Rhinal–hippocampal theta coherence during declarative memory formation: interaction with gamma synchronization?

Abstract

The hippocampus and the rhinal cortex, two substructures of the medial temporal lobe, together play a crucial role in human declarative memory formation. To investigate in detail the mechanism connecting these two structures transiently during memory formation we recorded depth EEG in epilepsy patients from within the hippocampus and the rhinal cortex. During this recording, patients performed a single-trial word list-learning paradigm with a free recall memory test following a distraction task. Rhinal-hippocampal EEG coherence and spectral power at both locations in the time interval up to 2 s after onset of word presentation were analysed in the frequency range 1-19 Hz. Successful as opposed to unsuccessful memory formation was associated with a general rhinal-hippocampal coherence enhancement, but without alterations in spectral power. Coherence increases in the theta range were correlated with the previously reported memory-related changes in rhinal-hippocampal gamma phase synchronization. This correlation may suggest an interaction of the two mechanisms during declarative memory formation. While theta coherence might be associated with slowly modulated coupling related to an encoding state, rhinal-hippocampal gamma synchronization may be more closely related to actual memory processes by enabling fast coupling and decoupling of the two structures.

Human memory strength is predicted by theta-frequency phase-locking of single neurons

“…Here, we show that spike-timing accuracy is predictive of encoding success in the absence of such power differences. Between-area synchronization can similarly predict memory success in the absence of any power differences 7 . However, these observations do not rule out the possibility that oscillatory power is different in areas upstream of the MTL neurons we recorded from (thus influencing the timing of propagated spikes).…”

Novel insights into false recollection: A model of déjà vécu

“…Klimesch, 1999: Fell, Klaver, Elfadil, Schaller, Elger, & Fernandez, 2003Sederberg, Kahana, Howard, Donner & Madsen, 2003;Jacobs, Hwang, Curran & Kahana 2006;Guderian & Duzel, 2005;Osipova et al, 2006). Moreover, theta synchronization over spatially distributed networks is associated with experiential qualities of consciously remembering (Klimesch et al, 2001;Barbeau et al, 2005), and with retrieval that incorporates correct source judgements (Guderian & Duzel, 2005).…”

Cortical EEG correlates of successful memory encoding: Implications for lifespan comparisons

“…Neuroimaging studies point to the importance of the medial-temporal lobes (MTL) and lateral prefrontal cortex (PFC) in encoding-related activities e.g., (Brewer et al, 1998;Canli et al, 2000;Fell et al, 2003;Ferna`ndez et al, 1999b;Kelley et al, 1998;Kirchhoff et al, 2000;Mangels et al, 2001;Otten et al, 2001Otten et al, , 2002Paller et al, 1987;Sanquist et al, 1980;Stebbins et al, 2002;Tulving et al, 1994;Wagner et al, 1999Wagner et al, , 1998aWeiss and Rappelsberger, 2000). Differential contributions of various structures in the MTL (e.g., hippocampus, parahippocampus, rhinal cortices) and the PFC (e.g., lateral, medial, ventral, dorsal PFC) to successful encoding appear to depend on the type of learning materials (e.g., verbal vs. pictorial), encoding intentionality (incidental vs. effortful), and the nature of the task (e.g., item vs. associative memory).…”