Event Abstract Back to Event The temporal dynamics of human decision under risk Laurence Hunt1, 2*, Matthew Rushworth3 and Tim E. Behrens3 1 University of Oxford, FMRIB Centre, United Kingdom 2 University of Oxford, Experimental Psychology, United Kingdom 3 University of Oxford, United Kingdom The functional neuroanatomy of human value-based decision has been widely investigated using functional MRI (fMRI). A ventromedial portion of prefrontal cortex (VMPFC) has been isolated as especially important, although its precise contribution remains debated. One view holds that VMPFC codes stimulus values (1) that are then compared elsewhere in the brain (2). Another suggests VMPFC codes the value of the option that will be become chosen (3), perhaps to enable subsequent computation of a prediction error (4). In yet other studies, VMPFC signals the difference in value between chosen and unchosen options specifically at choice, not outcome, time (5,6) - arguing for a central role for VMPFC in the formation of a decision. This would suggest that an initial representation of stimulus value in VMPFC may evolve into chosen and unchosen value signals as an option is selected. However, the temporal evolution of this signal may occur too quickly to be resolved using fMRI. To test this hypothesis, we measured simultaneous magnetoencephalography and electroencephalography (M/EEG) in 19 subjects as they performed a simple economic decision task. Subjects weighed the probability of receiving monetary reward on two options with the magnitude of reward that could be attained. Subject choices matched predictions from descriptive economic models of behaviour, and reaction times correlated negatively with the difference in value between the two options. We used trial-by-trial regression to investigate the effects of several variables on M/EEG data at different timepoints through the trial. At the time decision stimuli were presented, and at the time of button-press, we looked for the effect of ’value difference’ (chosen-unchosen values) but also of overall ’stimulus value’ (chosen+unchosen values). We employed the multiple sparse priors (MSP) source reconstruction algorithm implemented in SPM8 to investigate the neuroanatomical basis of these signals. Aligned to the presentation of the decision, ’value difference’ signals localised to VMPFC, peaking approximately 800ms after stimulus onset. Crucially, however, this signal was immediately preceded by a ’stimulus value’ signal, also localised to VMPFC, representing the sum of all options available to the subject (500-800ms). Aligning instead to the time of subject response, ’value difference’ signals were found preceding the button-press, approximately 300ms before the action was executed. In contrast to the stimulus-aligned signals, these localised to SMA/cingulate motor area and primary motor cortex, but were not preceded by a ’stimulus value’ signal, and so might relate to motor preparation rather than the decision process per se. These results suggest that as a decision is made, the signal within VMPFC evolves from representing the stimulus value of all available options to representing the decision that has been made. The role of VMPFC may therefore go beyond the signalling of the chosen option for subsequent computation of a reward prediction error, and may be fundamental to the process of selecting an option during value-based decision.