Frontal-midline theta from the perspective of hippocampal "theta"

Electrical recordings from the surface of the skull have a wide range of rhythmic components. A major task of analysis of this EEG is to determine their source and functional significance. The hippocampal "theta rhythm" has been extensively studied in rats and its rhythmicity has recently been shown to be functionally significant, per se. Here, we use relevant aspects of the hippocampal literature to provide perspective on one of the most studied human EEG rhythms: frontal-midline theta. We review its electrographic features, localization, prevalence, age distribution, behavioural modulation (particularly in relation to working memory, spatial navigation, episodic memory, internalised attention and meditation), relationship to personality, drug interactions, neurochemical relationships, and coherence with rhythmic activity at other sites. We conclude that FM-theta, like hippocampal theta, appears to play a role in (or at least occur during) processing of memory and emotion. It is correlated with working memory and/or sustained attention; but this does not entail a role in function since clear behavioural correlates of hippocampal theta have been demonstrated that are not sensitive to hippocampal damage. FM-theta is increased by anxiolytic drug action and personality-related reductions in anxiety, whereas hippocampal theta is decreased by anxiolytic drugs. In animals, frontal theta and hippocampal theta can be phase-locked or independent, depending on behavioural state. So, the cognitive functions of FM-theta, and their relationship to hippocampal theta, are unclear and definitive evidence for functional involvement in cognitive or emotional processing is lacking. One possible solution to this problem is analysis of FM-theta in animals-provided homology can be determined. The issues of sporadicity and low incidence of FM-theta also need to be addressed in the future. Changes in functional connectivity, indicated by changes in coherence, are also a largely untapped resource. We suggest that the most hopeful path to assessing the functions of FM-theta will be through the use of drugs, and the variation of their effects depending on baseline levels of FM-theta. Finally, we review some theories of theta function. Despite the apparent richness of the current data, we conclude that it is difficult (and may ultimately be impossible) to formulate a theory that attributes a specific cognitive function to FM-theta. However, the theories share some general computational assumptions and these should be a useful guide to future work and, ultimately, a definite theory of the function or functions of FM-theta.