On the interpretation of mouth asymmetry.

Greater right-side mouth opening during propositional speech has now been observed in at least eight studies. All current data are consistent with the interpretation that left-hemisphere control of articulation of propositional speech results in greater activity of the right-side lip opening muscles. In addition, expression of automatic speech such as singing, emotion, and possibly prosody can also influence mouth asymmetry, but in the opposite direction, suggesting a relatively greater right-hemisphere role for these types of expression. Further work is needed to confirm these interpretations and to elaborate on the exact neuro-muscular mechanisms and task dependencies responsible for mouth asymmetry.