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

After 25 free-operant avoidance training sessions, a 1-min signal followed by a brief shock was presented on the average of once every 4 min. During the signal, the avoidance schedule was suspended (20 sessions). Response rates during the signal were markedly reduced. Shock rates during non-signalled periods increased. Fifteen additional sessions were given during which the signal was presented without shock. Response rates during signalled periods were greater than previously observed during signalled periods, indicating that signalled shock had suppressive control over a previously acquired avoidance response rate.