Responding maintained under fixed-interval and fixed-time schedules of electric shock presentation

Following initial histories under a schedule of electric shock postponement, lever pressing in squirrel monkeys was maintained under fixed-interval and fixed-time schedules of electric shock presentation. No difference in either rate or pattern of responding was obtained when these schedules were presented as components of a multiple schedule. When they were presented singly for long periods of time, the fixed-interval schedule consistently maintained a higher response rate than the fixed-time schedule. The pattern of responding under both schedules was similar, typically consisting of a pause at the beginning of each interval followed by either a steady or a positively accelerating rate of responding. The results suggest that the response-shock dependency is of critical importance in the maintenance of high rates of responding under schedules of electric shock presentation, and support the general view that such responding may be conceptualized as operant behavior under control of many of the same variables that control responding under comparable schedules of food or water reinforcement.     

Preference for less frequent shock under fixed-interval schedules of electric-shock presentation.

Lever pressing by 2 squirrel monkeys was maintained under fixed-interval 6-min and fixed-interval 2-min schedules of electric-shock presentation. Preference for these schedules was assessed during three experimental phases. In all phases, responses on one lever produced shock according to one or the other fixed-interval schedule, and responses on a second, changeover, lever switched between schedules. The opportunity to change over was presented during separate choice periods (during which the fixed-interval schedules did not operate) that followed the first through fourth shocks in each schedule. If no changeover occurred during those choice periods, a changeover automatically occurred following the fifth shock. In Phase I, durations of the choice periods were fixed. In Phase II, the choice periods equaled a proportion of their respective fixed interval. During Phase III (completed with 1 monkey) a response on the changeover lever during a given choice period reinstated the most recent fixed interval, and a failure to respond resulted in a changeover. During each of these phases, distinct preferences developed for the 6-min schedule. These results suggest that the maintenance of lever pressing by fixed-interval presentation of electric shock may not be an example of positive reinforcement, and that the response-maintaining characteristics of shock presentation may derive from other properties of the schedule.     

Behavior simultaneously maintained by both presentation and termination of noxious stimuli

Lever pressing by two squirrel monkeys was maintained under a 3-minute variable-interval schedule of response-produced electric-shock presentation. At the same time, responding on a second lever was maintained under a 3-minute fixed-interval schedule of termination of the shock-presentation schedule and shock-correlated stimuli. Under the termination schedule, the first response after a 3-minute period produced a 1-minute timeout, during which no events occurred and responding had no scheduled consequence. Relatively high and constant rates of responding were maintained on the lever where responding produced shock. Lower rates and positively accelerated patterns of responding occurred on the lever where responding terminated the shock schedule. Thus, responding was simultaneously maintained by presentation of an event and by termination of a stimulus associated with that event. Rates and patterns of responding on each lever were reversed when the schedules arranged on each lever were reversed on two occasions. When shock intensity was increased from 0 to 10 mA, responding maintained both by presentation of shock and by termination of the shock schedule increased, but responding maintained by shock presentation increased to a greater extent. Positive and negative reinforcement, usually regarded as separate behavioral processes involving different events, can coexist when behavior is controlled by different contingencies involving the same event.     

Electric shock produced drinking in the squirrel monkey.

Squirrel monkeys were periodically exposed to brief electric tail shocks in a test environment containing a rubber hose, response lever, and a water spout. Shock delivery produced preshock lever pressing and postshock hose biting. Additionally, all subjects displayed licking responses following postshock biting-attack episodes. Further experiments showed that licking was: (1) influenced by hours of water deprivation; (2) drinking behavior; (3) the direct result of shock delivery; and (4) developed spontaneously in naive subjects with or without opportunities for hose biting or lever pressing. Removing the opportunity to attack increased postshock drinking. A noxious environmental stimulus that causes aggression can also produce drinking.     

Effects of d-amphetamine and caffeine on schedule-controlled and schedule-induced responding.

The effects of d-amphetamine and caffeine were studied on rates and patterns of lever pressing and schedule-induced licking under fixed-interval schedules of food pellet presentation. In addition, the effects of caffeine were studied on lever pressing and licking under a multiple fixed-ratio fixed-interval schedule. Caffeine reduced mean overall rates of licking at lower doses than it reduced mean overall rates of pressing under the fixed-interval schedules, but the effects of caffeine on both licking and lever pressing depended largely on the control rate of responding. d-Amphetamine reduced mean overall rates of lever pressing and licking at about the same dose, but the effects of d-amphetamine also were a function of the control rate of responding.     

Maintenance of responding by squirrel monkeys under a concurrent shock-postponement, fixed-interval shock-presentation schedule.

