Poster E76, Thursday, August 22, 2019, 3:45 – 5:30 pm, Restaurant Hall
Semantic predictability modulates cortical sensitivity to phonetic competition
Hannah Mechtenberg1, Xin Xie2, Emily Myers1;1University of Connecticut, 2University of Rochester
INTRODUCTION. A critical step in speech perception is the resolution of phonetic ambiguity from overlapping speech categories (Myers, 2007). Activation of multiple categories in tandem arises when a speech sound falls in the acoustic space between two or more categories, resulting in competition for recognition. Phonetic competition occurs in naturally produced speech (Bradlow, Torreta, & Pisoni, 1996) and can be quantified by calculating the distance in acoustic space between a given token to tokens in other phonetic categories. For instance, the vowel /i/ (as in “beet”) is rarely confused with other vowels and has low values of phonetic competition, while the vowel /ɛ/ (as in “bet”) falls in a densely-populated acoustic space occupied by many other vowel categories, and as such has higher values of phonetic competition. Contemporary neurobiological models debate the role of frontal regions in speech perception and whether these regions are necessary in naturalistic listening conditions (Hickock, 2012). Previous work by Xie and Myers (2018) found sensitivity in the left inferior frontal gyrus (LIFG) to natural variation in phonetic competition within semantically anomalous sentences. An open question is whether semantic context alters reliance on frontal regions for processing phonetic category ambiguity by providing predictive cues to lexical identity (and therefore decreasing demands on phonetic processing). The current experiment examined phonetic competition within sentences that varied in their semantic predictability (Kalikow et al., 1977; Bradlow & Alexander, 2007), as sentences with high semantic predictability are more likely to be heard in naturalistic settings compared to sentences with low semantic predictability. If phonetic competition effects disappear once listeners can rely on semantic context to anticipate upcoming words in the sentence, it suggests that frontal recruitment to resolve phonetic ambiguity is situation-dependent. METHODS. Participants passively listened to both high and low predictability sentences (normed for predictability using the Cloze procedure) during fMRI. Rare probe trials (not further analyzed) kept participants’ attention focused on the content of each sentence. The high and low predictability sets of sentences were highly intelligible and equated on pitch, duration, lexical frequency, and sentence-wide phonetic competition. RESULTS. Preliminary results indicate that phonetic competition modulates activity in the LIFG regardless of sentence predictability, partially replicating Xie and Myers (2018). Effects of phonetic competition differed by sentence predictability in the left posterior temporal/inferior parietal junction, with a greater effect of phonetic competition in high predictability sentences compared to low predictability sentences. CONCLUSION. These data replicate findings by Xie and Myers (2018), as similar neural regions show sensitivity to phonetic competition across investigations. Activity in the left posterior/inferior parietal junction implies that the relatively “easy” perception of anticipated words within highly predictable sentences may boost the phonetic competition processing signal compared to the higher semantic ambiguity of low predictability sentences. Taken together, these results suggest a neural system that flexibly recruits phonetic processing resources according to the predictability of the sentence context.
Themes: Speech Perception, Meaning: Lexical Semantics
Method: Functional Imaging