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Flexible combination of reward information across primates


A fundamental but rarely contested assumption in economics and neuroeconomics is that decision-makers compute subjective values of risky options by multiplying functions of reward probability and magnitude. By contrast, an additive strategy for valuation allows flexible combination of reward information required in uncertain or changing environments. We hypothesized that the level of uncertainty in the reward environment should determine the strategy used for valuation and choice. To test this hypothesis, we examined choice between risky options in humans and rhesus macaques across three tasks with different levels of uncertainty. We found that whereas humans and monkeys adopted a multiplicative strategy under risk when probabilities are known, both species spontaneously adopted an additive strategy under uncertainty when probabilities must be learned. Additionally, the level of volatility influenced relative weighting of certain and uncertain reward information, and this was reflected in the encoding of reward magnitude by neurons in the dorsolateral prefrontal cortex.

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Fig. 1: Experimental paradigms.
Fig. 2: Different strategies for combination of reward information under risk and uncertainty.
Fig. 3: Additive models explain choice under uncertainty.
Fig. 4: Adjustment of choice behaviour to volatility of the environment.
Fig. 5: Behavioural adjustments in response to changes in volatility of the environment in the ML task.
Fig. 6: Neural signature of behavioural adjustments to volatility in the dlPFC.

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Data availability

The data that support the findings of this study are available from the corresponding author upon request.

Code availability

Custom computer codes that support the findings of this study are available from the corresponding author upon request.


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We thank E. Chu, S. Nichols-Worley and L. Tran for collecting human data, and C. Strait and M. Mancarella for collecting monkey data in the gambling task. This work is supported by the National Science Foundation (CAREER Award no. BCS1253576 to B.Y.H. and EPSCoR Award no. 1632738 to A.S.), and the National Institutes of Health (grant no. R01 DA038615 to B.Y.H., grant nos. R01 DA029330 and R01 MH108629 to D.L., and grant no. R01 DA047870 to A.S.). The funders had no role in study design, data collection and analysis, decision to publish or preparation of the manuscript.

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Authors and Affiliations



A.S. conceived the project. C.H.D., B.Y.H. and D.L. designed the experiments in monkeys. S.F. and A.S. designed the human experiments. S.F. and A.S. performed model simulations and analysed the data. C.H.D. and S.F. conducted the experiments. C.H.D., S.F., D.L., B.Y.H. and A.S. analysed and interpreted the experimental data. D.L., B.Y.H. and A.S wrote the manuscript and all authors contributed to revising the manuscript.

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Correspondence to Alireza Soltani.

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The authors declare no competing interests.

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Supplementary Notes 1 and 2 and Figs. 1–7.

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Farashahi, S., Donahue, C.H., Hayden, B.Y. et al. Flexible combination of reward information across primates. Nat Hum Behav 3, 1215–1224 (2019).

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