While the role of auditory saliency is well accepted as providing insight into the shaping of phonological systems, the influence of visual saliency on such systems has been neglected. This paper provides evidence for the importance of visual information in historical phonological change and synchronic variation through a series of audio-visual experiments with the /f/~/θ/ contrast. /θ / is typologically rare, an atypical target in sound change, acquired comparatively late, and synchronically variable in language inventories. Previous explanations for these patterns have focused on either the articulatory difficulty of an interdental tongue gesture or the perceptual similarity /θ / shares with labiodental fricatives. We hypothesize that the bias is due to an asymmetry in audio-visual phonetic cues and cross-talker cue variability. Support for this hypothesis comes from a speech perception study that explored the weighting of audio and visual cues for /f / and /θ / identification in CV, VC, and VCV syllabic environments in /i/, /a/, or /u/ vowel contexts in Audio, Visual, and Audio-visual experimental conditions using stimuli from ten different talkers. The results indicate that /θ / identification is more variable than /f/, both in Audio and Visual conditions. We propose that it is this variability which contributes to the unstable nature of /θ / across time and offers an improved explanation for the observed synchronic and diachronic asymmetries in its patterning.
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