 To study this phenomenon, Lewis and colleagues (1969) taught rats a simple passive avoidance task, a fear-aggravated test used to evaluate learning and memory in rodent models.Some rats were allowed to familiarize themselves with the task while others were only allowed to experience the task once.After the rats had completed their task a pre-determined number of times, the experimenters administered an electroconvulsive shock to the rats meant to induce selective amnesia.