a two-compartment box used for avoidance-conditioning research with animals. An electric shock to the feet is scheduled to be delivered if the animal remains in the same compartment for more than a specified period of time (e.g., 30 seconds). The subject can avoid electric shocks by regularly moving from one compartment to the other (i.e., shuttling between them). There is sometimes a small barrier that the animal must jump over or push to get from one compartment to the other.