Extinction of Sidman avoidance behavior

Extinction of Sidman avoidance behavior by eliminating the noxious stimulus was studied in Sprague-Dawley rats with bar-pressing as the response. Each of three subjects was trained and extinguished on each of the following schedules in a different order: nondiscriminated, response-shock interval = 20 sec, shock-shock interval = 5 sec; nondiscriminated, response-shock interval = 40 sec, shock-shock interval = 5 sec; discriminated, response-white noise interval = 15 sec, noise-shock interval = 5 sec, shock-shock interval = 5 sec. Less than one 4-hr session was required for extinction for all procedures. When a warning stimulus was present, resistance to extinction increased. Subjects did not, however, respond to avoid the signal. Only small differences in extinction were found after training on different schedules with no warning signal.     

Response-shock delay as a reinforcer in avoidance behavior

After rats received preliminary training to avoid shock on a discrete-trial retractable-bar avoidance procedure, the procedure was changed such that responses retracted the lever but did not affect the rate of shock. Responses only delayed the onset of shock. About half of the animals under these procedures responded consistently on almost 100% of the discrete-trial cycles over days. When short latencies maximized the response-shock delay, animals tended to make short-latency responses. When long latencies maximized the response-shock delay, animals tended to make long-latency responses. When all response latencies produced the same response-shock delay, animals made differing average-latency responses. And, when responses did not delay shock, most of the animals primarily engaged in shock-elicited responding while the other animals engaged in preshock responding.     

Conditioned suppression of an avoidance response by a stimulus paired with food

Three food-deprived Long-Evans rats were exposed to a non-discriminated shock avoidance procedure. Superimposed upon this operant avoidance baseline were periodic presentations of a conditioned stimulus that was paired with food, the unconditioned stimulus. These pairings resulted in increases in the rate of shock over that recorded when the conditioned stimulus was not present. A traditional suppression ratio failed to reveal any differential effect of the conditioned stimulus on the overall rate of avoidance responding, although all subjects showed a consistent pattern of pausing and postshock response bursts during presentations of the conditioned stimulus. When food was withheld during a final extinction phase, the conditioned stimulus ceased to occasion increases in shock rates and disruptive postshock response bursts were eliminated. An analysis of conditioned suppression procedures is proposed that stresses not only operant-Pavlovian or appetitive-aversive incompatibility, but also the manner in which the baseline schedule of reinforcement affects operant behavior changes that are elicited by the superimposed Pavlovian procedure.     

Associative vs topographical accounts of the immediate shock-freezing deficit in rats: Implications for the response selection rules governing species-specific defensive reactions

Rats were placed in a novel chamber and were given a single shock, either immediately upon placement in the chamber or after a 2-m delay. During a postshock observation period, delayed shock animals froze while immediate shock rats did not (Experiments 1–5). Additionally, delayed shock animals showed an elevation in defecation, relative to unshocked controls, while immediate shock animals did not (Experiment 1). Freezing and elevated defecation were first found with a 9-s delay between placement and shock, and both responses increased linearly with increasing delays up to at least 81 s (Experiment 2). Immediate shock rats did not show escape-related behaviors when potential escape routes were available (Experiments 3–4). A 2-m preexposure to the chamber, that was separated in time from shock, did not alleviate the immediate shock deficit (Experiments 4–5). The immediate shock deficit was also apparent in a test given 24 h after shock receipt (Experiment 6). If immediate shock rats were later given delayed shock they began to freeze but showed no carryover effects of their prior shock experience (Experiment 6). It was concluded that the immediate shock deficit reflects a deficit in association formation between contextual stimuli and shock rather than a difference in defensive behavioral topography. It also appears that freezing is the dominant conditional response to shock-associated stimuli even though freezing is not unconditionally elicited by the shock itself.     

