Source-retrieval requirements influence late ERP and EEG memory effects

The present study examined whether event-related potential (ERP) memory effects and measures of ongoing EEG activity (power and phase locking) are sensitive to varying source retrieval requirements in recognition memory. ERP old/new effects were obtained in two distinct source-memory tasks. Functionally related EEG power and phase locking effects were found in the delta and theta frequency range. A late posterior negativity (LPN) was larger for old than new responses irrespective of source accuracy. It was also larger when participants were required to judge how they had previously interacted with a recognized picture as compared to judging its study location. This result is consistent with the view that the LPN reflects processes in the service of reconstructing previous episodes by integrating recognized items with task-relevant contextual attributes, and that LPN amplitude is related to the amount of contextual features available for forming such an integrated representation. Phase locking of ongoing delta and theta activity (but not EEG power) was functionally equivalent to LPN amplitude modulations, suggesting that stimulus-induced concentration of delta and theta phases without stimulus-induced power changes may be the neural mechanism of LPN generation. In addition, sustained enhancements of phase-locking precision in the theta range were observed for erroneous and delayed source judgments, suggesting that theta-phase locking is related to the coordination of multiple cortical assemblies in highly demanding task situations. (C) 2007 Elsevier B.V. All rights reserved.