Monkeys with inferotemporal (IT) lesions learn visual discriminations abnormally slowly. The evidence for various explanations of this defect is reviewed. It is concluded that the defect does not result from the disorders of perception, attention, or memory for either visual stimuli or visual associations that have so far been postulated. Two other explanations remain viable: that IT animals have a reduced capacity either to (a) categorize visual stimuli or (b) form associations with them. Since the existing evidence on these 2 hypotheses is inadequate, ways of testing them are suggested.(123 ref)(PsycINFO Database Record (c) 2016 APA, all rights reserved)