WITHIN-SESSION PATTERNS OF RESPONDING WITH CHANGES IN THE VARIABILITYAND PROBABILITY OF FOOD DELIVERY

The present study was an attempt to determine the factors to which sub jects sensitize and/or habituate within experimental sessions. Rats pr essed a lever and pigeons pecked a key for food reinforcers delivered during a 60 min session. In experiment 1, subjects initially responded on a simple variable-interval 30 s schedule that consisted of 25 inte rvals. In subsequent conditions, the number of intervals in the series was decreased until subjects responded on a simple fixed-interval 30 s schedule. In experiment 2, subjects always responded on a variable-i nterval 15 s schedule that resulted in a stimulus change. The probabil ity that a food reinforcer would accompany the stimulus change varied across conditions. Results showed that within-session patterns of resp onding did not change for either species with changes in the temporal pattern of reinforcer delivery (experiment 1). The within-session resp onse patterns generally became flatter with decreases in the probabili ty of food delivery for both species (experiment 2). The present resul ts indicate that subjects are sensitizing and/or habituating to the re inforcer itself and/or to some aspect of its delivery. They also help to highlight the strength of sensitization-habituation as an explanati on for within-session changes in responding. (C) 1997 Elsevier Science B.V.