(a) (left) Prior distribution from which the rewards for a pair of options are sampled. r1 and r2 indicate the reward magnitudes for individual options (for example, objects presented on left- and right-hand side of the screen in each trial). (right) A typical visual stimulus in each trial. (b) Reaction-time task. In each trial, the decision maker is presented with a pair of options. The decision maker reports her choice as soon as she has decided which of the two options she prefers. The reaction time (Ti) can vary among trials, and individual trials are separated by a fixed inter-trial interval.