Short-Term Memory Effects on Visual Global/Local Processing in Tufted Capuchin Monkeys (Sapajus spp.)

Nonhuman primates, differently from humans, are less proficient at processing global properties of visual compound stimuli. It has been suggested that humans preferentially process stimuli globally because this enables a more economical encoding of the stimuli. In this study we assessed the role of short-term memory (STM) in global/local processing by presenting tufted capuchin monkeys with Navon-type hierarchical figures in both simultaneous and delayed matching-to-sample tasks. Capuchins' ability to discriminate hierarchical stimuli was evaluated as a function of increasing delay intervals (0.0, 0.5, 1.0, 2.0 and 3.0 sec) between the disappearance of the sample and the presentation of the comparison stimuli. The results showed that recognition accuracy for local features was above chance level with delays of up to 3.0 sec, as previously reported when capuchins were faced with nonhierarchical stimuli. By contrast, the recognition of global configurations was above chance level in simultaneous, 0.0 and 0.5 sec delay conditions but not at delay intervals of 1.0 sec or longer. These findings indicate that capuchins' propensity to process the local properties of visual stimuli can be observed when a delay is interposed between the presentation of sample and comparison stimuli and was not reversed by increasing the delay. Moreover, our results show that capuchins' local propensity was not reversed by increasing stimulus size. Overall, our study confirms crucial differences between human and nonhuman primates and adds new insights into the comparative research on visual grouping functions of these species.