A comparison of escape and avoidance conditioning in wild and domesticated rats

In the first of two experiments, three cotton rats (Sigmodon hispidus) and three albino rats were exposed to instrumental escape, unsignaled avoidance, and signaled avoidance, in that order. All subjects learned the escape procedure quickly, with the albino rats having generally shorter latencies, higher response rates, and requiring fewer sessions to reach the criterion. When the avoidance contingency was introduced, the cotton rats continued to respond almost entirely in the presence of the shock, whereas the albino rats responded in its absence, thus displaying effective avoidance behavior. Introduction of a pre-aversive stimulus did not improve the performance of the cotton rats. In the second experiment, five cotton rats and four albino rats were exposed to a free-operant (Sidman) avoidance procedure with a shock-shock interval of 3 sec and a response-shock interval of 20 sec. The cotton rats initiated responding at lower shock intensities than the albino rats, but their asymptotic avoidance responding was far less effective.     

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.     

Effects of response-shock interval and shock intensity on free-operant avoidance responding in the pigeon

Two experiments investigated free-operant avoidance responding with pigeons using a treadle-pressing response. In Experiment I, pigeons were initially trained on a free-operant avoidance schedule with a response-shock interval of 32 sec and a shock-shock interval of 10 sec, and were subsequently exposed to 10 values of the response-shock parameter ranging from 2.5 to 150 sec. The functions relating response rate to response-shock interval were similar to the ones reported by Sidman in his 1953 studies employing rats, and were independent of the order of presentation of the response-shock values. Shock rates decreased as response-shock duration increased. In Experiment II, a free-operant avoidance schedule with a response-shock interval of 20 sec and a shock-shock interval of 5 sec was used, and shock intensities were varied over five values ranging from 2 to 32 mA. Response rates increased markedly as shock intensity increased from 2 to 8 mA, but rates changed little with further increases in shock intensity. Shock rates decreased as intensity increased from 2 to 8 mA, and showed little change as intensity increased from 8 to 32 mA.     

Summation of food and shock based responding

In a multiphase experiment, dogs first received discriminative, discretetrial, barrier-jumping training with two tones (S^D, S^Δ) in a shuttle box reinforced by either shock avoidance (Group I) or by food (Group II). Then the dogs were trained on free-operant barrier-jumping reinforced by the qualitatively opposite reinforcer—food in Group I and shock avoidance in Group II. Finally, test presentations of the tone stimuli were superimposed on the free-operant behavior. The tone S^Ds markedly facilitated responding in all animals. This experiment demonstrates a summation of responding maintained by shock avoidance and food reinforcement and casts doubt on explanations of conditioned suppression outcomes that appeal solely to incompatible motivational states within the organism.     

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.     

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.     

Acquisition and extinction of signaled avoidance as a function of intermittent reinforcement

Signaled, shuttle-box avoidance responding in female rats of the Fischer₃₄₄ strain was examined as a function of four separate contingencies of intermittent reinforcement. In Experiment 1, when avoidance responses during acquisition were reinforced 25% of the time with prompt CS termination, animals responded equally often during acquisition and significantly more often during extinction than animals who received such reinforcement on a 100% schedule. Similar results were found under a trace procedure in Experiment 2 when avoidance responses were reinforced 25% of the time with informational feedback stimuli. In contrast, during Experiment 3, when animals were shocked on only 25% of the trials on which they failed to respond, the level of avoidance responding during both acquisition and extinction was significantly less than it was when animals were shocked on a 100% schedule. Comparable results were found in Experiment 4 when avoidance responses during acquisition averted shock on only 25% of the trials. Thus, intermittent reinforcement contingencies involving response-contingent feedback stimuli and shock have differential effects on avoidance responding during both acquisition and extinction trials under the signaled avoidance procedure.     

Acquisition and retention of control of instrumental behavior by a cue-signaling airblast: How specific are conditioned anticipations?

The conditioned state evoked by a CS that had been paired with an aversive airblast was assessed by superimposing the CS on baselines of instrumental responding. The first experiment used a shock avoidance baseline; CS-airblast pairings were administered on the baseline, with the shock avoidance schedule in effect. The CS that signaled airblast came, over trails, to accelerate shock avoidance responding. As further CS-airblast trials were administered, however, the acceleratory effect of the CS decreased, producing an inverted-U-shaped acquisition curve. A second experiment used off-baseline CS-airblast conditioning, and the CS was tested on both shock avoidance and water reinforcement baselines. After 20 CS-airblast pairings the CS accelerated shock avoidance and suppressed water-reinforced responding. After 10o pairings, however, the acceleratory effect on shock avoidance was much attenuated, although the suppressions of the water-reinforced behavior did not differ between 20- and 100-trial groups. Two further experiments examined retention, over a 45-day interval, of the attenuation of the CS's effects on shock avoidance. Over the retention interval, this attenuation disappeared. It was, however, reinstated by giving the animals “reminder” presentations of the airblast. These results are interpreted as representing systematic changes in the specificity of the conditioned state evoked by a Pavlovian CS.     

