Responding of pigeons under variable-interval schedules of signaled-delayed reinforcement: effects of delay-signal duration.

Two experiments with pigeons examined the relation of the duration of a signal for delay ("delay signal") to rates of key pecking. The first employed a multiple schedule comprised of two components with equal variable-interval 60-s schedules of 27-s delayed food reinforcement. In one component, a short (0.5-s) delay signal, presented immediately following the key peck that began the delay, was increased in duration across phases; in the second component the delay signal initially was equal to the length of the programmed delay (27 s) and was decreased across phases. Response rates prior to delays were an increasing function of delay-signal duration. As the delay signal was decreased in duration, response rates were generally higher than those obtained under identical delay-signal durations as the signal was increased in duration. In Experiment 2 a single variable-interval 60-s schedule of 27-s delayed reinforcement was used. Delay-signal durations were again increased gradually across phases. As in Experiment 1, response rates increased as the delay-signal duration was increased. Following the phase during which the signal lasted the entire delay, shorter delay-signal-duration conditions were introduced abruptly, rather than gradually as in Experiment 1, to determine whether the gradual shortening of the delay signal accounted for the differences observed in response rates under identical delay-signal conditions in Experiment 1. Response rates obtained during the second exposures to the conditions with shorter signals were higher than those observed under identical conditions as the signal duration was increased, as in Experiment 1. In both experiments, rates and patterns of responding during delays varied greatly across subjects and were not systematically related to delay-signal durations. The effects of the delay signal may be related to the signal's role as a discriminative stimulus for adventitiously reinforced intradelay behavior, or the delay signal may have served as a conditioned reinforcer by virtue of the temporal relation between it and presentation of food.     

Humans' sensitivity to variation in reinforcer amount: Effects of the method of reinforcer delivery

Two experiments examined human subjects' sensitivity to variation in reinforcer amount under different methods of reinforcer delivery. Subjects chose between schedules varying in terms of amount and/or delay of reinforcement, the reinforcer being points exchangeable for money. In Experiment 1, reinforcer amount was manipulated by varying the monetary value of the points across conditions while the number of seconds of access to a consummatory response remained constant. Choice was strongly sensitive to reinforcer amount and indicative of self-control, as in previous experiments. In Experiment 2, reinforcer amount was manipulated by automatically delivering different numbers of points during the amount period, and the consummatory response was eliminated. Sensitivity to variation in reinforcer amount was significantly lower than in Experiment 1. Furthermore, the subjects in Experiment 2 exhibited significantly less self-control than did the subjects in Experiment 1. Humans' sensitivity to variation in reinforcer amount appears to be affected by factors that enhance the discrimi-nability of the consequences of responding.     

Responding of pigeons under variable-interval schedules of unsignaled, briefly signaled, and completely signaled delays to reinforcement

In Experiment 1, three pigeons' key pecking was maintained under a variable-interval 60-s schedule of food reinforcement. A 1-s unsignaled nonresetting delay to reinforcement was then added. Rates decreased and stabilized at values below those observed under immediate-reinforcement conditions. A brief stimulus change (key lit red for 0.5 s) was then arranged to follow immediately the peck that began the delay. Response rates quickly returned to baseline levels. Subsequently, rates near baseline levels were maintained with briefly signaled delays of 3 and 9 s. When a 27-s briefly signaled delay was instituted, response rates decreased to low levels. In Experiment 2, four pigeons' responding was first maintained under a multiple variable-interval 60-s (green key) variable-interval 60-s (red key) schedule. Response rates in both components fell to low levels when a 3-s unsignaled delay was added. In the first component delays were then briefly signaled in the same manner as Experiment 1, and in the second component they were signaled with a change in key color that remained until food was delivered. Response rates increased to near baseline levels in both components, and remained near baseline when the delays in both components were lengthened to 9 s. When delays were lengthened to 27 s, response rates fell to low levels in the briefly signaled delay component for three of four pigeons while remaining at or near baseline in the completely signaled delay component. In Experiment 3, low response rates under a 9-s unsignaled delay to reinforcement (tandem variable-interval 60 s fixed-time 9 s) increased when the delay was briefly signaled. The role of the brief stimulus as conditioned reinforcement may be a function of its temporal relation to food, and thus may be related to the eliciting function of the stimulus.     

