Mechanisms of same/different abstract-concept learning by rhesus monkeys (Macaca mulatta).

Experiments with 9 rhesus monkeys (Macaca mulatta) showed, for the first time, that abstract-concept learning varied with the training stimulus set size. In a same/different task, monkeys required to touch a top picture before choosing a bottom picture (same) or white rectangle (different) learned rapidly. Monkeys not required to touch the top picture or presented with the top picture for a fixed time learned slowly or not at all. No abstract-concept learning occurred after 8-item training but progressively improved with larger set sizes and was complete following 128-item training. A control monkey with a constant 8-item set ruled out repeated training and testing. Contrary to the unique-species account, it is argued that different species have quantitative, not qualitative, differences in abstract-concept learning.