HUMAN memory involves an interaction between transient working memory
and a long-term store. We found electrophysiological evidence supporti
ng the existence of two distinct mechanisms subserving these processes
. Stimuli held in memory for less than 4 s generate large, early laten
cy P300 potentials which may index activation of a frontally-mediated
rapid working memory system. Stimuli held in memory for over 4 s selec
tively generate N400 potentials which may reflect activation of mesial
temporal cortices involved in access to the long-term store. These re
sults suggest that memory processing in the initial 10 s after stimulu
s detection involves at least two distinct distributed cortical-limbic
systems.