Individual Risk Preferences Originate from Bayesian Inference on Parietal Magnitude Representations

Brisbane Convention & Exhibition Centre 

Risk preferences – the willingness to accept greater uncertainty to achieve larger potential rewards – determine many aspects of our lives and are often interpreted as an individual trait that reflects a general ’taste’ for risk. From a neural perspective, this subjective attitude towards risk is often proposed to be determined by properties of neural subjective value calculations. However, this perspective cannot explain why risk preferences can change considerably across contexts and even across repetitions of the identical decisions. Here we provide modelling, fMRI, and TMS evidence that contextual shifts and moment-to-moment fluctuations in risk preferences can emerge mechanistically from Bayesian inference on noisy magnitude representations in parietal cortex (rather than in subjective-value-coding areas like ventro-medial PFC). Our participants underwent fMRI while choosing between safe and risky options that were either held in working memory or present on the screen. In a second experiment, we applied continuous theta-burst magnetic stimulation over individually-defined parietal magnitude areas or a vertex control site, before fMRI of the same behavioural paradigm. Risky options that were held in working memory were less likely to be chosen (risk aversion) when they had large payoffs but more likely to be chosen (risk-seeking) when they had small payoffs. These counterintuitive effects are mechanistically explained by a computational model of the Bayesian inference underlying the perception of the payoff magnitudes: Options kept in working memory are noisier and therefore more prone to central tendency biases, leading small (or large) payoffs to be overestimated (or underestimated) more. Congruent with the behavioural modelling, fMRI population-receptive field modelling showed that on trials where intraparietal payoff representations were noisier, choices were also less consistent and less risk-neutral, in line with participants resorting more to their prior belief about potential payoffs. Finally, the same model could also account for behavioural and neural effects of continuous theta-burst magnetic stimulation over the individually-defined parietal magnitude representations: This led to lower choice consistency and a larger tendency towards risk-seeking choices, particularly when safe options were presented before risky options. These behavioural changes were accompanied by reduced fidelity of neural magnitude representations, as reflected by decreased nPRF amplitudes, noisier neural signals, and reduced decoding accuracy for payoff magnitudes. Individual estimates of the increase in noise of our computational decision-making model correlated with the reduction in nPRF amplitude after parietal cTBS across subjects. Our results highlight that individual risk preferences and their puzzling changes across contexts and choice repetitions are mechanistically rooted in perceptual inference on noisy parietal magnitude representations, with profound implications for economic, psychological, and neuroscience theories of risky behaviour.