Effects of cycle length on performance on a temporally defined avoidance schedule

Three rats were trained on a temporally defined avoidance schedule logically similar to a fixed-interval, limited-hold positive reinforcement schedule. This avoidance schedule was composed of time periods during which responses had no scheduled consequences alternating with time periods during which a response precluded shock. As with fixed-interval length and response rate on positive reinforcement schedules, an inverse relationship was obtained between the length of the no-consequence interval and response rate during the no-consequence interval. An inverse relationship was also obtained between the length of the no-consequence interval and the per cent of shocks avoided. A rate increase within the no-consequence interval, similar to that typically produced by fixed-interval positive reinforcement procedures, was displayed by one of the rats where the no-consequence interval was at intermediate values and frequency of shock was relatively high. The introduction of a discriminative stimulus correlated with the avoidance interval produced typical discriminated avoidance behavior as well as alterations in temporal patterning of responses during the no-consequence interval in the two rats exposed to this procedure. These alterations in temporal patterning disappeared when the discriminative stimulus was removed. The results were consonant with those reported in the literature involving food reinforcement and fixed-interval, limited-hold schedules.     

Active role of the shock-avoidance contingency in shuttlebox-avoidance learning in Fischer344 rats

Different groups of Fischer344 rats acquired and maintained shuttlebox-avoidance responses under one, two, all three, or none of the shock-avoidance, warning-signal (WS) termination, and shock-escape contingencies. The intertrial interval was fixed at either 30 or 60 s. Removal of the shock-avoidance contingency led to a marked disruption of avoidance behavior at both intervals. This result was not produced by a selective punishment effect of the avoidance response by an inevitable shock. Thus, the shock-avoidance contigency provided particularly effective control over the avoidance learning. The WS termination contingency appeared to have little overall effect at the 60-s intertrial interval. It is suggested that response-contingent stimulus change served only as a discriminative cue for the absence of a shock.     

Extinction of responding maintained by timeout from avoidance

The resistance to extinction of lever pressing maintained by timeout from avoidance was examined. Rats were trained under a concurrent schedule in which responses on one lever postponed shock on a free-operant avoidance (Sidman) schedule (response-shock interval = 30 s) and responses on another lever produced 2 min of signaled timeout from avoidance on a variable-ratio 15 schedule. Following extended training (106 to 363 2-hr sessions), two experiments were conducted. In Experiment 1 two different methods of extinction were compared. In one session, all shocks were omitted, and there was some weakening of avoidance but little change in timeout responding. In another session, responding on the timeout lever was ineffective, and under these conditions timeout responding showed rapid extinction. The within-session patterns produced by extinction manipulations were different than the effects of drugs such as morphine, which also reduces timeout responding. In Experiment 2 shock was omitted for many consecutive sessions. Response rates on the avoidance lever declined relatively rapidly, with noticeable reductions within 5 to 10 sessions. Extinction of the timeout lever response was much slower than extinction of avoidance in all 4 rats, and 2 rats continued responding at baseline levels for more than 20 extinction sessions. These results show that lever pressing maintained by negative reinforcement can be highly resistant to extinction. The persistence of responding on the timeout lever after avoidance extinction is not readily explained by current theories.     

Length of experimental session as a parameter of response rate under a non-discriminated

Following the stabilization of response rate under an avoidance schedule which was defined by two temporal parameters, the shock-shock interval and the interval by which each response postponed the onset of shock, the length of the experimental session was changed. It was found that after the subjects had been exposed to a longer session of avoidance schedule, their rates of response were considerably increased without a corresponding reduction in the number of shocks received.

In recent years considerable use has been made of an avoidance training technique in which the performance of the response functions to postpone the onset of an aversive stimulus, usually shock, by a fixed period. In the absence of the required response the aversive stimulus is programmed to occur at regular intervals. Experiments by Sidman (1953) have shown that the critical independent variable controlling the rate of avoidance response, is the shock postponement interval (R*S). All other things being equal, the rat in the lever pressing situation will respond at a rate which is inversely related to the R*S interval, low intervals generating high response rates and high intervals generating low response rates. However, under very low values of R*S, the response rate may break down altogether. The animal then receives shock at the rate determined by the shock-shock interval parameter.

As a result of an apparatus failure, Sidman, Herrnstein and Conrad (1957) discovered that the response rate can also be increased by occasionally shocking the animal in spite of its avoidance responses. An apparatus failure has also been responsible for the isolation of yet another parameter of response rate in the shock-postponement avoidance situation and is reported here. Briefly, it was found that a change in the duration of an experimental session influences the response rate on subsequent sessions.     

