Recursive sequence generation in monkeys, children, U.S. adults, and native Amazonians

Stephen Ferrigno et al.  

Science Advances  26 Jun 2020:
Vol. 6, no. 26, eaaz1002
DOI: 10.1126/sciadv.aaz1002 




The question of what computational capacities, if any, differ between humans and nonhuman animals has been at the core of foundational debates in cognitive psychology, anthropology, linguistics, and animal behavior. The capacity to form nested hierarchical representations is hypothesized to be essential to uniquely human thought, but its origins in evolution, development, and culture are controversial. We used a nonlinguistic sequence generation task to test whether subjects generalize sequential groupings of items to a center-embedded, recursive structure. Children (3 to 5 years old), U.S. adults, and adults from a Bolivian indigenous group spontaneously induced recursive structures from ambiguous training data. In contrast, monkeys did so only with additional exposure. We quantify these patterns using a Bayesian mixture model over logically possible strategies. Our results show that recursive hierarchical strategies are robust in human thought, both early in development and across cultures, but the capacity itself is not unique to humans.


Center-embedded sequence generation in adults, children, and monkeys

Subjects were first trained on a sequence generation task (Fig. 1A and movie S1). Participants were presented with four brackets in random locations and had to touch them in a specific order to receive positive feedback (Fig. 1B). Subjects were trained on two lists until they reached the training criterion of 70% correct (Fig. 1C). These training lists were consistent with a center-embedded structure but did not require subjects to learn the center-embedded nature of the lists. They could be encoded as two arbitrary lists of items (e.g., A -> B -> C -> D). Previous studies have shown that both children and monkeys can represent lists or arbitrary items that do not contain any internal dependencies or underlying structure (35). Once trained to criterion, a novel list, which was composed only of the center two elements from each of the training lists, was randomly mixed into training trials (Fig. 1D). Subjects received positive feedback regardless of the order produced on the transfer trials. These transfer trials were aimed at seeing whether the underlying center-embedded structure of the training lists was represented and then generalized to new combinations of items even when this was not required to represent or reproduce the training lists. Subjects from all groups reached criterion on the training lists and remained above chance on the training lists throughout testing (chance = 8%; mean: U.S. adults = 97%, Tsimane’ adults = 91%, children = 60%, monkeys = 68%).

  • Copyright © 2020 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. No claim to original U.S. Government Works. Distributed under a Creative Commons Attribution NonCommercial License 4.0 (CC BY-NC).

This is an open-access article distributed under the terms of the Creative Commons Attribution-NonCommercial license, which permits use, distribution, and reproduction in any medium, so long as the resultant use is not for commercial advantage and provided the original work is properly cited.

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