Extinction of responding maintained by timeout from avoidance.

The resistance to extinction of lever pressing maintained by timeout from avoidance was examined. Rats were trained under a concurrent schedule in which responses on one lever postponed shock on a free-operant avoidance (Sidman) schedule (response-shock interval = 30 s) and responses on another lever produced 2 min of signaled timeout from avoidance on a variable-ratio 15 schedule. Following extended training (106 to 363 2-hr sessions), two experiments were conducted. In Experiment 1 two different methods of extinction were compared. In one session, all shocks were omitted, and there was some weakening of avoidance but little change in timeout responding. In another session, responding on the timeout lever was ineffective, and under these conditions timeout responding showed rapid extinction. The within-session patterns produced by extinction manipulations were different than the effects of drugs such as morphine, which also reduces timeout responding. In Experiment 2 shock was omitted for many consecutive sessions. Response rates on the avoidance lever declined relatively rapidly, with noticeable reductions within 5 to 10 sessions. Extinction of the timeout lever response was much slower than extinction of avoidance in all 4 rats, and 2 rats continued responding at baseline levels for more than 20 extinction sessions. These results show that lever pressing maintained by negative reinforcement can be highly resistant to extinction. The persistence of responding on the timeout lever after avoidance extinction is not readily explained by current theories.