Motor response vigour and visual fixation patterns reflect subjective valuation during intertemporal choice

Value-based decision-making is of central interest in cognitive neuroscience and psychology, as well as in the context of neuropsychiatric disorders characterised by decision-making impairments. Studies examining (neuro-)computational mechanisms underlying choice behaviour typically focus on participants' decisions. However, there is increasing evidence that option valuation might also be reflected in motor response vigour and eye movements, implicit measures of subjective utility. To examine motor response vigour and visual fixation correlates of option valuation in intertemporal choice, we set up a task where the participants selected an option by pressing a grip force transducer, simultaneously tracking fixation shifts between options. As outlined in our preregistration (https://osf.io/k6jct), we used hierarchical Bayesian parameter estimation to model the choices assuming hyperbolic discounting, compared variants of the softmax and drift diffusion model, and assessed the relationship between response vigour and the estimated model parameters. The behavioural data were best explained by a drift diffusion model specifying a non-linear scaling of the drift rate by the subjective value differences. Replicating previous findings, we found a magnitude effect for temporal discounting, such that higher rewards were discounted less. This magnitude effect was further reflected in motor response vigour, such that stronger forces were exerted in the high vs. the low magnitude condition. Bayesian hierarchical linear regression further revealed higher grip forces, faster response times and a lower number of fixation shifts for trials with higher subjective value differences. An exploratory analysis revealed that subjective value sums across options showed an even more pronounced association with trial-wise grip force amplitudes. Our data suggest that subjective utility or implicit valuation is reflected in motor response vigour and visual fixation patterns during intertemporal choice. Taking into account response vigour might thus provide deeper insight into decision-making, reward valuation and maladaptive changes in these processes, e.g. in the context of neuropsychiatric disorders. Author summary Value-based decision-making is a process of particular interest in cognitive neuroscience. Impairments in decision-making are hallmarks of several neuropsychiatric disorders, and specifically temporal discounting, i.e. the devaluation of future rewards, is discussed as a potential marker for psychiatric disorders. Subjective utility during value-based decision-making is commonly inferred on the basis of participants' choices only. Here were investigated if motor response vigour and visual fixation patterns may serve as additional and implicit measures of subjective utility during intertemporal choice. We demonstrate that motor response vigour and visual fixation patterns are related to the reward magnitudes and subjective value differences between options. Further, we show that the choice and response time data are well accounted for by a drift diffusion model including a non-linear scaling of the drift rate by the subjective option value differences. Our results suggest that measures of motor response vigour and visual fixation patterns may provide further insight on valuation during decision-making when combined with choice and response time data