Abstract
Expressive language is complex and involves processing within a distributed network of cortical regions. Functional MRI and magnetoencephalography (MEG) have identified brain areas critical for expressive language, but how these regions communicate across the network remains poorly understood. It is thought that synchronization of oscillations between neural populations, particularly at a gamma rate (>30 Hz), underlies functional integration within cortical networks. Modulation of gamma rhythms by theta-band oscillations (4–8 Hz) has been proposed as a mechanism for the integration of local cell coalitions into large-scale networks underlying cognition and perception. The present study tested the hypothesis that these oscillatory mechanisms of functional integration were present within the expressive language network. We recorded MEG while subjects performed a covert verb generation task. We localized activated cortical regions using beamformer analysis, calculated inter-regional phase locking between activated areas, and measured modulation of inter-regional gamma synchronization by theta phase. The results show task-dependent gamma-band synchronization among regions activated during the performance of the verb generation task, and we provide evidence that these transient and periodic instances of high-frequency connectivity were modulated by the phase of cortical theta oscillations. These findings suggest that oscillatory synchronization and cross-frequency interactions are mechanisms for functional integration among distributed brain areas supporting expressive language processing.
Highlights
Expressive language refers to the cognitive processes involved in language production and transmission
GAMMA-BAND SYNCHRONIZATION AMONG ACTIVATED REGIONS Task-dependent increases in gamma-band synchronization were observed between multiple source pairs within the activated expressive language network
Beamformer analysis of focal changes in power during covert expressive language processing localized a distributed network of cortical areas congruent with neural regions known to be involved in expressive language (Salmelin, 2007; Price, 2010), providing seed locations for the synchronization analysis based on regional activity changes within the present data
Summary
Expressive language refers to the cognitive processes involved in language production and transmission. Experimental manipulations of expressive language usually require subjects to name an object, generate a verb, or read a word These are simple tasks, they still require the coordination of a number of cognitive processes such as word selection, word retrieval from memory, syntactic and phonological encoding, articulation, and auditory feedback (Levelt et al, 1999). This seemingly simple phenomenon is in reality very complex and involves multiple specialized cortical regions. While classical models of language (e.g., Geschwind, 1970) have linked distinct brain areas with specific functions, current research models (e.g., Hickok and Poeppel, 2004; Indefrey and Levelt, 2004; Golfinopoulos et al, 2010) have shifted focus away from ones that segregate language function to ones that explain how disparate neural areas are integrated to produce function
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