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Relative encoding of speech intensity in the human temporal cortex
Poster Session D, Saturday, October 26, 10:30 am - 12:00 pm, Great Hall 3 and 4
Ilina Bhaya-Grossman1,2, Yulia Oganian3, Emily Grabowski2, Edward Chang1; 1University of California, Berkeley, 2University of California, San Francisco, 3University of Tübingen
Lexical stress, or the emphasis placed on syllables within words, critically facilitates word recognition and comprehension processes. For instance, it enables listeners to distinguish between the noun “a present” (PRE-sent) and the verb “to present” (pre-SENT). In English, lexical stress is prominently signaled by relative speech intensity, with the stressed syllable exhibiting the greatest intensity relative to other syllables in the word. Prior work has shown that the human speech cortex on the superior temporal gyrus (STG) encodes speech intensity as a series of discrete acoustic landmarks marking moments of peak intensity change (peakRate). Building on this finding, a key question arises: Is there a neural encoding of relative intensity in the STG that supports the perception of lexical stress? To address this question, we performed intracranial recording (n=9 ECoG patients) while English speaking participants performed two experiments. In Experiment 1, participants performed a forced choice task, identifying whether the first or second syllable was stressed in a set of synthesized two syllable pseudo-words (e.g. hu-ka, ma-lu). The intensity of the first syllable in each pseudo-word varied while the intensity of the second syllable was fixed, allowing us to experimentally test whether neural responses to the second syllable depended on the intensity of the first. We found that a subset of cortical sites on the human STG encoded relative intensity, that is, these sites showed activation in response to the second syllable only when its intensity was greater than that of the first syllable. Critically, we found that cortical sites that encoded relative intensity were distinct from those that encoded peakRate. Neither population encoded which syllable participants perceived as stressed when they were presented with ambiguous pseudo-words, where both syllables had identical intensity. In Experiment 2, we used a passive listening paradigm to extend our findings to a naturalistic speech stimulus. Our results indicate that relative and absolute intensity of speech is encoded in two distinct neural populations on the STG and further, that these populations do not encode stress percepts in cases where the intensity cue to lexical stress is removed. Our results reveal the multiple, distinct neural representations that work in concert give rise to lexical stress perception.
Topic Areas: Speech Perception, Computational Approaches