Neural correlates of semantic and phonemic variants of verbal fluency tasks: A combined MEG and fMRI study.

Abstract

Objective: The neural correlates of verbal fluency tasks (VFT) have been characterized by functional magnetic resonance imaging (fMRI). Still, the spatio-spectral neural oscillatory dynamics elicited by VFT and the differences between their semantic and phonologic variants are unsettled. We investigate, using fMRI and magnetoencephalography (MEG), the neural correlates of VFT and the differences in neural oscillatory dynamics between phonological (PFT) and semantic (SFT) fluency tasks.

Methods: Thirty right-handed healthy adults underwent MEG and fMRI recordings while performing covert PFT and SFT.

Results: fMRI showed different neural networks for PFT (left-dominant lexical-semantic control network) and SFT (nodes of the left-dominant semantic network). MEG showed beta-band power suppression in the left operculum in both VFT, with no difference between PFT and SFT.

Conclusions: MEG and fMRI detect distinct task-induced neural activity changes during VFT. MEG findings likely reflect the neural consequences of covert word production initiated at the inferior/middle frontal gyri, as identified by fMRI.

Significance: This study demonstrates the added value of combining MEG and fMRI to fully characterize VFT network dynamics. It paves the way for the use of VFT for non-invasive presurgical language mapping using a method free of neurovascular uncoupling.