A chain-pulling response was initially developed under a shock-postponement (avoidance) schedule with two squirrel monkeys. Few responses occurred on a lever where responding initially had no scheduled consequence or, subsequently, when a 3-minute fixed-interval shock-presentation schedule was concurrently arranged for lever responses. Appropriate rates and patterns of lever responding developed and were later maintained under the fixed-interval 3-minute shock-presentation schedule alone when the chain and shock-postponement schedule were removed. When both the shock-postponement and shock-presentation schedules were again simultaneously in effect, steady rates of chain pulling were maintained by the shock-postponement schedule and positively accelerated rates and patterns were maintained on the lever by the shock-presentation schedule. Response rates under both schedules were directly related to shock intensity. A history of exposure to a shock-postponement schedule, even though with a topographically different response and manipulandum, was sufficient for the development and eventual maintenance of responding by the presentation of shock. Further, differential performances can be maintained simultaneously by the presentation and postponement of electric shock.     

Effects of ratio contingencies on responding maintained by schedules of electric-shock presentation (response-produced shock)

Squirrel monkeys' lever pressing was established under fixed-interval schedules of electric-shock presentation (response-produced shock). After appropriate temporal patterns of lever pressing were engendered, either fixed-ratio schedules of shock presentation were added to the fixed interval, or yoked variable-ratio schedules were substituted for the fixed-interval schedules. When fixed-ratio schedules were added, there was an initial rise in response rate and schedule-appropriate patterns of responding developed. After many sessions, however, responding ceased abruptly, in some cases with remarkable quickness. When variable-ratio schedules were substituted, responded declined gradually and eventually was poorly maintained. Ratio contingencies may not support responding as well as interval contingencies when electric shock is the maintaining event.     

Effects of response-dependent and independent electric shock on schedule-induced polydipsia

In Experiment I, lever pressing by rats was maintained by the delivery of food pellets under a 45-sec fixed-interval schedule. Fixed-time 180-sec and fixed-interval 180-sec schedules of shock delivery were systematically superimposed on the baseline food schedule to study effects on schedule-induced water intake. Response-dependent shock had little, if any, effect on water intake, whereas shocks independent of lever pressing attenuated fluid intake. In Experiment 2, rats received food pellets under a fixed-time 60-sec schedule. Electric shock delivered concurrently under a variable-time 180-sec schedule, but never while the animal was licking or within 5 sec after licking terminated, led to similar attenuation of water intake. These findings suggest that schedule-induced polydipsia is sensitive to differences in the functional properties of response-independent and dependent electric shock.     

Effects of long-term shock and associated stimuli on aggressive and manual responses

Squirrel monkeys were exposed to response-independent, fixed-frequency shock that produced biting attack upon a pneumatic hose. Attacks decreased within and across sessions at low intensities and high frequencies of shock, but increased within and across sessions at higher intensities and lower shock frequencies. Stimuli paired with shock, when presented alone, came to produce biting, and stimuli correlated with shock parameters that produced increases in responding within sessions produced similar increases when presented alone. Further experiments showed that continuing exposure to shock also produced lever pressing or chain pulling, with longer shock exposure again producing higher response rates. Whereas biting generally decreased throughout the intershock interval, manual responding generally increased as shock time approached, but immediately before shock was often suppressed. Following shock, biting attack predominated over manual behavior. The results suggest a possible explanation for the extreme resistance of avoidance behavior to extinction, and may also partially explain the persistence of responding during schedules of response-produced shock. Relationships of the present findings to naturalistic observations of relations between fleeing, freezing, and fighting performances are discussed.     

Responding under fixed-ratio and multiple fixed-interval fixed-ratio schedules of electric shock presentation

Squirrel monkeys, initially trained under a schedule of electric shock postponement and then under fixed-interval schedules of electric shock presentation, were studied under multiple fixed-interval fixed-ratio and under fixed-ratio schedules of shock presentation. Under the fixed-interval (10-min) component of the multiple schedule, a pause was followed by a gradual increase in responding to a rate maintained until shock presentation; under the fixed-ratio (3-, 10-, or 30-response) component of the multiple schedule, a brief pause was typically followed by a relatively high and uniform rate of responding until shock was presented. When the 60-sec timeout periods, which usually followed shock presentation, were eliminated from the multiple schedule for one monkey, responding was only transiently affected. In the one monkey studied, responding was maintained under a fixed-ratio schedule alone (with timeout periods), but rates of responding were lower than under the fixed-ratio component of the multiple schedule. Characteristic patterns of responding, similar to those engendered under schedules of food presentation or shock termination, can be maintained under fixed-ratio schedules of shock presentation; further, patterns of responding can be controlled by discriminative stimuli in multiple schedules.     

