Can monkeys and apes use sequence order to resolve perceptual ambiguity? A lack of evidence in two rhesus macaques and one language trained bonobo

Abstract

In human language, word order impacts how words are categorized and understood (as nouns, verbs etc.). We have demonstrated that this effect extends to non-linguistic stimuli, suggesting that the impact of sequence order on stimulus categorization is a domain-general, rather than language-specific cognitive process. Here, we ask if this process is evolutionarily conserved in other non-human primates. We developed a non-linguistic experiment and tested rhesus macaques (Macaca mulatta) and bonobos (Pan paniscus). Participants first learned to categorize shapes (as rounded, squared or pointed). We next assessed how they categorized ambiguous morph shapes by combining exemplars from each category. We provided sequential information by training the animals to select shapes in a specified order: rounded, then squared and then pointed. Finally, we assessed whether presentation in specific positions in this learned sequence would impact subsequent categorization of ambiguous morph shapes. In humans, an ambiguous morph was more likely to be categorized as the shape it replaced in a sequence. Non-human primates, however, showed no such effect, categorizing stimuli independent of their position in a sequence. This suggests that the tendency to infer category membership from sequence order is not shared across primates, and may have evolved more recently in the human lineage.