Phonological processes in speech perception

Abstract

The goal of my research was to identify the cortical bases of phonological processing during speech perception. Phonological processes transform the vocal noises that we hear into discretely perceived symbols that can convey information. Despite accumulated evidence for phonological processes mediated by posterior superior temporal regions, questions persist about the laterality of these processes in cortex and whether phonological computations can be delineated into lexical and sublexical processing. Lexical and sublexical phonological processes are demonstrated using effects related to phonological manipulations at the whole word level, versus those defined by syllabic or sub-syllabic ones. Lexical and sublexical activities are distinguished by current theories on the functional neuroanatomy of speech (see Hickok & Poeppel, 2004; 2007) and cognitive models of speech perception (see McClelland & Elman, 1986). I constructed a publicly available database to study the effects of phonotactic frequency and phonological neighborhood density manipulations on words and pseudowords, the Irvine Phonotactic Online Dictionary (Chapter 2). In Chapter 3, I conducted two short term memory experiments to establish independent facilitative effects of phonotactic frequency and density in pseudoword recall, and observed that syllable structures constrain word distributions among phonotactic frequency and density value ranges. In Chapter 4, I manipulated density and phonotactics during word recognition and observed lexical phonological processing in bilateral superior temporal regions, as in Okada and Hickok (2006), contradicting the left angular gyrus activity reported by Prabhakaran et al. (2006). We also observed that activity increased in left Inferior Frontal Gyrus for words with high phonotactic frequency compared to low, adding evidence of distinct cortical resources for sublexical phonological processes. In chapter 5, I found phonological repetition-suppression effects in bilateral Superior Temporal Sulci that resulted from short lists of words with that shared parametrically varied numbers of phonemes. Bilaterally, Superior Temporal Sulci had lower responses to word lists that contain repetitive phoneme sequences, demonstrating that both participate in speech perception. In summary, these experiments added new evidence that phonological processing occurs bilaterally in Superior Temporal Sulci, with distinct sublexical phonological processing in left Inferior Frontal Gyrus.