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Abstract
Phonological processing refers to the ability to derive the sounds of words from their written representations. It is a critical skill required by both beginning and experienced readers, and difficulties in phonological processing are strongly associated with reading impairment. Thus in order to gain insight into developmental and acquired reading disorders it is important to build a comprehensive understanding of phonological processing in non-impaired readers. Though phonological processing is considered to be composed of input and output phonological systems, previous functional neuroimaging studies of this cognitive ability have not specifically addressed its two-component nature. Using fMRI, we sought to develop a more comprehensive knowledge of the neurobiological basis of phonological processing by modulating demands on input and output phonology using differences in stimulus lexicality (pseudowords versus words) and response modality (aloud versus silent), respectively. However, the sensitivity of fMRI to subject motion and the loud acoustic noise of the gradients have limited its use in studies involving overt speech. We describe a method of fMRI data acquisition, known as the behavior interleaved gradients (BIG) technique, which circumvents these limitations by temporally interleaving periods of task performance with periods of image acquisition. A comparison of BIG with a conventional method of continuous data acquisition yielded fewer motion-related artifacts and revealed signal increases in several reading-related areas, including the left inferior frontal gyrus and left superior temporal gyrus, with BIG relative to the conventional method. With a confirmation of its validity as a neuroimaging technique BIG was used to acquire data for the above-described investigation of phonological processing. Here our primary finding was the discovery of the involvement of left posterior fusiform cortex in phonological processing. This region, located in left posterior fusiform cortex at Brodmann's Area 19, was uniquely modulated by varying phonological processing demands. The results suggest that in mature readers word forms are subject to phonological processing at the point they are first recognized as alphabetic stimuli. This dissertation demonstrates the utility of fMRI for tasks involving overt speech, and illustrates the importance of considering diverse ways of asking questions about and seeking answers to questions of cognitive functions using functional neuroimaging.
