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Patients with anterior limbic lesions, in particular of the posterior orbitofrontal cortex, often act on the basis of memories that do not relate to ongoing reality and justify their behavior with invented stories that can mostly be traced back to real events (spontaneous confabulation). Recent studies demonstrated that the patients fail to suppress activated memory traces that do not pertain to ongoing reality. In the present study, we used a similar paradigm and high-resolution event-related potentials to explore when this suppression happens. Healthy subjects made two runs of a continuous recognition task, composed of the same set of pictures, and were requested to indicate picture recurrences only within the ongoing run. Thus, performance in the first run depends on new learning, whereas the second run requires the ability to realize whether a picture is solely familiar from its occurrence in the previous run ('distracter') or whether it has already appeared in the ongoing second run ('target'). We find that correct rejection (suppression) of currently irrelevant pictures (distracters of run 2) is associated with absent negative deflection of a frontal potential and absence of a specific cortical potential map configuration after 220-300 ms. By contrast, learning and recognition of repeatedly presented information is associated with cortical amplitude modulation after 400-480 ms. These findings indicate that by the time the content of a mental association is recognized and consolidated, its cortical representation has already been adjusted according to whether it relates to ongoing reality or not. This sequence may also explain the ability to distinguish between the memory of a true event and the memory of a thought.

Recent studies indicated that the human medial temporal lobe (MTL) may not only be important for long-term memory consolidation but also for certain forms of short-term memory. In this study, we explored the interplay between short- and long-term memory using high-density event-related potentials. We found that pictures immediately repeated after an unfilled interval were better recognized than pictures repeated after intervening items. After 30 min, however, the immediately repeated pictures were significantly less well recognized than pictures repeated after intervening items. This processing advantage at immediate repetition but disadvantage for long-term storage had an electrophysiological correlate: spatiotemporal analysis showed that immediate repetition induced a strikingly different electrocortical response after 200-300 ms, with inversed polarity, than new stimuli and delayed repetitions. Inverse solutions indicated that this difference reflected transient activity in the MTL. The findings demonstrate behavioral and electrophysiological dissociation between recognition during active maintenance and recognition after intervening items. Processing of novel information seems to immediately initiate a consolidation process, which remains vulnerable during active maintenance and increases its effectiveness during off-line processing.