Left dorsal speech stream components and their contribution to phonological processing

Abstract

Models propose an auditory-motor mapping via a left-hemispheric dorsal speech-processing stream, yet its detailed contributions to speech perception and production are unclear. Using fMRI-navigated repetitive transcranial magnetic stimulation (rTMS), we virtually lesioned left dorsal stream components in healthy human subjects and probed the consequences on speech-related facilitation of articulatory motor cortex (M1) excitability, as indexed by increases in motor-evoked potential (MEP) amplitude of a lip muscle, and on speech processing performance in phonological tests. Speech-relatedMEPfacilitationwasdisrupted byrTMSof the posterior superior temporal sulcus (pSTS), the sylvian parietotemporal region (SPT), and by double-knock-out but not individual lesioning of pars opercularis of the inferior frontal gyrus (pIFG) and the dorsal premotor cortex (dPMC),andnot byrTMSof the ventral speech-processing stream or an occipital control site.RTMSof the dorsal stream but not of the ventral stream or the occipital control site caused deficits specifically in the processing of fast transients of the acoustic speech signal. Performance of syllable and pseudoword repetition correlated with speech-relatedMEPfacilitation, and this relation was abolished with rTMSofpSTS,SPT,andpIFG.Findingsprovidedirectevidencethatauditory-motormappingin the left dorsalstreamcausesreliableandspecific speech-related MEP facilitation in left articulatory M1. The left dorsal stream targets the articulatory M1 through pSTS and SPT constituting essential posterior input regions and parallel via frontal pathways through pIFG and dPMC. Finally, engagement of the left dorsal stream is necessary for processing of fast transients in the auditory signal.