Summation of conditioned suppression.

Four lab rats first learned to press a lever for food. Then a tone was paired with mild shock. Later, a light was also paired with shock. Each cue soon stopped the rats from pressing — a simple conditioned suppression setup.

On test day the researchers turned on both the tone and the light at the same time. They wanted to know if the two scary cues together would freeze the rats more than either cue alone.

When the tone and light came on together the rats almost stopped pressing entirely. Their suppression score was bigger than the score for the tone alone or the light alone.

In plain words, two warning cues added up. The compound stimulus produced stronger fear than either single stimulus — a clear summation effect.

Schwarz et al. (1970) ran the same kind of test but with pigeons working on a fixed-interval food schedule. The birds pecked faster when two cues were on together, showing the same additive rule in a reward setup. The two studies back each other up: summation works for both punishment and reward.

Davis et al. (1972) moved the idea to free-operant avoidance. Rats avoided shock more when a tone-plus-light compound signaled the period. This extends the 1970 finding from simple suppression to active avoidance.

Dove et al. (1974) looked like they contradicted the summation rule. They used chained schedules and saw suppression instead of addition when the compound sat in the early link. The key difference is schedule place. In the first link the compound acted as a chain cue, not a pure fear signal, so rates dropped. Same science, different context.

If you pair two warning stimuli — pictures, words, or sounds — the client may react more strongly than to either alone. Use this when you want a big, fast pause (e.g., stop before a transition). If you need to avoid over-suppression, present cues one at a time or separate them in a chain. The 1970 rat data give you a simple rule: combined cues add their punch.