Food and intracranial stimulation responding suppressed with regular-interval shock.

Attenuation of conditioned suppression during intracranial stimulation was compared with that during food reinforcement. Response rates controlled by food and by brain stimulation were equalized on a multiple schedule by adjusting the stimulating current. When foot shock was delivered during timeout periods separating response components, responding for food was significantly more suppressed than responding for brain stimulation. When components were shortened from 10 to 2 minutes, responding maintained by either food or brain stimulation showed a similar temporal pattern of suppression preceding each shock, but responding in the component involving food remained significantly more suppressed. Explanations for the attenuated suppression during brain stimulation based on neural disruption, stimulus blocking, and analgesic properties were questioned. The increased responding during brain stimulation seemed to reflect greater response strength relative to food reinforced responding.     

Responding in the squirrel monkey under fixed-ratio schedules of stimulus termination

Key pressing was maintained under a fixed-ratio schedule in which electric shock was scheduled for delivery at a fixed time (t seconds) after each stimulus onset, and every n^th response terminated the stimulus and initiated a timeout from shock. Under this procedure, the higher the rate of responding, the briefer the duration of the stimulus presentation and the lower the frequency of shock delivery. The effects of several schedule parameters were studied to determine whether the maintenance of responding was dependent on an inverse relation between response rate and shock frequency. Shock rate and shock frequency were made independent of response rate by decreasing the value of t to 0.5 second and delivering shock only during the first presentation of the stimulus after a fixed time, including stimulus and timeout durations, had elapsed since the previous shock. The experiments showed that shock frequency and response rate are inversely related when t is of relatively long duration compared to the value of the fixed-ratio parameter, but that a decrease in shock rate or frequency due to a high rate of responding is not necessary for the maintenance of responding under a fixed-ratio schedule of stimulus termination.     

Schedule-induced biting under fixed-interval schedules of food or electric-shock presentation

Squirrel monkeys pressed a lever under fixed-interval schedules of food or of electric-shock presentation. Both schedules induced repeated biting on a latex hose. Whether lever pressing was controlled by food or by electric shock, a pattern of decreasing hose biting and increasing lever pressing occurred within fixed-interval cycles. As the fixed-interval duration was increased from 6 to 600 sec, average rates of lever pressing decreased under both schedules. Average rates of hose biting first increased with increasing parameter value, reaching a maximum at values that varied from 60 to 337 sec in different monkeys, and then declined at higher values. d-Amphetamine at appropriate doses increased overall rates of lever pressing maintained by food or by shock, but either did not affect or decreased overall rates of hose biting. When no timeout period occurred between fixed-interval cycles, the monkeys bit most frequently immediately after food or electric shock was presented. When there was a timeout period, hose biting began shortly after the start of the fixed-interval cycles, with little or no hose biting immediately after food or electric shock was presented. Most hose biting appeared to be schedule-induced rather than food- or shock-induced.     

The effects of two response-elimination procedures on reinforced and induced aggression.

Pecks against a stuffed pigeon were reinforced according to a fixed-interval schedule for one group of pigeons and a variable-interval schedule for a second group. Red and green stimulus lights were alternately illuminated. Subsequently, food deliveries no longer occurred during one color (extinction). In the presence of the other color, food was presented only when no attack occurred for 30 sec. When attack produced food, all pigeons generally exhibited characteristic fixed-interval or variable-interval response patterns. Two birds in each group frequently exhibited postreinforcement schedule-induced aggression. Attack was reduced to low levels at approximately the same rate by extinction and differential reinforcement of other behavior. For birds that had previously exhibited schedule-induced aggression the initial reduction of attack during the second experimental phase was followed by induced attack immediately after food delivery in the differential-reinforcement-of-other-behavior component and upon onset of the extinction component, Either extinction or differential reinforcement of other behavior may eliminate reinforced aggression but may be relatively ineffective for reducing induced attack.     

Differential effects of cocaine and pentobarbital on fixed-interval and random-interval performance

Reports have indicated that the behavioral effects of a drug can be related to the nondrug control rate of behavior in the absence of the drug. To investigate the purported relationship between control rate and drug rate, squirrel monkeys were trained under a fixed-interval 300-s schedule of stimulus-shock termination, a procedure that engendered a wide range of response rates. A light illuminated the experimental chamber during the fixed interval, and the first lever press after 300 s had elapsed terminated the light for 30 s and precluded an electrical stimulus to the tail. Following acute intramuscular administration of cocaine (0.03-0.56 mg/kg), overall rate increased and different control rates of responding, during different parts of the fixed interval, converged toward a common rate. Subsequently, the schedule was changed to a multiple fixed-interval 300-s random-interval 300-s schedule; performance during the random-interval component was characterized by steady responding at a uniformly high rate. Analysis of fixed-interval and random-interval performances following acute cocaine administration revealed convergence of response rates toward a common, uniform rate. Pentobarbital (0.3-10.0 mg/kg) only decreased overall rate, and different control rates of responding during the fixed interval did not converge toward a common rate. The results indicate that this type of analysis can be useful in comparing pharmacological agents from different classes and that the rate at which responding becomes uniform can provide a quantitative behavioral end point for characterizing drug effects on behavior.     

