Effects of differential reinforcement expectancies on successive matching-to-sample performance in pigeons.

A series of experiments employed a symbolic variant of Konorski's delayed successive matching-to-sample task in order to determine whether differential reinforcement expectancies affect discriminative responding. One of two sample stimuli (S1 or S2) was followed, after a delay (0, 5, or 10 sec), by one of two test stimuli (T1 or T2). Pigeons' key pecking during test periods could produce food only on S1-T1 and S2-T2 (positive) trials; nonreinforcement invariably occurred on S1-T2 and S2-T1 (negative) trials. Differential reinforcement was scheduled by following the two positive trial sequences with different probabilities of reinforcement (.2 and 1.0); nondifferential reinforcement was scheduled by following the two positive trial sequences with a single, intermediate probability of reinforcement. (.6). Subjects given differential reinforcement acquired the conditional discriminaton more rapidly and reached higher terminal levels of performance than nondifferential controls (Experiment 1). Moreover, the magnitude of these differences increased as the delay between sample and test stimuli was lengthened. Reversing the probabilities of reinforcement in the differential problem produced a substantial and durable disruption of conditional discrimination performance (Experiment 2). The same general pattern of results was obtained when differential sample key pecking was eliminated (Experiment 3). These results can be parsimoniously interpreted by postulating the existence of learned reinforcement expectancies, and they detract from the merits of trace theory as a complete account of animal memory.