One functional aspect of theta oscillations that recent studies have begun to explore in humans is their role for the recollection of personal events. A specific role of theta for recollection but not for stimulus-familiarity would support links between cortical theta oscillations and hippocampal functioning, given that the hippocampus seems to be more critical for recollection than for stimulus-familiarity. During recollection, theta oscillations might mediate a dynamic link between hippocampal and neocortical areas, thereby allowing to recruit and bind distributed cortical representations. We recorded theta oscillations using whole-head magnetoencephalography while nine healthy subjects made recognition memory judgments on previously studied and unstudied pictures of faces. For each recognized face, subjects indicated whether they also recollected the background image in front of which that face was studied. Theta oscillations were higher in amplitude during recollection than during recognition in the absence of accurate memory for the background. These theta oscillations were induced in nature, meaning that they showed considerable phase variability from trial to trial. To nevertheless extract the field distribution of coherent theta oscillations from single trials, we calculated phase differences between sensor pairs at each time point of each single trial. This field information was used to localize the brain sources of synchronized theta-generators. The results suggest that recollection is associated with induced activity increase in a distributed synchronous theta network, including prefrontal, mediotemporal, and visual areas. These findings are compatible with the notion that theta oscillations are related to the binding of distributed cortical representations during recollection.
Copyright 2005 Wiley-Liss, Inc.