Simultaneous matching-to-sample: In this animation, trials begin with the presentation of a red or green sample stimulus on the center key.  After some period of time, typically 5 s, red and green choice stimuli are presented on the outer keys.  Selection of the matching choice stimulus produces reinforcement (typically, 3-s access to grain).  An intertrial interval, typically 10 s in duration, separates trials within a session. This version of matching is often referred to as identity matching because the correct choice stimulus is a physical match to the sample.