Free operant avoidance as a function of the response-shock=shock-shock interval

Two temporal parameters of free operant or Sidman avoidance behavior are the interval by which responses postpone shocks (Response-Shock interval) and the interval between shocks when no responses occur (Shock-Shock interval). Avoidance behavior was examined in three white rats under conditions where the Response-Shock and Shock-Shock intervals were always equal. With intervals from 10 to 60 sec response rates and shock rates were similar, decreasing, negatively accelerated functions of increasing Response-Shock=Shock-Shock interval. Over this range, response and shock rates were linearly related to the reciprocal of the Response-Shock=Shock-Shock interval. It was shown, however, that this relation cannot hold at extremely long intervals. Both the ratio of responses emitted to shocks received and the percentage of shocks possible which were avoided increased at long Response-Shock=Shock-Shock intervals. These findings may be related to the fact that long intervals provide optimal conditions for conditioning avoidance behavior in the rat.     

What is both necessary and sufficient to maintain avoidance responding in the shuttle box?

To learn what maintains the frequency of shuttle box avoidance responses, male rats from the Berkeley S₁ strain, after 200 trials of standard discriminative avoidance training, were given 100 additional trials under one of four different conditions. Responding at the maximum rate was maintained when animals performed under the training conditions or when responses continued to terminate the warning signal immediately, even though shock was never given for failing to respond. In contrast, avoidance responding was reduced markedly if, and only if, trials were given in which the signal ceased to terminate immediately (i.e. it shut off either well before or well after a response). This decrement occurred even though avoidance responses continued to avert shock. Thus, under the conditions of this experiment prompt signal offset was both necessary and sufficient to maintain the occurrence of well-established shuttle box avoidance responses.     

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.     

Continuous punishment of free-operant avoidance in the rat

Three groups of albino rats were trained under a free-operant avoidance (Sidman) procedure with equal shock-shock and response-shock intervals. After stable performance was achieved, the animals were concurrently exposed to a brief electric shock after each response. The procedures were as follows: Punishment Schedule I: punishment shock was introduced at an intensity approximately one quarter that of avoidance shock; increments of nearly this same size were made as stable performance was achieved at succeeding punishment shock intensities. Punishment Schedule II: punishment shock was introduced at approximately one-half the intensity of avoidance shock; after stable performance, punishment shock was increased to the same intensity as avoidance shock. Punishment Schedule III: punishment shock was introduced and maintained at the same intensity as avoidance shock. Punishment was continued for all groups until one of two suppression criteria was attained. All animals made fewer responses and received more avoidance shocks as a function of increasing punishment shock. Half of the animals under Punishment Schedule I required punishment shock higher than avoidance shock to meet their assigned suppression criterion. A comparison of all procedures showed that suppression was greater when punishment shock was initially at high intensity.     

Rapid acquisition of a sidman avoidance response

Rats were trained on a Sidman avoidance task with a 22-sec R-S and a 6-sec S-S interval for six daily 30-min training sessions. Disabling the response lever during and 0.5 sec after shock termination represented one factor in the 2 by 2 factorial design and the presentation of a feedback light cue accompanying an effective response was the other. Rats in the lever-disable condition with feedback light made significantly more avoidance responses than the other treatment groups. In a second experiment, daily 60-min sessions were observed. Using a lever-disable plus feedback light treatment, rats made more avoidance responses and received fewer shocks during the second 30-min as opposed to the first 30 min period.     

The effects of cocaine on behavior maintained by timeout from avoidance.

