Response-dependent shock in second-order fixed-ratio schedules of food presentation.

Three pigeons key pecked under second-order schedules in which the completion of two successive fixed-ratio 50 components constituted a reinforcement cycle. Tandem, chained, and brief-stimulus second-order schedules were studied when completion of the initial fixed-ratio 50 component delivered brief intense electric shock in every nth reinforcement cycle and n assumed values between one and nine. During sessions without shock, the brief-stimulus (unpaired with food) schedule generated higher rates of responding in the initial component than did the tandem schedule. Electric shock engendered increased time to the fifth response and a repeated pause-run pattern of not responding and responding, particuuarly in the initial component, even with shock scheduled in every ninth reinforcement cycle. The results were consistent with those reported for shock of a shorter duration scheduled in every reinforcement cycle. The overall rate of responding decreased as a function of increasing shock density and was lower in brief-stimulus than in tandem schedules.