Symmetrically-reinforced delayed conditional discrimination (DCD): 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 vertical test stimulus is reinforced and not pecking (for 5 s) the horizontal test stimulus is reinforced.  If the initial stimulus is green, pecking the horizontal test stimulus is reinforced and not pecking the vertical test stimulus is reinforced.