Determining the functional neuroanatomy of speech perception and reading.

Abstract

Brain imaging studies are advancing our knowledge of the anatomy of language. In this investigation, the spatiotemporal nature of phonological and semantic processing was explored along with the influence of working memory (WM) on these functions. Semantic comprehension was the common theme across three experiments, each of which involved either speech perception or reading. Neural activity was imaged using event-related brain potentials (ERPs) and functional magnetic resonance imaging (fMRI). The first experiment examined ERP correlates of phonology and semantics in a visual to auditory semantic priming task. The results supported the role of phonological processing in the formulation of lexical candidates; a process that likely utilized WM to provide 'bottom-up' input for semantic analysis. Spatial analyses confirmed that the phonological mismatch negativity (PMN) and semantic N400 response had specific activation patterns, with intracranial generators localized to different regions of the left perisylvian cortex. The second experiment examined WM influences on the N400 in a visual semantic priming task in which WM load was also varied. Increased WM load delayed the response latency (and reduced the relative amplitude). Intracranial generators were localized to left perisylvian language areas as well as memory regions in the bilateral anterior temporal lobes and left inferior parietal lobe. The effects of increased WM load were observed within these memory regions (particularly the latter). The third experiment explored fMRI correlates of the N400 in a reading task that used contextually constrained sentences. While a number of foci corresponded with left perisylvian N400 generators, the activation patterns varied across individuals. The findings demonstrated that hemodynamic correlates of the N400 were detectable in fMRI, but are likely at or near threshold levels. Overall, the investigation provided convergent evidence across stimulus and imaging modalities for a distributed neural architecture; one that supports cognitive processes essential to semantic comprehension.

Thesis (Ph.D.)--Dalhousie University (Canada), 2002.