Figure 1.

The slot-machine task uses two-reels, with the same six icons displayed on each reel, and a horizontal ‘payline’ across the centre of the screen. On trials with a white screen background, the volunteer selects one ‘play icon’ on the left reel, using two buttons to scroll through the icons, and one button to select. On trials with a black screen background, the computer selects the play icon. Following icon selection, the right-hand reel spins for a variable duration (2.8–6 s), and decelerates to a standstill. During outcome (4 s), if the right reel stopped on the selected icon (i.e. matching icons displayed in the payline), the subject was awarded £0.50; all other outcomes won nothing. Following the outcome phase, there was an inter-trial interval of variable duration (2–7 s). In the fMRI version of the task, two ratings were taken on intermittent (1/3) trials: following selection, subjects were asked ‘How do you rate your chances of winning?’, and following outcome, subjects were asked ‘How much do you want to continue to play the game?’. Reprinted from Clark et al. (2009).