Conceptual thresholds for same and different in old-(Macaca mulatta) and new-world (Cebus apella) monkeys

Learning of the relational same/different (S/D) concept has been demonstrated to be largely dependent upon stimulus sets containing more than two items for pigeons and old-world monkeys. Stimulus arrays containing several images for use in same/different discrimination procures (e.g. 16 identical images vs. 16 nonidentical images) have been shown to facilitate and even be necessary for learning of relational concepts (Flemming et al., 2007; Wasserman et al., 2001; Young et al., 1997). In the present study, we investigate the threshold at which a new world primate, the capuchin (Cebus apella) may be able to make such a discrimination. Utilizing a method of increasing entropy, rather than conventional procedures of decreasing entropy, we demonstrate unique evidence that capuchin monkeys are readily capable of making 2-item relational S/D conditional discriminations. In another experiment, we examine the supposed level of difficulty in making S/D discriminations by rhesus monkeys (Macaca mulatta). Whereas pigeons (Columba livia) and baboons (Papio papio) have shown marked difficulty simultaneously discriminating same from different arrays at all when composed of fewer than 8 items each, rhesus monkeys seem to understand that pairs of stimuli connote sameness and difference just the same (Flemming et al., 2007). With sustained accurate performance of 2-item S/D discriminations, both experienced and task-naïve rhesus monkeys appear quite certain in their conceptual knowledge of same and different. We conclude that learning of the same/different relational concept may be less dependent upon high levels of entropy contrast than originally hypothesized for nonhuman primates.