Dreher et al. 10.1073/pnas.0605569104. |
Fig. 7. Representative mean spatially normalized, unsmoothed echo-planar images from 20 scanning sessions included in this study.
SI Methods
Experimental Paradigm.
Stimuli representing slot machines were projected on a screen in front of the subjects. Experimental trials were divided into two phases: reward anticipation and outcome. During reward anticipation, a slot machine was presented on the screen and the words: "Chance to win $XX" (where XX stands for $0, $10, and $20) remained visible on top of each slot machine with a pie chart indicating in red the probability of winning the indicated amount of money and in white the probability of receiving nothing.There were four slot machines (A, B, C, and D) designed to vary reward probability, magnitude, and expected reward value (reward probability ´ magnitude): slot A, P = 1/4 for $20 and P = 3/4 for $0; slot B, P = 1/2 for $20 and P = 1/2 for $0; slot C, P = 1/2 for $10 and P = 1/2 for $0; slot D, P = 1 for $0 (sure to get no reward). During the delay phase, each of three spinners of the slot machine rotated successively before stopping
on a fixed image that was shown until the end of the trial. The delay duration was fixed (15 s). In the outcome phase, "$0," or pictures of $10 and $20 bills were projected for 2 s, the latter two surrounded, respectively, by a small and a large stack of gold pieces to produce visual experience of distinct reward magnitudes and reinforce the pleasantness of winning money. To equalize visual similarity between stimuli, the "$0" outcome was presented in a gray rectangle having the same dimensions as the bills. The intertrial interval between slot machines varied between 4 s and 16.5 s with a geometric distribution of mean = 6.8 s.Subjects indicated which slot machine was presented by pressing a specific response button on a diamond-shaped four-response button device at the time of slot presentation and again at the time of the outcome (regardless of winning or not). The association between each slot machine and a specific response button was learned during a training session before scanning. These motor responses ensured that subjects were attending to the specific types of slot machines as well as their outcomes and enabled us to keep the motor component equal between slot presentation and outcome. Importantly, the stimuli presentations were not contingent on the subject's response. There were a total of six runs, each consisting of 16 trials (four trials for each type of slot machine). Each of the four possible slot machines occurred pseudorandomly during each run. To prevent fluctuations between runs, the probability of each potential outcome was exact and reached at the end of each run for each slot machine. In addition, the sequential order presentation of each slot machine was controlled by equalizing the probability transition for each pair of slot machines. The order of the runs was randomized between subjects.
fMRI Data Acquisition.
Signal dropout in the orbitofrontal cortex due to susceptibility artifact was reduced by using a local high-order z-shimming performed in the axial direction with an oval-shape region that included the orbitofrontal cortex and the basal ganglia. In addition, we tilted the head of the subject 30° angle relative to the AC-PC line because this simple head positioning procedure improves the shim in this area (1). Indeed, a region of high field distortion is located above the nasal cavity and a region of low field is located behind the nasal cavity. Because these field effects are dependent on the direction of the main magnetic field relative to the head, it is possible to direct the distortions away from the inferior frontal lobe by using this simple positioning method for acquiring para-axial slices (1). The head position was obtained by slightly increasing the padding on the base of the neck and reducing padding for the back of the head using an air pressure-inflatable pillow. The angle was obtained by using a protractor to measure the angle between the vertical and the AC-PC line and was then confirmed with the sagittal localizer.1. Heberlein K, Hu X, International Society of Magnetic Resonance in Medicine Ninth Annual Meeting, Glasgow, 2001, p 1157.