Fig. 1: The task and animal behavior.

a, Schematic of the task. On each trial, rats initiated the trial by fixating in the center port. At the onset of fixation, a tone was played, indicating the magnitude of the lottery. The tone remained on until the response in the choice port (see Methods for details). b, An example sequence of trials. For trial type, white diamonds, yellow triangles and blue triangles represent choice trials, forced lottery trials and forced surebet trials, respectively. The sine waves in the ‘tone freq’ illustrate that the lottery sound varied from trial to trial. Animalsʼ responses are marked in diamonds, with yellow for lottery and blue for surebet. The reward received (μl) on each trial is shown in light blue circles, whose size represents the relative amount. The red cross indicates a ‘lottery-lose’ trial. c, Example subject performance from rat 2154 (from the muscimol experiments). The probability of choosing lottery is plotted as a function of the EV of lottery minus the EV of surebet in μl of water. The circles with error bars are the mean and 95% binomial CIs. The lines are psychometric curves generated by a logistic fit; the thin gray lines are fits to each session; and the thick gray line is the fit to all the sessions combined (n = 1,135 trials, 16 sessions). d, Subject performance from control sessions for all experiments. For muscimol animals, control sessions were 1 d before an infusion event. For optogenetic animals, we include the control trials from sessions with laser stimulation. The lines are logistic fits from each animal, with the color of the line indicating the experiment that animal participated in (n =33,511 trials, 319 sessions, 8 rats for infusion, 8 rats for optogenetics and 6 rats for electrophysiology).