Event-Related fMRI of Facial and Lexical Stimuli

An event-related fMRI paradigm was employed. Participants read the task instructions before entering the scanner. Once inside, a two-button response pad was fixed under their right hand. They were asked to keep their gaze fixed on the center of the screen throughout the experiment. Each trial started with a white fixation cross on a black background followed by a stimulus (face or word). Each stimulus was presented within a thin white frame whose left or right side was of a darker (gray) shade and remained on the screen for 2 s (see Figure 1). Participants had to push the left button with their index finger or the right button with their middle finger if the gray sidebar appeared to the left or to the right of the stimulus, respectively. The inter-stimulus interval ranged pseudo-randomly between 2 and 18.8 s. Each participant performed four functional imaging runs; each run consisted of 72 trials, of which 36 faces (12 happy, 12 angry, 12 neutral) and 36 words (12 positive, 12 negative, 12 neutral). The AFNI (Cox, 1996 (link); Cox and Hyde, 1997 (link)) make_random_timing.py function¹ was used to simulate a series of randomized timing sequences for the trials of each stimulus category, from which the sequence with the best statistical power for the effects of interest was then identified with the 3dDeconvolve program.² Within each category, stimuli were presented in a pseudo-random order, with the constraint that no more than three consecutive stimuli belonging to the same class could occur. Two passive rest blocks were included at the beginning and at the end of each session (range 22.1–24.3 s). Each functional run lasted 8 min and the MRI session included 4 of them.
Participants performed a few practice trials inside the scanner before the experiment started. E-Prime 3.0 software (Psychology Software Tools, Pittsburgh, PA) was used to present the stimuli via the ESys functional MRI System³ remote display, and to collect behavioral responses.
At the end of the experiment, and outside the scanner, participants were asked to rate all the experimental stimuli for their valence and arousal on the Self-Assessment Manikin scale (SAM) (Bradley and Lang, 1994 (link)). The two rating questionnaires were delivered and completed on an Excel spreadsheet displayed on a tablet.