The comparator hypothesis arose primarily in response to so-called “cue competition” effects.
First proposed by Ralph Miller,[1] the comparator hypothesis is a model of Pavlovian associations which posits that cue competition effects arise at the time of test, that is during performance, not during learning.
The model assumes, essentially, that during conditioning the subject acquires both CS-US and context-US associations.
The model was initially proposed to account for unexplained variations in cue competition effects such as recovery from blocking, but has been expanded to apply more broadly to learning phenomena.
They concluded that "...all versions of comparator theory make a number of surprising predictions, some of which appear hard to reconcile with empirical data."