Effects of lick-contingent timeout on schedule-induced polydipsia

Rats bar pressing on a 1-min fixed-interval schedule for 45-mg food pellets became polydipsic when water was concurrently available. They were then exposed to conditions in which each lick on the drinking tube produced a timeout period during which the food-schedule lever was retracted and the fixed-interval timer either did or did not continue to operate. Licks occurring within a timeout period extended its duration. As the duration of the lick-initiated timeout period was increased logarithmically through four values from 10 sec to 80 sec, lick rates as well as water intake rates generally decreased for all three subjects. As timeout duration was progressively increased, the rate of licks occurring in the absense of, but producing, timeouts decreased for all three rats, whereas the rate of licks occurring in the presence of timeout periods remained essentially constant. Water-intake rates and, with one exception, lick rates were suppressed more by timeout periods during which the fixed-interval timer did not continue to operate. These results indicate that lick-contingent timeout from positive reinforcement reduces schedule-induced drinking, and this suppressive effect is greater when the timeout period necessarily increases the interreinforcement interval beyond its minimum duration than when it does not.     

Choice of timeout during response-independent food schedules

Rats' lever pressing terminated visual or auditory stimuli associated with fixed-time or variable-time schedules of food delivery and produced a timeout period during which food delivery could not occur. Lever pressing during a timeout period reinstated the food-associated stimuli and again permitted food delivery according to the fixed-time or variable-time schedules. The mean interfood interval ranged from 1 minute to 16 minutes (variable-time schedules) or 32 minutes (fixed-time schedules); the timer controlling schedule intervals did not stop during timeout periods. The percentage of session time spent in timeout increased when the mean interfood intervals were lengthened and decreased when the mean interfood intervals were shortened. Timeouts were initiated most frequently about half way between successive food deliveries (fixed-time schedules) or after 15 seconds or more had lapsed since the last food delivery (variable-time schedules). Elimination of food delivery increased the percentage of session time spent in timeout, and elimination of the timeout contingency decreased lever press rates. When timeout was produced only when the lever was held in the depressed position, little time was spent in timeout. The main determinants of timeout initiation and termination appeared to be the rate of food delivery, freedom of movement during timeout, and the stimulus change associated with initiation and termination of timeout.     

Responding under sequence schedules of electric shock presentation

Lever pressing by squirrel monkeys was examined under second-order schedules of electric shock presentation in which different discriminative stimuli were associated with consecutive components (sequence schedules). Components were always two-minute fixed-interval schedules, and three different overall schedules were studied. Under an overall eight-minute fixed-interval schedule, the first component completion after at least eight minutes had elapsed produced electric shock. The number of components actually completed ranged from one to four; thus, different discriminative stimuli were occasionally associated with electric shock presentation. Under an overall “yoked” variable-ratio schedule, electric shock was presented after completion of a variable number of components; the required number and the distribution of components were matched to those obtained under the overall eight-minute fixed-interval schedule. Under an overall fixed-ratio schedule, electric shock was presented after completion of four components (chained schedule). Under all three sequence schedules, responding in early components was characterized by a pause followed by a single response after the end of the two-minute interval; responding in later components was characterized by a shorter pause followed by positively accelerated responding. Manipulation of overall schedules of shock presentation in these complex behavioral situations produced changes in responding comparable to those ordinarily obtained after similar manipulation of dependencies under both single and second-order schedules of food presentation. These experiments extend the range of conditions and levels of complexity under which responding can be maintained by presentation of electric shock.     

Attack produced by intermittent reinforcement of a concurrent operant response

Squirrel monkeys pressed a lever to produce food on several fixed-ratio schedules; they could also bite a rubber hose. Biting attack occurred during the postreinforcement pause and in early portions of the ratio response run. Also, biting attacks increased after transitions to higher values of the fixed-ratio requirement and in extinction. The results show that extinction-induced aggression effects occur in primates.     

An analysis of rats' drinking-tube contacts under tandem and fixed-interval schedules of food presentation

Rats' lever presses and drinking-tube contacts were studied under fixed-interval schedules of food presentation and under a tandem schedule composed of three fixed intervals. One group of rats was exposed first to the tandem schedule, next to fixed-interval schedules of comparable interpellet intervals, and once again to the tandem schedule; a second group of rats was exposed first to a fixed-interval and then to the tandem schedule. Under the tandem schedule, lever presses occurred at a higher rate and were more uniformly distributed in time than under the fixed-interval schedule. Tube contacts emitted by rats exposed first to a fixed-interval schedule consisted mostly of tongue contacts, which occurred at a high rate shortly after food; tube contacts emitted by rats exposed first to the tandem schedule consisted mostly of paw contacts, which occurred at a lower rate at times other than shortly after food. Changing the schedule from fixed interval to tandem decreased the frequency of tongue contacts for all rats. Under schedules of food presentation with comparable interpellet intervals, the schedule of food presentation, rather than the rate of food delivery per se, determined the topography and temporal locus of drinking-tube contacts.     

