2.3. fMRI threat task

The fMRI threat task and laboratory procedures described here have been used previously by our group (Gorka et al., 2020; Radoman et al., 2021). During the task, participants were administered brief, mild electrical shocks to their left foot at a level that they described as ‘highly annoying but not painful’ (between 1 and 5 mA). The electrodes were placed on the left foot to minimize movement during the task and to limit potential scan artifacts. This level of shock was reached via a work-up procedure before the start of the actual task. Ideographic shock levels were used to ensure equality in perceived shock aversiveness (Rollman and Harris, 1987). The shock stimuli lasted 400 ms and were delivered using a Biopac MP150 with an STM100C module (Biopac Systems, Inc., Goleta, CA) connected to a 200 V maximum stimulus isolation unit (STMISOC, Biopac System, Inc., Goleta, CA). Task stimuli were administered using Presentation software package (Neurobehavioral Systems, Inc., Albany, CA).

To examine the neural correlates of temporally unpredictable threat, we used a modified version of the original NPU-threat task developed by Grillon and colleagues (Schmitz and Grillon, 2012). Participants experienced three, within-subject conditions: no shock (N), predictable shock (P) and unpredictable shock (U). A numeric countdown was displayed during each condition ranging between 3 and 8 s, jittered (M = 5 s). Text displayed at the bottom of the monitor indicated the current condition. During N trials, no shocks were delivered and the text read ‘No Shock’. During P trials, participants received a shock only when the countdown reached ‘1′ and the text read ‘Shock at 1′. During U trials, participants received a shock at random, regardless of the countdown and the text read ‘Shock at Anytime’. Following each countdown, individuals saw a fixation cross for 5–7 s, jittered (M = 6 s). N, P and U countdowns were presented in blocks of 6, and each condition/block was administered in a randomized order (counterbalanced) 6 times over the course of two runs. Participants received 10 electric shocks during P and 10 electric shocks during U, during each run. The rate of ‘Shock at 1′ during the P condition was 60%, consistent with the NPU version used by Grillon and colleagues (Schmitz and Grillon, 2012).