Symmetrically-reinforced delayed simple discrimination (DSD): In this procedure, all trials terminate in reinforcement, although the behavior required to obtain reinforcement changes across trials.  In this animation, if the initial stimulus is red, pecking the test stimulus is reinforced, regardless of whether it is a vertical or horizontal line.  If the initial stimulus is green, not pecking the test stimulus for 5 s is reinforced, regardless of whether it is a vertical or horizontal line.