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Poster B75, Wednesday, November 8, 3:00 – 4:15 pm, Harborview and Loch Raven Ballrooms

Inferior frontal gyrus activation is modulated by phonetic competition: An fMRI study of clear and conversational speech

Xin Xie1, Emily Myers2;1University of Rochester, 2University of Connecticut

The speech signal varies naturally in phonetic ambiguity. For instance, conversational speech, is spoken with less articulatory precision than clear speech, leading to greater potential for confusability at the phonetic level (Ferguson & Kewley-Port, 2007). Current psycholinguistic models assume that ambiguous speech sounds activate more than one phonological category, and that competition at prelexical levels cascades to lexical levels of processing. Previous research (e.g., Myers et al., 2009; Poldrack et al., 2001) suggests that phonetic competition modulates activity in the left inferior frontal gyrus (LIFG), yet these studies have often used artificially manipulated speech and/or metalinguistic tasks that may tap a different set of cognitive processes than those necessary for natural language processing (Hickok & Poeppel, 2007). In the current study, we exploited the natural variation in phonetic competition (PC) in the speech signal in order to investigate neural sensitivity to PC as listeners engaged in a receptive language task. Fifteen healthy adults heard nonsense sentences spoken in either a Clear or Conversational register (all highly intelligible; pre-equated in pitch and duration) as neural activity was monitored using fMRI. Conversational sentences contained greater PC, as estimated by measures of vowel confusability (p < .001). A post-scanner probe matching test revealed longer RTs to Conversational sentences than to Clear sentences (p = .05), suggesting that higher PC led to greater perceptual difficulty. Critically, Conversational sentences elicited greater activation in a region in the LIFG, whereas the opposite pattern was observed in the temporal lobe (left STG and Heschl’s gyrus; p < 0.05, FWE corrected). Sentence-level PC metrics also uniquely correlated with LIFG activity (p < 0.05, FWE corrected), explaining variance not shared by RT or intelligibility or lexical properties of words (word frequency and neighborhood density). Conclusion: Our findings reveal a critical role of LIFG in the resolution of phonetic competition that is inherent to spoken language processing, consistent with the notion that recruitment of this region does not require an explicit phonological judgment.

Topic Area: Perception: Speech Perception and Audiovisual Integration

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