Summation of response rates to discriminative stimuli associated with qualitatively different reinforcers.

Thomas et al. (1968) worked with lab rats pressing a lever.

Each rat first learned two simple cues.

A light meant sugar water was coming. A tone meant grain pellets were coming.

Next the team turned on both light and tone together. They wanted to see if the rat would press faster than with either cue alone.

The rats pressed at a middle speed, not faster.

The combined cue gave a rate that sat between the light rate and the tone rate.

More responding did not add together. The summation idea failed.

Lattal (1974) ran a near-copy design but added mild shock. In that setup the light-plus-tone mix did raise pressing above single levels. The extra aversive stimulus flipped the outcome, showing danger can make combined cues stronger.

Podlesnik et al. (2017) moved the idea into extinction. They paired the target cue with an extra safety cue during extinction. Responding dropped faster, proving combined cues can also help when you want less behavior.

Rose et al. (2000) used the same logic in functional analysis. They painted rooms different colors and swapped therapists. Clear separate cues helped kids show sharper patterns, turning the old lab trick into a clinical shortcut.

When you mix SDs you do not automatically get more behavior. Expect middle strength unless other factors like punishment or extinction are in play. In practice, keep your cues clean during assessment and only blend them on purpose. If you need a fast drop in problem behavior, pair the trigger with a strong alternative cue and keep the pair consistent.