In 1972, Robert A. Rescorla and his colleague Allan R. Wagner at Yale University, published the Rescorla–Wagner model of associative learning. This model conceptualizes learning as the development of associations between conditioned (CS) and unconditioned (US) stimuli, with learning occurring when these stimuli are paired on discrete trials. The change in the association between a CS and an US that occurs when the two are paired depends on how strongly the US is predicted on that trial – that is, informally, how "surprised" the subject is by the US.