Figure 1. Experimental Design.

(A) Color pictures of objects were presented in groups of four constituting an object quartet. Objects were presented consecutively for 1 s each, with a 200-ms gap between objects, over a background scene that was displayed continuously throughout the presentation of each object quartet (first presentation: FIRST). Subjects were pre-exposed to the set of background scenes used, prior to the scanning experiment. There followed a 2-s period during which a fixation cross was displayed.

(B) Following this 2-s period, the same object quartet was re-presented in one of three sequence types (S_rep, S_half, or S_new) on either the same or different background scene (C_same [as illustrated in (B)] or C_diff) (second presentation; a 3 × 2 factorial design). The sequential order of objects during the second presentation was A-B-C-D (S_rep), A-B-D-C (S_half), and either C-A-D-B (as illustrated) or B-D-A-C (S_new) (with the first presentation of the object quartet denoted by A-B-C-D). Importantly, each object was trial-unique and only presented twice during the experiment (i.e., in first and second presentations). Subjects performed an incidental 1-back target detection task throughout the experiment (see Materials and Methods). Note that the stimuli shown here are visually similar (but not identical, for copyright reasons) to those used in the experiment.