Early parietal response in episodic retrieval revealed with MEG

Recent neuroimaging and lesion studies have led to competing hypotheses for potential roles of the left lateral parietal lobe in episodic memory retrieval. These hypotheses may be dissociated by whether they imply a role in preretrieval or postretrieval processes. For example, one hypothesis is the left parietal cortex (particularly in more ventral subregions) forms part of an "episodic buffer" that supports the online representation of the retrieved target, a role that is, by definition, postretrieval. An alternate view maintains parietal activity (particularly in more dorsal subregions) contributes to top-down orientation of attention to retrieval search, a preretrieval role. The present investigation seeks to reveal the earliest onset of lateral parietal activity in three anatomically-defined subregions of the left lateral parietal cortex to identify any preretrieval activation. Subjects performed a pair-cued recall task while neural activity was recorded with magnetoencephalography (MEG) at millisecond temporal resolution. MEG data were then mapped to each subject's cortical surface using dynamic statistical parametric mapping (dSPM). Both dorsal and ventral regions showed retrieval-related activations beginning within ∼100 ms of the cue to retrieve and lasting up to 400 ms. We conclude that this early and transient pattern of activity in lateral parietal cortex is most consistent with a preretrieval role, possibly in directing attention to episodic memory retrieval.