Categorization of natural stimuli by monkeys (Macaca mulatta): effects of stimulus set size and modification of exemplars.

The concept humans was studied in two experiments on rhesus monkeys, in each of which a two-choice simultaneous discrimination procedure was used. In Experiment 1, the choice was between scenes with humans and scenes without humans, with the slide set sizable enough that a large number of trials could be given without repeating any individual slide. Speed of categorization learning was faster and final level of performance was higher than in prior research in this laboratory involving a much smaller slide set. Experiment 2 was an attempt to obtain some information about the basis for the categorization by means of a series of probe trials. Probe trials involved slides of humans that were modified in one of several ways and slides in which monkeys or apes were present instead of humans. When paired with slides with humans, probe slides were seldom chosen, except when they showed a human rightside up in an upside down scene. In the latter case, choices were at the chance level. When paired with a slide with no humans in the scene, probe slides were usually chosen, except when they showed monkeys or apes or silhouettes of humans, in which case choices were again at the chance level. Possible reasons for the differences in results of category learning tests with pigeons and monkeys are discussed as are the implications of the probe tests for a concept interpretation of these results.