Shadow recognition in human infants

Seventy-five children of five different age groups (ranging from 13–18 to 37–42 months of age) were tested using various nonverbal indices for (a) their ability to respond to shadowed information about objects other than themselves, (b) their ability to manipulate their own shadow, and (c) their ability to recognize their own shadow as evidenced by the explicit use of shadowed information about themselves to respond to themselves. The results show that although infants are aware of shadows and show signs of having mastered the concept of shadow permanence at a fairly early age, compelling evidence of shadow self-recognition is developmentally delayed relative to what has been reported for the onset of mirror self-recognition, with most infants failing to show positive evidence prior to 3 years of age. The results are discussed in light of psychological differences that may exist between shadows and mirrors. Prior involvement and/or tutoring by parents about shadows had no effect on the emergence of shadow recognition.     

Response-reinforcer relations and the maintenance of behavior

The effects on pigeons' key pecking of unsignaled delays of reinforcement and response-independent reinforcement were compared after either variable-interval or differential-reinforcement-of-low-rate baseline schedules. One 30-min session arranging delayed reinforcement and one 30-min session arranging response-independent reinforcement were conducted daily, 6 hr apart. A within-subject yoked-control procedure equated reinforcer frequency and distribution across the two sessions. Response rates usually were reduced more by response-independent than by delayed but response-contingent delivery of reinforcers. Under both schedules, response rates were lower when obtained delays were greater. These results bear upon methodological and conceptual issues regarding comparisons of contingencies that change the temporal response–reinforcer relations.     

The effects of delay on neonatal retention of habituated head-turning

Recovery of neonatal head orientation following auditory habituation was used to examine the effects of intertrial delay. One of two words served as stimuli with criterion orienting and habituation established before the delay. Fifty-six full-term neonates were assigned randomly to one of four delay groups: 10 (control), 55, 100, or 145 s. The percentage of head-turns toward the sound source (with one turn per trial) for blocks of three trials served as the principal dependent variable. Recovery of localized head-turning increased as length of delay increased, with the 100- and 145-s groups showing greater recovery than the 10- and 55-s groups. Infants presented with control trials systematically turned away from the sound source following habituation. Selective sensory adaptation, spontaneous recovery, and short-term memory interpretations of these data are considered.     

Effects on preference of reinforcement delay, number of reinforcers, and terminal-link duration

Three experiments used concurrent-chains procedures to examine the effects of reinforcement delay, number of reinforcers, and terminal-link duration on preference. In Condition 30 of Experiment 1, food was delivered after 30 seconds in each 150-second terminal link, with four additional food deliveries occurring at 30-second intervals in one of the links. In Condition 5, food was delivered after 5 seconds in each 25-second terminal link, and the four additional reinforcers were delivered at 5-second intervals. Preferences for the multiple-food chain were greater in Condition 30. In Experiment 2, the terminal link(s) providing only one reinforcer terminated immediately after delivery of the reinforcer. Preferences for the multiple-food chain were smaller than in Experiment 1. In Condition 5 of Experiment 3, food was delivered after 5, 75, 100, 125, and 150 seconds in one 150-second link and after 5 seconds in the other. Condition 50 differed only in that the first (or only) reinforcer in each link was delivered after 50 seconds instead of after 5 seconds. Preferences for the multiple-food chain were greater in Condition 50. Results of Experiments 1 and 2 do not correspond to results obtained by Moore (1979).     

Responding during reinforcement delay in a self-control paradigm.

Eight pigeons chose between a small, immediate reinforcer and a large, increasingly delayed reinforcer. Responding during the large-reinforcer delays was examined. During large-reinforcer delays, pecks on one key produced the small, immediate reinforcer; pecks on the other key had no effect. Thus, a pigeon could reverse its initial choice of the large, delayed reinforcer, or it could maintain its original choice. Pigeons that made a relatively high number of initial large-reinforcer choices tended to maintain these choices, and those pigeons that actually received a relatively high number of large reinforcers, tended to respond more frequently on the ineffective key during the delay periods. The findings suggest that some previous studies of self-control training in pigeons may have resulted in increased self-control partially due to a lack of opportunity for the pigeons to change their choices.     

A temporal parameter influencing choice between signalled and unsignalled shock schedules

The present study investigated whether choice of a signalled variable-time shock schedule over an unsignalled one was influenced by the average intershock interval. Eight rats were given a choice between signalled and unsignalled shock schedules in a series of conditions with average intershock intervals of 510, 270, 150, 90, 60, and 45 sec. Each test condition was preceded by a training-baseline condition, and schedule values were arranged in an ascending (four subjects) or descending (four subjects) order. Choice of the signalled conditions was directly related to the average intershock interval of the variable-time schedule for six of the eight subjects. The per cent of time in the signalled condition was highest when the average intershock interval was 150 sec or longer and lowest when the average intershock interval was 45 sec. The findings were interpreted as being due to changes in the safety features of the signalled schedule, rather than to changes in the average intershock interval per se.