Potency of Conditioned Reinforcers Based on Food and on Food and Punishment.

Pigeons peck more frequently on the left key of two keys when food is presented more frequently during the stimulus that reinforces pecks on that key than during the stimulus that reinforces pecks on the right key. This preference can be annulled and reversed by punishing each peck on the left key during the stimulus that reinforces pecks on the left key.