# Two rhesus macaques underproduced counted actions more often than overproduced them in a numerical task

- Published: 2026-11-11 06:45 UTC
- Section: Life & Health › Neurotechnology
- Tags: numerical production, rhesus macaques, counting sequences, underproduction, accumulation-to-bound model, Weber–Fechner scaling, reaction times, sequential stopping
- URL: https://realtech.media/news/rt-18f842f76c99f734c4a1dc05_two_rhesus_macaques/

**Monkeys trained to produce a set number of actions showed a systematic bias toward stopping early. The authors explain it with an accumulation-to-bound model of termination.**

![Two rhesus macaques underproduced counted actions more often than overproduced them in a numerical task](https://realtech.media/media/2026/11/dacb02965e2b2840.jpg)
*Charles J. Sharp, Wikimedia Commons, CC BY-SA 4.0*

Researchers trained two rhesus macaques to produce an instructed number of actions in response to sign numerals or dot arrays, according to a report in Proceedings of the National Academy of Sciences [1]. Action sequences were controlled for temporal parameters. Both monkeys performed accurately with both stimulus formats [1].

Accuracy showed a systematic asymmetry: underproduction occurred more often than overproduction. The authors report that this runs contrary to predictions from perceptual numerosity judgments governed by Weber–Fechner scaling [1]. Reaction times mirrored the asymmetry, with longer latencies for underproduced sequences and shorter latencies for overproduced ones [1].

The authors describe a stochastic accumulation-to-bound model in which a decision variable evolves across counting steps and triggers termination when a threshold is reached. According to them, the model accounts for both the accuracy and the reaction-time patterns [1]. They interpret the results as evidence for a sequential stopping process that contributes to numerical production, linking counting behavior to general principles of decision formation [1].

## Sources

1. [Proceedings of the National Academy of Sciences](https://doi.org/10.1073/pnas.2620933123)

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Source: RealTech, realtech.media — https://realtech.media/news/rt-18f842f76c99f734c4a1dc05_two_rhesus_macaques/