Relative durations of conditioned stimulus and intertrial interval in conditioned suppression.

The effects of the relative durations of the conditional stimulus and the intertrial interval on bar pressing during a conditioned-suppression procedure were examined as a function of two additional variables--type of operant baseline schedule and rate of shock presentation. In Experiment 1, response suppression was compared across components of a multiple fixed-ratio, random-ratio, fixed-interval, random-interval schedule, at relative conditioned-stimulus/intertrial-interval durations of 1/1, 1/4, and 1/9. In Experiment 2, relative conditioned-stimulus/intertrial-interval duration (1/5, 3/3, or 5/1) was manipulated across groups, while shock frequency (2, 6, or 10 shocks/hr) was manipulated within groups. In both experiments, suppression during the signal was virtually complete at all relative durations. Responding was also suppressed during the intertrial interval, but that suppression varied as a function of experimental manipulations. In Experiment 1, intertrial-interval response rates were higher when relative signal duration was 1/9 than when it was 1/1, although both relative signal duration and shock frequency, which covaried, could have contributed to the difference. In Experiment 2, the patterning of response rates between successive shocks was affected by relative duration, absolute rates during the intertrial interval varied as a function of shock frequency, and differences between suppression during the signal and suppression during the intertrial interval were affected by both relative duration and shock frequency. The data support an analysis based upon relationships between shock-correlated and intertrial-interval stimuli and, as assessed by the relative-delay-to-reinforcement metric, are comparable to results that have been reported from experiments using similar manipulations under the autoshaping paradigm.     

Brief exposure to contingent reinforcement produces a sustained increment in the strength of an elicited response.

Squirrel monkeys' lever pressing produced by response-independent shock was measured before, and for an extended period following, exposure to a shock-avoidance procedure. Following avoidance training, the frequency of responding increased by multiples of 10 to 50 across subjects and evidence little or no decrement for up to 6 months of postreinforcement testing. Manual responding produced by intense environmental stimulation may be substantially strengthened by a brief history of reinforcement, such that it is sustained over long periods without reinforcement.     

The effects of number of responses on the postreinforcement pause in fixed-interval schedules

The present study manipulated the number of responses in a modified fixed-interval schedule by imposing a blackout after each unreinforced response during the interval. The blackout duration was varied, and the duration of the fixed interval was held constant. The subjects were initially exposed to a fixed-interval 300-sec schedule. Blackout durations of 0, 10, and 50 sec were used. Following this, a fixed-interval 30-sec schedule was used with blackout durations of 0, 1, and 5 sec. Under the fixed-interval 300-sec schedule, the number of interreinforcement responses varied over a wider range than occurred under the fixed-interval 30-sec schedule. The duration of the postreinforcement pause decreased as blackout durations were increased and number of responses decreased on the fixed-interval 300-sec schedule, but pause length did not vary with changes in blackout duration and number of responses for the fixed-interval 30-sec schedule. The differences in the effects of blackout duration and response manipulation on the two fixed-interval schedules were attributed to relatively greater changes in the number of interreinforcement responses for the fixed-interval 300-sec schedule.     

Choice for signaled over unsignaled shock as a function of signal length.

Subjects chose between signaled and unsignaled shock conditions while signal length was varied between .5 and 2.0 seconds in steps of .5 seconds in both ascending and descending series. Preference for the signaled condition failed to develop initially for five of six subjects when signals were .5 or 1.0 seconds but became strong for all subjects when signals were 2.0 seconds (ascending series). Preference declined when signals were shortened, but for most subjects this decline was small (descending series). Since discriminable shock-free periods were present in the signaled condition at all signal lengths, these results suggest that safety may not be a sufficient condition for preference to develop for signaled shock.     

Chained schedules of avoidance: Reinforcement within and by avoidance situations

Four rats were exposed to chained schedules with variable-cycle avoidance in both links. Responding in the initial link cancelled shocks scheduled once per minute and, according to a conjoint fixed-ratio schedule, produced a terminal link where scheduled shock rates varied from 0 to 8 shocks per minute in different conditions of the experiment. Response rates in the terminal link increased as a function of the scheduled shock rate. Response rates in the initial link, on the other hand, decreased as a function of the shock rate actually received (rather than scheduled) in the terminal link. While consistent with other studies of aversive control, these results differ from those obtained in chained schedules of positive reinforcement in that increases in reinforcement within the terminal link of the chain did not systematically increase the reinforcing value of that link.