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Multi-modal imaging of brain networks subserving speech comprehension
[Thesis]. Manchester, UK: The University of Manchester; 2013.
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Abstract
Neurocognitive models of speech comprehension generally outline either the spatial or temporal organisation of speech processing and rarely consider combining the two to provide a more complete model. Simultaneous EEG-fMRI recordings have the potential to link these domains, due to the complementary high spatial (fMRI) and temporal (EEG) sensitivities. Although the neural basis of speech comprehension has been investigated intensively during the past few decades there are still some important outstanding questions. For instance, there is considerable evidence from neuropsychology and other convergent sources that the anterior temporal lobe (ATL) should play an important role in accessing meaning. However, fMRI studies do not usually highlight this area, possibly because magnetic susceptibility artefacts cause severe signal loss within the ventral ATL (vATL). In this thesis EEG and fMRI were used to refine the spatial and temporal components of neurocognitive models of speech comprehension, and to attempt to provide a combined spatial and temporal model.Chapter 2 describes an EEG study that was conducted while participants listened to intelligible and unintelligible single words. A two-pass processing framework best explained the results, which showed comprehension to proceed in a somewhat hierarchical manner; however, top-down processes were involved during the early stages. These early processes were found to originate from the mid-superior temporal gyrus (STG) and inferior frontal gyrus (IFG), while the late processes were found within ATL and IFG regions.Chapter 3 compared two novel fMRI methods known to overcome signal loss within vATL: dual-echo and spin-echo fMRI. The results showed dual-echo fMRI outperformed spin-echo fMRI in vATL regions, as well as extra temporal regions.Chapter 4 harnessed the dual-echo method to investigate a speech comprehension task (sentences). Intelligibility related activation was found in bilateral STG, left vATL and left IFG. This is consistent with converging evidence implicating the vATL in semantic processing.Chapter 5 describes how simultaneous EEG-fMRI was used to investigate word comprehension. The results showed activity in superior temporal sulcus (STS), vATL and IFG. The temporal profile showed that these nodes were most active around 400 ms (specifically the anterior STS and vATL), while the vATL was consistently active across the whole epoch.Overall, these studies suggest that models of speech comprehension need to be updated to include the vATL region, as a way of accessing semantic meaning. Furthermore, the temporal evolution is best explained within a two-pass framework. The early top-down influence of vATL regions attempt to map speech-like sounds onto semantic representations. Successful mapping, and therefore comprehension, is achieved around 400 ms in the vATL and anterior STS.