Networks for phonological and semantic processing in auditory language comprehension

Introduction: The aim of our study was to explore the cortical network structure of phonological and semantic processing during auditory language comprehension.

Methods: 33 healthy subjects (mean age 34±16 y; 22 male; 17 right-handed) performed an event-related fMRI comprehension task. Participants were asked to listen to a total of 270 aurally presented sentences of normal speech (SP), pseudo speech (PS) and reversed speech (REV). Contrasting PS vs. REV was expected to activate areas mainly responsible for phonological processing (C1) while contrasting SP vs. PS to activate areas involved in semantic processing (C2). Data were analysed in random effects analyses in SPM5. From peak voxels, time series were extracted and a newly developed directed partial correlation method (dPC) was used to explore the network structure of auditory phonological and semantic processing.

Results:

(i) C1 revealed a left temporo-frontal network with peak activations in the anterior (aSTS) and posterior (pSTS) superior temporal sulcus, the frontal operculum (FOP), the inferior frontal gyrus (BA 44) and the dorsal premotor cortex (dPM). dPC-analyses containing these peaks revealed effective connectivity between the following nodes: (a) temporo-frontal: aMTG-PMd, pMTG-FOP, pMTG-BA44, pMTG-PMd; (b) frontal: FOP-BA 44, BA 44-PMd; temporal: aMTG-pMTG.

(ii) C2 revealed a bilateral but left-lateralized temporo-frontal network with peak activations in the anterior (aMTG) and posterior (pMTG) middle temporal gyrus, the fusiform gyrus (FUS), the pars orbitalis (F3orb) and triangularis (F3tri) of the inferior frontal gyrus. dPC-analyses containing theses activation peaks demonstrated effective connectivity between the following nodes: (a) temporo-frontal: aMTG-orbIFG, pMTG-orbIFG, pMTG-triIFG; (b) temporal: aMTG-FUS, pMTG-FUS; frontal: orbIFG-triIFG.

Discussion: Using an auditory language comprehension task we were able to isolate temporo-frontal networks attributable to auditory phonological and semantic processing. Computation of effective connectivity using dPC-analyses demonstrated that (i) pSTG, BA44 and PMd are the central nodes within the phonological network while (ii) pMTG and orbIFG (BA 47) were found to be the central nodes within the semantic processing network. We suggest that this functional model of auditory language comprehension should be investigated with structural methods e.g. DTI to identify interconnecting fibre pathways within both networks.