Conditional acceleration and external disinhibition of operant lever pressing by prereward, neutral, and reinforcing stimuli.

The team put rats on a DRL 20-second schedule. Each lever press had to wait 20 seconds after the last press to earn food.

A 20-second light came on right before each free food delivery. The rats could not earn this food by pressing.

They tracked how fast the rats pressed during the light versus during silence.

The rats pressed the lever more during the 20-second light. The schedule still punished rapid presses, but the light lifted that brake.

The authors called this "external disinhibition." The light did not reward pressing; it simply undid the schedule's suppression.

Hamilton et al. (1978) saw the same speed-up, but called it "conditioned reinforcement." The 1982 study says the light is not a reinforcer—it just unlocks behavior the schedule had locked down.

Rey et al. (2020) later showed the same lift in humans on a DRO schedule. Free rewards also undid schedule suppression, proving the rat finding holds across species and tasks.

Hursh et al. (1974) found the opposite: free food failed to keep pigeons pecking. The key difference is the task. Autoshaping asks birds to peck for food they already get free. The DRL task asks rats to press despite a delay rule. The light lifts the rule, not the need to respond.

When you use DRL, DRO, or any schedule that suppresses behavior, watch what happens right before unearned rewards. A simple signal can spike the very response you are trying to slow. Check your data for these blips and either remove the signal or add extra safeguards.