Neurophysiological correlates of memory for experienced and imagined events

Changes in slow cortical potentials within EEG were monitored while autobiographical memories of experienced and imagined event were generated and then held in mind for a short period. The generation of both kinds of memory led to significantly larger negative dc shifts over left versus right frontal regions, and this was interpreted as a reflection of substantial left frontal activation. The generation phase was also associated with greater right versus left negative dc shifts over posterior occipital regions. This pattern replicates and extends previous findings from our laboratory. In addition, however, experienced memories were associated with significantly larger negative dc shifts over occipito-temporal regions than imagined events. Furthermore, during the hold-in-mind period, imagined events led to larger negative dc shifts over left frontal regions than experienced events. These findings suggest that memories for imagined and experienced events may share control processes that mediate construction of memories but that they differ in the types of content of the memories: memories of experienced events contain sensory-perceptual episodic knowledge stored in occipital networks whereas memories for imagined events contain generic imagery generated from frontal networks.