Graded differential reinforcement: Response-dependent reinforcer amount.

After key pecking had been autoshaped, six pigeons were exposed to a condition in which the duration of grain availability at the end of an 8-second trial depended on the number of responses emitted during the trial (0.25-second access to grain per response). This procedure, called correlated reinforcement, alternated across conditions with the automaintenance baseline in which the 8-second trial terminated with a constant 2.5-second access to grain. Two control procedures were run; in both, the reinforcer durations were yoked to those obtained in the last correlated session. In the yoked control no responses were required, but in the single-response yoked control at least one response was required to receive the yoked duration. The correlated condition maintained response rates above those produced by the two control conditions. These results may be accounted for by differential reinforcement.