Rats were trained on concurrent schedules under which responses on one lever postponed shock (avoidance) and responses on the other lever produced brief (2-min) periods of signaled timeout from avoidance. For 6 rats, timeout from avoidance was programmed on a variable-interval 45-s schedule that generally resulted in rates that were lower than those on the avoidance lever. For another 6 rats, timeout was arranged on a variable-ratio 15 schedule that produced higher baseline rates. Cocaine (3 to 40 mg/kg) produced large, dose-dependent increases in behavior maintained by timeout in both groups of rats. Avoidance responding was also generally increased by cocaine, but the increases were of lesser magnitude. Increases in response rates were seen across a broad range of doses on behavior maintained by either interval or ratio schedules, an outcome that was unexpected on the basis of most studies of cocaine on food-maintained behavior. These results were similar to those of previous studies of the effects of amphetamine on behavior maintained by timeout from avoidance and suggest that stimulant drugs affect behavior maintained under a shock-postponement schedule differently than they affect behavior maintained by timeout from avoidance.     

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.     

Effects of partial avoidance and punishment contingencies on shuttlebox-avoidance learning in Fischer 344 rats

An unsignaled, escapable shock was presented contingent on an avoidance response. Fischer 344 rats responded less to the warning signal in proportion to its temporal distance from the avoidance response. Partial contingency effects were further obtained by variation in the instrumental conditioning space for an aversive stimulus. However, the arbitrary omission of an imminent shock on half the trial in which the rats failed to avoid a shock, led to little avoidance acquisition, and shock-frequency reduction was thus not sufficient to produce the acquisition of the avoidance response. Because early avoidance responses were initiated by escape from shock, a stimulus contingency may be essential for response initiation, and an explicit response contingency is important in maintaining successful avoidance responses.     

Parameters affecting the acquisition of Sidman avoidance

Four studies were conducted to investigate the source of reinforcement of Sidman avoidance. First it was found that the acquisition of avoidance was seriously impaired if there was no immediate stimulus consequence of responding, i.e., proprioceptive and auditory feedback. The second study showed that if the shock-shock interval was split so that only responses occurring in one half of the interval were effective in postponing shock, then learning was impaired. It made little difference which half of the interval was used. The third study demonstrated that Ss learn Sidman avoidance more quickly with a variable shock-shock interval than with the usual fixed-interval procedure. The results of the second and third experiments argue against the view that avoidance is reinforced by the decrease in conditioned aversiveness that occurs at long post-shock times. The final experiment indicated that Ss do not learn Sidman avoidance if the response-produced delay of shock is preceded by a shock, hence it seems unlikely that the crucial source of reinforcement is merely an overall reduction in shock density. All of the findings are consistent with the hypothesis that the avoidance response is reinforced by the decrease in conditioned aversiveness of stimuli at short post-response times. This seems to be the case even at the beginning of acquisition.     

Lick-shock contingencies in the rat

Hungry rats were allowed to lick an 8% sucrose solution and then one of four lick-shock contingency conditions was superimposed on the licking baseline. These conditions were: free-operant avoidance, free shock, punishment, and no shock. From highest to lowest response rates, the groups fell in the order-avoidance, no shock, free shock, and punishment. Lick rates adjusted rapidly to introduction and removal of the contingencies. Post-shock responding was lowest in the punishment condition and highest in the free shock condition. No method was found simultaneously to equate shock frequency and separate response rates for the three shock contingency conditions. Only small, or no, reductions in shock rate occurred over sessions under the free-operant avoidance schedule when the shock-shock interval was 10 sec but large reductions occurred when the shock-shock interval was reduced to either 1 or 2 sec.     

Methylphenidate and stimulus control of avoidance behavior

The introduction of a warning signal that preceded a scheduled shock modified the temporal distribution of free-operant avoidance responses. With response-shock and shock-shock intervals held constant, response rates increased only slightly when the response-signal interval was reduced. The result is consistent with Sidman's (1955) findings under different conditions, but at variance with Ulrich, Holz, and Azrin's (1964) findings under similar conditions. Methylphenidate in graded doses increased response rates, modifying frequency distributions of interresponse times. Drug treatment may have disrupted a "temporal discrimination" formed within the signal-shock interval. More simply, methylphenidate influenced response rates by increasing short response latencies after signal onset; this effect was more prominent than the drug's tendency to increase the frequency of pre-signal responses. When signal-onset preceded shock by 2 sec, individual differences in performance were marked; methylphenidate suppressed responding in one rat as a function of increasing dose levels to a greater degree than in a second animal, but both subjects received more shocks than under control conditions.