Additive and suppressive response summation with a chain schedule.

Rats responded in a two-segment (variable-interval variable-interval) chain schedule. In one experiment, three subjects had either clicker, light, or clicker plus light as terminal-segment stimuli. All three responded at the highest rate when clicker plus light were present, thus showing additive summation. For three other subjects, initial-segment stimuli were either clicker, light, or clicker plus light. Two subjects responded at the lowest rate when clicker plus light were present, thus showing suppressive summation. In a second experiment, three subjects had either clicker, light, or neither clicker nor light as terminal-segment stimuli. None of these subjects showed reliable additive summation. Three other subjects had clicker, light, or neither as the initial-segment stimulus, and all three showed suppressive summation. Additive and suppressive summation both can be demonstrated with chain schedules, but stimulus parameters may be major variables in producing the effect.