Escape responding in a shuttlebox

Measures of response latency are a primary tool for those who investigate escape responding in rats. Unfortunately, the single term “latency to escape” has been applied to several different measures of latency. The present study was designed to demonstrate that two different measures of the “latency to escape” tap different aspects of escape responding. To that end, rats were given escape training using a Sidman Avoidance Schedule with brief inescapable electric shock as the aversive stimulus. The latency to escape the shock-shock interval, as measured from the onset of the last shock in a shock period, did not change across trials. However, the latency to escape the shock period, as measured from the onset of the first shock in a shock period, decreased across trials. In addition, the presentation of a feedback stimulus contingent upon escape responding did not affect the latency to escape either the shock period or a shock-shock interval. The results show that these two latency measures, typically not recognized as unique, measure different characteristics of the strength of escape responding in a shuttlebox. Alternative accounts of this pattern of data were considered.     

Free-operant avoidance conditioning in individual and paired human subjects

Male, medical and graduate students were subjected to a non-discriminated avoidance regimen with shock-shock and response-shock intervals of 10 sec. Using a yoked-chair procedure it was found that acquisition of the button-pressing avoidance response was influenced by the social environment in which the conditioning occurred. There was a significantly greater number of “learners” among subjects conditioned individually than among those exposed to the conditioning procedures in the presence of a second person.     

Key pecking as a function of response-shock and shock-shock intervals in unsignalled avoidance

Five pigeons were exposed to an unsignalled avoidance procedure where key pecks were maintained through shock postponement. Functions obtained showed an inverse relationship between rate of responding and length of the response-shock interval, while changes in the shock-shock interval had no systematic effect on response rates. The rate of shocks delivered generally decreased with increases in length of both response-shock and shock-shock intervals. Results show that key pecking in pigeons, maintained through an unsignalled avoidance procedure, was affected by changes in response-shock and shock-shock intervals in the same manner as other responses in pigeons and in rats.     

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.     

Nondiscriminated avoidance of shock by pigeons pecking a key

Four pigeons were trained to avoid shock by pecking a key on a free-operant avoidance schedule in which no exteroceptive stimulus signalled impending shock. Response rate was an inverse function of response-shock interval when shock-shock interval was held constant at 2 sec and response-shock intervals varied from 5 to 40 sec. Amphetamine increased response rates in two subjects and reserpine markedly reduced responding in one.     

Discrimination learning by macaca mulatta with option to switch between S+ and S−

Three rhesus monkeys were trained to press either of two response keys. A response on the reinforcement key during presentation of the reinforced stimulus produced a sucrose pellet followed by an intertrial interval, but during presentation of the unreinforced stimulus produced only the intertrial interval. A response on the switching key changed the discriminative stimulus from reinforced to unreinforced or from unreinforced to reinforced. The reinforced stimulus was presented automatically on half the trials, but could be produced only by a switching response on the other half. Switching tended to occur in three distinct stages during acquisition of discriminative behavior. The first stage was identified as "nondiscriminative switching"; the second as "nonswitching"; and the third as "discriminative switching".     

Children's acquisition and reversal behavior in a probability learning situation as a function of programed instruction,internal-external control,and schedules of reinforcement

A three-choice, contingent task was used with 192 fifth and sixth graders in a 2 × 2 × 3 factorial design combining instruction (programed instruction on selected probability concepts vs no programed instruction), locus of control (internal vs external), and schedules of reinforcement (33, 66, and 100%). The dependent measures were the percentage of correct acquisition responses, of correct reversal responses, and of pattern responses, as well as posttests on probability concepts. The major findings of the study were associated with schedule of reinforcement. In acquisition and reversal, Ss under 100% reinforcement during acquisition tended to maximize the greatest, followed by the 66 and 33% conditions, in that order. The ordinal relationship among schedules was the exact reverse of the maximizing approach for the pattern responses. A partial reinforcement effect was obtained in reversal. Evidence indicated that programed instruction and locus of control affected maximizing behavior, patterning behavior, and resistance to extinction (though these variables did not interact with reinforcement schedule in the predicted direction). Finally the posttest data showed that instructed Ss did learn more relative to noninstructed Ss.     

