2.4. fMRI Behavioral Paradigm: Mnemonic Discrimination Task

Participants were given a verbal explanation of the task, and, completed pre-training with mock stimuli outside the scanner. As shown in Figure 1, the task consisted of an explicit 3-alternative forced choice task (for more details see Kirwan and Stark (Kirwan and Stark, 2007)), in which participants viewed novel (new), repeated (old), and lure (similar) stimuli. Stimuli were colored photographs of common objects. Each participant completed a single run consisting of 96 similar pairs, 96 identical pairs and 192 unrelated novel items (foils), totaling 576 trials. All trial types were fully randomized throughout the run. Each stimulus was presented for 3 seconds with a 0.5 second inter-stimulus-interval. The number of trials separating similar and identical pairs randomly varied between 10 and 40 trials. Participants were instructed to make a judgment as to whether the object seen was new (i.e., novel items), old (i.e., repeated items) or similar but not identical (i.e., lure items). Of critical interest were the participants’ responses on the lure items. A response of “old” to a lure (i.e., similar) item would constitute a failure of discrimination (possibly indicative of reduced capacity for pattern separation), whereas an accurate response of “similar” to a lure would constitute a successful discrimination (Yassa et al., 2010, 2011a). As in prior work, a Lure Discrimination Index (LDI) was calculated as p(“Similar”|Lure) – p(“Similar”|Foil)], which accounts for response bias.

An example of the mnemonic discrimination behavioral task. Each item was presented for 3 seconds with a 0.5 second inter-stimulus-interval. Novel (new), repeated (old), and lure (similar) items were fully randomized throughout the run. Examples of a repeat pair (left) and a lure pair (right) are shown.