Suppression of operant behavior and schedule-induced licking in rats

The first experiment studied the effects of punishment on rats' lever pressing maintained by a fixed-interval schedule of food reinforcement and on the associated schedule-induced licking. When licking was followed by shock, licking was suppressed but lever pressing was largely unaffected. When lever pressing was followed by shock, lever pressing was suppressed but licking was unaffected. In both cases, the punished behavior recovered its previous unpunished level when the shocks were discontinued. In a second experiment, the rats' lever pressing was maintained by a variable-interval schedule of food reinforcement under which polydipsic licking also developed. Both lever pressing and licking were partially suppressed during a stimulus correlated with occasional unavoidable electric shocks. With a higher shock intensity, both behaviors were suppressed further. Both lever pressing and licking recovered their previous levels when shocks were discontinued. These results show that schedule-induced licking, which has been described as adjunctive behavior, can be suppressed by procedures that suppress reinforced lever pressing, an operant behavior.     

Variable-ratio schedules of timeout from avoidance: effects of d-amphetamine and morphine.

Rats were trained on concurrent schedules in which pressing one lever postponed shock and pressing the other lever produced periods of signaled timeout from avoidance on variable-ratio schedules. These procedures generated high rates of timeout-reinforced responding and provided a baseline for studying the effects of drugs on behavior maintained by different types of negative reinforcement (shock postponement vs. timeout). Morphine (2.5 to 10.0 mg/kg) reduced behavior maintained by timeout at doses that increased or had no effect on avoidance responding. In contrast, d-amphetamine (0.125 to 2.0 mg/kg) produced large increases in timeout responding at doses that had minimal effect on avoidance in rats trained on variable-interval and variable-ratio schedules. Thus, the event-dependent effects of morphine, observed in previous studies in which timeout responding was maintained at low rates by interval schedules, were replicated with high timeout rates maintained by variable-ratio schedules. The effects of d-amphetamine could also be described as "event dependent" because timeout responding was stimulated more than avoidance regardless of the maintenance schedule or baseline rate.     

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.     

Interactions in multiple schedules: negative induction with squirrel monkeys

In Experiment I, lever pressing by squirrel monkeys was maintained under a sequence of variable-interval, multiple variable-interval variable-interval, and multiple variable-interval extinction schedules of food presentation. Negative induction (decreased responding in the unchanged component) occurred when one component of the multiple variable-interval variable-interval schedule was changed to extinction. Negative induction was transient over sessions; responding in the unchanged component usually recovered to a rate similar to that under the multiple variable-interval variable-interval schedule. Negative induction was not accompanied by consistent changes in the patterns of local responding within the unchanged component, and did not depend on whether component schedules were associated with localized (lever lights) or diffuse visual stimuli (houselights), or on whether the unchanged component was a 60- or 180-sec variable-interval schedule. In Experiment II, responding was maintained under a sequence of variable-interval and multiple variable-interval timeout schedules of food presentation. Negative induction occurred when responding declined gradually in the timeout component but not when responding declined abruptly. The nature of interactions in multiple schedules may depend on the species; negative induction was observed with squirrel monkeys under conditions similar to those that produce positive contrast with pigeons.     

Second-order schedules: discrimination of components

Pigeons were exposed to a series of second-order schedules in which the completion of a fixed number of fixed-interval components produced food. In Experiment 1, brief (2 sec) stimulus presentations occurred as each fixed-interval component was completed. During the brief-stimulus presentation terminating the last fixed-interval component, a response was required on a second key, the brief-stimulus key, to produce food. Responses on the brief-stimulus key before the last brief-stimulus presentation had no scheduled consequences, but served as a measure of the extent to which the final component was discriminated from preceding components. Whether there were one, two, four, or eight fixed-interval components, responses on the brief-stimulus key occurred during virtually every brief-stimulus presentation. In Experiment 2, an attempt was made to punish unnecessary responses on the brief-stimulus key, i.e., responses on the brief-stimulus key that occurred before the last component. None of the pigeons learned to withhold these responses, even though they produced a 15-sec timeout and loss of primary reinforcement. In Experiment 3, different key colors were associated with each component of a second-order schedule (a chain schedule). In contrast to Experiment 1, brief-stimulus key responses were confined to the last component. It was concluded that pigeons do not discriminate well between components of second-order schedules unless a unique exteroceptive cue is provided for each component. The relative discriminability of the components may account for the observed differences in initial-component response rates between comparable brief-stimulus, tandem, and chain schedules.