Rhesus macaques show metacognitive sensitivity to artificial grammars

Human language, and cognition more generally, is subserved by two interacting systems: an implicit system that acts rapidly and automatically and an explicit system that allows conscious access to knowledge.^1,2 While grammatical rules are initially acquired implicitly, explicit awareness develops over time supporting error monitoring and more flexible learning.^3,4,5,6 Critically, even preverbal infants demonstrate metacognition,^7,8 or sensitivity to their internal cognitive states. Therefore, this form of metacognition may represent a developmental, and possibly evolutionary, precursor to the more explicit representations seen in humans. Indeed, nonhuman animals have been shown to have metacognitive sensitivity in perceptual and working memory tasks.^9,10,11,12 Moreover, artificial grammar learning paradigms have demonstrated that nonhuman animals can learn simple analogs of the grammatical rules of language.^{13,14,15,16,17,18} Here, we combine an artificial grammar paradigm with a test of metacognitive sensitivity^7,12,19 in nonhuman primates. Six rhesus macaques were presented with three static visual stimulus sequences conforming to different artificial grammars and were required both to select the "correct" sequence and to maintain their response (touching the stimulus on the screen) for a variable waiting time (5-12 s) to receive a reward. Monkeys waited significantly longer following correct than incorrect responses, demonstrating metacognitive sensitivity during an artificial grammar task.