ReviewFrontal midline theta oscillations during working memory maintenance and episodic encoding and retrieval
Introduction
Recent work in neuroscience has indicated that neural oscillations may play a fundamental role in human cognition (Fell and Axmacher, 2011, Siegel et al., 2012). In particular, theta oscillations have received considerable attention from researchers, including studies of hippocampal theta in rodents (for review, see Buzsáki, 2002) and cortical theta in humans (Kahana et al., 2001, Mitchell et al., 2008). Scalp electroencephalography (EEG) studies in humans have consistently reported prominent theta power enhancement over frontal regions during various working memory (WM) and episodic memory tasks (Klimesch, 1999), but the functional significance of these oscillations in memory processes remains unclear.
In this review, we aim to provide a concise overview of theta involvement in WM and episodic memory, with a particular emphasis on scalp-recorded theta oscillations that are prominent around Fz electrode site, also known as “frontal midline theta” (FMT).1 We begin by discussing the electrophysiological signatures of scalp-recorded FMT and its potential neural sources. Next, we summarize recent studies relating FMT modulations to working memory and episodic memory tasks. Finally, we conclude by reviewing current models regarding how FMT might contribute to memory.
Section snippets
Electrophysiological features and measures of FMT
The observation of scalp-recorded theta oscillations can be dated back to at least Arellano and Schwab (1950), but the term “frontal midline theta” was not introduced until Ishihara and Yoshi (1972). Using a set of strenuous mental tests such as continuous arithmetic calculations, Ishihara and Yoshi (1972) reported increased occurrence of EEG fluctuations centering around 6.5 Hz that was maximal over frontal sites while participants were actively engaged in the tests. Since then, several studies
Working memory and FMT
WM processes support the active maintenance of information so that it can be manipulated or quickly accessed at a later time (Baddeley, 1986, Baddeley, 2003). Several electrophysiological studies have also reported persistent FMT oscillations during WM tasks (Gevins et al., 1997, Hsieh et al., 2011, Jensen and Tesche, 2002, Meltzer et al., 2007, Meltzer et al., 2008, Raghavachari et al., 2001, Scheeringa et al., 2009). For instance, Gevins et al. (1997) used a n-back task that allowed them to
Episodic memory and FMT
Episodic memory refers to the ability to remember the details of past events (Tulving, 2002). It is well established that the hippocampus, parahippocampal cortex, and PFC play important roles in episodic memory (Eichenbaum et al., 2007, Wheeler et al., 1997), and as noted above, theta oscillations are evident in these regions. Accordingly, many studies have sought to find links between FMT and episodic memory by comparing theta power elicited on trials that were associated with successful
Functional interpretations of FMT
Thus far, we have reviewed evidence demonstrating the relationship between FMT oscillations and WM, episodic encoding and retrieval. The evidence strongly suggests that FMT oscillations are meaningfully related to cognition. The wide range of phenomena to which theta has been linked make it challenging to ascribe any simple functional interpretation. Adding further complexity to this issue is the fact that FMT oscillations are probably not exclusively related to performance on traditional
Acknowledgments
We thank the anonymous reviewers for their helpful comments and suggestions. L.-T.H. and C.R. contributed equally to this work. This work was supported by an NIMH grant awarded to C.R. (RO1MH0068721). Liang-Tien Hsieh is a Howard Hughes Medical Institute International Student Research fellow.
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