Some factors involved in the comparison of response systems: acquisition, extinction, and transfer of head-poke and lever-press Sidman avoidance

Head poking, a suggested natural escape reaction to shock for the rat, was compared to lever pressing in a Sidman avoidance study. Both responses could be emitted at any time, but only one was effective in a given session. Acquisition and extinction of the two responses were compared under both signalled and unsignalled avoidance. Then, a test for transfer was conducted in which acquisition conditions were re-instated, but the effectiveness of the responses was reversed. Three differences between responses were noted: (a) head poking was superior in reducing shock rates under signalled conditions; (b) head poking was more resistant to extinction, especially under signalled conditions; (c) under unsignalled conditions, animals were unable to learn to head poke if they had previously learned to lever press. Findings a and c were pursued in later experiments. Finding a depended on the location of the warning signal with respect to the response system. When the lever press required approach to the warning signal, the head poke was superior. But when the head poke required approach to the warning signal, the two responses were equally effective. Finding c depended on the absence of feedback for head poke during transfer. Two conclusions are offered: first, the two responses appear to obey the same laws when their topographical differences are taken into account. Second, response feedback appears to be more critical in transfer than in original acquisition.     

Postacquisition injection of tetrodotoxin into the parabrachial nuclei elicits partial disruption of passive avoidance reaction in rats.

The recent discovery that post-trial functional blockade of the parabrachial nuclei by intracerebral injection of 10 ng tetrodotoxin (TTX) disrupts acquisition of conditioned taste aversion (CTA) (Ivanova & Bures, 1990a,b) has prompted attempts to ascertain the role of this structure in other types of inhibitory learning. In Experiment 1, rats with implanted parabrachial cannulae were trained in a step-through avoidance task and received bilateral TTX (2 x 10 ng) immediately after the acquisition trial; they displayed significantly weakened avoidance of the shock compartment 2 days later. In Experiment 2, rats were anesthetized with pentobarbital (50 mg/kg) immediately after passive avoidance acquisition and received parabrachial TTX 15 min later; whereas anesthesia alone left the passive avoidance reaction (PAR) unaffected, TTX elicited similar disruption as in unanesthetized animals. In Experiment 3, TTX was injected in anesthetized animals 0, 1, 2, or 4 days after PAR acquisition. The amnesic effect was significant when the acquisition-TTX delay had been prolonged to 24 but not to 48 or 96 h. Since CTA is disrupted by reversible blockade of parabrachial nuclei and of the adjacent reticular formation elicited up to 4 days after acquisition (Ivanova & Bures, 1990b), PAR seems to be impaired to a lesser degree and for a shorter time than CTA by similar TTX treatment.     

Long-Term conditioned fear modification in the dog as measured by changes in urinary 11-hydroxycorticosteroids. Heart rate and behavior

Heart rate (HR), operant key pressing, and urinary 11-hydroxycorticosteroids (11-OH-CS) were recorded while dogs were subjected to various aversive conditioning schedules over a six-month period. The schedules consisted of Sidman avoidance, followed by a Sidman schedule which paired unavoidable shocks (US) with offset of seven discrete conditioned stimuli (CS). Introduction of the Sidman avoidance schedule led to an increase in HR. Changes in the experimental schedule always results in an increase in urinary 11-OH-CS. Neither HR nor urinary 11-OH-CS were correlated with rate of operant key pressing. Differences in the dependent variables reflected the different topological characteristics of the dogs studied. This study demonstrated: (a) a causal independence of physiological and behavioral responses conditioned to the same stimulus complex, and (b) that longterm increases in HR can be maintained if the experimental situation is manipulated to maintain the fear eliciting characteristics of the conditioning situation.     

The fear potentiated startle effect

A differential conditioning study examined whether an acoustic startle probe, presented during extinction of an aversively conditioned visual stimulus, potentiated the reflex eyeblink response in humans and whether this potentiation varied with the change in affective valence of the conditioned stimulus. Sixty college students were randomly assigned to view a series of two slides, depicting either unpleasant/highly arousing, unpleasant/moderate arousing, neutral/calm, pleasant/moderate arousing or pleasant/highly arousing scenes and objects (duration: 8 sec). During preconditioning (8 trials) and extinction (24 trials) acoustic startle probes (white noise bursts [50 ms; 95 dBA] were administered during and between slide presentation). During acquisition (16 trials) CS+ was reinforced by an electric shock. Startle response magnitudes significantly increased from preconditioning to extinction and were substantially larger to CS+. Conditioned startle reflex augmentation linearly increased with the pleasantness of the slides. Furthermore, subjects showed a greater post-conditioning increase of judged aversiveness to slides that they had previously reported to be more pleasant, exactly paralleling the startle reflex results.