Processing negation may demonstrate compositionality in rhesus macaques

Compositionality, the ability to flexibly combine meaningful words into higher-level representations, is a critical feature of human language. However, the evolutionary origins of compositional processing are unclear, and evidence for this ability in nonhuman primates is largely lacking. Compositionality is inherent in processing negative clauses, such as 'not red'. Here, we conducted a series of three experiments using a novel, nonlinguistic 'negation' paradigm, which provide evidence of compositional processing in nonhuman primates. Macaques learned to associate an abstract visual stimulus with the process of negation (i.e., 'not'), and to combine this compositionally with novel images (e.g., a dog) on a trial-by-trial basis (i.e., to select a stimulus that was not a dog). Our results suggest compositional processing in monkeys, raising the possibility that the core of this language-critical ability may be present in monkeys, and thus may not be unique to humans or to language.