Stimulus-driven incidental episodic retrieval involves activation of the left posterior parietal cortex

Recent reviews have highlighted the important role that the posterior parietal cortex (PPC) serves during episodic memory retrieval. A handful of studies have also noted that the PPC is active when old information is present on tasks that do not require overt episodic retrieval. Based on this observation, we examined whether incidental study-phase retrieval, cued by the repeated presence of stimuli, was sufficient to activate the PPC and whether this activation would be modulated by the lag between the initial and repeated presentation of those stimuli. Blood flow was measured with positron emission tomography (PET) while subjects classified pictures that were either new, repeated following a short lag, or repeated following a long lag. Activity in the left inferior parietal lobule (IPL, BA 40), amongst other regions, was greater for repeated than new pictures, and was greater following a long lag than a short lag, even though intentional retrieval was not required. These results suggest that the presence of repeated stimuli is sufficient to initiate left PPC mediated episodic retrieval.