Figure 1. Time line representation of the contingency conditions.

A) During instrumental training (positive contingency), the lever becomes inactive for a variable interval (white rectangle) following each reward delivery. The first lever press after this interval triggers an immediate reward. No reward occurs in the absence of lever press (positive contingency). B) During omission training, rewards are delivered following a 20 s delay without lever press (black rectangle). A lever press during the delay resets the delay. Consecutive rewards are delivered at 20 s intervals in the absence of lever press activity (negative contingency). C) During yoked training, rewards are synchronized to the rewards of another rat trained in omission, regardless of the yoked rat's activity. Rewards may occur at any time with respect to lever presses (null contingency).