Pecks on an operant key were reinforced on either multiple variable-interval variable-interval or multiple variable-interval extinction schedules of reinforcement. The stimuli that signalled the multiple-schedule components were located on a second key (signal key), and a changeover delay prevented reinforcement of signal key-peck-operant key-peck sequences. No behavioral contrast was observed on the operant key, and appreciable responding to the signal key occurred during the variable-interval component of the multiple variable-interval extinction procedure. Peck durations on the signal key were markedly shorter than peck durations on the operant key. Moreover, most responses on the signal key occurred just after the multiple-schedule components changed. These data support an account of behavioral contrast in terms of the summation of pecks that are separately controlled by stimulusreinforcer and response-reinforcer dependencies, and suggest that the stimulus-reinforcer dependency is responsible primarily for local contrast. In addition, the data suggest that pecks that are controlled by these two dependencies may belong to topographically different classes.
Thirteen master pigeons were exposed to multiple schedules in which reinforcement frequency (Experiment I) or duration (Experiment II) was varied. In Phases 1 and 3 of Experiment I, the values of the first and second components' random-interval schedules were 33 and 99 seconds, respectively. In Phase 2, these values were 99 seconds for both components. In Experiment II, a random-interval 33-second schedule was associated with each component. During Phases 1 and 3, the first and second components had hopper durations of 7.5 and 2.5 seconds respectively. During Phase 2, both components' hopper durations were 2.5 seconds. In each experiment, positive contrast obtained for about half the master subjects. The rest showed a rate increase in both components (positive induction). Each master subject's key colors and reinforcers were synchronously presented on a response-independent basis to a yoked control. Richer component key-pecking occurred during each experiment's Phases 1 and 3 among half these subjects. However, none responded during the contrast condition (unchanged component of each experiment's Phase 2). From this it is inferred that autoshaping did not contribute to the contrast and induction findings among master birds. Little evidence of local contrast (highest rate at beginning of richer component) was found in any subject. These data show that (a) contrast can occur independently from autoshaping, (b) contrast assays during equal-valued components may produce induction, (c) local contrast in multiple schedules often does not occur, and (d) differential hopper durations can produce autoshaping and contrast.
Machine language subroutines can be integrated with the SKED system. These subroutines can shorten lengthy programs that could otherwise be handled by SKED, and can provide complex decision functions, data recording schemes, and software for new peripheral devices. Rules and examples for each function will be presented.
Responding was maintained in squirrel monkeys under variable-interval schedules of electric shock presentation when a period of timeout followed each response-dependent shock. Response rate decreased when timeout duration was decreased, and responding ceased wheni timeout was eliminated. These results in(licate that under certain conditions, a shock-free period following each response-produced shock is necessary to maintain responding.Recent studies have demonstrated that animals with specific types of behavioral hiistories will continue to respond when the only consequence of responding is the occasional presentation of a brief, intense electric shock (Byrd, 1969(Byrd, , 1972Kelleher and Morse, 1968;McKearney, 1968McKearney, , 1969McKearney, , 1970McKearney, , 1972Morse, Mead, and Kelleher, 1967;Stretch, Orloff, and Dalrymple, 1968; Stretclh, Orloff, and Gerber, 1970 ation of the traditional distinction between "positive" and "negative" reinforcers, i.e., between events such as food, whose onset normally acts as a reinforcer, and electric shock, whose offset normally acts as a reinforcer.In some studies that have demonstrated maintained responding on schedules of response-produced shock, both shock and a period of time relatively free from shock were contingent on responding: the typical, periodic (fixed-interval) schedules generated discontinuous response rates, with the animals pausing for about half the fixed-interval before responding resumed. This pause may be taken as an indicator of a "functional timeout". Timeout (TO) from schedules of shock presentation has been shown to be reinforcing (e.g., Sidman, 1962;Verhave, 1962). The maintenance of responding on response-dependent shock schedules could therefore be due to the response-contingent period of time free of shock, rather than to the shock that precedes it. In the present experiment, the effect of timeout duration on responding under a variableinterval schedule of shock presentation was studied. METHOD SubjectsFour experimentally naive adult squirrel monkeys (Samiri sciureus) were housed individually and treated according to the general specifications described by Kelleher, Gill, Riddle, and Cook (1963). Food and water were available at all times in the monkeys' home cages.
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