Event-related potentials time-locked to the onset of the Sternberg probe item, as a function of Task (single, dual), Electrode (six midline electrodes), Probe (old, new), and Group (controls, PTSD). The ERP old/new effect – the positive shift for previously presented (old) probes that are correctly recognized, relative to new probes that are correctly rejected – was observed beginning at 300 ms for both groups in the single-task condition, but only for the controls in the dual-task condition.