Examining the discriminative and strengthening effects of reinforcers in concurrent schedules

Abstract

Reinforcers may increase operant responding via a response-strengthening mechanism whereby the probability of the preceding response increases, or via some discriminative process whereby the response more likely to provide subsequent reinforcement becomes, itself, more likely. We tested these two accounts. Six pigeons responded for food reinforcers in a two-alternative switching-key concurrent schedule. Within a session, equal numbers of reinforcers were arranged for responses to each alternative. Those reinforcers strictly alternated between the two alternatives in half the conditions, and were randomly allocated to the alternatives in half the conditions. We also varied, across conditions, the alternative that became available immediately after a reinforcer. Preference after a single reinforcer always favored the immediately available alternative, regardless of the local probability of a reinforcer on that alternative (0 or 1 in the strictly alternating conditions, .5 in the random conditions). Choice then reflected the local reinforcer probabilities, suggesting some discriminative properties of reinforcement. At a more extended level, successive same-alternative reinforcers from an alternative systematically shifted preference towards that alternative, regardless of which alternative was available immediately after a reinforcer. There was no similar shift when successive reinforcers came from alternating sources. These more temporally extended results may suggest a strengthening function of reinforcement, or an enhanced ability to respond appropriately to "win-stay" contingencies over "win-shift" contingencies.

Keywords: alternation; choice; key peck; pigeon; reinforcer effect; switching-key.

Fig 1

Log (just-productive/not-just-productive) response ratio in successive 2-s time bins after a food for all individual pigeons in all conditions with random alternation of foods where p_{(P on)} was varied across conditions. A value of ± 3.5 indicates exclusive preference in that time bin. The thick line plots the group mean preference pulse and the horizontal line is at 0, indicating the indifference point.

Fig 2

Log (just-productive/not-just-productive) response ratio in successive 2-s time bins after a food for all individual pigeons in all conditions with strict alternation of foods where p_{(P on)} was varied across conditions. A value of ± 3.5 indicates exclusive preference in that time bin. The thick line plots the group mean preference pulse and the horizontal line is at zero, indicating the indifference point.

Fig 3

Lag reinforcer effects and log k (preference not due to any of the 8 preceding reinforcers) for all individual pigeons in all random-alternation conditions where p_{(P on)} was varied across conditions. The lag reinforcer effects (connected by a straight line) are relative to the left ordinate and log k (single point, not connected to others) is relative to the right ordinate.

Fig 4

Lag keylight-onset effects and log k (preference not due to any of the eight preceding keylight onsets) for all individual pigeons in Conditions 3 and 4 where p_{(P on)}  =  0.5. In Condition 3 foods strictly alternated across the side-keys and in Condition 4 they randomly alternated. The lag keylight-onset effects (connected by a straight line) are relative to the left ordinate and log k (single point, not connected to the others) is relative to the right ordinate.

Fig 5

Group mean log (L/R) response ratio in successive interreinforcer intervals in all random-alternation conditions where p_{(P on)} was varied across conditions.

Fig 6

Group mean log (L/R) response ratio as a function of the number of successive discontinuations in conditions with random alternation of foods where p_{(P on)} was varied across conditions.

Fig 7

Group mean log (L/R) response ratio after a series of reinforcers (from either alternative) followed by the same keylight onset or by a keylight onset which differed from the preceding keylight-onset sequence for all of the conditions where p_{(P on)}  =  0.5.