There is recent evidence that the visual concomitants, not only of the articulation of phones (consonants & vowels), but also of tones (fundamental frequency variations that signal lexical meaning in tone languages) facilitate speech perception. Analysis of speech production data from a Cantonese speaker suggests that the source of this perceptual information for tones involve rigid motion of the head rather than non-rigid face motion. A perceptual discrimination study was conducted using OPTOTRAK output in which rigid or non-rigid motion of the head could be presented independently, using two conditions: one in which words to be discriminated only differed in tone, and another in which they only differed in phone. The results suggest that non-rigid motion is the critical determinant for successful discrimination of phones, whereas both non-rigid and rigid motion are required for the discrimination of tones.
Bibliographic reference. Burnham, Denis / Reynolds, Jessica / Vignali, Guillaume / Bollwerk, Sandra / Jones, Caroline (2007): "Rigid vs non-rigid face and head motion in phone and tone perception", In INTERSPEECH-2007, 698-701.