Table 1 Procedure: Tests of behavioral impulsivity

CARROT [44]

Participants complete four trials of sorting a pack of cards, each card with five digits, into three corresponding trays. The first trial (T1) involves sorting 60 cards while being timed, with this time used as the time limit for subsequent trials. In trial two (T2), the participant sorts a pack of 100 cards until told to stop. The third trial (T3) involves sorting 100 cards again with the time restriction of the previous trial, but with a small monetary reward offered for every five cards correctly sorted. A 20 cent coin is placed in front of the participant as the fifth card is sorted into the correct trays. The fourth trial (T4) is identical to T2 and controls for fatigue or practice effects on response speed. After T4, the participant is given the money earned during T3.

CARROT score of reinforcement sensitivity, calculated by subtracting the mean of the number of cards sorted in T2 and T4 from the number of cards sorted in T3. CARROT = T3 - ((T2+T4)/2).

TCIP [31]

A forced-choice, reinforcement-directed computerized task, modeled on delay discounting and delay of gratification tasks. Participants press a mouse button to select one of two shapes (a square and a circle), each associated with either a short delay (in this case, 5 seconds) followed by a small reinforcer (in this case, 5 points) or a longer delay (15 seconds) followed by a larger reinforcer (15 points). For this experiment, the parameters were set to include 10 training trials followed by 40 session trials using the "Reward Feedback" option. Pairing of shapes with immediate/delayed conditions was counterbalanced within each experimental induction group. Reinforcement contingencies were not made explicit, with participants implicitly learning the relationship between the number of reward points displayed on the screen and each preceding geometric shape choice.

1. Proportion of more immediate reinforcer choices (higher = more impulsive)

2. Reaction times when making these more immediate reinforcer choices (faster = more impulsive).

GoStop [31]

Like other stop response inhibition procedures, participants are required to attend to a series of visual stimuli, respond when a target "go" signal appears, and withhold responding when a "stop" signal or non-target stimulus appears. In the GoStop, the stimuli are a series of five-digit numbers presented in black font one at a time on the screen. The "go" signal is a number that matches the previous number identically and is also presented in black. The "stop" signal is a matching number that changes color from black to red font some time after the stimulus onset. In addition to No-Stop (only the "go" signal) and Stop trials, at least half of the trials are Novel trials, with randomly generated non-matching numbers presented in black. For this experiment, the parameters were the default option of two blocks, seven stop trials (default is 10), 28 non-stop trials (default is 40), and 56 novel trials (minimum of one Novel stimulus following every Stop and No-Stop Trial). Stop Interval settings (ms from stimulus onset, SOA) were set as default (four intervals of 50 ms, 150 ms, 250 ms, and 350 ms, quasi-randomized throughout the session). Stimuli were presented for 500 ms each followed by 600 ms blackout between stimuli presentations.

1. Percent inhibited responses (proportion of Stop trials where no response occurs) (lower = more impulsive).

2. Stop Latency (time in ms between the Stop Signal onset and response) (quicker = more impulsive).