What inhibits centralization of unstressed full vowels along the F1 dimension in American English?

The current study reports that lip rounding inhibits centralization of “unstressed” full vowels (e.g., [æ] in activity, [ɔ:] in audition) along the F1 dimension in American English. In the previous ASA meeting, I reported that unstressed [æ] and [ɛ] underwent centralization along the F1 dimension, whereas unstressed [ɔ:] and [oʊ] did not. The former are [lax, unrounded, front], while the latter are [tense, rounded, back]. It remained unclear whether tenseness, roundedness, or backness was responsible for inhibiting the F1-related centralization of the latter. In the present study, six native speakers of American English produced words containing full vowels ([iː, ɪ, eɪ, ɛ, æ, ʌ, ɑː, ɔː, oʊ, uː]) in initial syllables with varying stress levels (P: primary, S: secondary, U: unstressed). F1 values at the vowel midpoint were measured, and absolute differences were calculated between P and U (P–U), and between S and U (S–U). Multiple linear mixed models were fit separately to P–U and S–U as dependent variables, with each model including one of the following as a fixed effect: tenseness, lip rounding, or backness. Model comparisons revealed that, for both P–U and S–U, the model with lip rounding as the fixed effect provided the best fit.