MECHANISMS UNDERLYING THE EFFECTS OF UNSIGNALED DELAYED REINFORCEMENTON KEY PECKING OF PIGEONS UNDER VARIABLE-INTERVAL SCHEDULES

Three experiments were conducted to test an interpretation of the resp onse-rate-reducing effects of unsignaled nonresetting delays to reinfo rcement in pigeons. According to this interpretation, rates of key pec king decrease under these conditions because key pecks alternate with hopper-observing behavior. In Experiment 1, 4 pigeons pecked a food ke y that raised the hopper provided that pecks on a different variable-i nterval-schedule key met the requirements of a variable-interval 60-s schedule. The stimuli associated with the availability of the hopper ( i.e., houselight and keylight off, food key illuminated, feedback foll owing food-key pecks) were gradually removed across phases while the d ependent relation between hopper availability and variable-interval-sc hedule key pecks was maintained. Rates of pecking the variable-interva l-schedule key decreased to low levels and rates of food-key pecks inc reased when variable-interval-schedule key pecks did not produce hoppe r-correlated stimuli. In Experiment 2, pigeons initially pecked a sing le key under a variable-interval 60-s schedule. Then the dependent rel ation between hopper presentation and key pecks was eliminated by arra nging a variable-time 60-s schedule. When rates of pecking had decreas ed to low levels, conditions were changed so that pecks during the fin al 5 s of each interval changed the keylight color from green to amber . When pecking produced these hopper-correlated stimuli, pecking occur red at high rates, despite the absence of a peck-food dependency. When peck-produced changes in keylight color were uncorrelated with food, rates of pecking fell to low levels. In Experiment 3, details (obtaine d delays, interresponse-time distributions, eating times) of the trans ition from high to low response rates produced by the introduction of a 3-s unsignaled delay were tracked from session to session in 3 pigeo ns that had been initially trained to peck under a conventional variab le-interval 60-s schedule. Decreases in response rates soon after the transition to delayed reinforcement were accompanied by decreases in e ating times and alterations in interresponse-time distributions. As re sponse rates decreased and became stable, eating times increased and t heir variability decreased. These findings support an interpretation o f the effects of delayed reinforcement that emphasizes the importance of hopper-observing behavior.