Coding of stopping and economic values in OFC
The task followed standard stop signal task paradigm (Logan, 1994; Logan and Cowan, 1984). Subjects were placed in front of a computer monitor (1920x1080px) with black background. Following a brief (300 msec) central fixation on a white circle (radius 25px, Figure 1A), the fixation spot disappeared on the appearance of eccentric saccade target (90px white square, positioned at 288px in left or 1632px in right of screen, 50% chance). A go trial (67% of trials, randomly selected) was indicated by a go signal which is the peripheral target, whereas a stop trial (33% of trials, randomly selected) was indicated by an additional appearance of a stop signal—a central gray square delayed relative to the go signal presentation. Stop signal delays (SSD) in the task were set to stabilize at a delay causing 50% successful inhibition out of all stop trials recorded for the task in that day; SSDs were modulated through a staircase procedure with intervals of 16 msec. On go trials, subjects were rewarded for a saccade to the go signal and fixating on it for 50 msec; and on stop trials, subjects were rewarded for inhibiting their saccade to go signal and fixating at the stop signal for 400 msec. Water rewards were provided as feedback, and they were contingent on subject’s choice. Rewards were always 125 μl. The inter trial interval was 800 msec.
The neuroeconomic stopping task had a similar task framework to stop signal task, interleaved with the other in an interval picked randomly from range 1-3 trials. In go trials (random 67% of the total), a peripheral target called go offer (90px white square, positioned at 288px in left or 1632px in right of the screen, 50% chance) was presented, and it was randomly associated with low (15μl), medium (125μl), or high (250μl) reward offers, as indicated by yellow, blue and magenta colored squares, respectively. In stop trials (random 33% of the total), a center stop offer delayed with respect to the appearance of go offer was presented in addition. The stop-offer was also randomly associated with yellow, blue and magenta colors to indicate low, medium and high reward sizes. The go offer in stop trials was always in blue color to represent medium reward sized offer. This setup allowed the subject to make a choice through reward comparison in case of stop trials, and through a forced choice in case of go trials. All other parameters were the same as stop signal task.
