide range did not attenuate responding. Terminal performance was characterized by a consistent pattern of shock-elicited responses. Responses were also elicited by a tone following repeated tone-shock pairings. Finally, responding that was maintained by response-independent shocks was quickly suppressed by response-contingent shocks of the same intensity.     

ASSESSING THE ROLE OF EFFORT REDUCTION IN THE REINFORCING EFFICACY OF TIMEOUT FROM AVOIDANCE

Rats responded on concurrent schedules of shock‐postponement or deletion (avoidance) and timeout from avoidance. In Experiment 1, 3 rats' responses on one lever postponed shocks for 20 s and responses on a second lever produced a 1‐min timeout according to a variable‐interval 45‐s schedule. Across conditions, a warning signal (white noise) was presented 19.5 s, 16 s, 12 s, 8 s, or 4 s before an impending shock. Raising the duration of the warning signal increased both avoidance and timeout response rates. Timeout responding, although positively correlated with avoidance responding, was not correlated with the prevailing shock rate. In Experiment 2, 3 rats' responses on one lever deleted scheduled shocks according to a variable‐cycle 30‐s schedule and responses on a second lever produced a 2‐min timeout as described above. After this baseline condition, the avoidance lever was removed and noncontingent shocks were delivered at intervals yoked to the receipt of shocks in the baseline sessions. Timeout responding decreased when the avoidance lever was removed, even though the shock‐frequency reduction afforded by the timeout remained constant. These results suggest that a key factor in the reinforcing efficacy of timeout is suspension of the requirement to work to avoid shock, rather than the reduction in shock frequency associated with timeout.     

Enhancement of progressive-ratio performance by chlordiazepoxide and phenobarbital

The key pecking of two pigeons was reinforced with food on a progressive-ratio schedule, which required an increasing number of responses for each successive reinforcement: 8, 16, 24, 32, etc. When the subject failed to complete the next ratio in the sequence within 60 min, the session terminated. The number of responses in the final completed ratio was defined as the "breaking point". After the breaking point had stabilized (60 sessions), it served as a baseline to assess the effects of varying doses (5 to 80 mg/kg) of chlordiazepoxide and phenobarbital, administered intramuscularly 30 min before the sessions. Both drugs increased the breaking point. The dose-effect curves were inverted U-shaped, with maximum enhancement of performance occurring at 20 mg/kg for chlordiazepoxide and at 40 mg/kg for phenobarbital. A comparable enhancement was not obtained during a non-drug "probe" session, which was conducted after the subjects' body weights had been temporarily reduced from 80% to 70% of their free-feeding weights. The drug-induced enhancement of breaking point was related to the initial values of the performance and may represent a reduction in the aversiveness of the schedule.     

Conditioned suppression, punishment, and aversion

Three experiments were conducted to assess the aversive properties of a visual stimulus in the presence of which one group of birds received response-contingent shock (discriminated punishment) while a yoked group of birds received non-contingent shocks (conditioned suppression). In Experiment 1, presentation of the visual stimulus contingent on key pecking reduced the response rate (conditioned punishment effect) for birds under the conditioned suppression procedure but did not reduce the response rate of birds under the discriminative punishment procedure. Non-contingent shocks also produced greater suppression of responding maintained by positive reinforcement in the presence of a visual stimulus than did response-contingent shocks. In Experiment 2, a greater shock intensity (2 mA) was used. All the differences between the two groups found in Experiment 1 were also found in Experiment 2. Experiment 3 demonstrated that response-contingent shock did not result in a conditioned punishment effect even when positive reinforcers were unavailable during the discriminative punishment schedule. The exteroceptive stimulus that was paired with shock in the conditioned suppression procedure acquired the ability to punish behavior. The exteroceptive stimulus in the discriminative punishment schedule did not acquire this ability.     

Conditioned suppression by a stimulus associated with nalorphine in morphine-dependent monkeys

Three rhesus monkeys, physically dependent on morphine, were trained to press a lever for food on a fixed ratio of 10 responses. A tone, initially a neutral stimulus, was aperiodically presented every third or fourth session, 5 min before and after the intravenous injection of nalorphine, a morphine antagonist which produces an immediate withdrawal syndrome in morphine-dependent monkeys. After several sessions, conditioned suppression of food-lever response rate was observed. Conditioned bradycardia, emesis, and excessive salivation also occurred. In 40 to 45 sessions the conditioned suppression of food-lever response rate and the conditioned autonomic changes were extinguished by presenting pairings of a tone and saline injection. The monkeys were then reconditioned by presenting the tone aperiodically, every third or fourth session, 5 min before and after the intravenous injection of nalorphine. Results were similar to the initial conditioning sessions. Two rhesus monkeys not dependent on morphine were stabilized on a food schedule similar to that used for the first three monkeys. These monkeys showed no change in food-lever response rate during or after nalorphine injections.