Abstract
Frontal midline theta rhythm (Fmθ) appears widely distributed over medial prefrontal areas in EEG recordings, indicating focused attention. Although mental calculation is often used as an attention-demanding task, little has been reported on calculation-related activation in Fmθ experiments. In this study we used spatially filtered MEG and permutation analysis to precisely localize cortical generators of the magnetic counterpart of Fmθ, as well as other sources of oscillatory activity associated with mental calculation processing (i.e., arithmetic subtraction). Our results confirmed and extended earlier EEG/MEG studies indicating that Fmθ during mental calculation is generated in the dorsal anterior cingulate and adjacent medial prefrontal cortex. Mental subtraction was also associated with gamma event-related synchronization, as an index of activation, in right parietal regions subserving basic numerical processing and number-based spatial attention. Gamma event-related desynchronization appeared in the right lateral prefrontal cortex, likely representing a mechanism to interrupt neural activity that can interfere with the ongoing cognitive task.
Highlights
Frontal midline theta rhythm (Fmθ) is a distinct train of focal 5–7 Hz theta waves which appears over medial frontal areas in the EEG of normal subjects when performing a broad range of cognitive tasks demanding mental concentration (Ishihara and Yoshi, 1972; Iramina et al, 1996; Sasaki et al, 1996; Ishii et al, 1999)
A striking finding was the identification of significant gamma power www.frontiersin.org changes, in particular gamma event-related synchronization (ERS) in the right intraparietal sulcus (IPS) and adjacent cortex, and gamma event-related desynchronization (ERD) in the inferior frontal cortex that appeared concomitantly with Fmθ (Figure 5). This clearly shows that focusing attention on mental calculation results in Fmθ generation and in the activation of neural networks involving the parietal and lateral prefrontal cortex, likely associated with the arithmetic processing of the task, with power changes in the gamma band representing the underlying neural activity
Using MEG and SAMpermutation analysis, our results confirm and extend those of previous EEG and MEG studies indicating that Fmθ is generated in medial prefrontal cortex and dorsal anterior cingulate cortex (ACC) during cognitive tasks requiring focused attention and working memory process
Summary
Frontal midline theta rhythm (Fmθ) is a distinct train of focal 5–7 Hz theta waves which appears over medial frontal areas in the EEG of normal subjects when performing a broad range of cognitive tasks demanding mental concentration (Ishihara and Yoshi, 1972; Iramina et al, 1996; Sasaki et al, 1996; Ishii et al, 1999). Earlier studies of Fmθ using scalp EEG reported widespread distribution of this activity in midfrontal sites, but an accurate identification of its cortical generators within the medial frontal cortex was lacking (Mizuki et al, 1980; Laukka et al, 1995; Iramina et al, 1996) This is mainly due to the low spatial resolution of EEG. Despite the fact that fMRI scanner noise may affect mental concentration in some individuals when engaged in mental reasoning tasks (Pripfl et al, 2006), these fMRI studies clearly visualized Fmθ activity localized to the anteromedial frontal cortex (Gevins et al, 1997; Mizuhara et al, 2004; Sammer et al, 2007)
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