The effect of a pre-shock signal on a free-operant avoidance response.

The team placed rats on a free-operant avoidance schedule. Every 15 seconds without a lever press, a brief shock arrived.

After the rats learned to avoid, the researchers added a 1-minute tone. The tone always ended in shock. They watched what happened to lever pressing.

The tone cut lever pressing almost to zero. More shocks now reached the animals because they stopped avoiding.

When the tone no longer ended in shock, pressing slowly returned. The signal had acted like a brake on the whole avoidance system.

Sanders et al. (1971) ran the same setup one year later and saw the same brake effect, showing the finding is reliable.

Liberman et al. (1973) later let rats choose between signalled and unsignalled shock. The animals picked the signalled schedule even when it gave eight times more shocks. This seems opposite to the 1970 brake, but the difference is choice. When the rat can move away, the safety period after the signal is valuable. When the rat must stay on one schedule, the signal simply predicts pain and stops the response.

Glover et al. (1976) showed the same twist: long gaps between shocks make the signal helpful; short gaps make it useless. Together these papers show that a warning stimulus is not good or bad by itself—its value depends on what the animal can do and on the timing of the aversive events.

If you use warning stimuli with clients, check what the signal really predicts and what responses are available. A buzzer that always precedes a reprimand can shut down useful avoidance behavior and accidentally increase the very events you want to reduce. Test the contingency: remove the aversive outcome from the signal and see if appropriate responding returns. Safe periods must be real and reachable for the signal to help rather than harm.