Summation of responding maintained by fixed-interval schedules.

Schwarz et al. (1970) worked with pigeons on fixed-interval (FI) schedules. Each bird pecked for food on two separate FI schedules. Each schedule had its own colored light.

Next the team turned on both lights at once. They wanted to see if response rates would add up when the two stimuli were compounded.

When both lights came on together, the birds pecked almost exactly the sum of their single-stimulus rates. The compound cue did not average or suppress responding.

The result held across early, middle, and late segments of the FI. Additive summation was steady, not a fleeting artifact.

Dove et al. (1974) extended the same question to chained FI schedules. They saw additive summation when the compound sat in the terminal link, but saw suppression when it sat in the initial link. Position in the chain flips the effect.

Davis et al. (1972) moved from food to avoidance. They also found additive summation: tone-plus-light produced more avoidance responses and fewer shocks. The rule holds across reinforcer types.

Cherek et al. (1970) flipped the valence to conditioned suppression. Two fear cues together created stronger suppression than either alone. Summation works in both appetitive and aversive domains.

When you blend two SDs that already control FI behavior, expect the rates to stack, not cancel. Use this to build high-energy compound cues for skill drills or to maintain momentum during long waits. Just watch where in the chain or session you place the blend—early links can flip the effect to suppression.