In a matching-from-sample experiment with pigeons they were trained with matching stimuli which were identical (matching) to the correct stimulus, which were identical with the incorrect stimulus (not-matching) or which were entirely different than either (amatching). Nonmatching was superior to either matching or amatching. Animals tested under two drive levels demonstrated "that increased motivation facilitated the learning of the easier problem, but hindered the learning of the more difficult problems." (PsycINFO Database Record (c) 2006 APA, all rights reserved)